Turbo Tech: Calculating Compressor Flow
Turbo Tech: Size Matters gave a quick and easy way to get in the ballpark for picking the right size turbo for an application. For those of you comfortable with crunching a few numbers in a spreadsheet, there are math equations to calculate compressor mass flow and pressure ratio. They are the same equations that go into the Garrett Boost Advisor and Borg Warner MatchBot. To perform the calculations, a few critical pieces of information are required: power, air/fuel ratio, BSFC (brake specific fuel consumption), volumetric efficiency, air filter pressure drop, and intercooler pressure drop.
Volumetric efficiency is very important here and is a significant character of an engine. As volumetric efficiency is in the denominator of the equation, an engine with higher VE (such as a late 90’s/early 2000’s B/K/F-series Honda) requires much less intake manifold pressure (boost pressure) for the same compressor mass flow as an engine with low VE (like an old SR20).
Let’s say I hypothetically want to turbocharge Project S2000 and make about 50% more power. I don’t have all the data required by the equations but I have a little bit and I can make reasonable estimations. The only hard data I have is my dyno plot from when I installed the Hondata KPro. I only have a vague memory of the A/F ratio, so I have to estimate there too.