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post #1 of 29 (permalink) Old 05-09-2013, 06:22 AM Thread Starter
 
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how to choose exhaust size

Hi!

How do you decide which exhaust pipe size will be best for the application?
I read that probably the smallest diameter that can handle the cfm will be best for performance.
For example my 450hp GTO would need a dual 2,5" (the size I use at the moment).

But there are other opinios that say "bigger is better" and a 3" dual would be best.

Is there a way to determine what size is best for performance?

The formula I got says that you need 2,2cfm for each hp. That is 990cfm for my engine. A 2,5" has a total flow of 510cfm, a 3" a total of 750cfm and a 2,25" has 410cfm. So a 2,5" (2x510=1020cfm) would be best. What do you think?
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post #2 of 29 (permalink) Old 05-09-2013, 07:13 AM
 
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It is my understanding that unless your engine is producing more than 550 hp, a 2 “exhaust is sufficient. If your exhaust is too big for the hp rating, you lose low end performance. Your engine is essentially an air pump that moves some maximum CFM. With engines having CFM outputs very much less that what 550 horse can produce, the velocity of the moving column of air in a larger pipe (3”) is too slow to contribute to what is known as the scavenging effect. Scavenging helps “pull” the expanding exhaust gasses away from the exhaust header collector and keep it moving down the pipe.

Also consider a cross-over “X’ style exhaust. This helps relieve parasitic back pressure pulses that build up in standard separate left/right dual exhaust.
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post #3 of 29 (permalink) Old 05-09-2013, 09:08 AM Thread Starter
 
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That sounds good, I also think that 3" duals are overkill for my engine (I don't want too loose low end as it is mainly a street car!)

I use a 2,5" dual Flowmaster with H pipe.. an X pipe would be better I guess, but if the difference isn't too big I like the sound of the H pipe more.

So I'll keep this exhaust.. maybe I'll change to matching turbo style mufflers like Magnaflows. the sound is very good now, but maybe the drone is better with Magnaflows and they may flow a little better because of the design (no Chambers). A complete Magnaflow setup with 2,5" and x pipe would be pretty nice as well I guess, but why should I replace the tubing.. the Flowmasters are only 2 years old... or is stainless much better than the aluminized steel? (performance and sound.. I don't question that stainless is the better quality)
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post #4 of 29 (permalink) Old 05-09-2013, 09:35 AM
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3" will rob you of power if you are producing under about 700hp. The best size for street/strip cars seems to be 2.5".
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post #5 of 29 (permalink) Old 05-09-2013, 10:30 AM
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go 2.5 inch on your car. The reason is flow velocity as you've read/heard. Too much cross-sectional area kills flow velocity resulting in a loss of the inertia effect to help exhaust the cylinders, so that the engine now has to "push" the gasses out - and that requires energy that could have otherwise gone to the crankshaft to make torque.

This is one area where the whole "bigger is always better" idea definitely does NOT apply.

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post #6 of 29 (permalink) Old 05-09-2013, 10:41 AM Thread Starter
 
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I'm glad to hear that from you too, thanks

A friend of mine wants to use a single 3" exhaust for a 305cui 170hp Chevy... he says that's just perfect but I think thats too big as well and he will loose low end torque (and "win" more noise). Is there some reason to go this big or should he really think of 2,5" single?
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post #7 of 29 (permalink) Old 05-09-2013, 10:47 AM
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Depends on how good of a friend he is....

Single vs. dual does change things a little, but on an engine that small I'd still be hesitant to go that large. But then it's not like chevy's have any bottom-end torque to begin with, so... what's it gonna hurt?

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post #8 of 29 (permalink) Old 05-09-2013, 11:15 AM Thread Starter
 
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good argument!
I didn't drive the car yet, but he also don't want to use the whole rpm range.. it's for cruising and rpm's from idle to 4000... that's why I really wanted to convice him not to use a 3" performance exhaust.

I'll keep my 2,5" duals.. can you give me some advice on mufflers? Some say the Flowmasters don't flow very good.. but I found a test that showed that they are just a little more restrictive than Magnaflows. (5hp difference).
I have some Magnaflows on my Cadillac (4,6 Northstar) and they sound great. Would they be a good choice for my GTO as well instead of the Flowmaster 40's?
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post #9 of 29 (permalink) Old 05-09-2013, 12:29 PM
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Back in the day, I worked on a few customer Scarabs, which were Datsun 240Z's with Chevy 350 V8 conversions. They all seemed to run a single big exhaust, but it was 2.5" if memory serves. The only reason for a single exhaust on these cars was for clearance and fitment to the car. With a 'V' engine especially, dual exhausts are always better than a single for a performance boost.
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post #10 of 29 (permalink) Old 05-09-2013, 12:46 PM
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Nice! Back in the day I built a `77 Toyota celica fastback with a 400hp sbc. Had it on a shortened dodge d50 pick up frame and a posi rear end. Full duel exhaust with huge mufflers you could hardly hear it running. Sleeper of the year!! Made a lot of money with that car.
I traded it for the car I traded for the GTO !!
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