3in is plenty for 90% of 4cyl turbo cars that are in the 200-300 power range. 400+ is where you would probably want something as big as a 4in, and if u have that much power, you probably wouldnt be on here looking for exhaust advice...lol

Doubt alot of ppl heard of no shoes 'round here.

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Sorry, you'll have to type a long response now, cause I have to agree with his statement (depending on the size of the motor).

Oversizing the exhuast can hurt due to the density, weight and velocity of the air traviling through it. Smaller exhausts will maintain higher velocity, which keeps the temps higher (less dense, lighter). If the exhaust is too big, the velocity slows, temps decrease and the exhuast pulses collide.

The key is to have the correct size pipe - not too big, not too small. I have 2.5" on my wife's car, 3" on my car due to the orginal T04 60-1 ( seems to be good with the 20G L2 as well).

2.5" was fine on the 16G powered DSM without any problems, and we hit low 12's with that car.

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Exactly correct, for N/A applications.

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I am glad you have had good results. However, this anecdotal evidence does not disprove that turbo applications improve as back pressure decreases.

I have never seen one serious tuner suggest that someone should use a smaller exhaust in a forced induction application. By this logic, a 2.5" exhaust could potentially be better than a 3' exhaust with a small turbo. That has been disproven as myth time and again by real world dyno testing. The well accepted rule of thumb is the bigger, the better (on FI applications). That said, as I typed above, there aren't many people on this board that will benefit from larger than 3" piping. I wouldn't even consider it if my goals were under 400whp.

The addition of the turbo doesn't effect the basic principles of exhaust flow - it only makes it more important since a poorly flowing exhaust will hinder the turbos ability to make boost.

Looking back at the previous posts:
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Right, because a 6" long exhaust means the gases are instantly out of the way. No worry about pulses or scavaging or back pressure. Though no back pressure will hinder performance before the turbo spools up.

The next best thing to having no exhaust is having an MR2 - thier full exhaust is as long as our cat backs. That or an exhaust big enough to seem like there no exhuast - like a 12" diameter! I have no idea where anyone will find a muffler to fit it though.

Can you post some dyno graphs for 2.5" versus 3"? I spent a few minutes on google and only found debates on the DSM, 200/240, SRT, and Volvo clubs on the same subject! I guess this makes 6gc a real club!!!!!!

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sorry i should have been more clear..that was a long night..
4" is pretty utterly pointless on a motor thats not supporting the HP for it..like the above stated oversized pipe without enough power to move the air isn't helping at all...yea "noshoes" can run 4" cause his pipe is 3' long in a "2" and he has a pretty freakin nice HP #......a stock 3sgte in a 10' long celica with 4" i doubt would see any gains..so "noshoes" motor and setup really doesn't apply in a celica situation..

and j...even on an FI motor if your exhaust is too big and long the exhaust velocity will eventually have slowed by the end of the pipe..you'll have increased back pressure from having to actually push the spent and slowed gases out regardless of FI or NA

its about correct size for your correct horsepower and CFM...you have to keep restritctions to a minimum and velocity to a maximum

This post has been edited by playr158: Mar 12, 2006 - 4:04 PM

about the 4" exhaust

one of the big supra shops, either titan or horsepower freaks (i think it's titan actually) sells a 4" supra catback ...

if u go to their webpage they'll provide their '1st hand' commentary on why/when you'd need it...

i think to4r also had a dyno run of most supra exhausts starting with the oldschool greddy SP and working up to the newer titanium/stainluss catbacks (so from 2.38" up to some 3" and maybe slightly over 3" catbacks) with dyno data for all of it...

as far as titan goes they saw something like a 40hp gain pulling off a 'high flow' air filter on a 600 hp car... their exhaust (4") was worth over 120hp or so on some 700+ hp car that came in with a 3" if i remember correctly...

i just want to say this though...exhaust has to do with area which is related to diameter squared

2^2 = 4
2.25^2=5.0625
2.5^2=6.25
3^2=9
3.5^2=12.25
4^2=16

so the next step up if u had a high hp 3sgte isn't a 4" yet...

alright thats a inline 6 supra.....not a 4cyl celica
so the obvious reasons why 4" works there is pretty clear..

but defenitly right on the area

This post has been edited by playr158: Mar 12, 2006 - 4:24 PM

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Actually, its even MORE applicable. Try this experiment. Blow as hard as you can through a regular straw. Now cut the straw down to 2 inches and blow as hard as you can. Tell me which one flows more air. A: the shorter one of course. Flow is increased when the pipe length is decreased. The benefits from a larger diameter exhaust are more apparent in cars requiring longer piping - not less. Noshoes actually recommended to me to go with a 4" exhaust for just these reasons. Since my goals were less than most of his customers, I stayed with 3".

