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Discussion Starter · #1 ·
I know someone did an exhaust comparison on here but I can't remember who? Ken, Sound Performance??

If someone knows the link by all means post it up.

My question is regarding exhaust diameters. I am having a custom exhaust built but am stumped in size. What HP range do most jump to 4"? The shop building it says that 3.5" is the biggest I would ever need to go and that 4" is a downfall. SO I am looking for some graphs of the differences.

Engine mods: BW S366 .91AR
P&P +1mm head
GSC S2 cams
3.0L
 

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There is such a thing as too little back pressure. It all depends on your setup. In the February 2011 issue of Modfied magazine PHR did a test of the 3" vs. 3.5" exhausts on a MKIV. The test car gained 66 RWHP at 19 psi and 52 RWHP at 29 psi. The gains were after 500 RWHP was achieved it seems.
 

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There is such a thing as too little back pressure. It all depends on your setup. In the February 2011 issue of Modfied magazine PHR did a test of the 3" vs. 3.5" exhausts on a MKIV. The test car gained 66 RWHP at 19 psi and 52 RWHP at 29 psi. The gains were after 500 RWHP was achieved it seems.
There is no such thing as not enough back pressure. Back pressure is completely useless on a turbo car and even useless on a properly designed NA car.
 

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keep in mind where different exhaust diameters start to become meaningfully restrictive. a 3" will start to become restrictive around 700 or so whp. that doesn't equate to saying "at 700whp you NEED to go to a 4" exhaust."; it's simply the approximate point, at which, the 3" starts to represent a significant restriction. iirc, i remember Geoff from FR posting that in his testing, a 4" exhaust was not a meaningful restriction until approx 1400whp.

also keep in mind that the noise level increases dramatically between 3" and 4" (in my experience at least, having run both). the best street setup is probably a 3" or 4" dp/mp with a cut-out, running into a 3" cat-back. and ground clearance is a final concern - if i were starting over, and going larger than 3", i would use oval tubing this time around to gain a little extra.


craig
 

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Discussion Starter · #8 ·
Yeah I totally got away from the 3" exhaust. It is either 90mm/3.5" or 100mm/4". I was told that the exhaust pressure wouldn't be there and in return create a laggier turbo-pushing the power higher in the RPM.
I would be curious to see where the gains were from going 3.5" to 4". If someone had around an 800hp car to check. The other think I find is crappy is there really isn't a way to tell if you are restricting the motor unless you change and that is expensive.
 

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Would I be correct in saying that a larger diameter exhaust will increase turbo lag but also increase horsepower while a smaller exhaust will decrease lag but reduce horsepower? I'm wanting to make my power early and only have a 600 RWHP goal on pump + meth. If I were making 600 RWHP and going to a 3.5" exhaust would shift my power band to the right I'd rather stick with the 3" exhaust.

On a related note I was told that 2.5" IC piping would be better than 3" piping for quicker spool time.
 

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Would I be correct in saying that a larger diameter exhaust will increase turbo lag but also increase horsepower while a smaller exhaust will decrease lag but reduce horsepower? I'm wanting to make my power early and only have a 600 RWHP goal on pump + meth. If I were making 600 RWHP and going to a 3.5" exhaust would shift my power band to the right I'd rather stick with the 3" exhaust.

On a related note I was told that 2.5" IC piping would be better than 3" piping for quicker spool time.
no, larger diamater exhaust won't hurt your turbo performance like that.

the 2.5" piping on the intake tract will be slightly less laggy because you've got a smaller volume to pressurise.
 

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Discussion Starter · #12 ·
So I went on a search looking for Oval tubing and HOLY SHIT that is expensive. Burns sells it for $61.00/foot. I am afraid that the 4" is going to rub on everything. Do you guys have problems with it rubbing?
 

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Would I be correct in saying that a larger diameter exhaust will increase turbo lag but also increase horsepower...
no, larger diamater exhaust won't hurt your turbo performance...
right. generally speaking, the rule with turbo charged cars is: "the best exhaust, is NO exhaust."

one caveat regarding that rule - i have heard knowledgable people speculate that an unrestrictive exhaust can hurt bottom end power, before the turbo is even spooled (when the car is basically in "NA" mode); but this is generally considered irrelevant, as there is really no reason to be concerned with a turbo car's power output in NA mode.


So I went on a search looking for Oval tubing and HOLY SHIT that is expensive. Burns sells it for $61.00/foot. I am afraid that the 4" is going to rub on everything. Do you guys have problems with it rubbing?
depends on ride height and how well the exhaust is tucked up under the car. my car is lowered (not slammed) on coilovers, and running 245/40 18f and 295/35 18r tires; i have about 4" of ground clearance at the v-band between the dp & mp (the v-band hangs about 1/2" lower than the piping). and my 4" exhaust was fab'd by a professional supra builder who tucked it pretty tightly. when i mount the 295/45 17 DR's on the back, i get another 1/2" or so of clearance.


craig
 

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The best set up for noise would be the 4 inch DP/MP and an SP boost actuated cut out along with a 3 inch cat back system (HKS, Tanabe, Greddy, Apexi, whatever). This is what I run on my car. The car only gets loud under full boost as the wastegate and cut out open.

