How Does an Air Filter Make Power

The following was posted by Jim Conforti on the E36Digest:

How does an air filter make power?

I was recently asked this very question and I thought it might be a good bit of info for the group.

First.. filters don't really MAKE power.. they ROB it, but that which robs least is best ;)

I'm going to gloss over the math a bit.. if you don't understand this, I'd suggest reading the citations at the end before you ask me things..

First a basic equation.. HP = Trq * RPM/5252

So, to increase HP we need to increase Torque..

The best way to do that "mechanically" is to increase the Volumetric Efficiency of the entire engine "system".

Since Ve is effectively the percentage of existing charge density to theoretical maximum charge density, if we can increase charge density, we increase Ve.

So let's do that..

Intake air density is effected by temperature and pressure (good old n=PV/RT)... if we decrease temperature or increase intake pressure within the same volume of air you have more "molecules" of air.

Every intake "device" (HFM, filter, etc) acts as a restriction according to Bernoulli's laws (no, I won't torture you with them) specifically, that the "orifice" reduces the mass flow thru the device. (yes, pilots we're gettin' really glossy here ;)

In the end, if you reduce the restrictions between the cylinder and the real world.. you increase the power the motor produces..

If you can do this, and still keep the air/fuel ratios where they belong.. You "make" power over what was there before.

You might ask.. why I'm rambling about this.. well because someone who I thought would know this, and they didn't because they asked me some rather miscued questions.

Apparently someone was telling them that only Temperature and Velocity effect HP..

I don't know who the "someone" is.. but I think they should grab the Chem 101 book and look up PV=nRT (again and again).

Rearrange to find P/RT = n/V or P/RT ~ density.

Increase PRESSURE or Decrease TEMPERATURE and you make MORE POWER.. this is, BTW, the same way that Turbo's, Superchargers, Intercoolers, Ram Air, and "free flow filters" all work.

There are other "factors" at work.. sometimes you see a system make MORE power with some filter, than without one at all...

This is usually due to metering efficiencies.. suffice it to say that if we TUNED each of the systems as it existed, we COULD GET more WITHOUT the filter, than we could WITH the filter. Since we live in a real (dusty) world.. we have to settle for the best flowing filter we can find.

If you had any questions, hopefully this will answer some of them.

If, as I suspect.. the BS artists are out there spinning tales in the background.. this should help you out in filtering the nonsense from the science ;)

Jim C.

PS: Oh yeah, the refs..

Bosch Automotive Handbook, 4th Ed. 0-8376-0333-1

Internal Combustion Engine Fundamentals
John B. Heywood
0-07-028637-X

The Internal-Combustion Engine in Theory and Practice
Charles Fayette Taylor
0-262-70026-3 Vol. 1
0-262-70027-1 Vol. 2