Ok, theres debates about this all the time.
please try not to turn this thread into another one, id like to keep it civil, and keep it to factual evidence as much as possible. everyone has their own opinions on it. and thats fine.
but none of us is going to agree until we see it on paper.

in my opinion, based on what ive seen so far, I dont believe the top end gained by a 3 inch, will make up for the lost torque, for a cobalt making approx 250whp....
but i would be more than happy to be proven wrong.
hell i would be the first to go buy an electic 3 inch cutout for my downpipe right now.

but we need some proof.
I really would love to see some dyno proof of 3 inch vs 2.5

 

I dont see the need for a 3 inch on a 4 cylinder. My 2.5 inch exhaust and the rest of my setup still ran slightly faster than the hahn setup with the 3 inch exhaust. I dont think it hurts but i also dont think 3 inch is neccesary.

 

see thats exactly how i feel too.
but theres a good amount of people who strongly believe the opposite.

Id just love to see some real hard facts on it, instead of just theories, so we can all make a more informed decision.

 

what about 2.75?

 

thats in the mix too for sure.

 

i have the 2.4 ss, however i purchased the magnaflow exhaust for the 2.2/2.4 way back in the day. I thought it was a 2.5" exhaust system but its only 2.25. Now that i'm supercharged im kinda disappointed because i want to go larger, but im not sure what size. But honestly i could afford to lose some low end torque because i have no way of controlling it in the lower rpms (no boost controller ability such as the boost bypass on the LSJ)

 

tq or boost limiting in tuning

 

not with the LE5 (which is what i was just trying to explain lol)

 

okay heres what I have....

supercharged cars dont spool up like turboed cars...as we all know...With turboed cars you want more flow so that the turbo can spool up quicker. Hence why when you get a 3" exhaust on a turboed car you get more gains.. Heck look at say a STI ... they can gain up to 30-45 whp with just a 3-3.5" exhuast.... I mean its crazy.. but since we have a supercharged car.. we want flow but we also want alittle backpressure... we run on pulleys and our boost has nothing to do with the exhaust system directly like turboed cars.. the best way we can get gains as far as exhaust goes is probably a 2.5-2.75" exhaust... that way you dont loose as much low end torque... so anyway.. thats my .02

 

backpressure is the enemy of flow, you do not want backpressure. scavaging is what your after. here is a basic idea of what happens with the exaust flow

when the exaust leaves the manofold, it creates a vacume on the back of the other valves. the idea behind the theory is that when the pulse leaves the manifold it will create a vacume on one cylinder during the overlap period of the cam (when both the intake and exaust valves are open in that cylender). this pulls extra air/fuel into the cylinder, more than the piston itself could.

now this is were pipe size comes into play. a larger diameter pipe will tend to make scavaging more efficient at higher rpm, were a smaller pipe will be more efficient at lower rpm, but top end performance will suffer.

now throw a header into the mix. a header is designed to do all the scavaging. the primary diameter and length, aswell as the collector diameter length all play a huge part as to were the header is most efficient. long and/or small diameter primaries will be very efficient down low, were short and large diameter is better up high. but no matter what header design you have, any exaust after the collector is a restriction, this is why you make more power up high with an open header. now with a V engine, an X or H pipe can help scavaging a lot.

to throw a supercharger into the mix, basicly once you hit boost, the header does nothing. the engine doesnt scavage as instead of sucking air into the cylinder, its being forced in.

basicly as i see it, with a supercharged 4 cyl, you are making no power bellow 3000rpm, and if you want power you drop a gear and step on it. you are better off going with a short tube large primary header and 3" exaust to aid power when your under boost. out of boost you will see a bit of a gain with a proper header.

 

see both explainations of why/why not make sense.
but theres still no way to prove which one applys specifically to our engine until we test it.

maybe i should just buy a cheap 3 inch cutout and dyno with it open and closed heh.

