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1.15 A/R T4 Divided vs 0.82 A/R V-band- 6466 ApexTurbo Dyno Testing
Here is the first round of back-to-back dyno comparisons, on McKenzie Lange’s AWD ’90 Eagle Talon. 2.0LR, 10.5:1, wet block, GSC S3 camshafts on pump e85, with dedicated v-band and T4 divided turbo manifolds. Each setup purpose built to share the same components- same turbo, same turbo placement, wastegates (two, not necessary with the v-band, but keeps boost control more consistent between setups), 3.5” downpipe/full exhaust, same runner size- even the same primary lengths. All done on the same day with the test car never unstrapped from the Dynojet chassis dyno. Runs were done in 3rd gear with similar stabilized fluid and hot parts temps, and each run starting from a steady state/no load 3000rpm.
The turbo is an Apex 6466 with either the1.15 A/R T4 divided or .82 A/R v-band turbine housing, tested across three boost levels, and matched within ~1psi to each other using a manual boost controller (air regulator to the top wastegate port to effectively increase/augment the WG spring rating).
DSPORT’s 2016 twin scroll vs single scroll test, “The Great Divide,” was a fantastic example of what to expect from Garrett divided/non-divided v-band turbine housings when paired to the same divided v-band turbo manifold. Today, most turbine housing options fall between v-band or T4 divided, using completely different flanges and manifolds. It’s not uncommon to hear that a non-divided v-band option will out-spool their older 4 bolt flanged counterparts like T3 and specifically T4 housings- even T4 divided. There are instances where that’s true, but we’d argue quicker spool/response would come from incremental, secondary improvements. Improvements perhaps stemming from a more recent redesign, more ideal casting material such as stainless steel with its lower thermal conductivity, smoother investment cast internal volutes vs rougher sand cast housings, potentially tighter tolerances and blade clearance, a round flange that matches your round crossover, or just tending to be paired with newer aero/tech turbo options- skewing the perception. The turbo manifold design can play a surpringly large role as well, as we’ve seen through various tests over the years.
There are certainly some things we like to take advantage of with v-band turbine housings, but when it comes to spool- does the turbo care about what fastens the flanges together? Does it care more about how many volutes/scrolls there are? What are some considerations you have when choosing which turbine housing?
You may notice the A/R are quite different, with the divided T4 A/R being ~30% larger than the 0.82 A/R non-divided v-band. It’s tempting, and seems logical to use the same A/R in a back-to-back test like this to eliminate a variable, but arguably the divided turbine housing requires it due to how the individual exhaust events are split up to function.
Side-by-side and cross section of a non-divided and divided housing (not the exact housings used)
This is likely due to a few reasons:
-The total volute area of the divided housing is larger at 1.15 A/R, but each cylinder/exhaust pulse only sees half of that volute/nozzle area at a time- and only half of the total cylinders travelling through it, with no overlapping exhaust events. In the non-divided example, each cylinder sees the entire 0.82 A/R volute/nozzle of the v-band housing, shared by all cylinders and their exhaust events.
-The divider wall in the divided turbine housing will also come with a little more frictional losses, which can add to the exhaust manifold pressure (EMAP) by itself.
-If the A/R were the same 0.82 A/R between both housings, the divided would almost certainly spool sooner, but due to the above, be much more likely to choke/nose over in power due to higher EMAP at higher power/rpm levels. A 0.8x A/R in a divided housing can yield great results, and often paired with smaller turbos, but would arguably be undersized for this comparison. Conversely, if both divided and non-divided were 1.15 A/R, the non-divided v-band housing would likely be oversized for this combination, losing appreciable spool rpm and at the point of diminishing returns where it’s unlikely to pay off elsewhere with this turbo size/combination.
So they are different, but arguably for good reason, and why many turbine housing offerings share this same trend. Including the options available for this turbo. Comparing the exhaust pressure readings later, they seem pretty well matched.
There are plenty of other factors in deciding A/R in general, and apparently some manufacturers who already upsize to compensate (potentially Garrett’s divided v-band in the 2016 DSPORT test?), but we’ve heard/found ~25-40% larger than a comparable non-divided A/R turbine housing is a good rule of thumb. Especially if you already have a good reference A/R to begin with, since A/R is more of a balancing act with diminishing returns if you stray too far away.
In general, increasingly larger turbos tend to use increasingly larger A/R’s, and larger displacement engines tend to use a larger A/R. The ‘formula’ starts to fall apart when comparing different manufacturers and turbine wheels/designs, etc.
In anticipation of seeing what happens on the dyno, we set a benefit/loss benchmark to help avoid bias in the interpretation of the results. A rough estimate, but every ~100whp (or ~10lb/min) of turbo flow potential gained tends to lose ~300-500rpm worth of spool on a 2.0L 4G63. Let’s call it 400rpm. So something like a 400whp capable turbo setup that comes in at 4200rpm or a now 600whp capable turbo setup that comes in closer to 5000rpm, for example. Or a turbine housing A/R change that lands somewhere in between that give and take for potential power and spool.
So, if the more costly/complex divided configuration didn’t exceed that benefit/loss ratio by a good margin, ~25whp for every 100rpm, there wouldn’t be much to leverage over a simple turbo or A/R change to do the same thing.
The results-
Horsepower curves, blue 1.15 A/R T4 divided, red 0.82 A/R V-band:
Torque curves:
All runs matched within ~1psi or less between each setup at full boost. First three are horsepower at the wheels, bottom three are torque at the wheels, each showing the ~35psi, ~45psi, ~50psi boost comparisons and their respective boost curves.
Boost in the bottom portion to show any slight variances that could influence the power/torque comparisons. When both were at full boost, the v-band setup sometimes had a 0.5psi boost advantage after 7000rpm, and as high as 1psi at the end of the highest boost run, tending to hold boost a little better with the manual boost controller used. An identical boost curve at every rpm point would be nice, and could have been dialed in with electronic boost control (and kept more flat/consistent), but the slight boost curve variances with rpm added another layer to each setup’s characteristics.
