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How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust Videos
"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:
Pipe Dia. (inches).....Total CFM (est.)....Max HP for 1 Pipe.... Max HP For Dual System
1 1/2 .......................171.................... 78........................... 155
1 5/8 .......................203 ....................92 ..........................185
1 3/4 .......................239 ...................108 .........................217
2 ............................ 318 ...................144 ........................289
2 1/4........................ 408 ...................185 ........................371
2 1/2 ........................ 509 ...................232 ......................463
2 3/4 .........................622 ...................283....................... 566
3.............................. 747 ....................339...................... 679
3 1/4........................ 882................... 401 ........................802
3 1/2.........................1029 ..................468 ......................935
NOTE: 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.
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