HOW MANY POUNDS OF AIR DOES IT TAKE TO MAKE ONE HORSEPOWER?

By 460-BBF-Turbo-In-CC (From the legendary Car Craft turbo blog)



“He lives most life whoever breathes most air.” – Elizabeth Barrett Browning

The concept is simple: your engine has to take in enough air in order to burn enough fuel to reach your horsepower goal. But for many Car Crafters, the “experts” all seem to be speaking different languages.

At times, it all sounds like an alphabet soup. The tuning experts talk of “AFR” (air/fuel ratio). Then the carburetor and cylinder head guys talk of “CFM” (cubic feet per minute). Yet most of us measure fuel in gallons or liters, not CFM. And the CFM recommendations seem wildly inconsistent. (i.e. “You’ll need a 650 CFM carb and some 200 cfm heads . . . .”) Making it worse, the supercharger and turbo folks talk about “pounds of boost” or other arcane measurements.

Many Car Crafters just throw up their hands and copy what the next guy is doing. And sadly, too many never really understand why what they’re copying works or doesn’t work.

Turbo Car Crafters, however, don’t usually have anyone to copy. So to be successful, we’ve got to understand how to compare and measure air and fuel.

We’ll start with some of “rules of thumb” of power production. They’re not a substitute for working a PLAN in detail, but they’ll get us “in the ballpark” and hopefully give us a basis to understand what the “experts” are talking about.

According to the turbocharger engineers at Garrett, it takes one pound of air per minute to make nine horsepower (See, Julian Edgar, 21st Century Performance, at pg. 161).

Turbo expert Jay K. Miller (Turbo: Real World High-Performance Turbocharger Systems) makes the estimates a little simpler by suggesting that it takes one pound of air mass for every ten horsepower. (Miller at pg. 40).

Engineer Mark Warner also uses the easier 1:10 rule of thumb, stating that once you know the mass flow rate into an engine, you can multiply it by 10 to get a horsepower estimate. (Warner, Street Turbocharging at pg. 29)

Others suggest that at the torque peak, each pound of air will support about 9.5 h.p. and at the horsepower peak each pound of air supports about 10.5 h.p. The popular 1:10 “rule” seems to split the difference.

In actual practice, each of these “rules of thumb” will be varied by many factors. But the basic concept of combining a “mass” of air with a smaller “mass” of fuel remains fundamental.