Wideband readings are also added for reference, with each sharing the same AFR target (richer with higher boost levels), and much of the “difference” due to the divided coming into boost sooner (and commanding a richer AFR)
Overview:
The divided setup consistently came into boost ~380rpm sooner, reaching 30psi by ~4830rpm vs the v-band by ~5220rpm, even with the divided’s larger 1.15 A/R vs the 0.82 A/R of the v-band housing. At full boost, the 0.82A/R v-band setup saw 1-2% more power in certain spots when overlayed, but both closely matched overall in dyno power and torque numbers. Ranging from 3-15whp and 5-14wtq, tending to show more variance as boost levels increased. The power curve shapes at full boost were almost identical. with neither setup nosing over, even during later runs to 9500rpm.
Zooming in on boost curves and looking at EMAP:
During the highest boost setting tested:
At 9000rpm the 0.82 A/R v-band was at a 1.15:1 ratio of exhaust pressure to boost pressure.
At 9000rpm the 1.15 A/R divided was at a 1.20:1 ratio of exhaust to boost pressure.
Also referenced as exhaust pressure ratio, drive pressure ratio, engine pressure ratio, EMAP/IMAP, EMAP/MAP, etc. Lower exhaust pressure at the same boost, resulting in a lower ratio, is generally a good sign when it comes to turbos and the setup in general.
Blue divided, red v-band, solid boost, dashed exhaust pressure. The zoomed in boost curves better illustrate the slight variances between the two configurations. The EMAP sensor signal dropped out in some runs, but other similar boost runs could be referenced for the ratios earlier.
EMAP overview:
Before 7000rpm the divided had lower exhaust pressure (EMAP in psig) for the same boost, at ~7200rpm both were nearly the same, and by 9000rpm the divided was a few psi higher EMAP than the v-band. Since EMAP influences boost control through the wastegate valve, the v-bands lower EMAP lines up with it holding boost slightly better up there.
EMAP was still impressively low overall for the turbo size and power levels. There are many factors, but much of that credit goes to the turbo itself- requiring less average exhaust pressure to maintain the boost pressure and airflow throughout most of the testing. A 1:1 ratio wasn’t seen until closer to 900whp and 50psi of boost.
EMAP trends and the EMAP/IMAP relationship give valuable insight when analyzing anything turbocharged. Particularly when it comes to explaining the difference in turbos themselves; such as why one turbo will tend to make more power at the same psi than another, gauging how hard a given turbo is being pushed, fine tuning/matching A/R size, etc.
It still carries important information in a test like this, with the trend of each EMAP flipping/crossing each other being very interesting, but falls short in fully explaining what drove the divided configuration into boost sooner.
When trying to understand the “why” behind these results, it’s easy to focus on the turbo side of things. After all, the only components swapped in this test were the turbo’s hotside and a matching turbo manifold. But the characteristics between each setup may become more intuitive by considering what’s happening to the engine with these changes. Looking through the engine’s data logs from this test gave strong indications that the potential spool differences, and perhaps much of what’s driving them, are coming from the engine side. Things like engine VE/cylinder filling and specifically exhaust blowdown interference. Negative effects that can be eliminated if certain cylinders can be paired/separated up to the turbine wheel, and profound enough to be easily felt from the driver’s seat.
What is exhaust blowdown interference?
A nice diagram from Mazda showing the exhaust path between cylinders with a common (4 into 1) collector, and the affected rpm range. Their example is naturally aspirated, but the effect is likely even more pronounced with a turbine wheel partially blocking the exit.
At its simplest- a negative effect that can happen when two or more cylinders have their exhaust valves briefly open at the same time.
What can happen- the exhaust from one cylinder during its blowdown phase, with the exhaust valve initially opening and sending out a large pulse/rush of exhaust, travels through another cylinder’s open exhaust valve and into that cylinder.
Exhaust reversion caused by another cylinder.
In a 4 cylinder (or 2 rotor that shares the same 180deg blowdown interference), that offending exhaust pulse arrives at a very inopportune time. The blowdown interference happens near the end of the exhaust stroke, as the exhaust valve is closing, leaving little or no time to evacuate the exhaust contamination back out.
What we truly care about is the following effect: any exhaust gas that remains in the cylinder will displace intake air charge during the following intake stroke. Exhaust gas displaces your engine’s displacement, hurting cylinder filling, and we have a great way of seeing that through changes in engine volumetric efficiency (eVE, or just VE).
Side note:
We’re talking about engine VE that compares what is in the intake manifold (measured by a MAP and IAT sensor) to what % of the cylinder is filled with that awaiting air mass after each complete engine cycle. This isn’t VE that compares to outside/atmospheric air, where “VE” values could exceed 100%, 200%, 300%, etc., heavily skewed upwards by the boost level, but engine VE with all the temperature and density/boost variables already accounted for. More boost actually tends to lower “true” VE, but that could probably be its own article. This is what many speed density tuning programs base their calculations on, and generate a VE map from, by calculating/inferring by how much extra fuel it takes to maintain the same measured air fuel ratio, and in-turn, how much extra airflow is going through the engine, per complete cycle, at those points. That makes the VE map an excellent reference to see how efficiently your engine is filling the cylinder at various rpm and load points, and invaluable to trace where certain components affect the efficiency of the engine, and by how much.
What does that mean to you?
Since engine VE what % of your engine’s cylinders, and total displacement, is being utilized during various operating conditions- another way to think about it is what displacement the engine is acting like at each of those points. For example, at 80% VE, this 2.0L would only act like a 1.6L at that operating condition/rpm and load point of the VE map. That’s very noticeable and, thinking of the VE map as an “effective engine displacement” map, makes the VE% changes much more tangible. And those changes come with the exact same knock-on effects that come with a large displacement engine when it comes to boost, too. More airflow to drive the turbo into boost sooner, often more power per psi, etc.
Back to blowdown interference.
The other part of that Mazda diagram, showing one way to mitigate blowdown interference and the rpm range (less) affected.
There are many ways to mitigate the negative effect of blowdown interference, but a divided turbo setup can completely eliminate it. The cylinders are intentionally paired so no exhaust valves will be open at the same time, separating them from the cylinders that do, and cutting off the shared exhaust path entirely. No more blowing exhaust into another cylinder, no more engine acting smaller due to it.
For an inline 4 cylinder, 2 rotor, and 6 cylinder, the cylinder’s exhaust runners can be properly paired and sent into each side/scroll of a divided turbine housing to avoid negative cross talk/interference on the exhaust side.
With more cylinders however, or an odd count, you can’t effectively separate/organize the cylinders into two scrolls where no exhaust valves are simultaneously open. So not all engines can effectively utilize a divided turbine housing.
