Exhaust Size

[Focker420]

I want to find out what the best size of exhast to go with for my set-up would be.
I have heard 3" was the way to go when used with a turbo applacation, And i have also been told that 3" was to big and 2.5" was the way to go. And if the down pipe should be bigger or smaller than the cat back?

Keep in mind I want the best for the engine with the new turbo, But don't want it to be super loud either. So somthing quite till i pucnch it then sound like a bat outta hell.

 

[DSMcrazy3]

3" is never wrong for a turbo application. 2.5" will be fine for you, assuming you'll be running the 8> psi most 420a-t owners run. Heck, it's not necessarily recommended but I know of 2 certain 420a-t owners running stock 2.25" even.

I recommend the Tsudo 3" catback for the GS-T though, for the price, you really can't go wrong.

 

[4UH8ERS]

If you got the cash: Greddy evoII or Greddy sp2

BTW: It's spelled--Exhaust--Please correct the title for future searchers.

 

[Blitzeclips]

Exhaust are all about velocity. Remember that, if you can, apply it to general physics of the exhaust set-up, or volume efficiencies. Should be great for reference in future set-ups..

You're fine with 2.5" n/t, but I'd go with a full 3" when you become turbo.

I got the Tsudo 3" cat-back exhaust for $212 shipped.

 

[totaleclipse_05]

Did some searching and found this info, thought I would share:

N/A cars: as most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

for turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.

downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.

as for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”

"as for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.

a large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.

if an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow.

necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.

also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems.

another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.

Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc

comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however.

there's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.

as for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.”

"here's a worked example (simplified) of how larger exhausts help turbo cars:

say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

so here, the turbine contributed 19.6 psig of backpressure to the total.

now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).

so in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.

*this is why larger exhausts make such big gains on nearly all stock turbo cars*-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.

A=as you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would. As for output temperatures, I'm not sure I understand the question. Are you referring to compressor outlet temperatures?

the advantage to the bellmouth setup from the wg's perspective is that it allows a less torturous path for the bypassed gases to escape. This makes it more effective in bypassing gases for a given pressure differential and wg valve position. Think of it as improving the VE of the wastegate. If you have a very compromised wg discharge routing, under some conditions the wg may not be able bypass enough flow to control boost, even when wide open. So the gases go through the turbine instead of the wg, and boost creeps up.

the downside to a bellmouth is that the wg flow still dumps right into the turbine discharge. A divider wall would be beneficial here. And, as mentioned earlier, if you go too big on the bellmouth and the turbine discharge flow sees a rapid area change (regardless of whether the wg flow is being introduced there or not), you will incur a backpressure penalty right at the site of the step. This is why you want gradual area changes in your exhaust.

 

[clipsit]

If you got the cash, I would say Espeilir or Tanabe exhaust. RRE sells them for one of the cheapest. Hopupracing is also another carrier of both i believe and have great prices.

 

[totaleclipse_05]

I would go with the 3 in. GST Tsudo exhaust. Cheap and many people have them and like them a lot.

 

[4UH8ERS]

Let me restate my previous opinion. Greddy for best overall sound and quietness with performance. Thermal R&D if you just want good quality and performance.

 

[BigRand]

P.I.T. Exhaust 3" GST should still be onsale at horsepowerfreaks.com I got mine for $350+shipping, looks and sounds beautiful. they have sound clipse as well.

Just another idea.

 

[TalonESIT]

Focker, decide whether you feel 2.5 or 3 inch suits your needs, I can't see a problem just going the route of a 3'' system ( you'll never ask yourself "what if I went the route of the 2.5'' system").
I have a 2.5 turbo back system, but I wish I would have found out sooner that the GST bolted right up because then I would have went the route of a 3'' system.

Look into v-band clamps as well, for your downpipe, will save you a lot of trouble later down the line

 

[VelocitàPaola]

Alright kids, play nice in the pit or have your threads locked...

 

[Blitzeclips]

OP- Keep in mind, a bigger exhaust isn't always a better exhaust. It's about efficiently utilizing the exiting gases, and scavenging the cylinder for obtaining the most power out of your motor. A good example offhand is the Hahn Racecraft record breaking Sunfire. It's running the 16G set-up, almost identical to the kits available for the 420a, yet it has a 2 1/4" exhaust. For the motor, turbo set-up, and car in general, this is the ideal size for it's exhaust diameter. Just something to keep in mind when you're talking about exhaust upgrades..

 

[dr1665]

OP- Keep in mind, a bigger exhaust isn't always a better exhaust. It's about efficiently utilizing the exiting gases, and scavenging the cylinder for obtaining the most power out of your motor. A good example offhand is the Hahn Racecraft record breaking Sunfire. It's running the 16G set-up, almost identical to the kits available for the 420a, yet it has a 2 1/4" exhaust. For the motor, turbo set-up, and car in general, this is the ideal size for it's exhaust diameter. Just something to keep in mind when you're talking about exhaust upgrades..

