Another handy notation to have concerns airflow, which is simply cubic inches multimlied by rpm and divided by 3,456. That means a 360 cid engine (can you tell we like Cummins?) spinning at 3,000 rpm would consume about (360 x 3,000 / 3,456) 312 cfm of air when operating at 100-percent volumetric efficiency. However, volumetric efficiency is usually pretty far from 100 percent; it’s usually closer to 80 percent. So that means we now get an answer of (312 x 0.8) 249.6—let’s just say 250—cfm of air. The great thing is, with these simple formulas you don’t even have to be that good at math. You just plug in a couple numbers and there’s your answer. If you’re wondering what airflow has to do with anything, we’re getting to that.
Turbos and Engines
Let’s say you have your Cummins spinning at 3,000 rpm and you want to add some boost. Every atmosphere above outside air adds another bar to the pressure ratio, so if ambient is 14.7 psi (it varies by elevation) then 14.7 pounds of boost would be a 2:1 pressure ratio. That means 29.4 psi would be a 3:1 pressure ratio, and 73.5 psi would be a 6:1 pressure ratio. If we take our earlier theoretical 250 cfm figure, that means to make 14.7 pounds of boost you’d need a turbo that flows 500 cfm, 750 cfm for 3:1, and a whopping 1,500 cfm
When it comes to selecting a fuel system to make horsepower, it’s best to talk to someone who’s already done it to get some advice. Also keep in mind that one company’s “100% over” injectors might not be the same size as another manufacturer’s 100%...
Figuring out exactly how much a turbo will flow requires a bit of help from the manufacturer. Companies like Garrett and Borgwarner provide access to compressor maps, which show the maximum pressure ratio, flow and compressor speed.
Compound turbos can be a bit more difficult to size than single turbos and can be geared toward response, all-around performance or peak power. In general, the larger turbo should flow roughly twice what the small turbo will flow.