Anyone running an exhast larger than a 3"?

[slowgsr]

Nice!!! Do you have any pics of the set-up? Did you use a 3" on your current set-up then switch to 4"? If so did you notice a gain with your butt dyno or actual dyno?

I changed setups, stock engine with a 58mm turbo to a built engine with a 72mm turbo. So i went from 3 to 4". I have pics on here.. I started a thread in this fabrication section. I can't do side exit because its street car, full exhaust/muffler etc. I used vbands and run a small stainless race muffler right at the rear.

 

[laser90]

I think large exhaust is seriously over rated.

I'm in no way saying that a 3" is a restriction. I'm just curious to see if others are using larger exhausts to help eek out every bit of power they can out of there set-ups

 

[pboglio]

I recently datalogged my EVOIII equipped ride running a 2.5" downpipe/3" catback, then back to back tested against a 3" open downpipe. Power was in the +400 w.h.p. range. Nearly a zero difference in power and torque between them. I made significant power gains with other mods that weekend, but swapping the exhausts wasn't one of them. Save your money and your ears and stick with a 3" catback.

 

[1992_Talon_tsi]

Well im not a big turbo guy im on my way tho, but let me talk about my experienced. The day i installed my flowtech exhaust system witch i ordered the first time 3.5" when i was done i turn on the key and the car started normally but then it started idling rough, i found out that because it was 3.5" too much back compression was used for a b16g so i just took it back and got a 3" and it worked perfectly.

 

[delta448]

I thought you could get away with smaller piping right after the turbo and the air expanded as it cooled. That's why a lot of DPs go from 2.5 to 3in.

The only thing I know of that expands as it cools is water as it becomes ice.

The Ideal Gas Law, PV=nRT

A little back pressure is a nice thing to have.

For a turbo car, this is just plain not true.

http://www.dsmtuners.com/forums/frequently-answered-dsm-questions/168578-exhaust-straight-scoop-backpressure.html

 

[RipperXX]

I'm in no way saying that a 3" is a restriction. I'm just curious to see if others are using larger exhausts to help eek out every bit of power they can out of there set-ups

I think I worded it wrong, I wasn't so much referencing the flow but noise of the larger systems. As I mentioned there are a couple ways to keep things quiet but flow well and not mis out on any power.


As noted earlier in this thread a shorter smaller diameter exhaust will flow as well as a slightly larger longer exhaust system. So for instance you think I there is any real difference in a full length 4" exhaust vs my 3" cutout setup? It dumps just a few inches after the DP. I HIGHLY doubt theres any real difference in a full 4" setup and mine. Except mine is reasonably quiet when I want it to be.

 

[laser90]

I recently datalogged my EVOIII equipped ride running a 2.5" downpipe/3" catback, then back to back tested against a 3" open downpipe. Power was in the +400 w.h.p. range. Nearly a zero difference in power and torque between them. I made significant power gains with other mods that weekend, but swapping the exhausts wasn't one of them. Save your money and your ears and stick with a 3" catback.

That's some interesting info there. Isn't the Evo III nearing its limit at that power level. I'm still curious about the larger frame turbos though. I'm not discounting you results either, just wondering since a larger turbo moves alot more air would it see similar results as yours or possibly an increase in power?

EDIT posted this before reading rippers reply

 

[1gDSM4g63]

That's some interesting info there. Isn't the Evo III nearing its limit at that power level. I'm still curious about the larger frame turbos though. I'm not discounting you results either, just wondering since a larger turbo moves alot more air would it see similar results as yours or possibly an increase in power?

You're right on, the flow rate for a 16g is like 38 lb/min whereas a large frame turbo is more than double the flow rate like my Holset HX52 which is 89 lb/min. Thus going to a larger diameter exhaust will make a significant difference. Just imagine the performance difference when going from stock exhaust to a 3" on a 16g. It's not like I'm expanding the exhaust from 2" to 3" on my larger frame turbo like the 16g, my turbo already has a 4" vband hotside.

