My compound turbo set-up

[95GSXracer]

Originally Posted by TimIacobucci

How much more headroom do you have on the primary still? You're going 8's and not even leaning on either turbo at all yet huh?

I'm actually maxed out on the primary (big) compressor. People often ask why the car is only going 8.80s with the S475 when others are going mid 8s and in rare cases (kiggly) low 8s. The reason is that the PR is so low. If you look at the map, this ~105 lbs wheel maxes out at closer to 90 lbs at a low PR of 2:1. So from a PR perspective, the turbo is out for a Sunday drive (17 psi), but is giving all the flow it can. Running off the map over 8500 or so is evidenced in the turbine drive pressure spike above that rpm. Running this same compressor at a higher PR will move more air more efficiently. I'm hoping to step up to an 80 or 88mm compressor this year.

So let me get this right, your saying the more flow you can get before the primary turbo will make it spool faster?

I was wondering if modifying an existing 3" ss exhaust from the secondary turbo to a remote mount in the hatch would slow the gas velocity too much and hurt the primary spool. But if that's a happy size for the secondary turbine exit and I taper it back down smoothly to meet the primary turbo turbine inlet you think will just help?

Anything to reduce post secondary (small) turbine pressure will improve secondary turbo spool, which is also your overall turbo spool since it spools first. In the case of a remote mount setup you could step it up right at the turbine discharge, then gradually reduce it to a smaller size for the long run back to the big turbo if you're worried about velocity. Some OEMs are going to this type of downpipe to improve spool, look up expansion chamber downpipes, there is very little info out there, but there is some. As far as whether 3" all the way back or reducing it to 2.5" is better, I can't say.

I am thinking this works because the primary turbo spool time is basically just based on the gas energy increase from the secondary turbo spooling. The secondary like any turbo will spool faster with a larger pressure differential across the turbine so the least back pressure between the stages in the exhaust the faster the spool for the secondary and consequently the primary.

I guess it seems obvious now but it's sort of counter intuitive. That's pretty cool actually, with a normal setup pre turbine flow is usually a compromise for gas velocity and spool time, with this you actually spool the big turbo faster if the exhaust before it flows better.

Running with that idea I am wondering if it might actually be beneficial to open the primary turbo wastegate to bypass the exhaust restriction the primary turbine causes to spool the secondary faster, then have it close at a low boost pressure. I am thinking if the primary is sized properly it will be pretty large and need a decent amount of boost from the secondary going into the engine before it really starts to wake up, it would require some experimentation but I bet a delay to a relatively high boost pressure than most would anticipate would actually still spool the primary faster than not.

Until the big turbo starts to make boost, there is very little back pressure from it (expansion ratio near zero), so I don't think it gets in the way. This can be seen in the spool data I collected early on, timing the spool up on the small turbo with and without the big turbo installed. There was no noticeable difference.

[LandSpeed-DSM]

I am in the process of fabbing up the exhaust side of a compound system on a 1.5L SOHC Honda. Using a 61mm Garret in a 1.15 A/R open T4 with 3 inch V-band discharge as the primary and a MHI 14B in the 6cm2 housing with the wastegate blocked off..

Because I want to be able to run as little as single digit boost should I choose, and as much as 3 Bar or more on kill mode, I have opted to go with a 60mm TiAl off the exhaust manifold and a 44mm gate on the inter-stage pipe between the 14B and the Garrett.

The plan is to eventually move 55+ lbs/min and wind out as high as 11k rpm on a 309/292 cam.

Basically I am wondering if I have the wastegates laid out properly... With the bigger 60mm gate used to bypass all that exhaust around the small turbo, and the 44mm to regulate boost on the primary?


(Crappy hand-sketch because the stage I am at doesn't give much of an idea of the room/layout I have to work with)

I am curious to hear what others experienced in this field have to say.

[bastarddsm]

That looks like your headed in the right direction.

[95GSXracer]

Those WG sizes should work fine. I have two V44s on the manifold and one on the big turbo and they bypass enough for a 75mm compressor. Then again I don't run low boost, so it may not be a good comparison.

[bastarddsm]

Originally Posted by 95GSXracer

I'm actually maxed out on the primary (big) compressor. People often ask why the car is only going 8.80s with the S475 when others are going mid 8s and in rare cases (kiggly) low 8s. The reason is that the PR is so low. If you look at the map, this ~105 lbs wheel maxes out at closer to 90 lbs at a low PR of 2:1. So from a PR perspective, the turbo is out for a Sunday drive (17 psi), but is giving all the flow it can. Running off the map over 8500 or so is evidenced in the turbine drive pressure spike above that rpm. Running this same compressor at a higher PR will move more air more efficiently. I'm hoping to step up to an 80 or 88mm compressor this year.

