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Discussion Starter #1
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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It is my understanding that unless your engine is producing more than 550 hp, a 2 ½ “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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Discussion Starter #3
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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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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:agree 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.

Bear
 

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Discussion Starter #6
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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Depends on how good of a friend he is.... :D

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? :rofl:

Bear
 

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Discussion Starter #8
:lol: 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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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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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. :D
I traded it for the car I traded for the GTO !! :cheers
 

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I'm using an H-pipe instead of an "X" too, for the same reason...sound quality. My cousin has an X-pipe on his 350cu.in. 68 Camaro, and it sounds like a stock 80's 5.0 Mustang.(which IS a great sound for the Fords.) Being a diehard Chevy guy, he hates it when I say that, but he hates it even more when I blow his doors off with my H-piped nastier sounding 400 Pontiac!
 

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Discussion Starter #14
Did someone test the difference between H and X pipe on his car? Sound and Performance?

Or the difference between Flowaster 40's and Magnaflows? I like how my GTO sounds, but maybe the Magnaflows would sound good as well but with less drone?!
 

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Flowmaster 40s and Magnaflow will be a significant difference. Flowmasters 40s have that old muscle car growl and are known as some of the loudest "off-the-shelf" mufflers. Magnaflow has a much tamer, almost video game like racy sound depending on the application. Much less drone also with magnaflows.

Look at youtube samples between H, X and flowmaster and Magnaflows. There are tons of them.
 

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Flowmaster anything has never floated my boat. Not then, not now. Too loud, too 'in your face'. Tacky. I've run Walker Turbo mufflers for decades in my performance GTO's, and stock in my convertible. Sometimes, a little less 'edge' makes a more powerful statement.
 

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Discussion Starter #18
I'll search for some videos at YouTube, thanks!

Is there any realtionship between exhaust size and the camshaft? (duration, type, overlap...) For example does some kind of cam like a bigger sized diameter?! A friend of mine told me so but I cannot see why this should have a big effect (if we talk about header size maybe.. but the exhaust pipe size??)
 

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Yeah.....if it's a two stroke :lol:.



Honestly a decent dual setup with 2" or 2 1/2" (max) mandrel bent setup is gonna be perfect for you. Mandrel bends because it will help the smoother all the bends are and the pipe size should have plenty of volume for you. I don't think you are putting a lot of HP on the table either way with a good flowing setup and most of it will be how the mufflers flow and sound. :cheers
 

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Interesting thread. Last year when I put the headers on my 474 I went from 2.5 x pipe to a 3.0 x pipe from the collectors back to the 2.5 in mufflers and tailpipes. This combo I was told would increase torque. Hard to compare with adding the headers, but it does run stronger. Maybe just the HP increase from the headers but curious on others opinions on this combo.
 
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