Exhaust Pipe Size Calculator – Horsepower vs Diameter
This exhaust pipe size calculator will tell you what exhaust pipe size (diameter) you need for the horsepower you are running. Just enter the horsepower, the calculator does the rest. (Calculations from Corky Bell’s legendary book, Maximum Boost, buy on ebay, if available, any mistakes are mine, not Corky Bell’s)
NOTE: Whichever calculator use, also look at exhaust speed versus recommend max. Probably not ideal to be too close to limit (190ft/sec MotorTrend, 250ft sec Corky Bell)
QUICK LINKS – Corky Bell based calculator – MotorTend adjusted calculator (gives slightly bigger bore results) – Downpipe Info – Background
Don’t know your horsepower? Check the engine horsepower calculator here.
Corky Bell Based Calculator
eBay
Buy the book Maximum Boost on ebay – Click Here
The Calculation & Considerations
This exhaust pipe size calculator uses information from Corky Bell’s book, Maximum Boost as a reference. The book was published in 1997 when large exhaust pipe diameters were harder to find, due to their price and being harder to work with due to the high price of large diameter pipe benders at the time.
These factors may have made assessing the benefit of large diameter exhausts harder (when the book was written) because large bore exhaust pipes (eg 3, 4 and 5 inch) were rare, so dyno results for cars running these diameters was also rare.
To put it another way, the benefits of large diameter pipes may have been underestimated. For this reason we have added results which are 20% larger than those recommended in the book.
It also pays to look at the exhaust flow speed as well as the diameter. If the exhaust speed is high, use the larger diameter.
As a rule, Corky Bell states exhaust gas velocity should not exceed 250 ft/sec
StrikeEngine TV Highlights
A word about downpipes…
In contrast to the exhaust system the rule seems to be for turbo downpipes, the bigger the better.
Koracing have compared 2.5 inch downpipes to 3 inch downpipes. They observed power comes in sooner but peak power is around the same, although on the 3 inch downpipe boost was 1 PSI less.
Stav Tech has an interesting post here, the sum up being the larger the downpipe the better, because the gas exiting the turbo is so turbulent. The larger downpipe improves spool and power. The exhaust system itself can be a smaller diameter because the gases become more homogeneous as they go down the exhaust.
Header Primary Diameters
A bit off topic but a very good article here comparing large bore primaries vs small bore primaries on a 4G63 motor. To illustrate how big is not always better.
And going into the weeds even further, here is an article going into exhaust manifold design
Use this calculator, like all the calculators on strikeengine.com, as a guide only.
Alternative Calculator – MotorTrend
MotorTrend have done a test comparing a dual 2.5 inch exhaust system vs a dual 3inch exhaust system. You can read about the test here.
The calculator below slightly tweaks the formulas used in the above calculator to give results closer to what MotorTrend found.
Generally, from the numbers I have put into the calculators, I believe the 20% plus size from the original calculator and the 20% plus size from this alternative calculator will put you in a good spot.