Although this argument about exhaust gases cooling and slowing down has some logical appeal, I am just saying that I have never seen any evidence it is true in terms of real world experience or reading what recognized experts have posted in the past.

This post has been edited by jgreening: Mar 12, 2006 - 4:40 PM

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I know for sure that ATS did testing to show that the KO downpipe (2.5") yielded lower gains than a 3" downpipe on the same exhaust with the tiny ct-26. It was about 5hp difference I believe and was for levels way under 300whp. I know a downpipe is not a full exhaust but the principles are the same. I have also seen comparison dynos of full exhausts on other forums which have shown similar results but I am not sure I could find them.

we've already established shorter is better? thats what we all agree on....

"it is easy to get overeager on fitting large-diameter pipes into an exhaust system. "the larger the better" is not always the case....there is an exhaust gas velocity that ought NOT TO BE EXCEEDED, this velocity is approximately 250 ft/sec.....to size a tailpipe, you can adhere to this exhaust gas velocity or to simple the guildeline of selecting a tube diameter approx. 10% larger then the turbine outlet diameter."
corky bell

a 3sgte @ 15psi makes 321cfm ruffly
just to make sure i'm correct
i got it by....((122.04cid x 5000rpm x .5stroke x .90ve)/1728)x(14.7x15/14.7) = 321cfm

a 2.5" pipe can flow more then 600cfm(ruffly 293ft.sec) adiquetly
but we all know that a 3" exhaust does prove gains over a 2.5"
but at the point of 4" it would seem too large for a celica

This post has been edited by playr158: Mar 12, 2006 - 5:01 PM

Interesting post by Dave Buschar on the evo forums with Dyno numbers on an 04' Evo:

snip----

3. BR 2.5" axleback muffler. 267.1 whp, increase of 15 whp!

4. BR 2.5" downpipe. 270.4 whp, increase of 3.3 whp.

5. Super AFC with BR settings, 289.3 whp, increase of 19 whp.

This with the addition of a boost gauge and the mount for the boost gauge completes our Stage 0 and costs $995. Overall gains with this kit while remaining emmision legal with the stock cat and with a very quiet car was 52.2 whp and 29 ft. lbs of torque. HP per dollar spent is $19.06.

This package really puts the car into a great spot to do further upgrades from. Actually I was surprised by the next test:

We left the 2.5" downpipe on, added a 2.5" offroad pipe and the 3" catback with the Magnaflow muffler. The horsepower jumped to 313 whp and 292.9 ft lbs. For further comparison we ran an open exhaust system from the back of the offroad pipe, in other words no catback at all, the car only made 1 more whp. The open exhaust numbers were 314.4 whp.

We then went to the full 3" turbo back exhaust using the Magnaflow muffler and an offroad pipe. The car layed down 314.9 whp.

--- snip

Out of curiosity Scott, what lessons do you take from that post? Somehow I suspect we may have different interpretations.

That 3" exhaust is better then 2.5"
I was wrong when I said that 2.5" is only good for 400 HP, now I'd say its fine for sub 300 HP range.

It does go along with velocity and cool-down expansion effect I mentioned earlier, since replacing the rear section with 3" gained a good amount of power and replacing the turbo back didn't make much a difference. Unfortunately the dyno graphs on his web site are for buschars kits - which includes more then just the exhaust changes. I'd like to see what the effect was on low/mid range torque with the larger DP.

This post has been edited by scothaniel: Mar 12, 2006 - 9:31 PM

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In this post, I am playing devil's advocate for purposes of discussion. Lets assume that there are no factors in these runs like improper cool down times, variations between runs, margin of error, etc. Under your theory, wouldn't the power have been less with the 3" downpipe since the exhaust gas velocity would have decreased? I recognize that it only made 2 more horsepower with the 3" downpipe but the fact that it didn't lose hp seems to contradict your theory. The big 16G is not exactly a large turbo.