Since the 180deg blowdown interference experienced by our 4 cylinder happens near the end of the exhaust stroke, one way to mitigate its negative effect is by shortening the time available for the exhaust to travel and narrowing the time window that it can occur. This naturally occurs as engine rpm is increased, while the exhaust pulse is more or less fixed/limited by the local speed of sound, now unable to reach another cylinder before the door is shut on it.
So, now we have a case where blowdown interference should be practically eliminated (divided), and one where it should be prevalent at lower rpm (non-divided) but eventually fade away with increasing rpm until there isn’t enough time to interfere at all. Overlaying the dialed-in/calculated VE curves during the run of each setup as they sweep across their VE maps gives an elegant comparison. Highlighting the scale/magnitude, and at what point they converge as the effect naturally fades away- divided or not.
*This relies on a well dialed-in VE map, stable fueling (no fuel pressure drop off, etc.), stable/controlled air-fuel curve, and accurate wideband O2 readings for feedback throughout. All of which were handled by Haltech with any slight fuel trims being accounted for, using ‘Base Fuel Tuning’ + ‘O2 Control Output’ for the final VE values shown. The absolute values could still be potentially skewed, but it’s the % change between setups that truly matters.
VE vs boost and rpm. Blue divided, red v-band/non-divided. Top graph portion is VE, bottom portion boost in psig:
The initial shape of the red v-band VE curve, the one that would be experiencing blowdown interference, is increasing at a satisfyingly linear rate as rpm increases, which is exactly what you’d expect if there is less and less time for the negative cylinder-to-cylinder effects to take place.
22% difference is significant; on the scale of ~0.4 liter difference in engine displacement when it comes to driving the turbo and noticeable off-boost torque around 3000-3500rpm. Whether you imagine it as the non-divided acting closer to a 1.6L at that rpm, or requiring a 2.4L+ engine to match the divided 2.0L setup due to the negative effects caused by blowdown interference. At 4000rpm a scale of ~0.3L displacement difference, 4500rpm ~0.14L, 5000rpm ~0.05L. (a better visual would be ideal here)
During that affected range:
Divided=less negative interference->less residual exhaust gas->better VE/cylinder filling of air/fuel->engine effectively acts larger->more airflow generated through the engine at the same rpm->quicker into boost.
The differences between these VE curves- where they merge, how much initial difference there is, overall shape, rpm dependence- strongly suggest exhaust blowdown interference between cylinders. If that’s the case, there will be some implications. A significant one, one that should be somewhat predictable in how things shift around, are the camshaft(s) being used. Specifically the exhaust duration. Longer exhaust duration will open the door for more blowdown interference and cause it to happen sooner in the exhaust stroke/event, requiring even higher rpm to navigate out of the negatively affected range. More duration, higher rpm before the negative effects can go away on their own. The initial drop in VE could be even more pronounced as well, since with more duration comes more valve overlap (when the intake and exhaust valves in the same cylinder are briefly open at the same time). Valve overlap also occurs at the end of the exhaust stroke, giving the exhaust blowdown interference the opportunity to travel up into the intake tract if things get bad enough- no longer constrained to the size of the combustion chamber alone.
McKenzie’s car in these tests was using GSC S3 camshafts, which have a longer exhaust duration vs something like the milder S2 option, meaning his S3 leave more available for the divided configuration to “clean up,” offering more to improve on at low rpm, which will tend to show a greater difference between divided and non-divided setups than the S2 option. Or, for factory camshafts, the negative effects could be rpm’d out of well before that, and show little difference during spool on a turbo that comes on closer to 5000rpm.
Lots of factors, but the differences a divided setup can make are likely to scale with camshaft duration and overlap. This lines up with something we’ve noticed- a divided setup tends to be less sensitive to camshaft duration and overlap, and mitigates the usual “softening” of the bottom end that usually comes with it. You are often able get away with more aggressive camshafts with fewer ill-effects/trade-offs in the lower rpm range. This seems to apply to intake and exhaust cam timing as well, which could result in some interesting future comparisons.
You may notice that the VE differences were minimal by the time each turbo/setup was fully into boost around ~5500rpm, potentially keeping the spool differences between divided/non-divided closer than anticipated at ~380rpm. Maybe a smaller turbo that reaches full boost before 5000rpm could see a greater rpm split, for example. Maybe not. We have seen greater differences in spool, closer to 600 to 1000rpm, but the test results here are likely more representative of the difference between two configurations that are already well optimized. A/R size, arguably manifold design, etc.
The benefits that can be seen on the exhaust side can quickly diminish depending on how well the baseline setup is doing, and how much residual exhaust gas there is to clean up. After installing our test v-band manifold/turbo setup, there was already a significant change in VE/fueling requirements at the onset of boost compared to the prior v-band manifold/turbo setup that was on the car (not the setup some may be thinking). Already indicating ~15% higher VE during spool up, similar to what I expected from the divided setup but against another v-band manifold/turbo, it made me question how much was left in the VE/spool department to improve on with the divided.
There are other factors at play too, outside of just the measured VE changes that we focused on. The housings are different materials and different castings/surface finishes on the inside from that list which may have played an incremental role. There could be something to the fact that each divided exhaust pulse only “sees” half of the turbo manifold and half of the turbine housing, reducing the total volume for the exhaust pulse to expand into. That can increase boost response by itself. There could be something to what happens to turbine efficiency, pulsed vs constant flow, whether or not the pulses/events are ordered or overlapping when they hit the turbine wheel, or if certain turbine wheel designs tend to favor one or the other, etc. Many things we can’t personally speak to, but also put a lot of focus on the turbo/turbine side when there was a significant measured effect on the engine/VE/cylinder filling side that is missing in many open vs divided conversations. At least in our 4 cylinder applications. Effects that may explain the characteristics/mechanisms behind each and why some engines/configurations may not see similar results.
You may also be asking, what about a smaller A/R v-band housing? Wouldn’t a smaller 0.63A/R offer quicker spool than the 0.82A/R and do the same thing as the divided setup? It could close the gap, but a key difference comes down to how each go about increasing spool/response. A smaller A/R shrinks the nozzle/volute size in the turbine housing, increasing the velocity of the exhaust gas driving the turbine wheel, which generates boost sooner but at the expense of higher EMAP across the board. It would certainly make for another great addition/data point, but likely coming back to that ~25whp loss for every 100rpm gained in spool in our 2.0 4G63 application. The divided setup works fundamentally differently- allowing the engine to work more efficiently by increasing VE to achieve the same result.