Is that really true that the compressor maxes out at lower flowrates at less psi? Its sort of counter intuitive to me. I know the maps make it look like you have less mass flow available at lower pr's, but I assumed that the choke line was vertical, and it just has shit for efficiency at low pr's.

I guess I was under the assumption that the wheel can only take in a certain volume of air (which has a mass determined by ambient conditions), and that is defined by the area of the compressor inlet. What comes out of the compressor is that same mass of air, but at a much reduced volume and higher pressure.

[95GSXracer]

When I say maxed out I really mean efficiency is in the toilet, drive pressure is climbing exponentially, and I don't want to push it any further since it's at the point of deminishing return. The maps only go to ~60-65% effciency, and they'll run lower than that. I'm already off the map. For comparison, my 75mm wheel is at 60%, an 80mm would run at 66%, and an 88mm at 77%! Add to that the fact that I'm expecting ~13 lbs/min more flow, and the current turbo isn't going to cut it. I'm also focusing on reducing drive pressure, so compressor efficiency becomes important.

[rrussell]

Well I should know how my rear mount compound setup works with a FP 68HTA and a BW S400SX 75mm in about three months.

Turbo System:
Rear-Mount BW S400SX4-75mm, Atmospheric Primary Turbo,
1.32 A/R Turbine T6 Flange.
Engine-mount FP 68HTA, Second Stage Turbo.
DN Performance manifold, with two Tial 44mm gates and a 32mm internal waste gate.
Primary Stage will have one 60mm wastegate in the 3"exhaust 5" before the BW turbo.
TurboWerks Exa-Pump Electric Oil Scavenge Pump w/sump for Primary Turbo Return.
Ebay God-Speed FMIC for now.

The BW turbo is on the passenger side rear right in front of the tail light mounted on top of steel plate(sandwich hatch floor) that will be part of the cage. The intake is facing towards the passenger quarter panel. It will draw air through the factory quarter panel vents.

The exhaust will exit out the drivers side right in front of the tail light and drop through the floor pan. It will be cut flush at bumper height.

I'm going to build a box to contain all this.

I will have a new build thread on it after I get done going through the transmission.
Transtar/IPT 5 disk front clutch kit in stock basket.
Transtar/IPT end clutches
Kiggly flexplate
John Deere HyGard fluid
Foreign Muscle DSM (Transmission Control Unit) EPROM Upgrade


Kevin and Paul have been the ones that have inspired me on this.

[TimIacobucci]

This sounds extremely interesting and very similar to what I was contemplating.
I can't wait to see your build thread and some pics of the layout.

Do you have any plans for primary stage intercooling at all?

How does the area of the t6 inlet compare to a 3" pipe?

Overall it sounds like a very efficient high boost setup using some very nice turbos. I hope your transmission will hold up to the torque. I want to try out the manual route first as I do enjoy driving it more but I think I dislike breaking my car more than I enjoy shifting.

Originally Posted by 95GSXracer

When I say maxed out I really mean efficiency is in the toilet, drive pressure is climbing exponentially, and I don't want to push it any further since it's at the point of deminishing return. The maps only go to ~60-65% effciency, and they'll run lower than that. I'm already off the map. For comparison, my 75mm wheel is at 60%, an 80mm would run at 66%, and an 88mm at 77%! Add to that the fact that I'm expecting ~13 lbs/min more flow, and the current turbo isn't going to cut it. I'm also focusing on reducing drive pressure, so compressor efficiency becomes important.

That makes sense, well I guess you found one of the downsides, you will have to use a much larger compressor to flow efficiently at a lower rpm than all the other traditional setups on a comparable level. hahaha, that is awesome.

I guess it also makes sense that if the turbine is sized such that it would not even spool without the secondary turbo there until extremely late then it isn't going to provide much of a restriction until it comes on line as you said.

Also if the 75mm flows more efficiently at a higher pressure ratio why not just turn the boost up rather than move to a larger compressor? Lack of intercooling on the primary?

Or is it you simply want to keep it very efficient at lower boost so as to minimize complexity and weight by not going to dual stage intercoooling? Flowing more air more efficiently at a lower boost on a bigger turbo just suites your design goals better?

Originally Posted by LandSpeed-DSM

I am in the process of fabbing up the exhaust side of a compound system on a 1.5L SOHC Honda. Using a 61mm Garret in a 1.15 A/R open T4 with 3 inch V-band discharge as the primary and a MHI 14B in the 6cm2 housing with the wastegate blocked off..

The plan is to eventually move 55+ lbs/min and wind out as high as 11k rpm on a 309/292 cam.

I think that will certainly spool very quick and make probably more torque than any other 1.5L at high boost, I just don't know if the overall range of 11K rpm on big cams is reasonable with the 14B.