Its a turbo that pushes over 500 CFM at 20 psi though, which is a lot more then a stock ct26 btw. 300+ HP. In fact, Buschar was almost able to hit the 400 mark with the stock evo 16GH turbo! A ct20b cant even touch that.

Like I said, I'd like to see the effect on low / mid range torque.

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That would be if you are running a rectangular shaped exhaust. For area calculations it would be diameter * Pi (3.14159...) or volume=area * legnth of exhaust.

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sorry but you're wrong ...and i was wrong too...

circumferance (sp?) = 2pi R = pi D
area = pi r^2 ... i just dropped pi because and forgot to halven D to get R

so 2" exhaust ~ 1*1
2.5" exhaust ~ 1.25*1.25
...etc .. .they all share pi so if you're looking for % difference it doesn't matter (Why i dropped it)

the guy who said that's an inline 6 ... forget that

my point WAS ... if you are making 700 hp or so (i don't care if it came from a single cylinder or 700cylinders) ...as long as it's a 4 stroke, then 700hp is going to be about 1050cfm of air no matter what type of motor (b4, I4,I6,v6,v8...etc) it came from...and if you are moving that kind of air i'd think about using more than a 3" exhaust.

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Actually, that would be a square (not a rectangular) shaped exhaust. Diameter * pi would be the circumfrance. To determine area of the exhaust, you use radius squared * pi. Therefore, the following would be the correct calculations to determine area of the cross section of the pipe:

2 inch piping = 3.14 sq. inches of area of cross section of pipe
2.25 = 3.97
2.5 = 4.91
3 = 7.07
3.5 = 9.62
4.0 = 12.56

EDIT: Nuke beat me.

This post has been edited by jgreening: Mar 13, 2006 - 5:18 PM

Heh I see some people have forgot high school math

Do you guys know how to calculate the area of an oval (ellipse)?

Its useful when comparing oval mufflers to round ones. The bigger the volume; the quieter the muffler.

Lets say you have a

5" (height) x 8" (width) x 14" (body length) muffler - $128.42 MSRP
Oval Muffler

and a

6" (diameter) x 14" (body length) muffler - $127.47 MSRP
Round Muffler


Which of these would have the bigger volume?


Volume of Oval Muffler: 2.5 x 4 x 3.14 x 14 = 439.6 cubic inches

Volume of Round Muffler: 3 x 3 x 3.14 x 14 = 395.6 cubic inches

In this case the oval muffler has the larger volume for the same money! If you guys already knew this then take no offence

we need to work on pipe size before we get to the muffler part lol

so a 3sgte 5000 rpm at 15psi flows 321cfm (5.35 cf per second)

what is the pipe size that will keep the velocity of 321cfm exhaust gas around 400 ft/sec or so

This post has been edited by playr158: Mar 14, 2006 - 1:11 PM

i like my math better

3sgte + 3in exhaust = the right thing to do


hopefully ill be getting mine installed soon. going today to get an estimate.

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What downpipe and muffler are you getting?

thats my scan!

(and it came from "maximum boost" by Corky Bell) to give credit where it be due

This post has been edited by playr158: Mar 14, 2006 - 3:00 PM

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That must be what HP level the stated pipe would "support". It surely does not reflect the ideal size to use since a single straight 2.5" exhaust is represented as 500 bhp.

This is going to seem completly random and all, but has anyone run a dual-exhaust on a 3sgte? Im sure its not recommended, i could see it more on a mr2, but i was just curious.

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i already have a 3in stainless DP on the car and ill be using a 3in magnaflow muffler.

looks like my exhaust will have to wait some more. went to get a quote today, lets just say that it was insane! i have another shop that is willing to give me a great deal, but the guy has almost no time to fit me in... sucks.

How much was the quote? Mandrel bent? Stainless/aluminized?

What happened to the topic ... you know that sentence at the top of the page that is supposed to be what all discussion under it is about HAHA
"swappers, what did you do for exhaust?" is no more!

We should rename the topic to "swappers... what do you think about different exhaust sizes?"