How To Calculate Exhaust Pipe
If you’re a maths genius and/or an engineer, you’re probably going to like this page. However if you find yourself getting stuck (or bored) with the info below, here are the key take-always:
1. The factory exhaust pipe diameter is usually a good choice for most vehicles.
2. The exhaust manufacturers are doing all the maths for us – no need to reinvent the wheel. If they say it will work for your vehicle, it will probably work for your vehicle.
3. Also here there,s an easy-to-read exhaust system size table that is good for quick calculations.
Breaking Down The Problem
While we’re not going to go through and list out all the formulas and calculations you need to figure this exactly, we will break down the problem, explain how you would go about figuring things out scientifically, and then leave you with some good exhaust system maths.
The science goes like this…
That’s it! Of course, when you sit down to figure it, you’ll find that getting a good scientific estimate takes a lot of work (which is why we don’t bother with it here).
Exhaust System Maths
Easy Way To Estimate: Your intake system needs to flow 1.5 CFM per engine horsepower, and your exhaust system needs to flow 2.2 CFM per engine horsepower.
Good Way To Estimate: Take engine RPM x engine displacement, then divide by two. This is the intake volume. Use this same volume of air for the exhaust system, but then correct for thermal expansion (you need to know exhaust temps to figure things out).
Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:
NOTE: These numbers are estimates and that other performance mods must be taken into account
|Tube Diameter (inches)||Tube Area (in2)||Total CFM (est.)||HP N/A||HP Turbo|
All pipes are assumed to be 1.5mm gauge stainless steel.
You can see that up to 400HP most exhaust systems only need to be 2 1/2 inch in diameter. Anything larger is overkill !!.