 

[pboglio]

Come back when your flowing 89 lb/min. Better yet, run what you have right now and swap to a 4" catback on a dyno vs. a 3" catback and if you pickup +5 h.p. in the 6000-7000 rpm band I'll mail you a $10 bill. Then start investing some stock in ear plugs. Worry about what you need to worry about. This is like staying up at night worrying about needing titanium connecting rods for 300 h.p.

 

[TSimage]

The only thing I know of that expands as it cools is water as it becomes ice.

The Ideal Gas Law, PV=nRT



For a turbo car, this is just plain not true.

http://www.dsmtuners.com/forums/frequently-answered-dsm-questions/168578-exhaust-straight-scoop-backpressure.html

That article says a lot and nothing all at the same time. The idea of needing his "vacuum" aside from the horrific Nascar analogy spends back to what I was saying. A little back pressure is ok to have. You don't want a lot, its quite obvious thats bad but at the same time, having an exhaust being too big is bad as well. Thats all I was saying.

 

[pboglio]

Yes, a little backpressure (i.e. 2-2.5 psi) isn't going to hurt power an ounce, but any backpressure affects knock. So ideally, you'd still want zero backpressure, but on high octane racing gas its not as critical.

That backpressure article has snippets of correct info, and some pretty serious problems. It assumes the gas velocity is assisting the exhaust extraction, when in fact the waves that assist exhaust extraction are many times faster than the bulk exhaust gas speeds. Its the length and diameter of the tubing that times a reflected wave that bounces back and forth until the exhaust valve opens, helping to lower exhaust backpressure at the valve, IF the length and diameter is the "correct" size for the target rpm band. His "vacuum" theory is the "cadence" theory and that was disproved about 60 years ago.

Just pick up "Scientific design of intake and exhaust systems". Desktop Dyno software also had a companion softcover book that explained exhaust gas rarefaction waves (i.e. exhaust pulse/tuning) better than anything else I've ever seen.

 

[habitatguy187]

I have absolutely no idea what thread it was in but someone claimed to have picked up around 30whp on the dyno going from a full 3" exhaust to 4". It wasn't on a small turbo though, maybe a 3052??? It was forever ago when I read it.

 

[TSimage]

My 4":

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But then again thats on a BW s475 turbo with a 4" DP so that is a lot different. I actually NEED an exhaust that big. BTW yes thats full Aluminum V banned in 2 spots for easy removal.

 

[RipperXX]

Yes, a little backpressure (i.e. 2-2.5 psi) isn't going to hurt power an ounce, but any backpressure affects knock. So ideally, you'd still want zero backpressure, but on high octane racing gas its not as critical.

That backpressure article has snippets of correct info, and some pretty serious problems. It assumes the gas velocity is assisting the exhaust extraction, when in fact the waves that assist exhaust extraction are many times faster than the bulk exhaust gas speeds. Its the length and diameter of the tubing that times a reflected wave that bounces back and forth until the exhaust valve opens, helping to lower exhaust backpressure at the valve, IF the length and diameter is the "correct" size for the target rpm band. His "vacuum" theory is the "cadence" theory and that was disproved about 60 years ago.

Just pick up "Scientific design of intake and exhaust systems". Desktop Dyno software also had a companion softcover book that explained exhaust gas rarefaction waves (i.e. exhaust pulse/tuning) better than anything else I've ever seen.

I personally don't think any type of scavenging or "pulse wave tuning" etc. is very relevant in turbo applications. With out exhaust manifolds being so short, and the exhaust turbine in the mix with the back pressure (sometimes more than the boost pressure in mismatched poorly designed setups). Closest to it in a turbo app IMO would be a twin scroll exhaust housing and manifold setup.

All that crap applies to NA engines. Any amount of back pressure is undesirable period. Some old people confuse high exhaust velocity at low rpm due to small diameter headers / exhaust. AS back pressure being good and helping with low end torque. It's not the back pressure thats choking the engine at higher revs helping at low. It's the high exhaust velocity helping at low rpm's, before it starts to choke the engine.