I'd give you points for this reply, but I've got to spread it around or something some more. Good job. I would advise the OP hit up Google and try finding outside articles talking about exhaust theory. It's all about the velocity.

What happens if you hook a fire hose up to the spigot on your house? How fast does that water come out the other end? Now, what happens when you hook your garden hose up to the fire hydrant? What happens to all that excess water?

Unless you're making more than 300hp or so, I would say that 3" exhaust is:
A) Overkill.
B) Just something to impress people who don't know what they're talking about.
C) An intentional effort at modifying spool characteristics.
D) The foundation for soon-to-happen installs and tuning which will take you past 300hp.

I'll let you guys discuss why.

 

[BigRand]

I'm not going to directly quote the article that was posted from 2gnt.com but correct me if I'm wrong. I thought it said that scavenging was useless for a turbo car after the turbo. And that the gains assciated with it are minimal compared to the back pressure that is caused by having a pipe diameter to small. (diminsihing returns). And don't get me wrong I understand you can have to big of an exhaust.

I'm not saying hahn doesn't have its reasons, but It goes against what I just learned from that post, and I would just have to think they would have another explanation for only using the 2 1/4" piping. Enlighten me please
;

 

[totaleclipse_05]

I'm not going to directly quote the article that was posted from 2gnt.com but correct me if I'm wrong. I thought it said that scavenging was useless for a turbo car after the turbo. And that the gains assciated with it are minimal compared to the back pressure that is caused by having a pipe diameter to small. (diminsihing returns). And don't get me wrong I understand you can have to big of an exhaust.

I'm not saying hahn doesn't have its reasons, but It goes against what I just learned from that post, and I would just have to think they would have another explanation for only using the 2 1/4" piping. Enlighten me please
;

That's what I was thinking as well BigRand. It doesn't quite make sense to me either. I'm not too keen on exhaust and sizing and all that, but on a turbo car, I always thought you wanted as less backpressure as possible after the turbo and a 2.25 in. exhaust would just seem to not follow that rule of thumb.

 

[clipsit]

I'm not going to directly quote the article that was posted from 2gnt.com but correct me if I'm wrong. I thought it said that scavenging was useless for a turbo car after the turbo. And that the gains assciated with it are minimal compared to the back pressure that is caused by having a pipe diameter to small. (diminsihing returns). And don't get me wrong I understand you can have to big of an exhaust.

I'm not saying hahn doesn't have its reasons, but It goes against what I just learned from that post, and I would just have to think they would have another explanation for only using the 2 1/4" piping. Enlighten me please
;

I would also like to be enlightened more, but i tend to agree a little w/ dr1665 that if you arent going above maybe 300+ then, yeah, maybe 3" inch isnt for you, but it still will flow well at the top end. you def will lose some torque and low end power, but you usually see and feel those loses more when you go to a bigger DP on a stock setup as compare to the stock one. Like I have a buddy w/ an evo who bought the works DP and was like, I feel like I lost power, which isnt necessarily true, but he def prob lost some low end power, but gained it up top, so IDK, it has alot to do w/ opinions and what not, but I think, why not go 3" and maybe a smaller DP...? Thats unless you obviosly want to go w/ something big. I know when i had my Hayame on my gs when it was turboed, I love that exhaust...Rand knows what I am talking about...and it spooled well, but I wanted to go bigger but never went bigger, before selling the car. So I am not really trying to make a point but if it sounds like I am, then its late and I am tired......anyway, I guess it depends on the setup and opinion and what not whether you think you should or not, but I guess I dont see anything wrong w/ going 3" on most if not all turbo applications regardless...? Just my opinion...? DISCUSS...?

 

[TalonTsi31]

i perssonally would gowith a 3'' catback VRS if you got the cash one of the best.

 

[dr1665]

I would also like to be enlightened more, but i tend to agree a little w/ dr1665 that if you arent going above maybe 300+ then, yeah, maybe 3" inch isnt for you, but it still will flow well at the top end.

You're getting there. The thing to keep in mind is that each turbo is capable of flowing a specific amount of air at peak effciency. Sometimes, we get caught up in how turbochargers typically respond very well to exhaust mods, but you still have the potential for overkill.

Additionally, if you went with too large an exhaust (downpipes, specifically), you stand more likely to experience boost spikes. Who wants that?

 

[Focker420]

Just to clairify i am building this motor to be around 350 whp daily driving. And try to hit the 400 whp when on the track with slicks or on the dyno. I like the 3 inch idea already from what i have read, So the down pipe should be smaller than the intermediate pipe if i read that right, to stay away from boost spikes.

Also the gst exhast fits right in u say? the routing is the same and the mounts?

