Thread: Exhaust size and velocity

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65cayne's post got me thinking - watch out!

What is the rule of thumb for header/exhaust diameter?

Too small - air throughput is restricted.
Too big - velocity is reduced.

I would think the ideal diameter is a function of actual CFM througput.
Or perhaps the target is an ideal velocity that is a function of CFM and diameter.

Thanks for throwing this out there, SBC. I've been wondering the same if there is an actual formula to figure the ideal sizing. My mechanic suggested the 3" for my 402 but I always wondered if that hurt the velocity/scavenging. Switching from 2.5" with glass-pack side pipes to 3" with flowmaster 40's took 0.3-0.4 seconds off my 1/4mi ET but how much from the mufflers and how much from the plumbing? I don't know.

>I would think the ideal diameter is a function of actual CFM >througput.
>Or perhaps the target is an ideal velocity that is a function >of CFM and diameter.

Here's something I've always wondered about. There's a variable that complicates the calculation, and that's the decrease in exhaust gas temperature. As the exhaust gas cools, it reduces in volume, and as we all know, temperature and volume are proportional. The cooler the gas the less the volume. Using that reasoning, as the temperature drops, so could the cross sectional areas of the pipe. Does that mean the pipe diameter can get smaller downstream and still handle the volume? What about density? As the volume drops, density goes up, doesn't it? So how does that figure into the equation?

Bob

I believe there is a formula for figuring all that out in one of my books...I will see if I can find it and post it tonight. The word "marriage" comes into play again in terms of your combination of parts.
I believe it relies a lot on your operating range (RPM) and cid...The unfortunate thing about all parts combinations is that there is only one RPM value where your setup will perform at peak efficiency. I guess this is a "non-answer" really...but an interesting topic. I'll see what I can find...

Here's a place to start; http://www.carcraft.com/techarticles...ics/index.html

Originally Posted by SBC

65cayne's post got me thinking - watch out!

What is the rule of thumb for header/exhaust diameter?

Too small - air throughput is restricted.
Too big - velocity is reduced.

I would think the ideal diameter is a function of actual CFM througput.
Or perhaps the target is an ideal velocity that is a function of CFM and diameter.

First you need to know what your Intake to exhaust ratio is.
Go rent a chassis dyno with you buddies and try different tube dia's,collecter length,step headers etc.

Here ya go,this may help in what size.How to do the formula,I have no idea.
http://www.wallaceracing.com/header_length.php

Try http://www.headerdesign.com/

I remember seeing a scale for HP=dia.Under 300 2" and smaller ,300- 400 2 1/2" ,400 or more 3".Something like that

How about this according to your formulas, this must make heaps of HP