This exhaust pipe size calculator will tell you what exhaust pipe size you need for the horsepower you are running. Just enter the horsepower, the calculator does the rest.
Don’t know your horsepower? Check the engine horsepower calculator 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 the price and harder to work with due to the pricing of pipe benders. This may have made assessment of large diameter exhausts harder when the book was written ie smaller data set versus today.
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.
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 of 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.