Steve
 

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my car spools better with open DP and AFR are a tad different, really noticed it around 3-5k rpm

consider going stock exhaust with a 4" downpipe and 4" cutout

thats going to be the rage soon in my opinion. they are cracking down on loud exhaust hard here in vancouver island/bc canada. in usa the limit is 92 dB and they just changed it here to 84 dB. (loud motorcycles basically no longer exist here)
i just got a ticket for too loud and my 4" exhaust with ported head/+1mm valves was 94 dB at part throttle with a hks super drager that is "so quiet they dont even make a 5" silencer for the tip"

i realize this isnt totally relevent to sizing choices but it is a solid choice if you want the benefits of wide open exhaust and little backpressure, but also want to cruise around secretly with the flip of a switch.
and theres no better time to invest in a silent +cutout setup than at the beginning.

4" is super loud. my car wakes up my neighbours. ive had 2 of them complain.
but if your an enthusiast it sounds soooo good. :)

i would venture a guess that volume is the only CON with going larger exhaust.
go with a 6" oval pipe and call it done.
 

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I really like the idea of a 4" DP with a cutout and then have the rest of the exhaust in 3". Gives you the best of both worlds and is a great way to get someones attention only when you want to.
 

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Would I be correct in saying that a larger diameter exhaust will increase turbo lag but also increase horsepower while a smaller exhaust will decrease lag but reduce horsepower? I'm wanting to make my power early and only have a 600 RWHP goal on pump + meth. If I were making 600 RWHP and going to a 3.5" exhaust would shift my power band to the right I'd rather stick with the 3" exhaust.

On a related note I was told that 2.5" IC piping would be better than 3" piping for quicker spool time.
This logic is correct when talking BEFORE the turbine (eg turbine housing sizing), not after.

my car spools better with open DP and AFR are a tad different, really noticed it around 3-5k rpm

consider going stock exhaust with a 4" downpipe and 4" cutout

thats going to be the rage soon in my opinion. they are cracking down on loud exhaust hard here in vancouver island/bc canada. in usa the limit is 92 dB and they just changed it here to 84 dB. (loud motorcycles basically no longer exist here)
i just got a ticket for too loud and my 4" exhaust with ported head/+1mm valves was 94 dB at part throttle with a hks super drager that is "so quiet they dont even make a 5" silencer for the tip"

i realize this isnt totally relevent to sizing choices but it is a solid choice if you want the benefits of wide open exhaust and little backpressure, but also want to cruise around secretly with the flip of a switch.
and theres no better time to invest in a silent +cutout setup than at the beginning.

4" is super loud. my car wakes up my neighbours. ive had 2 of them complain.
but if your an enthusiast it sounds soooo good. :)

i would venture a guess that volume is the only CON with going larger exhaust.
go with a 6" oval pipe and call it done.
Perhaps you mean 3 inch exhaust with a 4 inch cutout? Either way, my car should be back in the next week or two with that setup. I'll be sure to make plenty of videos of it.

PS Making the exhaust piping any more than say .5" bigger than the turbine outlet is gonna be an exercise in futility.
 

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right. generally speaking, the rule with turbo charged cars is: "the best exhaust, is NO exhaust."

one caveat regarding that rule - i have heard knowledgable people speculate that an unrestrictive exhaust can hurt bottom end power, before the turbo is even spooled (when the car is basically in "NA" mode); but this is generally considered irrelevant, as there is really no reason to be concerned with a turbo car's power output in NA mode.
I thought this was more related to header design - given that in an NA car the exhaust pulse tuning benefits are mainly in the header - and teh presence of the turbo means you aren't going to get the benefits of exhaust pulse tuning beyond it, it'd behave a bit like a pipe with a damper at the end of it?
 

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Discussion Starter · #19 ·
The best set up for noise would be the 4 inch DP/MP and an SP boost actuated cut out along with a 3 inch cat back system (HKS, Tanabe, Greddy, Apexi, whatever). This is what I run on my car. The car only gets loud under full boost as the wastegate and cut out open.

Steve
How easy is it for that actuator to open? What kind of power are you running? If you have the 3" wouldn't that make your exhaust pressure pretty high?

This logic is correct when talking BEFORE the turbine (eg turbine housing sizing), not after.



Perhaps you mean 3 inch exhaust with a 4 inch cutout? Either way, my car should be back in the next week or two with that setup. I'll be sure to make plenty of videos of it.

PS Making the exhaust piping any more than say .5" bigger than the turbine outlet is gonna be an exercise in futility.
I am looking forward to seeing/hearing some videos. Will you post up some pictures as well?
 
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