# how to choose exhaust size



## Chris-Austria (Dec 23, 2010)

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?


----------



## 68Resto-mod (Nov 10, 2009)

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.


----------



## Chris-Austria (Dec 23, 2010)

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)


----------



## geeteeohguy (Feb 2, 2008)

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".


----------



## BearGFR (Aug 25, 2008)

: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


----------



## Chris-Austria (Dec 23, 2010)

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?


----------



## BearGFR (Aug 25, 2008)

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? :rofl:

Bear


----------



## Chris-Austria (Dec 23, 2010)

: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?


----------



## geeteeohguy (Feb 2, 2008)

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.


----------



## Rukee (Feb 8, 2007)

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 !! :cheers


----------



## ALKYGTO (Mar 29, 2010)

The AlkyGTO has dual 3 1/2" pipes..........


----------



## BearGFR (Aug 25, 2008)

ALKYGTO said:


> The AlkyGTO has dual 3 1/2" pipes..........


Probably has something to do with that funny looking mailbox thing you've got poking through your hood.... 

Bear


----------



## Tropical Goat (Jan 30, 2011)

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!


----------



## Chris-Austria (Dec 23, 2010)

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?!


----------



## KingJacobo (Jul 6, 2011)

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.


----------



## ALKYGTO (Mar 29, 2010)

Consider the Hooker Aerochamber mufflers. They keep my car suprisingly quiet until I open it up. I really like them and the quality of them is top notch.

Holley Performance Aero Chamber Products


----------



## geeteeohguy (Feb 2, 2008)

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.


----------



## Chris-Austria (Dec 23, 2010)

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??)


----------



## ALKYGTO (Mar 29, 2010)

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


----------



## kilkm68 (May 5, 2011)

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.


----------



## KingJacobo (Jul 6, 2011)

kilkm68 said:


> 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.


Longtube headers will definitely add some power, not sure how much HP you are putting down but with a 474 you might benefit from 3 in piping. Even if 3' hurt your low end, the decent gains from headers may still put you ahead of where you started. 

But as already stated, 2.5 is as big as most want to go unless you are pushing some serious power..which is highly likely with a 474.


----------



## Chris-Austria (Dec 23, 2010)

I'm already convinced that 2,5" duals are best for me, but when I told my friend that 3" single on his 170hp Chevy is too big he said that it depends on the camshaft how big you can go...


----------



## BearGFR (Aug 25, 2008)

Well, yes and no on the cam. If you "cam the heck" out of an engine so that peak volumetric efficiency is shifted far into the high-rpm range, then it'll be able to make use of the larger passages _because flow velocity will be high._ However, that same engine is going to be a dog at low rpm. The original conversation had to do with the effects of header/exhaust tubing size _at low rpm_. TAANSTAAFL - "There ain't no such thing as a free lunch." - more torque/power at higher rpm always comes at a cost - in the form of a LOSS at lower rpm. (Disclaimer: _UNLESS_ the engine has some form of variable valve timing mechanism that effectively alters the cam profile at various RPM - but that doesn't include us.),

Bear


----------



## Chris-Austria (Dec 23, 2010)

I'll give him this advice once again but if he wants to make it run poor he has to try the 3" on the 178°intake cam and max rpm of 5000...
He said something like a cam with the same intake and exhaust duration can handle a bigger exhaust pipe size and a cam like mine (dual pattern) is more sensitive. Makes no sense to me, but maybe he's right.. I also believe that it is like you said.. if the "air pump" runs at higher rpm (because of a more radical cam) it needs more air into the carb and logically it will pump more "air" into the exhaust where it may need a bigger sized pipe. But a 178° intake and about 190° exhaust duration isn't very radical (I think they called it the "peanut" cam).
The variable valve timing is something like the "vanos" that my BMW had I guess. The only thing I know that might do something like this are the Rhoads lifters.. some said I should use them if I want the RAIV cam to have more low end torque.. but I choose the lifters that came with my Comps cam and I don't feel a need for more low rpm torque (my tires already can't handle the torque I have)


----------



## geeteeohguy (Feb 2, 2008)

Chris, do yourself a favor and spend your energy other than trying to reason with your friend. It will get both of you nowhere. Let him 'do his thing' and learn from the results. He is so far off base that he would benefit from some education gained from the experience. That's how a lot of us, me included, learned some of the most important lessons!


----------



## Chris-Austria (Dec 23, 2010)

That's what I thought.. maybe he has to learn it the hard way.. but I guess everybody had to at some point in his life!
I wanted to beware him, but maybe he'll be happy even with less torque.. it's like some say a steak from a chicken tastes better than an angus steak.. Imagination makes some things better than they are


----------



## tpa_steve (Oct 6, 2012)

*3" exhaust too big and terrible drone*

I've got a 1972 455 in my 67 GTO with pretty big cam (230 intake duration, 236 exhaust). I seem to be experiencing 2 problems similar to others in this thread. My setup including Doug's headers a large 3" exhaust and flowmasters. My problems appear to be as follows:
1. less than optimal low end performance and need to increase requirind increased fuel flow to offset
2. a terrible exhaust drone

Sensing that an increase in backpressure might be advantageous, I briefly considered supertrapp exhaust tips to minimize required modifications and cost while providing for optimal tunability - then I saw their price$$$$.

Are there any reasonable options to improve exhaust velocity to optimal level while avoiding a complete swapout? Can desired result be achieved by stepping down the exhaust size to 2 1/2 outlet? If so, best done before, after or at muffler or near tailpipe? Any suggestions appreciated


----------



## BearGFR (Aug 25, 2008)

The problem isn't a lack of back pressure, it's low flow velocity - especially in the sections closest to the engine. Adding restrictions at the end of the system isn't going to change that, in fact that would probably just make it worse. (BTW, 230/236 @ .050 lift in a 455 really isn't "big". In a 455 I'd call it "mild to moderate".) 
If you want to experiment by adding sections of smaller diameter tubing, then do it as close to the engine as possible. Before you go swapping parts though, please tell us more details about your combination (cam details, intake, carburetion, heads, transmission, rear gear, etc.) and the problems you're having?

Bear


----------



## EP Goat (Dec 18, 2008)

*428 that sounds stock*

Looking for a stock exhaust system for my 428 with a little larger cam. I want that stock sound. Any ideas?


----------