I think either scaling down the cam and rpm limit or upping the secondary turbo size will make the whole thing more efficient. As I have been saying i would rather limit rpm and just try and maximize the efficiency of both turbos, this will give you a large torque band and you won't need 11k rpm.

I think the big cams will run like crap on the small turbo, it will likely hurt the spool time of the 14B and thus the primary as well. Just as your cams are coming into their own you will be running off the 14B compressor. It won't be so bad if you taper the pressure ratio off the secondary and let the primary fill in the flow, but that would require some complex boost controls to get it to boost quick but then taper down when you run off the map and even at that it's a band aid.

I would say decide your torque band with the ve of the cams you want and look at what compressors are efficient at the pressure ratios you will be running them to match that flow. Or, if you have the turbos or are set on that turbo combo I would compromise on the cam and rpm to have the ve and engine flow match the turbos better. I think the latter will make a car that's allot more fun to drive.

It will probably still run awesome and work regardless of what I have said if you build it that way. I just think from a design standpoint things could be matched better to work more efficiently if you have not started buying and fabricating stuff yet.

[LandSpeed-DSM]

Thanks for your input guys, WG sizing and placement was my only real concern at this point.

I feel I will need the 11k redline because of how short the gearing is. There will be two EBC solenoids and because of that crazy rev-range and the small internal gate with a 55+ lbs/min goal I opted for a 60mm gate before the small turbine.

So I can bypass all that exhaust gas up top and keep boost in check as much as is possible.

I am not super worried about spool considering my goals are for top speed (not drag) with how light the car is, and it is FWD. Good points though and I am considering a larger compressor at some point.

The reason I am going compound is because of the small displacement and tiny ports on the cylinder head, I am very much volume limited so I need to run PRs through the roof to make power.

This is only a 1.5L so were I sticking with a 14B as a single AND revving past 9k I would totally agree with you, but from what I have been working on this will let me take advantage full advantage of the 61mm compressor.

I will have a 4"x30"x12" FMIC and nearly 15 feet of charge piping, as well as two meth nozzles (one after the primary compressor outlet, and another at the throttle body)

[WES_393]

That was an awesome 2 days of reading! lol. Awesome build though, I envy your fab skills!

Quick question though 99 GST, what thickness/type of metal did you use for the side exit exhaust bumper gaurd? And did you just use pop rivets?

[95GSXracer]

Originally Posted by TimIacobucci

How does the area of the t6 inlet compare to a 3" pipe?

The T6 inlet is huge. IIRC, a 3.5 inch pipe will squish into the shape of the inlet when made square.

Also if the 75mm flows more efficiently at a higher pressure ratio why not just turn the boost up rather than move to a larger compressor? Lack of intercooling on the primary?

Or is it you simply want to keep it very efficient at lower boost so as to minimize complexity and weight by not going to dual stage intercoooling? Flowing more air more efficiently at a lower boost on a bigger turbo just suites your design goals better?

I'm running 45 psi plus already. A PR of 2:1 on both turbos gives you a total PR of 4:1. So going less than 45 psi requires really low PRs.


Russell, that setup sounds awesome. I recognize your screen name from YB but I think this is the first time I've seen the full setup listed. I can't wait to see how it does.

[TimIacobucci]

Originally Posted by 95GSXracer

I'm running 45 psi plus already. A PR of 2:1 on both turbos gives you a total PR of 4:1. So going less than 45 psi requires really low PRs.

I didn't mean to say you were running low boost. I just meant specifically how you were talking about the compressor efficiency of your 75mm relative to the PR others run them at to achieve higher mass flow. I meant what if you leave the secondary at it's current PR and up the primary?

Originally Posted by 95GSXracer

The T6 inlet is huge. IIRC, a 3.5 inch pipe will squish into the shape of the inlet when made square.

Just looked it up and you were spot on.



So the area of a t6 is around 9.2 sq. in

A 3" pipe is around 7 sq. in.

A 3.5" pipe is around 9.6 sq. in.

So if a t6 turbine is used on the primary a 3" feeding it would actually be too small and a 3.5" about perfectly matched.

[95GSXracer]

The PRs of the turbo are multiplicitive, but boost pressures are additive. If I raise the boost of either turbo, manifold boost goes up. The only way to go up on primary PR and keep the same total PR would be to lower secondary PR, which is already really low at ~1.9. That turbo has the same problem as the big one. Really low efficiency thanks to the low PR and high flow.

I run a 2.5" pipe from the 50 trim to the T6 flange. When the WG tubes merge in it makes up some more of the flange area, but I still had to add filler plates to square it up. If I had the room I'd do 3" right off the turbo. I also thought about sending the main exhaust to one half of the divided flange and the WGs to the other half, but gave up when I actually tried it. Didn't like the fit for my layout situation.

