Quick and Dirty Exhaust System MathEasy 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:Pipe Diameter (inches)Pipe Area (in2)Total CFM (est.)Max HP Per PipeMax HP For A Dual Pipe System1 1/21.48171781551 5/81.77203921851 3/42.0723910821722.763181442892 1/43.554081853712 1/24.435092324632 3/45.4162228356636.497473396793 1/47.678824018023 1/28.951029468935NOTE: These numbers are just estimates. All pipes are assumed to be 16 gauge steel.The table above is probably over-estimating pipe size, but you can see that a 400 hp vehicle with a dual exhaust system only needs 2 1/4 – 2 1/2 inch pipes. Anything larger is overkill.
When The Flag Drops.,. The Bull ***t Stops.,. P. Engineer, Engine Builder
I always wondered why everybody thinks they need 3" or more. I thought a more sensible pipe size actually helped along the exhaust flow, even going to maybe a 1/4" smaller pipe out the muffler, because the gasses are cooler?
jim hand did some exhaust testing with his 455 powered wagon at the drag strip that was impressive.
76 455/4spd TRANS AM69 GRAND PRIX 455/5SPD