P.S. Check the back pressure at the manifold and compare it to your boost readings. If there around the same, or back pressure is higher, you need to upgrade IF your goal is just more power (like for a drag car) Also TSimage, nice...

 

[91stocker]

Heres a post i put up a while back, hope its useful to someone.

okay, I found the article by David Vizard, and it was actually in Super Chevy magazine. It's a long article, so I'll supply you with his exhaust math below:

according to Mr. Vizard (who's theories I happen to value) you need to flow 2.2 CFM (cubic feet per minute) of exhaust per 1 HP. Therefore you first need to know what your flywheel HP is, and multiply it by 2.2

So for an example if your engine produces 500 HP at the crankshaft, then that's 500x2.2=1,100CFM. Now remember that because you're using dual exhaust and therefore will have two exhaust pipes (left and right) then you need to divide this total CFM figure by2. So that's 1,100 CFM cut in half.... or 1,100 divided by 2=550 CFM. So you'll need an exhaust pipe diameter that will flow 550 CFM. with that in mind here are the pipe diameters and their flow rates (according to Mr. Vizard)...

2.5" diameter=560CFM

3" diameter=672CFM

3.5" diameter=784CFM

4" diameter=896CFM

so just do the math above using your engine's HP and you'll be able to determine how big of an exhaust pipe diameter you'll need. I hope this helps you.

or

In general 1 inch Hg backpressure = 1 HP lost

For reference, we have the following conversions factors:

1 ATM = 14.7 PSI = 76 cm of Hg = 29.921 inches of Hg = 1.013 bar"

seems to be no need for larger then 3" untill 500+hp

 

[1995gsx20g]

Heres a post i put up a while back, hope its useful to someone.

okay, I found the article by David Vizard, and it was actually in Super Chevy magazine. It's a long article, so I'll supply you with his exhaust math below:

according to Mr. Vizard (who's theories I happen to value) you need to flow 2.2 CFM (cubic feet per minute) of exhaust per 1 HP. Therefore you first need to know what your flywheel HP is, and multiply it by 2.2

So for an example if your engine produces 500 HP at the crankshaft, then that's 500x2.2=1,100CFM. Now remember that because you're using dual exhaust and therefore will have two exhaust pipes (left and right) then you need to divide this total CFM figure by2. So that's 1,100 CFM cut in half.... or 1,100 divided by 2=550 CFM. So you'll need an exhaust pipe diameter that will flow 550 CFM. with that in mind here are the pipe diameters and their flow rates (according to Mr. Vizard)...

2.5" diameter=560CFM

3" diameter=672CFM

3.5" diameter=784CFM

4" diameter=896CFM

so just do the math above using your engine's HP and you'll be able to determine how big of an exhaust pipe diameter you'll need. I hope this helps you.

or

In general 1 inch Hg backpressure = 1 HP lost

For reference, we have the following conversions factors:

1 ATM = 14.7 PSI = 76 cm of Hg = 29.921 inches of Hg = 1.013 bar"

seems to be no need for larger then 3" untill 500+hp

doesn't say a word about turbocharged.
also "dont forget dual exhaust" kind of sounds like v8 world to me.

 

[pboglio]

Let me ask a question, if exhaust scavenging is such a minor event in turbo cars, then why is everyone falling over themselves to build an equal length header, of the proper length AND runner diameter? Its there, guys who've exhausted all other options go after it, but the physics doesn't cease to exist because a turbine wheel is in the way, we are talking about pressure waves that are hugely more powerful than any sounds waves. If air can pass thru the turbine wheel just fine, an exhaust pulse wave can as well.

Let me put it this way, our intake systems are heavily dependant on intake pulse tuning, and the throttle plate could be 75% close and we still benefit from it.

Having said all that, I noticed a minor low end powerloss without the full exhaust system on, but it wasn't anything that bothered me since I was expecting the car to be gutless.

 

[1995gsx20g]

im pretty sure all of the equal length manifolds and runner diameter all make it a more smoothed out and balanced power band.

is the pressure drop from pre-turbine to post-turbine very large?