 

2.5in for 250hp...id consider a 3in at 300hp...depending on my driving style
btw, the above post is referring more to tuning your exhaust, which relates less to the diameter of your exhaust and instead depends on the length....the rest of the info he left out would state that these pulses of high/low pressure move at the speed of sound until the end of the pipe, and then reverse direction (the same thing happens for the intake), so you do some basic math to determine how long your exhaust needs to be so that your low pressure point (a vacuum) returns to the cylinder during valve overlap, to assist in scavenging as stated. however, this will apply to one exact rpm only, making it a tool used more in CVT equipped cars that can run at this one 'tuned' rpm

there is a set diameter exhaust for a set HP number, and any deviation will hurt power. However, note that at different RPMs, your making less HP than your max HP. So while a 3in exhaust might be a great size for the 260hp your making at redline, the same 3in exhaust will be preventing you from making more power at every point below that redline (the closer you get to the redline, the less the exhaust inhibits power)

the best exhaust would be one that changes from a 1.5", to a 2", to a 2.5", to a 3" as your revolutions (and resulting horsepower figures) increase. this is the concept of VVT...where your not changing the size of the entire intake/exhaust plumbing, but you ARE changing the size of the entry/exit points into the cylinder to match the HP your making at the specified RPM

using a VVT system that uses one exhaust valve at low RPM, and 2 at high RPM as an example, your not going to be utilizing a 2" exhaust down low and a 3"in exhaust up high, but you WILL be doubling your exhaust valve surface area at high RPM vs low RPM as soon as your VVT system opens the second exhaust valve.

all of this is aimed at keeping your exhaust gasses moving as fast as possible without your piping being so small that its actually a restriction. changing your valve surface area will help in doing this, but thats only one part....effectively similar to replacing your exhaust with a wider diameter one but leaving your headers/DP stock. yes, it helps (as we all know...), but changing it all would be best. The day someone figures a way to constantly change piping diameter based on RPM is the day the perfect exhaust is invented

NOTE: the above information is regarding N/A cars and S/C cars...T/C cars work differently, being that theyre actually driven by the exhaust, and thus have different rules to follow when determining proper exhaust sizing

and thats my write-up for the day! hope you guys learned something...cause that was a bitch to type! lol

 

I can provide you #'s for a 3.5" TBE on an Evo by next weekend if that helps.

 

I just dumped my 3 inch exhaust in favor of a 2.5 inch. 3 inch exhausts are loud and I didn't get any more HP than cobalt's with 2.5 inch exhaust at the dyno meet here in gaithersburg.

When you guys talk about the 300 HP mark, are you talking BHP or WHP? I bet the actually output of the LSJ is 320 HP before supercharger/draintrain loss on a stage 2 cobalt.

 

Well, Im gonna keep building on my 2.5 inch quiet corsa exhaust until I see a problem.

 

I agree. my loss on bottom end is not making up for it on top end. I regret my 3". But yet im also untuned yet as well.

 

i personally was referring to whp when talking about specific cobalt applications

 

do you perhaps have any dyno graphs showing the two?

id kill to see that.

and ty for the input.

I think a 3 inch would be beneficial at 300WHP...definately not BHP

 

Are you helping to prove my point or to dis prove it???

 

I might get a dyno with the different exhausts some point in the future, I have plenty of 3 inch exhaust dynos with and without nitrous. None with the 2.5 inch yet.

 

im on stock catback now with my car and im not sure where i wanna go with it yet, i know once im done the car will be over 300 whp, but this does not necessarily mean the car will need a 3". Look at 03/04 cobras alot of them run 2.5" catbacks and make 450+ whp. Im not sure but i may do a before and after dyno that day to see what the car gained. As for now i am thinking 3" unless there is a nice flowing 2.5" setup

 

there is ABSOLUTELY no need for 3 inch exhaust on a small displacement motor. In fact you will loose torque when 4 cylinders most need it with 3 inch exhaust.

 

Trying to help you prove your point of bigger exhausts on Turbo'ed cars increasing power compared to bigger exhausts on SC cars. Unfortunatly, I never got to the dyno yet

 

you cant use a turbo car as a comparison


put a 3 inch downpipe cutout on a turboed car. and torque goes up.
put a 3 inch downpipe cutout on a S/C car, torque goes down.

 

what does tq matter though when you are pretty much around 4500in every gear after you shift. Rob ran his 12's on a 2.5 catback, dwk150 had a 3.0 vibrant, im just torn between the two. Also i run a 2.5 pullied car tq won't get hurt that bad