And someone please enlighten me in what a v band clamp is.

 

[Blitzeclips]

Look guys, you have to remember bigger isn't always better. I know that's something most of you know, but you need to step out of the DSM box for a minute and think about this. For that Sunfire motor, you wouldn't want to treat it as a DSM, with the oh so big 3" exhaust for "quicker spool".

The primary use of an exhaust is to efficiently extrude charged/used (what have you) gas from the motor. The reason this sunfire has a smaller exhaust is probably because it's most efficient, and most effective for that specific motor. Although it is turbocharged, the velocity derived from efficiently exiting these gases is what creates an effective exhaust system. It's all about velocity, yes, but efficiency is a key aspect also. Just because it's a 2 1/4 doesn't meen it's not utilizing the exhaust velocity at an optimum level. That exhaust just so happens to be the most efficent, power producer for the exhaust, turbo and complete motor set-up.

Think about this: An intercooler gives you pressure drop, which creates more time to build boost. This is because you essentially created more space for this air to fill. By doing so, the duration of charge is increased, therefore dropping the efficiency you once had before adding the intercooler. Although exhausts and intercoolers have different purposes, I'm trying to illustrate the idea behind velocity and efficiency with this example.

Hope you all see what I'm trying to preach.


OP- Slicks will NOT add horsepower. It will simply assist putting this HP to the ground. The GS-T exhaust is a direct bolt up. You may have to kick the shit out of it to fit the rear piece but it will fit. Trust me.

I have no clue what the V-Band is, I have an idea but I'd rather not sway you the wrong direction.

 

[Focker420]

I understand that its all about the velocity and finding the fine line to use it to the best efficientcy of our motor, And the sunfire is a good example of how a smaller exhaust is more efficient for that motor but lets keep this strickly about what is best for the 420a motor when turboed.

And ya i know that slick won't give you horsepower but how else do you suggest i put 400 whp to the cement without LSD. (Dam i want the neon retrofit LSD).

Im picking up what your throwing down, Blitz Keep up the helpful relpy's

So how does using a GS-T exhaust (same year) with a custom down pipe say 2.5 inchs then goes to the 3 inch GS-T exhaust after the flex pipe at the bottom of my down pipe sound about right? PS ( no cat but using 2 resonators right now, keep same setup or no)

 

[Blitzeclips]

So how does using a GS-T exhaust (same year) with a custom down pipe say 2.5 inchs then goes to the 3 inch GS-T exhaust after the flex pipe at the bottom of my down pipe sound about right? PS ( no cat but using 2 resonators right now, keep same setup or no)

Sorry if that came out the wrong way about the slicks, the way you worded it must have thrown me off..
Yeah, that exhaust set-up would be fine. I actually have about the same set-up, but I opted for the Megabomb section on my downpipe. It's basically a slow expansion of 2.5"--> 3", about 6" after the exhuats housing, it's already 3" all the way back.

Please stick with the flex section idea! It's DEFINATELY something you will want with those goals in mind. I'm sure BigRand will tell you.

I'm not sure EXACTLY where the best place to put the flex section would be, but I had mine put on ribght around where the oil pan sits. It's that curve where the exhaust bends under the block that worried me the most. As long as it has something there to flex when the motor/car is under load, you should be fine.

 

[DSMcrazy3]

Why do we keep referring to it as a "GS-T" exhaust. If I'm not mistaken, those are 2.25" stock as well. Sure they mount up the same, but there's no need to refer to a 3" exhaust for our car as a 'GS-T' exhaust. It's not important, was just bugging me

Edit:When searching to purchase you must refer to it as a GS-T exhaust if you want to go above 2.5 inch, sure but there isn't really a need to here.

As for the 400 HP remark, to put that on the ground, especially with slicks you'll need to beef up that drivetrain if you don't have an LSD. Those stock axles came on a, little over 100 whp car.

 

[Blitzeclips]

The axles have be proven to handle around 400HP. Search AWDDYNAMITE running the 11 sec 1/4 and you'll see all he damaged was a CV boot.

Although, I will be upgrading my axles just to be on the safe side. An LSD really isn't necassary if you slip the clutch properly while using slicks. I don't plan on buying one.

 

[Focker420]

I was thinking of going more towards the oil return line on the oil pan. Right befor the 90 deg bend to go under the pan that way it has more horizontal movement. I'm guessing but i'd say about 5-6 inches after my o2 sensor.

Also this is kinda hard to explain but don't have pic right now to show. So on the dp were the wastegate dumps on the exhaust manifold (external wastegate) how should i connect the wastegate dump into the dp for optimal velocity. Should i use a t - type weld kind of like a t vaccum line. Or should i have the dump welded to the dp at the 90 deg bend from off the turbo exhaust housing, hope that makes sense.