I find it interesting that this also makes a divided setup less likely to experience compressor surge at the same boost/rpm, due to the engine ingesting more airflow at the same rpm (which is driving the turbo quicker into boost), which moves the operating point further away from its surge line on the compressor map. A smaller A/R alone tends to move things closer (up/left on the compressor map) to the surge line during spool up, making compressor surge more likely.
Only partially related, but I really wanted to mention it somewhere: the divided setup was able to hit 30psi at 4050rpm when messing around in 5th gear (and no surge). Pretty phenomenal for a 64mm turbo on a 2.0L engine; with many components that are tailored more towards high rpm to boot.
We came into this test with some theories and expectations, but more importantly just wanted to see what really happened. To see a solid comparison and overlays between two dedicated v-band and T4 divided setups with minimal variables. On the same car, same day, same turbo.
Still to be done are the T4 open/non-divided comparisons, which I expect the non-divided .96A/R T4 to behave very similar to the non-divided v-band housing, just with a larger split in spool rpm due to the larger A/R. That larger A/R will likely end up with less EMAP for the same boost (and a better EMAP/MAP), potentially resulting in a slightly higher peak power number. Hoping to find out very soon, and we have some other test approaches to verify VE trends while teasing out more mechanisms/effects that could still be at play.
Takeaways:
The non-divided v-band setup showed slightly lower peak EMAP, tended to hold boost slightly better at high rpm, and slightly higher power numbers (1-2%) when at full boost when compared to the divided combination.
The divided setup showed ~380rpm quicker spool and immediately noticeable off boost torque, similar to a larger 2.4L engine at low rpm, with the differences fading away as rpm increased. Each run required a touch more from the manual boost controller to reach the same boost as the v-band setup
The dyno curves were remarkably close once full boost was reached, with neither nosing over- even during later pulls to 9500rpm
The larger A/R on the divided housing was more than offset by the VE benefits when it came to spool characteristics in this test, and likely helped offset losses at high rpm
Exhaust blowdown interference (and the subsequent residual exhaust gas hurting engine VE) looks to be extremely prevalent at low rpm in non-divided inline 4 turbo setups, and likely a large factor in the spool differences seen in this test. Camshaft duration is likely to have a large influence on the differences as well.
The results of this test, and the exact rpm/spool/power differences, won’t apply to every turbo/engine/manifold. Many push-pull factors involved, with this test showing the difference between two arguably well optimized configurations. Although this test showed the largest difference to date with a single design change, we could see things skewed further in either direction due to other turbo manifold design changes we’ve tested over the years.
The differences from a divided turbine housing, or if it even ‘works’ at all or not, are highly dependent on the engine, cylinder count, and correct pairing.
2 rotor engines are likely to respond similarly due to sharing the same 180deg exhaust blowdown interference. 6 cylinder engines will have 120deg blowdown interference, likely influencing the mechanisms behind spool/response/power characteristics. Even 4 cylinder boxer engines, with their inherently longer runner lengths and larger overall runner/manifold volume could influence how large of an effect each mechanism mentioned here has.
8 cylinder engines don’t allow correct cylinder firing order/pairing to see any change or benefit, unless it’s a ‘Hot V’ with 2x divided turbos, etc.
For the opposite reason, with so few exhaust events and built in 240deg pulse separation, an even firing 3 cylinder already avoids blowdown interference and doesn’t require dividing anything up. Not that you really could anyway.
The Apex line-up is becoming well known to punch above its weight in power, and we were genuinely impressed by the 64mm setup overall in both power and response. Can't say I've seen a wider power band from a 900whp+ 2.0L 4g63.
Playing with a bellmouth inlet on the divided setup improved spool another ~120rpm, and yielded 922whp that was still slightly climbing at 9400rpm (46psi at that power level/point), likely with more improvement to be had with electronic boost control, less conservative timing/AFR, etc.
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The additional response and driving characteristics offered by the T4 divided was exactly what McKenzie was looking for from his street car, and is ultimately the setup he is sticking with. His previous setups embraced the simplicity of non-divided turbos, with very notable results, but constant experimentation keeps things interesting.
The increased response and better street manners now add the temptation of the next size up turbo, likely resulting in the same or still better spool/response than the 64mm v-band combo, while retaining the extra power potential on tap of a larger turbo (likely lowering EMAP further, too). This is how we like to leverage divided turbo setups when turbo size isn’t constrained.
For small displacement, automatic turbo cars that need to build boost on the converter, the spool/VE characteristics of a divided setup can make a large difference exactly where it's needed. And, based on what we saw from this test, without hindering high rpm capability like a 2.3L stroker setup for example.
For compressor limited classes, that don't have a need for spool down low, a non-divided/v-band housing could certainly be more ideal. The v-band tended to hold boost a little better, and made slightly more power when at full boost during this test. Less cost/complexity, generally lower drive pressure at high rpm, and potentially the most power out of a given turbo. Things like smaller outer dimensions on a v-band turbine housings can sometimes allow better packaging and/or fit a larger exhaust size right after the turbo, which could slightly improve overall power and spool as well.
It’s also worth mentioning some non-divided combinations that help close the gap in response through clever tech and design, such as the Xona Rotor UHF turbine wheel when paired with smaller than ‘typical’ A/R housing in our applications, while still maintaining low EMAP.
We genuinely appreciate what each configuration can offer, are fascinated by the characteristics/mechanisms behind them, and have wanted to share an example of what folks could expect from each for a long time- including potential variables behind the various experiences/results over the years.
Our inline 4 tends to prefer the divided setup, and the turbo tends to prefer a turbine housing without a divider wall.
What are the considerations you have when choosing a turbine housing? Did they change?
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Huge thank you to DSPORT, ApexTurbo and McKenzie Lange for all of their time, effort, and resources in this test, with the shared goal to let folks see what each turbine housing/configuration does, and align with what they are looking for from their turbo setup.
