This table below was supplied by Flowmaster. It’s intended as a basic reference guide to give you an idea of the general exhaust-pipe size needed to balance the prevention of excessive back-pressure while maintaining adequate exhaust-gas velocity.
Engine Displacement | H.P. | Pipe Diam, Single | Pipe Diam, Dual |
150-200 | 100 | 2.0 | 2.0 |
| 150 | 2.25 | 2.0 |
| 200 | 2.5 | 2.0 |
200-250 | 150 | 2.25 | 2.0 |
| 200 | 2.5 | 2.0 |
| 250 | 2.5 | 2.25 |
250-300 | 200 | 2.5 | 2.0 |
| 250 | 2.5 | 2.25 |
| 300 | 3 | 2.5 |
300-350 | 250 | 3.0 | 2.25 |
| 300 | 3.0 | 2.5 |
| 350 | 3.5 | 2.5 |
350-400 | 300 | 3.0 | 2.5 |
| 350 | 3.5 | 2.5 |
| 400 | 4.0 | 3.0 |
400-450 | 350 | 3.5 | 2.5 |
| 400 | 4.0 | 3.0 |
| 450 | 4.0 | 3.0 |
450-500 | 400 | 4.5 | 3.5 |
| 450 | 4.5 | 3.5 |
| 500 | 4.5 | 3.5 |
Quick Exhaust System Math
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 Diameter (inches) | Pipe Area (in2) | Total CFM (est.) | Max HP Per Pipe | Max HP For A Dual Pipe System |
| 1 1/2 | 1.48 | 171 | 78 | 155 |
| 1 5/8 | 1.77 | 203 | 92 | 185 |
| 1 3/4 | 2.07 | 239 | 108 | 217 |
| 2 | 2.76 | 318 | 144 | 289 |
| 2 1/4 | 3.55 | 408 | 185 | 371 |
| 2 1/2 | 4.43 | 509 | 232 | 463 |
| 2 3/4 | 5.41 | 622 | 283 | 566 |
| 3 | 6.49 | 747 | 339 | 679 |
| 3 1/4 | 7.67 | 882 | 401 | 802 |
| 3 1/2 | 8.95 | 1029 | 468 | 935 |
NOTE: These numbers are just estimates. All pipes are assumed to be 16 gauge steel.
Exhaust Back Pressure and Diameter
Learn about back pressure and how it works and why it is important to your engine. Al will also give you a break down of which diameter exhaust tubing you should choose depending on if you choose a single or dual exhaust and the horsepower of your engine.
