jon_0_3

OP said when u do the math with 311hp u need an exhaust size of 2.77" so anything under 300hp i would have to say 2.5" would be just fine. should know soon i have 2 guys with stage 2 one with a 2.5" exhaust and the onther getting a 3" so we will have to wait and see

Jackalope

That GOD someone gets it beside me and Nomy! 3" below 300hp is too damn big!

NJHK

I can understand the calculation but it doesn't factor in where the powerband is located in the RPM range.

Also there are two different types of powerbands: Peaky and Broad

These calculations might be correct but it might not factor in that this is for PEAK horsepower or where in your RPM band it's going to benefit you the most.

Regardless of pulley size relating to how much air pressure you're creating in the manifold, you're only going to push out as much as you ingest.

ItalianJoe1

iTrader: (10)

Has anyone ever actually dynoed any car to prove how much you lose by using to large of a system?

walrus

Great article. May help some of you out.
http://www.cobbtuning.com/info/?ID=3222

an0malous

+ rep for you my man.
thats exactly my point.

Kritter

Ok, maybe I'm way to over-analytical, or maybe just ot as knowledgeable about exhausts as I thought, but here's where I'm having confusion. The exhaust ports on the engine itself are only so big, hich means the exhaust manifold intake ports are the same size to match the engine, correct? Basically this tells me that only a set amount of exhaust is coming out of the engine, and regardless of 2, 2.5, 3.0, 4.0 inh exhausts, the amount of air leaving the engine is still only going to be as much as the exhaust ports on the block allow, unless you port/hone out the engine's exhaust ports, which will release more air. In that case you may as well hone the cylinders, and up your pistons so that the bigger ports you just made have an increased amount of exhaust leaving through them, which then gives way to needing a larger diameter exhaust.

In the end it just seems to me that unless you actually increase the air transfer WITHIN the engine itself, you're going to have the same amount exiting the engine as you did when it was still stock. Simply put, if you take a strw, and widen the diameter of one end, and blow air through the untouched end, you're only blowing as much air into the widened end as the untouched end will allow.

I just talked to my uncle, who builds race cars, and he agreed with me, but I don't know, just my thoughts.

N8s07SS

Well, the way I read this:

(1,25 inch (radius) x 1,25 inch (radius) x 3.1416 x 115 CFM per square inch) divided by 2.2 CFM per BHP = 256 BHP = 215 WHP. Plus the fact that this is unfactored with no bent and no CAT and no muffler.

Is that 215 whp is the limit of max. efficiency for a 2.5" system.

And therefore from this calculation:

(1,5 inch (radius) x 1,5 inch (radius) x 3.1416 x 115 CFM per square inch) divided by 2.2 CFM per BHP = 369 BHP = 310 WHP. Plus the fact that this is unfactored with no bent and no CAT and no muffler.

I get that a 3" pipe is most efficient at 310 whp. Am I understanding his calculation wrong or what?

an0malous

people are dynoing 12hp gains going 2.5 with about 240whp.....?
and why after this long, do we have not 1 single dyno before and after showing a 3 inch dyno doing anything other than kill low end.
ive talked to 2 people in person so far, who have told me to my face, that their 3 inch killed power with their current setup. 250-270whp
but both are planning on big turbo builds....so thats why they did it.

in all honesty, theres so many numbers in those equations that have not been proven, as well as the fact that alot of factors have been ignored....
Exit velocity...header type/shape/length/size.....

we can sit here and argue points of invented math all day.
but the facts show that 2.5 works with 250-280hp like a charm, dyno proven.
and 3 inch has yet to show anything, except quite a few people saying from experience that they felt power loss.

N8s07SS

Another thing I wonder too is why all this talk about 2.5" vs. 3"? What about the GMPP exhaust that is 2.25" and probably is more popular than any other cat-back? That's basically the reason I am so skeptical about the whole 3" is better argument. Why would GM engineer an exhaust that was too small to allow for more power? I have a feeling that they have a tad more experience designing exhaust systems than anyone on this site.

an0malous

now your asking the good questions.
why smaller? why?
why not go 4 inch? 5 inch? i mean bigger is better right?

G85Cobalt

Is there any company making exhaust systems that is between the 2.5 and 3.0..... if not that would be a smart idea...the slight loss of tq lowend but also some high end gains..

Jmc007

Firstly, when calculating flow for a 3" exhaust, I should have taken the inside diameter, which is close than 2.88". Then, it gives 750 CFM or 340 BHP or 290 WHP. Thse numbers ONLY means that this particular 3" exhaust will NOT cause any HP loss up to 340 BHP. HOWEVER, this particular pipe will (without any doubt) cause some low end torque loss for a couple of reasons. Yes because of the lower velocity. And surely also because the car has not been tuned properly. You know since this bigger pipe is waay less restrictive, then there is more flow entering the engine at the same rpm. So it shifts the PCM outside the tables and it just throw more gas and less timing than before, causing your buddies to say that that exhaust killed there power at low rpms.

