What size exhaust?

[eray92]

I have 3" exhaust pipes and flowmasters that dump before the rear end. The motor is a 383 with summit Long tube headers performer intake q-jet and a stockish cam. Would I benefit from 2 1/4 or 2 1/2 pipes?
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[bnoon]

Power-wise you might be giving up some low end. The real difference would be sound db level is quite a bit louder with 3" vs similar mufflers on a 2.25" system.

[eray92]

It's a crush bent setup and its not really in good shape. It also gets in the way for everything under the truck when I gotta work on it. I heard smaller exhaust is better for torque. True?
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[wilkin250r]

Originally Posted by eray92

I heard smaller exhaust is better for torque. True?

Sort of. It's close enough to use as a general rule. So let's get technical.

Air is not weightless. It has mass, and thus it has momentum when moving. The faster it is moving, the more force it has. Engine designers use this momentum of the moving exhaust gasses to help pull in the new air/fuel mixture, and it is significant.

So exhaust velocity is important. And for a given amount of air trying to move (set by the engine size and RPM), the smaller the exhaust, the faster it is moving. So smaller exhaust pipe results in higher exhaust velocity, and thus more power.

But there's a drawback. When you're trying to push LOTS of air (either a really big engine, or just high RPMs), bigger exhaust flows more air, it has less restriction.

So the end result is, smaller exhaust will improve mid-range RPM performance (often thought of as "torque"), while larger exhaust will improve high-rpm performance (peak horsepower).

So what's the best exhaust for you? It really depends on your engine design and overall application.

[INSIDIOUS '86]

A 2.5 mandrel bent system will support 550hp easy. No tailpipes on a 2.5 system is comparable to a full 3in system from mag tests if you choose to believe them.
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[TheOcho]

One of the best exhaust systems I've ever had was on my 67 Camaro. I know, it's not a truck and it certainly wasn't a 383. It was little 327, but that little SOB was worked and ran close to 400 HP @ 6200 RPM. I had 2 1/2" head pipes (with a balance pipe) into two 40 series flow masters and ending up in 2 1/4" tail pipes. The theory goes, that as the exhaust gasses cool, they occupy less space and the smaller tail pipes help to maintain a consistent velocity within the system, thereby increasing the scavenging effect at higher RPM without sacrificing low end and mid-range performance.

The system worked really well. As a matter of fact, when I ran open header, I didn't notice a difference on the top end (except for the noise and hot floor). The system was loud enough that you knew I coming, but not so loud as to wake up the neighborhood at 2 am (unless I really wanted to).

Again, this was on a little 327 in a Camaro, not too sure how it would perform on a pick-up with a 383. But thought I'd share it with you anyway.

[eray92]

I seen a 3" premade kit somewhere but can't remember
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[INSIDIOUS '86]

Pypes should have pre made kits
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[Pop's C-10]

If it dumps be for tha axle..i wouldn't think it would be that chokin..unless they crushed really bad..

[eray92]

No it's not real bad it's just old and starting to rust. Just trying to gather options before I get new exhaust put on
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[68Timber]

You can get 3" mandrel bent tail pipes from Summit for about $40 each. With your cam you'd probably be better off with 2.5" exhaust.

[eray92]

I'll probably end up with 2.5". My cam sound pretty close to a stock cam
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[piecesparts]

When I put my 383 in my 84 truck years ago, I called the Flowmaster people about exhaust sizes and their input was to this affect.

UNLESS you are racing the truck (high RPMs) or you are running a 454 or larger sized engine (large cylinder volume), then the size of the piping would be detrimental to run larger than a 2 1/2" exhaust. The larger piping would hurt the flow characteristics of the exhaust and hamper the affect that the cooling of the exhaust gases creates as they exit the headers. The gases cooling creates a certain amount of a vacuum effect and as it does that it draws the gases away from the cylinder heads. This enhances the removal of the gases. A larger pipe will slow that removal, by not giving the right vacuum effect. It will cause a reversionary effect and thus hamper your truck's ability to remove gases and then affect the filling of the cylinders with fresh fuel laden vapor.

Following their input, I run a full set of 2 1/2" dual exhaust on my truck, with headers and in the past Flowmaster mufflers. I now run Magnaflow muffs, due to Flowmaster always rusting out. The design is the same, just a different muffler maker.

[motornut]

stock 79 305-2B
went and cleaned out some manifolds a bit(didn't get any quieter lol)
2.25 dual pipes and mufflers with H pipe,flowmasters,out before rear wheels
sure don't sound stock anymore
my 78 has a 350
manifolds,2.25 dual pipes,no H,cherry bombs,out before rear axle

I find i get more sound out of the smaller pipe when stock
check the pipe size when you order ,some kits come 2.25

1979 JimmyG Boost from the 78 - YouTube