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Here is the first round of back-to-back dyno comparisons, on McKenzie Lange’s AWD ’90 Eagle Talon. 2.0LR, 10.5:1, wet block, GSC S3 camshafts on pump e85, with dedicated v-band and T4 divided turbo manifolds. Each setup purpose built to share the same components- same turbo, same turbo placement, wastegates (two, not necessary with the v-band, but keeps boost control more consistent between setups), 3.5” downpipe/full exhaust, same runner size- even the same primary lengths. All done on the same day with the test car never unstrapped from the Dynojet chassis dyno. Runs were done in 3rd gear with similar stabilized fluid and hot parts temps, and each run starting from a steady state/no load 3000rpm.
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The turbo is an Apex 6466 with either the1.15 A/R T4 divided or .82 A/R v-band turbine housing, tested across three boost levels, and matched within ~1psi to each other using a manual boost controller (air regulator to the top wastegate port to effectively increase/augment the WG spring rating).
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DSPORT’s 2016 twin scroll vs single scroll test, “The Great Divide,” was a fantastic example of what to expect from Garrett divided/non-divided v-band turbine housings when paired to the same divided v-band turbo manifold. Today, most turbine housing options fall between v-band or T4 divided, using completely different flanges and manifolds. It’s not uncommon to hear that a non-divided v-band option will out-spool their older 4 bolt flanged counterparts like T3 and specifically T4 housings- even T4 divided. There are instances where that’s true, but we’d argue quicker spool/response would come from incremental, secondary improvements. Improvements perhaps stemming from a more recent redesign, more ideal casting material such as stainless steel with its lower thermal conductivity, smoother investment cast internal volutes vs rougher sand cast housings, potentially tighter tolerances and blade clearance, a round flange that matches your round crossover, or just tending to be paired with newer aero/tech turbo options- skewing the perception. The turbo manifold design can play a surpringly large role as well, as we’ve seen through various tests over the years.
There are certainly some things we like to take advantage of with v-band turbine housings, but when it comes to spool- does the turbo care about what fastens the flanges together? Does it care more about how many volutes/scrolls there are? What are some considerations you have when choosing which turbine housing?
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You may notice the A/R are quite different, with the divided T4 A/R being ~30% larger than the 0.82 A/R non-divided v-band. It’s tempting, and seems logical to use the same A/R in a back-to-back test like this to eliminate a variable, but arguably the divided turbine housing requires it due to how the individual exhaust events are split up to function.
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Side-by-side and cross section of a non-divided and divided housing (not the exact housings used)
This is likely due to a few reasons:
-The total volute area of the divided housing is larger at 1.15 A/R, but each cylinder/exhaust pulse only sees half of that volute/nozzle area at a time- and only half of the total cylinders travelling through it, with no overlapping exhaust events. In the non-divided example, each cylinder sees the entire 0.82 A/R volute/nozzle of the v-band housing, shared by all cylinders and their exhaust events.
-The divider wall in the divided turbine housing will also come with a little more frictional losses, which can add to the exhaust manifold pressure (EMAP) by itself.
-If the A/R were the same 0.82 A/R between both housings, the divided would almost certainly spool sooner, but due to the above, be much more likely to choke/nose over in power due to higher EMAP at higher power/rpm levels. A 0.8x A/R in a divided housing can yield great results, and often paired with smaller turbos, but would arguably be undersized for this comparison. Conversely, if both divided and non-divided were 1.15 A/R, the non-divided v-band housing would likely be oversized for this combination, losing appreciable spool rpm and at the point of diminishing returns where it’s unlikely to pay off elsewhere with this turbo size/combination.
So they are different, but arguably for good reason, and why many turbine housing offerings share this same trend. Including the options available for this turbo. Comparing the exhaust pressure readings later, they seem pretty well matched.
There are plenty of other factors in deciding A/R in general, and apparently some manufacturers who already upsize to compensate (potentially Garrett’s divided v-band in the 2016 DSPORT test?), but we’ve heard/found ~25-40% larger than a comparable non-divided A/R turbine housing is a good rule of thumb. Especially if you already have a good reference A/R to begin with, since A/R is more of a balancing act with diminishing returns if you stray too far away.
In general, increasingly larger turbos tend to use increasingly larger A/R’s, and larger displacement engines tend to use a larger A/R. The ‘formula’ starts to fall apart when comparing different manufacturers and turbine wheels/designs, etc.
In anticipation of seeing what happens on the dyno, we set a benefit/loss benchmark to help avoid bias in the interpretation of the results. A rough estimate, but every ~100whp (or ~10lb/min) of turbo flow potential gained tends to lose ~300-500rpm worth of spool on a 2.0L 4G63. Let’s call it 400rpm. So something like a 400whp capable turbo setup that comes in at 4200rpm or a now 600whp capable turbo setup that comes in closer to 5000rpm, for example. Or a turbine housing A/R change that lands somewhere in between that give and take for potential power and spool.
So, if the more costly/complex divided configuration didn’t exceed that benefit/loss ratio by a good margin, ~25whp for every 100rpm, there wouldn’t be much to leverage over a simple turbo or A/R change to do the same thing.
The results-
Horsepower curves, blue 1.15 A/R T4 divided, red 0.82 A/R V-band:
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Torque curves:
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All runs matched within ~1psi or less between each setup at full boost. First three are horsepower at the wheels, bottom three are torque at the wheels, each showing the ~35psi, ~45psi, ~50psi boost comparisons and their respective boost curves.
Boost in the bottom portion to show any slight variances that could influence the power/torque comparisons. When both were at full boost, the v-band setup sometimes had a 0.5psi boost advantage after 7000rpm, and as high as 1psi at the end of the highest boost run, tending to hold boost a little better with the manual boost controller used. An identical boost curve at every rpm point would be nice, and could have been dialed in with electronic boost control (and kept more flat/consistent), but the slight boost curve variances with rpm added another layer to each setup’s characteristics.
Wideband readings are also added for reference, with each sharing the same AFR target (richer with higher boost levels), and much of the “difference” due to the divided coming into boost sooner (and commanding a richer AFR)
Overview:
The divided setup consistently came into boost ~380rpm sooner, reaching 30psi by ~4830rpm vs the v-band by ~5220rpm, even with the divided’s larger 1.15 A/R vs the 0.82 A/R of the v-band housing. At full boost, the 0.82A/R v-band setup saw 1-2% more power in certain spots when overlayed, but both closely matched overall in dyno power and torque numbers. Ranging from 3-15whp and 5-14wtq, tending to show more variance as boost levels increased. The power curve shapes at full boost were almost identical. with neither setup nosing over, even during later runs to 9500rpm.
