Topic: exhaust diameter CFM rate hmmmm

[hammy221]

im looking for a calculator to determine the difference in CFM flow rate between 2.25, 2.5 and 3" exhaust on our cars and what kinda power each can handle for the diameter and flow they can handle on a boosted application. now i know 3 will flow the most but what i wanna know i how much will a 3" flow vs a 2.25 or a 2.5. any help would be awesome as im not the greatest with a calculator and cant seam to find a calculator on line to help me out with this.

[SHiNY]

Mike -  I found this, it's not a calculator and most of what you just wrote it jibberish to me I hope it helps

The flow of the muffler must be equal to or greater than the pipe cfm sufficient to serve the hp of the engine to achieve zero loss through the system.  A large oversize system offers no advantage and is louder.

2.25 dia. flows  457 cfm
2.50 .............  564
3.00 .............  813
3.50 .............  1106
4.00 .............  1445

[hammy221]

thank you i really appreciate it

[kennysabarese]

I would read this article. It's about exhausts on turbos from a Garrett engineer. All principles applied to exhaust for NA cars DO NOT apply to turbo cars:

http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

[POS VETT]

That principle (of larger piping downstream turbocharger) was already used since the days I was playing with DSMs in the mid '90s.  I'm pretty sure the knowledge is much older.  The point of going with larger exhaust piping, like mentioned in Kenny's link, is the diminishing return.  Would 4" pipe better than 3" exhaust in turbo downstream application ?  Sure.  However 4" piping is much more expensive, heavier, and it takes up precious real estate.  Not to mention the increase in exhaust noises.  While exhaust noises are mostly irrelevant in racing environment, it becomes a significant factor in a street car.

I'd say a 3" exhaust is all you need with a 2.0l 4cyl engine in a street car; just make sure the transitions are smooth and that there is the least number of bends.  As straight as possible is the rule of thumb.

[kennysabarese]

I think this is all you need to know from the article.

"At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”

To me this means 3" for 300hp so 3" is perfect for a tuned stock turbo Kappa.

[POS VETT]

The rule of thumb seems to be "1 inch per 100 hp" then round up the diameter

[Kelu]

This means for my +400hp at crank I should try to build the downpipe at 4"?
I don't think so, I'm planing to match the turbine outlet at 3"

[Sol Asylum]

Sky888 is going to have to raise his car to get the 8 inch pipes under there.

[POS VETT]

This means for my +400hp at crank I should try to build the downpipe at 4"?
I don't think so, I'm planing to match the turbine outlet at 3"

Maybe.  My 505-hp naturally-aspirated Z06 has twin 3" exhaust pipes.

[kennysabarese]

Maybe.  My 505-hp naturally-aspirated Z06 has twin 3" exhaust pipes.

The inches to horsepower "rule" that is stated is for turbos. This DOES NOT apply to NA engines.  This is clearly stated in the article. You cannot compare the two.

[Kelu]

VETT: you couldn't mention about Z06??? ( every time when I hear about vette Z06 or up I go berserk)

[hammy221]

so if this affects turbo cars different the N/A cars cause of the turbo in the mix how would it apply to a supercharged car? i would assume the exhaust theory is the same as an N/A car cause the supercharger does not interrupt the exhaust flow right?