 

[TunersNation]

I agree with the posters that say you make more power with a larger downpipe than a smaller one. You however have to consider your full setup when putting the pieces together to maximize the outcome. So for people running 3.5" or larger dp & exhaust on a stock to 16g variant turbos, you're actually going to make less power than with a smaller dp/exhaust.

At ~550 whp, we upgrade to a 3.5" or 4" exhaust depending on the what the final goal for the car is. By-the-way, nice 4" aluminum exhaust......Back on topic. We have a customer's street car (1g gst) that we put a 4" aluminum full exhaust on. The car is close to 700 fwhp with goals for another 100whp before we squeeze the bottle for again another 100 whp. That should put us somewhere ~900whp. We're probably going to re-tune it again this week because we made some changes in the valvetrain .

A lot of people go straight to a 3" system because it saves them from upgrading agin later. Also as stated in this thread, all 3" downpipes start life @ the o2 housing in 2.5" because a 3" flange will not bolt up to a 1g/2g/e3 factory o2 housing.

To the original poster, if your goal for your track car is +550 whp, go for @ least 3.5".

 

[1gDSM4g63]

Come back when your flowing 89 lb/min. Better yet, run what you have right now and swap to a 4" catback on a dyno vs. a 3" catback and if you pickup +5 h.p. in the 6000-7000 rpm band I'll mail you a $10 bill. Then start investing some stock in ear plugs. Worry about what you need to worry about. This is like staying up at night worrying about needing titanium connecting rods for 300 h.p.

Who are you talking to? I obviously don't DD my car around. And yes, a 3" dp and exhaust is good enough. It is very well capable of at least 600 HP.

My 4":

But then again thats on a BW s475 turbo with a 4" DP so that is a lot different. I actually NEED an exhaust that big. BTW yes thats full Aluminum V banned in 2 spots for easy removal.

Does you car actually pass California smog with the 4" exhaust setup?

 

[dsm-onster]

Exhaust scavenging is important BEFORE the turbine on turbo cars.

Tuning exhaust diameter to power ranges has nothing to do with backpressure. If 2" exhaust matches your setup for 3500-5500rpms, then your actually seeing LESS back pressure at the header collector. The back pressure comes on after and that's why you see the power band look like it does. Turbo setups typically have turbine wheels at the collector disrupting flow and flowing different amounts at different times at the same rpm even.

Most of the top dogs have little to no down pipe. IF a little backpressure is OK then they would run more. It's easy to experiment with this. And, I believe the OP is asking about eaking out the most from their exhaust not what's OK to do.

The weight savings alone is impressive. The stock exhaust weighs 50lbs per my scale. And that is sub 2" piping.

I do agree that there is minimal benefit for most of us to run larger than full 3". The resistance to flow is minimal at 16g, 20g, s256 range. Which 99% of us run or want to run compressor flows in this range.

 

[delta448]

My 4":

But then again thats on a BW s475 turbo with a 4" DP so that is a lot different. I actually NEED an exhaust that big. BTW yes thats full Aluminum V banned in 2 spots for easy removal.

Looks awesome, I bet it sounds pretty impressive at full boost on that turbo too!

I personally don't think any type of scavenging or "pulse wave tuning" etc. is very relevant in turbo applications.

This is exactly what I was trying to imply.
Building backpressure post turbine primarily does one thing, it reduces the air pressure difference across the turbine, which directly affects how much air mass per time will flow through the turbine. More turbine airflow = more engine airflow.

Let me ask a question, if exhaust scavenging is such a minor event in turbo cars, then why is everyone falling over themselves to build an equal length header, of the proper length AND runner diameter?

Because that's what they see all the pros doing. Most of us won't ever be at the level where we need to spend the big money and have to worry over things like that.