Zooming in on boost curves and looking at EMAP:
During the highest boost setting tested:
At 9000rpm the 0.82 A/R v-band was at a 1.15:1 ratio of exhaust pressure to boost pressure.
At 9000rpm the 1.15 A/R divided was at a 1.20:1 ratio of exhaust to boost pressure.
Also referenced as exhaust pressure ratio, drive pressure ratio, engine pressure ratio, EMAP/IMAP, EMAP/MAP, etc. Lower exhaust pressure at the same boost, resulting in a lower ratio, is generally a good sign when it comes to turbos and the setup in general.
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Blue divided, red v-band, solid boost, dashed exhaust pressure. The zoomed in boost curves better illustrate the slight variances between the two configurations. The EMAP sensor signal dropped out in some runs, but other similar boost runs could be referenced for the ratios earlier.
EMAP overview:
Before 7000rpm the divided had lower exhaust pressure (EMAP in psig) for the same boost, at ~7200rpm both were nearly the same, and by 9000rpm the divided was a few psi higher EMAP than the v-band. Since EMAP influences boost control through the wastegate valve, the v-bands lower EMAP lines up with it holding boost slightly better up there.
EMAP was still impressively low overall for the turbo size and power levels. There are many factors, but much of that credit goes to the turbo itself- requiring less average exhaust pressure to maintain the boost pressure and airflow throughout most of the testing. A 1:1 ratio wasn’t seen until closer to 900whp and 50psi of boost.
EMAP trends and the EMAP/IMAP relationship give valuable insight when analyzing anything turbocharged. Particularly when it comes to explaining the difference in turbos themselves; such as why one turbo will tend to make more power at the same psi than another, gauging how hard a given turbo is being pushed, fine tuning/matching A/R size, etc.
It still carries important information in a test like this, with the trend of each EMAP flipping/crossing each other being very interesting, but falls short in fully explaining what drove the divided configuration into boost sooner.
When trying to understand the “why” behind these results, it’s easy to focus on the turbo side of things. After all, the only components swapped in this test were the turbo’s hotside and a matching turbo manifold. But the characteristics between each setup may become more intuitive by considering what’s happening to the engine with these changes. Looking through the engine’s data logs from this test gave strong indications that the potential spool differences, and perhaps much of what’s driving them, are coming from the engine side. Things like engine VE/cylinder filling and specifically exhaust blowdown interference. Negative effects that can be eliminated if certain cylinders can be paired/separated up to the turbine wheel, and profound enough to be easily felt from the driver’s seat.
What is exhaust blowdown interference?
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A nice diagram from Mazda showing the exhaust path between cylinders with a common (4 into 1) collector, and the affected rpm range. Their example is naturally aspirated, but the effect is likely even more pronounced with a turbine wheel partially blocking the exit.
At its simplest- a negative effect that can happen when two or more cylinders have their exhaust valves briefly open at the same time.
What can happen- the exhaust from one cylinder during its blowdown phase, with the exhaust valve initially opening and sending out a large pulse/rush of exhaust, travels through another cylinder’s open exhaust valve and into that cylinder.
Exhaust reversion caused by another cylinder.
In a 4 cylinder (or 2 rotor that shares the same 180deg blowdown interference), that offending exhaust pulse arrives at a very inopportune time. The blowdown interference happens near the end of the exhaust stroke, as the exhaust valve is closing, leaving little or no time to evacuate the exhaust contamination back out.
What we truly care about is the following effect: any exhaust gas that remains in the cylinder will displace intake air charge during the following intake stroke. Exhaust gas displaces your engine’s displacement, hurting cylinder filling, and we have a great way of seeing that through changes in engine volumetric efficiency (eVE, or just VE).
Side note:
We’re talking about engine VE that compares what is in the intake manifold (measured by a MAP and IAT sensor) to what % of the cylinder is filled with that awaiting air mass after each complete engine cycle. This isn’t VE that compares to outside/atmospheric air, where “VE” values could exceed 100%, 200%, 300%, etc., heavily skewed upwards by the boost level, but engine VE with all the temperature and density/boost variables already accounted for. More boost actually tends to lower “true” VE, but that could probably be its own article. This is what many speed density tuning programs base their calculations on, and generate a VE map from, by calculating/inferring by how much extra fuel it takes to maintain the same measured air fuel ratio, and in-turn, how much extra airflow is going through the engine, per complete cycle, at those points. That makes the VE map an excellent reference to see how efficiently your engine is filling the cylinder at various rpm and load points, and invaluable to trace where certain components affect the efficiency of the engine, and by how much.
What does that mean to you?
Since engine VE what % of your engine’s cylinders, and total displacement, is being utilized during various operating conditions- another way to think about it is what displacement the engine is acting like at each of those points. For example, at 80% VE, this 2.0L would only act like a 1.6L at that operating condition/rpm and load point of the VE map. That’s very noticeable and, thinking of the VE map as an “effective engine displacement” map, makes the VE% changes much more tangible. And those changes come with the exact same knock-on effects that come with a large displacement engine when it comes to boost, too. More airflow to drive the turbo into boost sooner, often more power per psi, etc.
Back to blowdown interference.
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The other part of that Mazda diagram, showing one way to mitigate blowdown interference and the rpm range (less) affected.
There are many ways to mitigate the negative effect of blowdown interference, but a divided turbo setup can completely eliminate it. The cylinders are intentionally paired so no exhaust valves will be open at the same time, separating them from the cylinders that do, and cutting off the shared exhaust path entirely. No more blowing exhaust into another cylinder, no more engine acting smaller due to it.
For an inline 4 cylinder, 2 rotor, and 6 cylinder, the cylinder’s exhaust runners can be properly paired and sent into each side/scroll of a divided turbine housing to avoid negative cross talk/interference on the exhaust side.
With more cylinders however, or an odd count, you can’t effectively separate/organize the cylinders into two scrolls where no exhaust valves are simultaneously open. So not all engines can effectively utilize a divided turbine housing.
Since the 180deg blowdown interference experienced by our 4 cylinder happens near the end of the exhaust stroke, one way to mitigate its negative effect is by shortening the time available for the exhaust to travel and narrowing the time window that it can occur. This naturally occurs as engine rpm is increased, while the exhaust pulse is more or less fixed/limited by the local speed of sound, now unable to reach another cylinder before the door is shut on it.