If we do the same calculation for a 2,75" exhaust, the ID is close than 2.63". Then, it gives 625 CFM or 285 BHP or 240 WHP. It also ONLY means that this particular 2,75" exhaust will NOT cause any HP loss up to 285 BHP, which is somewhere close of the LSJ with some bolt-ons without any major modifications. It could be a great size to use and i think someone should make some dyno testing on that one.

http://www.dezod.com/pd_dezod_motorsports_275.cfm

If we do the same calculation for a 2,5" exhaust, I should have also taken the inside diameter, which is close than 2.38". Then, it gives 510 CFM or 230 BHP or 195 WHP. It ONLY means that this particular 2,5" exhaust will NOT cause any HP loss up to 230 BHP, which is already somewhere close of the LSJ output without any mods ! So to me this size is a good trade-off for : Those who don't want to lose any low end torque, those who cannot tune anything inside their PCM, those who simply add a few minors bolt-ons (and I'm not talking about a 2,5" pulley with 20 PSI of boost).

Jackalope

You guys all sound as if your building to get that last 1 hp to net another 0.1 second in a 1/4 mile cause your down to you and one guy for the national championship!

The fact that 3"MAY FLOW better at the high high end of the rpm band is about as useful as a fart in a spacesuit when you drive your car on the street every day. The fact that you will never notice enough high end rpm range gain to justify how your low end will in fact suffer on the street is why this is a moot argument for an SS/SC.

Untill the day your dynoing 300 WHP 3"is a waste of money. Most racers are won or lost in the first 60' so why woud you want to reduce your of the line performance? Thats just plain silly!

Area47

i have a 3 inch, there IS a loss in low end. i can go back through old dyno charts that i have done and see this. is there a gain up top? yes, mostly felt above 5k rpms. if you have hpt you can gain some of the low end back via timing tables, but you will never gain all of it back. hi, science.

for every action there is an opposite and equal reaction.

why do i have a 3 inch? simple. i didn't have a magnaflow instock.

G85Cobalt

JMC007 is that the only company making the exhaust in 2.75.. if not i may go custom. i know there are many threads on here but what muffler will give me the deepest sound. i know regardless its going to be somewhat loud but i want deep without the i hear you down the street type i had on my civic

Jmc007

Well I should have said this would be the combined flow of all exhaust ports together, this at normal exhaust temperatures.

seekNdestroy

After arguing with my neighbor on this topic we came to an answer.valves The car is only going to release so much exhaust at once, he said the easiest way to figure exhaust size is your valve size. X2 (there's 2 exhaust vakves) I did some generic research ablnd found an upgraded head with 31mm exhaust valves. That's 62mm. I don't kow conversions so google told me that 62 mm is 2.44 inches, leaving us with a 2.5 exhaust. Yhis is for all around power, of course 3" will flow more on top end but you lose on bottom, so it makes the car more useful for hi rpm. But laggy at lower rpm . This is the same reason top fuel dragsters don't have 8 ricer fart cans attached to their engines, instead they use smaller tubing for max power. This is coming from someone who makes 600 hp on a n/a 1400cc drag bike. Bigger ia not always better or wed all be running open heads

On the mario andretti camaro build, in which they supercharged, they actually put a magnaflow exhaust on that has a SMALLER diameter than stock for an upgraded exhaust. They didn't go to a6" exhaust

stonny9

um this thread is 4 years old. A local member gained 5whp from 2.5 to 3. And he was on a 2.5 TVs with ported head

seekNdestroy

Lol it is from the release of the car but I think it needs some dyno numbers w a stock m62 w simple bolt ons then one w a tvs full monstern then stickied for those who are stuck in limbo on the subject.

P.s we gotta rerun. My ic pump was dead last time we ran.. lol

ItalianJoe1

iTrader: (10)

Tubing right off the head is way different than the cat-back. You can't look at the head and decide on an exhaust size that works. It depends on the airflow demands of the engine and its overall numbers.

The test on the TVS car was great, that was an engine moving a ton of air, and it still wasn't being held back by a smaller exhaust. A stock engine will make pleny of power on a stock size exhaust. The stock muffler is restrictive, but the piping isnt.

ebristol

iTrader: (4)

What that from a straight swap with no changes in the tune?

What was he running for a header/dp?

Was it the stock cat-back or an aftermarket 2.5"?

ItalianJoe1

iTrader: (10)

Clear Image header with no cat. GMPP cat-back to 3" was the only change.

Staged07SS

iTrader: (25)

GMPP catback is 2.25" (which I know you already know). So, I can see why he gained some horsepower.

I've found that a good 2.5" catback provides the best of both worlds (excellent exhaust gas velocity & flow).

Although, WSfraiser puts down around 350whp with a TVS & GMPP catback so, it's still debatable which setup is best.

07MetallicSC

iTrader: (16)

^ what was the rest of his build? curious as i cant remember