Equal length keeps the pulses incrementally equidistant to the volute; the largest exhaust restriction (after the exhaust valves) in most turbocharged cars is the volute. Equal length will reduce flow restriction by eliminating interference from colided pulses at specific engine speeds, which can also have an effect on force transferred to the turbine wheel, especially at low rpm. A non-equal runner length manifold can be length engineered/tuned to produce higher turbine pressure at a specifically desired spool-up rpm, but can also choke flow at high engine speed when the harmonics are down to second or first order. Perhaps the worst effect of non-equal length manifolds though, is that they will create big differences in the VE of their corresponding cylinders. Ever wonder why cylinder 2 is the most problematic on 4G63s? Individual cylinder tuning is found more frequently in the cars that spend the big money on equal length manifolds too, and not by coincidence.

Incidentally, this is why horizontally opposed engines (read: Subarus) have a "punchy" exhaust note at low rpm that flattens out somewhat at higher rpm, since the exhaust pulses are timed differently from each bank and will resonate differently.

Length is usually only intentionally increased when trying to make the runners as straight as possible since this can be more beneficial to flow than short runners that curve sharply.

Diameter is the same as referring to the cross sectional area (as a flow restriction), the most ideal way to make the transition from head exhaust port to manifold collector to turbine is by decreasing the cross-section gradually in a slow manner with no large changes in diameter, which could alter the force of each pulse by changing either it's pressure or velocity.

If air can pass thru the turbine wheel just fine, an exhaust pulse wave can as well.

Not always. This is relative to the turbine used. One that is restrictive to flow compared to it's compressor flow will destroy the pulse almost completely by taking a larger percentage of the energy and giving it to the turbine wheel, but also by choking the flow off at high rpm or boost pressure. A turbine that spools late and flows very well can conceivably preserve the pulses somewhat.

Having said all that, I noticed a minor low end powerloss without the full exhaust system on, but it wasn't anything that bothered me since I was expecting the car to be gutless.

I find this interesting, is it possible that the change in the exhaust could make enough effect that the ignition timing would need to be altered on your car? I imagine this is due to the car acting as a N/A car when under the spool threshold, where a larger exhaust will hurt scavenging and pulse strength.

 

[1995gsx20g]

To OP,

- We can all pretty much agree that a 3inch dp runs out of breath from 550-600whp.
- In most cases a setup with over 600whp won't really need a full exhaust anyway but if so then 3.5 to 4inch catback is needed like Tsimage's beautiful setup.

On a side note this should be a sticky.


The pressure drop from pre-turbine to post-turbine is this the reason scavenging does not matter post-turbo but matters pre-turbo?


In my 95 gsx with a n/t throttle body and a td05 20g at 9psi i found from personal experience when going from a full 3inch o2 housing open right at the oilpan to a 3inch exhaust with a highflow cat, 24inch long cherrybomb and a magnaflow muffler that has a 2.5inch outlet.
i did notice a gain in torque but i believe i gained alot of n/a torque(snappy responsive torque) but i was not logging anything at the time so it very well could have been changing ignition timing making it spool up slower with the o2 open. Also having the o2 housing open makes the turbo spool in a slower manner but once it gets going there is nothing stopping it.

For drag purpose i would choose open o2 anyday so i would say go with a 4inch dp at 600hp.

In simple terms i believe backpressure is a DD driveability concern if it is a concern at all.

 

[habitatguy187]

Having said all that, I noticed a minor low end powerloss without the full exhaust system on, but it wasn't anything that bothered me since I was expecting the car to be gutless.

I definitely noticed a low-end difference when I went from open 3" DP to bolting up a dual-chamber muffler to the DP and dumping under the car. It felt much more responsive in the lower rpms. This was back when I still had the 14b too.

 

[dsm-onster]

All this is because the o2 housing is open to tame boost during 3500-4500rpms. The flow has a direct path not affected by the turbine wheel where the turbine ONLY responds to pressure/temperature differentials to function at all.

So in such point in the rev range, the nice scavenging effect of a smaller diameter pipe may help draw gases out better and thuse VE goes up and you feel more torques since the VE curve closely resembles the torque curve. I'd rather have the flow there when I need it AFTER then for a max effort setup with whatever turbo I was using. Thus why folks like English Racing run an open downpipe with their 16g/fp68 setup. Same turbine wheel as the 14b with a slightly larger turbine housing. Works well with smaller turbos and much larger turbos alike.