So, now we have a case where blowdown interference should be practically eliminated (divided), and one where it should be prevalent at lower rpm (non-divided) but eventually fade away with increasing rpm until there isn’t enough time to interfere at all. Overlaying the dialed-in/calculated VE curves during the run of each setup as they sweep across their VE maps gives an elegant comparison. Highlighting the scale/magnitude, and at what point they converge as the effect naturally fades away- divided or not.
*This relies on a well dialed-in VE map, stable fueling (no fuel pressure drop off, etc.), stable/controlled air-fuel curve, and accurate wideband O2 readings for feedback throughout. All of which were handled by Haltech with any slight fuel trims being accounted for, using ‘Base Fuel Tuning’ + ‘O2 Control Output’ for the final VE values shown. The absolute values could still be potentially skewed, but it’s the % change between setups that truly matters.
VE vs boost and rpm. Blue divided, red v-band/non-divided. Top graph portion is VE, bottom portion boost in psig:
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At the start of the run at lower rpm, the difference in VE during every pull hovers between 20-22% VE around 3000rpm, getting closer to a 15% difference by 4000rpm, and each setup’s VE curve steadily converges by around 5200rpm at every boost level, with the v-band seeing ~1-2% higher VE after 6500rpm in the ~35psi and ~45/42psi comparisons. It’s small enough to still be a coincidence, but this 1-2% additional VE matches the power difference the v-band saw before. Possibly due to the ~3-4psi lower EMAP at higher rpm vs the divided housing.The initial shape of the red v-band VE curve, the one that would be experiencing blowdown interference, is increasing at a satisfyingly linear rate as rpm increases, which is exactly what you’d expect if there is less and less time for the negative cylinder-to-cylinder effects to take place.
22% difference is significant; on the scale of ~0.4 liter difference in engine displacement when it comes to driving the turbo and noticeable off-boost torque around 3000-3500rpm. Whether you imagine it as the non-divided acting closer to a 1.6L at that rpm, or requiring a 2.4L+ engine to match the divided 2.0L setup due to the negative effects caused by blowdown interference. At 4000rpm a scale of ~0.3L displacement difference, 4500rpm ~0.14L, 5000rpm ~0.05L. (a better visual would be ideal here)
During that affected range:
Divided=less negative interference->less residual exhaust gas->better VE/cylinder filling of air/fuel->engine effectively acts larger->more airflow generated through the engine at the same rpm->quicker into boost.
The differences between these VE curves- where they merge, how much initial difference there is, overall shape, rpm dependence- strongly suggest exhaust blowdown interference between cylinders. If that’s the case, there will be some implications. A significant one, one that should be somewhat predictable in how things shift around, are the camshaft(s) being used. Specifically the exhaust duration. Longer exhaust duration will open the door for more blowdown interference and cause it to happen sooner in the exhaust stroke/event, requiring even higher rpm to navigate out of the negatively affected range. More duration, higher rpm before the negative effects can go away on their own. The initial drop in VE could be even more pronounced as well, since with more duration comes more valve overlap (when the intake and exhaust valves in the same cylinder are briefly open at the same time). Valve overlap also occurs at the end of the exhaust stroke, giving the exhaust blowdown interference the opportunity to travel up into the intake tract if things get bad enough- no longer constrained to the size of the combustion chamber alone.
McKenzie’s car in these tests was using GSC S3 camshafts, which have a longer exhaust duration vs something like the milder S2 option, meaning his S3 leave more available for the divided configuration to “clean up,” offering more to improve on at low rpm, which will tend to show a greater difference between divided and non-divided setups than the S2 option. Or, for factory camshafts, the negative effects could be rpm’d out of well before that, and show little difference during spool on a turbo that comes on closer to 5000rpm.
Lots of factors, but the differences a divided setup can make are likely to scale with camshaft duration and overlap. This lines up with something we’ve noticed- a divided setup tends to be less sensitive to camshaft duration and overlap, and mitigates the usual “softening” of the bottom end that usually comes with it. You are often able get away with more aggressive camshafts with fewer ill-effects/trade-offs in the lower rpm range. This seems to apply to intake and exhaust cam timing as well, which could result in some interesting future comparisons.
You may notice that the VE differences were minimal by the time each turbo/setup was fully into boost around ~5500rpm, potentially keeping the spool differences between divided/non-divided closer than anticipated at ~380rpm. Maybe a smaller turbo that reaches full boost before 5000rpm could see a greater rpm split, for example. Maybe not. We have seen greater differences in spool, closer to 600 to 1000rpm, but the test results here are likely more representative of the difference between two configurations that are already well optimized. A/R size, arguably manifold design, etc.
The benefits that can be seen on the exhaust side can quickly diminish depending on how well the baseline setup is doing, and how much residual exhaust gas there is to clean up. After installing our test v-band manifold/turbo setup, there was already a significant change in VE/fueling requirements at the onset of boost compared to the prior v-band manifold/turbo setup that was on the car (not the setup some may be thinking). Already indicating ~15% higher VE during spool up, similar to what I expected from the divided setup but against another v-band manifold/turbo, it made me question how much was left in the VE/spool department to improve on with the divided.
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There are other factors at play too, outside of just the measured VE changes that we focused on. The housings are different materials and different castings/surface finishes on the inside from that list which may have played an incremental role. There could be something to the fact that each divided exhaust pulse only “sees” half of the turbo manifold and half of the turbine housing, reducing the total volume for the exhaust pulse to expand into. That can increase boost response by itself. There could be something to what happens to turbine efficiency, pulsed vs constant flow, whether or not the pulses/events are ordered or overlapping when they hit the turbine wheel, or if certain turbine wheel designs tend to favor one or the other, etc. Many things we can’t personally speak to, but also put a lot of focus on the turbo/turbine side when there was a significant measured effect on the engine/VE/cylinder filling side that is missing in many open vs divided conversations. At least in our 4 cylinder applications. Effects that may explain the characteristics/mechanisms behind each and why some engines/configurations may not see similar results.
