2 1/2 vs 3 inch exhaust

[cwhubb]

Is there a rule of thumb or formula ( head flow, cam, ci, rpm,etc) regarding engine output to exhaust pipe diameter selection? I've read the forums till my eyes got blurry but there was no real consensus on 2 1/2 vs 3 inch, all the info was anecdotal. I would think theres a point where the engines output couldnt support a larger exhaust, something along the lines of too big a carb or intake.

 

[lewtot184]

brain teaser! there isn't a concrete/definitive answer.

 

[cwhubb]

I was afraid of that

 

[RedShadow30]

Call your local muffler shop and ask them (if you have one). From what ive been told, it all depends on what and where your powerband is. The cylinders are dependant on backpressure to suck out all the burned stuff. Too large and you will rob HP, too small and youll restrict HP. Different mufflers also can affect what size you want or need.

 

[lewtot184]

a 2 1/2" system with low restriction mufflers will cover a lot of territory. i have a 440 with 2" headers and 3" exhaust and consider it overkill. for what i do 1 3/4" headers and a good 2 1/2" exhaust would work fine.

 

[cwhubb]

the car came with 2" headers they were beat so I cut em off and have 1 3/4 (2's were rubbing shock towers) on now, I'm thinking those dumping into a full length 3" exhaust with mufflers (flow master big block 70) will act like a stepped header and enhance the scavenging effect but thats just my theory. Like Red said I want to find the balance between back pressure and hp. my set up is 440 .30 over 10:5:1cr, rpm performer intake, holley 950 (830cfm),1 3/4 headers, heads will flow around high 200's when I get them done, I'm camming it with 533 solid mp cam, 4:10 gears, 3800lb 70 charger

 

[1969CoronetR/T]

I agree with lewtot184. I have TTI 3" exhaust and I am sure a smaller diameter would not not hurt performance. It does sound nice with the Dynomax mufflers although I am not sure if a smaller diameter would change the timbre.

 

[Rocketman]

Here is mine. I have 1 3/4 headers with 2.5" exhaust thru flowmaster 40's. My muffler guy advised against going to 3" as it I would lose low end.

[video]http://camshaftsounds.com/cam-video/comp-xe285hl-241247-050-110lsa-440/[/video]

 

[lewtot184]

i think you can forget scavenging when using mufflers. mufflers create back pressure. a 2 1/2" pipe with ultra-flows matched to the 1 3/4" headers is what i'd do.

 

[cwhubb]

I'm not too worried about bottom end, the 440 has gobs of torque plus with the 410's out back I think you'd need a dyno to see a difference, this is harder than I thought, I love the sound of the flowmasters deep rumble and when you stomp on it its still a deep rich sound just times it by 10. they're clamped to the extension tubes and end right under the front seats for now, but I was bugging the neighbors with the open headers so I slapped them on, they're 3 inch in and out

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thanks for your advice guys. untill I can find a definitive formula I think I'll drop back, punt, and go with 3" to match my mufflers

 

[bigmanjbmopar]

the old flow vs size rule, water flowing through a small diameter pipe will be restricted and have more resistance vs a larger pipe with the same amount of flow rate.

Air exhausting out each port them combined in the collector which is usually 3" on most headers, so why not keep the size consistent all the way to the mufflers, that is were your restriction will be. I run 3" flowbastards 40's.

 

[Car Nut]

Here is mine. I have 1 3/4 headers with 2.5" exhaust thru flowmaster 40's. My muffler guy advised against going to 3" as it I would lose low end.

[video]http://camshaftsounds.com/cam-video/comp-xe285hl-241247-050-110lsa-440/[/video]

Sounds real good, what kind of ignition system are you running?

 

[Rocketman]

Sounds real good, what kind of ignition system are you running?

MSD with 6AL box

 

[hunt2elk]

When I had my last motor built, I called TTi for their recommendation on a 520 hp 451. They told me to go with 1 7/8 headers and their 2 1/2 inch mandrel bent exhaust system along with Dynomax Ultraflows. Won't know till spring if all is good, but I don't anything to compare it to either.

 

[Mopar-Charger]

i have a 1 5/8 headers to a 3 inch exhaust to 40 series flowmasters.

i have a 360 big cam 750 holley. no problems might even be too restrictive. i have cutouts 8inch behind the headers and it hauls *** on the top end compared to running full exhaust.

again like said above, the mufflers make a big difference. probably going to buy ultra flows and go with a smaller system. drone is killer on the highway

 

[mopar 3 B]

I have 1 3/4"header 3" collector 3" pipe 40 flowmaster terminator. Was using 2 1/4" pipe before going to 3" lost bottom end but gained top end.

 

[cwhubb]

@ mopar3b could you actually feel the loss seat of the pants wise? is this on a bb?

 

[mopar 3 B]

Yes and yes. Did not make it lazy. Just more throttle to get it moving.

 

[Kern Dog]

Call your local muffler shop and ask them (if you have one). From what ive been told, it all depends on what and where your powerband is. The cylinders are dependant on backpressure to suck out all the burned stuff. Too large and you will rob HP, too small and youll restrict HP. Different mufflers also can affect what size you want or need.

Uhhh... NO.
Most muffler shop guys I've met are just guys that can bend pipe and weld it together. Most do not race or care to race. They only know what they read in the trade magazines.
Cylinders dependant on backpressure? NO again. All engines make more power with less restriction. I laugh when I hear old timers say that their 340s and 440s with Magnum manifolds make as much power as the same engine with headers. Faster in, faster out, MORE power.
A stock engine will lose a little bit of power at low speeds with large headers, but take that same engine with iron exhaust manifolds and do tests on power with then without mufflers. The engine will make more power with less restriction.
Getting back on topic: 2 1/2" exhaust works fine up to around 425-450 HP. Beyond that, you will see benefits from going to a 3" system. I had a 2 1/2" system put in my Charger when I swapped from the stock 318 to my first 440. The 440 made around 380 HP and the car ran great. Currently I have a 440 based 493 and my 2 1/2" pipes were too small for my combination. I went with a 3" mandrel bent center section with an X crossover along with 3" in/out mufflers, yet retained the 2 1/2" tailpipes. Power jumped up dramatically. You'll find that tailpipes can be smaller than the center section with no loss in power. The exhaust gases have cooled off some at that point to where large tailpipes are not as important.

 

[Meep-Meep]

One thing is for certain about gas or liquid flowing through a pipe. It's called the boundary layer. This is a portion of the gas that sticks to the wall due to friction and doesn't move as fast as the gas toward the center of the pipe. The result is the effective diameter of the pipe is reduced by about 1/8" and should be factored in.