[rrussell]

Originally Posted by TimIacobucci

This sounds extremely interesting and very similar to what I was contemplating.
I can't wait to see your build thread and some pics of the layout.

Do you have any plans for primary stage intercooling at all?

How does the area of the t6 inlet compare to a 3" pipe?

Overall it sounds like a very efficient high boost setup using some very nice turbos. I hope your transmission will hold up to the torque. I want to try out the manual route first as I do enjoy driving it more but I think I dislike breaking my car more than I enjoy shifting.

I'm going to try the primary with out intercooling first. If it works well I'll run just the air to air on the secondary. On the last rear mount turbo system I did I was very surprised how well the temps drop with just intercooling piping from the back of the car to the front.

On the 3" exhaust as Kevin pointed out a 3 1/2 is easy to merge. So I will go this route.

The transmission and drive-train is my biggest concern. In my case I actually want the stock converter to be as tight as possible for top end. This is one of the reason for running Hygard fluid. I lost about 200rpm worth of stall but it still makes great boost 25+ at the line and comes up on the converter quickly.

I might run some ignition retard at my shift points to try to make the transmission live a little longer.

[fosforito95]

im wondering how paul connected his boost controller to the wastegate or wastegates to control his boost? i dont see a clear pic that shows it

[Slo2gen]

I would love to see the formulas that you guys use (I know the compressor charts for the turbos will be different) to figure out your pressure ratios and what not when adding another turbo into the mix.

[95GSXracer]

With boost control done the way I do it, boost pressures are additive. PRs are always multiplicitive.The two PRs multiplied together will equal total PR. For example, I run my turbos at roughly 2.1 (17 psi) and 1.9 (28 psi; 45 psi outlet, 17 psi inlet, gauge) for a total PR of 4:1, or 45 psi. Divide airflow by primary PR to roughly plot airflow on the secondary's map (remember to use actual PR, outlet psia divided by inlet psia). You can plot airflow and PR on the primary compressor's map as usual.

[suprasalas]

hey man how did u routing your oil system?

[95GSXracer]

Mine was just a feed off the filter housing split with a Y to both turbos. Same for the drains, both tee'd into one port on the oil pan. I'd rather have had two seperate 10an bungs on the pan but I never had a problem with it done like this.

[caged]

So paul what boost/pr were you running on each turbo for the 47psi and what were you using for boost control and on what wastegate? Also what were your Ignition timing numbers like with the compound compared to just running it as a single?

Iv just got my 4g63 (evo4 TDO5HR twinscroll) 16g/hx40 pro combo going and so far running 18psi and 7psi(hx40) is got an insane torque curve. Started talking about it back in 09 and finally had the opportunity to do it and it hasn't disappointed.

[viperlp01]

Generally you want to put the small turbo at a good spot in the efficiency range. Then turn the boost up using the big charger.

[Sp1kE]

Is it ok to add pictures of my setup into this thread being there is so much talk about compounds? or should this remain Paul's?

[viperlp01]

We should just make a compound turbo-charged thread. It seems like we have 5 people in this thead with a compound setup

[RWD4G63]

Originally Posted by viperlp01

We should just make a compound turbo-charged thread. It seems like we have 5 people in this thead with a compound setup

Compound Turbo Thread

There we go.

[99gst_racer]

Originally Posted by caged

So paul what boost/pr were you running on each turbo for the 47psi and what were you using for boost control and on what wastegate? Also what were your Ignition timing numbers like with the compound compared to just running it as a single?

I was running 20 psi on the 16g and about 27 psi on the 60-1. The 16G was running a 44mm Tial on springs only. And the 60-1 was running a Tial 44mm with a Halman MBC. Timing was about 10-11 degrees toward the upper RPM range. I've never had a single turbo making that much power or running that much boost, so the timing numbers don't really compare.

[Kapok6]

Do you think one could get away with using a single -10an oil return fitting on the oil pan and having a BB turbo and an MHI turbo both draining into it via 10an "Y" fitting? Or would that be too much oil trying to drain into one -10an sized hole?

[LandSpeed-DSM]

Sounds like it would be asking for trouble.

[ExtremeTurboSystems]

^ agreed. I would do separate drains.

[Kapok6]

What about a single -12an, or would that still not be big enough?

[ExtremeTurboSystems]

It sounds like your trying to avoid modifying your oil pan. You can run a -20AN line but the hole in the oil pan is still to small to drain two turbos. Oil pan modification is going to have to happen. I wouldn't take any short cuts when it comes to a compound setup. Do it once and do it right with each turbo having its own drain. Those are my thoughts on it atleast.