You may also be asking, what about a smaller A/R v-band housing? Wouldn’t a smaller 0.63A/R offer quicker spool than the 0.82A/R and do the same thing as the divided setup? It could close the gap, but a key difference comes down to how each go about increasing spool/response. A smaller A/R shrinks the nozzle/volute size in the turbine housing, increasing the velocity of the exhaust gas driving the turbine wheel, which generates boost sooner but at the expense of higher EMAP across the board. It would certainly make for another great addition/data point, but likely coming back to that ~25whp loss for every 100rpm gained in spool in our 2.0 4G63 application. The divided setup works fundamentally differently- allowing the engine to work more efficiently by increasing VE to achieve the same result.
I find it interesting that this also makes a divided setup less likely to experience compressor surge at the same boost/rpm, due to the engine ingesting more airflow at the same rpm (which is driving the turbo quicker into boost), which moves the operating point further away from its surge line on the compressor map. A smaller A/R alone tends to move things closer (up/left on the compressor map) to the surge line during spool up, making compressor surge more likely.
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Only partially related, but I really wanted to mention it somewhere: the divided setup was able to hit 30psi at 4050rpm when messing around in 5th gear (and no surge). Pretty phenomenal for a 64mm turbo on a 2.0L engine; with many components that are tailored more towards high rpm to boot.
We came into this test with some theories and expectations, but more importantly just wanted to see what really happened. To see a solid comparison and overlays between two dedicated v-band and T4 divided setups with minimal variables. On the same car, same day, same turbo.
Still to be done are the T4 open/non-divided comparisons, which I expect the non-divided .96A/R T4 to behave very similar to the non-divided v-band housing, just with a larger split in spool rpm due to the larger A/R. That larger A/R will likely end up with less EMAP for the same boost (and a better EMAP/MAP), potentially resulting in a slightly higher peak power number. Hoping to find out very soon, and we have some other test approaches to verify VE trends while teasing out more mechanisms/effects that could still be at play.
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Takeaways:
The non-divided v-band setup showed slightly lower peak EMAP, tended to hold boost slightly better at high rpm, and slightly higher power numbers (1-2%) when at full boost when compared to the divided combination.
The divided setup showed ~380rpm quicker spool and immediately noticeable off boost torque, similar to a larger 2.4L engine at low rpm, with the differences fading away as rpm increased. Each run required a touch more from the manual boost controller to reach the same boost as the v-band setup
The dyno curves were remarkably close once full boost was reached, with neither nosing over- even during later pulls to 9500rpm
The larger A/R on the divided housing was more than offset by the VE benefits when it came to spool characteristics in this test, and likely helped offset losses at high rpm
Exhaust blowdown interference (and the subsequent residual exhaust gas hurting engine VE) looks to be extremely prevalent at low rpm in non-divided inline 4 turbo setups, and likely a large factor in the spool differences seen in this test. Camshaft duration is likely to have a large influence on the differences as well.
The results of this test, and the exact rpm/spool/power differences, won’t apply to every turbo/engine/manifold. Many push-pull factors involved, with this test showing the difference between two arguably well optimized configurations. Although this test showed the largest difference to date with a single design change, we could see things skewed further in either direction due to other turbo manifold design changes we’ve tested over the years.
The differences from a divided turbine housing, or if it even ‘works’ at all or not, are highly dependent on the engine, cylinder count, and correct pairing.
2 rotor engines are likely to respond similarly due to sharing the same 180deg exhaust blowdown interference. 6 cylinder engines will have 120deg blowdown interference, likely influencing the mechanisms behind spool/response/power characteristics. Even 4 cylinder boxer engines, with their inherently longer runner lengths and larger overall runner/manifold volume could influence how large of an effect each mechanism mentioned here has.
8 cylinder engines don’t allow correct cylinder firing order/pairing to see any change or benefit, unless it’s a ‘Hot V’ with 2x divided turbos, etc.
For the opposite reason, with so few exhaust events and built in 240deg pulse separation, an even firing 3 cylinder already avoids blowdown interference and doesn’t require dividing anything up. Not that you really could anyway.
The Apex line-up is becoming well known to punch above its weight in power, and we were genuinely impressed by the 64mm setup overall in both power and response. Can't say I've seen a wider power band from a 900whp+ 2.0L 4g63.
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Playing with a bellmouth inlet on the divided setup improved spool another ~120rpm, and yielded 922whp that was still slightly climbing at 9400rpm (46psi at that power level/point), likely with more improvement to be had with electronic boost control, less conservative timing/AFR, etc.
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The additional response and driving characteristics offered by the T4 divided was exactly what McKenzie was looking for from his street car, and is ultimately the setup he is sticking with. His previous setups embraced the simplicity of non-divided turbos, with very notable results, but constant experimentation keeps things interesting.
The increased response and better street manners now add the temptation of the next size up turbo, likely resulting in the same or still better spool/response than the 64mm v-band combo, while retaining the extra power potential on tap of a larger turbo (likely lowering EMAP further, too). This is how we like to leverage divided turbo setups when turbo size isn’t constrained.
For small displacement, automatic turbo cars that need to build boost on the converter, the spool/VE characteristics of a divided setup can make a large difference exactly where it's needed. And, based on what we saw from this test, without hindering high rpm capability like a 2.3L stroker setup for example.
For compressor limited classes, that don't have a need for spool down low, a non-divided/v-band housing could certainly be more ideal. The v-band tended to hold boost a little better, and made slightly more power when at full boost during this test. Less cost/complexity, generally lower drive pressure at high rpm, and potentially the most power out of a given turbo. Things like smaller outer dimensions on a v-band turbine housings can sometimes allow better packaging and/or fit a larger exhaust size right after the turbo, which could slightly improve overall power and spool as well.
It’s also worth mentioning some non-divided combinations that help close the gap in response through clever tech and design, such as the Xona Rotor UHF turbine wheel when paired with smaller than ‘typical’ A/R housing in our applications, while still maintaining low EMAP.
We genuinely appreciate what each configuration can offer, are fascinated by the characteristics/mechanisms behind them, and have wanted to share an example of what folks could expect from each for a long time- including potential variables behind the various experiences/results over the years.
Our inline 4 tends to prefer the divided setup, and the turbo tends to prefer a turbine housing without a divider wall.
What are the considerations you have when choosing a turbine housing? Did they change?
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Huge thank you to DSPORT, ApexTurbo and McKenzie Lange for all of their time, effort, and resources in this test, with the shared goal to let folks see what each turbine housing/configuration does, and align with what they are looking for from their turbo setup.
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