I’ve seen this discussed on a few threads now and I think it needs it’s own thread. Andy@ross-tech had posted some interesting air filter volume numbers and people have been speculating as to what they mean. Well, according to K & N, a 1600cc engine (slightly smaller than 98 cubic inch) turning at 10,000 RPM should have a minimum filter size of 47 square inches. The HAI is 119 and the AGS is 90 according to Andy’s calculations (see below). A 90 square inch filter is essentially the minimum requirements for a 350 Chevy V8. If anybody here believes that the MINI engine requires more airflow than a V8, I’ve got some beachfront Arizona property for sale. I like neilgj’s analogy best: “The whole air flow argument seems somewhat silly in the absence of a statement of how much air the engine can usefully process per minute. Ever drink from a firehose? Darned great flow on those firehoses.”

http://www.knfilters.com/facts.htm#RESULTS

I am not trying to stir the pot, just looking to put this stupid debate to bed with facts (not that they ever stand in the way.)

 

Don't know much about air filters, but I do know that when you use forced induction, you need to effectively increase the size of the engine for those kinds of calculations. As to how much, you got me. I would guess that for every bar over ambient, you might double the size of the engine. How many lbs of boost do we make? I just got my mini a few months ago, and really don't know too much about the details yet. For 15 lbs of boost, might it be equivalent to 2 x 1.6 , or 3.2L? that would put the AGS at about the minimum.

These are just guesses, but seem reasonable to me. Someone get out an engine building manual and educate us.

-Andy. No, not that Andy. Just another Andy.

 

That's a measure of the pumping efficiency of the motor. So if a 1.6 liter motor pumps 1.6 liters of air every two revolutions, that has a VE of 100%. A good way to calculate max air flow is

D * (RPM/2)*VE = Air Required.

Put in red line, and the displacement, and you get (for our little mini)

1.6 * 3500 * 2 = 11,200 liters/min or just shy of 400 cfm. I think the leading edge of boost is about 25-30 psi, so the last number could be three, raising total flow to 600 cfm, about the right size for a bit better than stock 350....

Also, pressure drops are cumulative, so even if the filter has the capacity to flow more than is needed, the pressure drop will be less with larger area. Always. That's not to say that the benefit will be felt, or if it's the largest restriction that should be addressed first. another point is that this all only really matters at WOT at the effect is most at high RPM. When the TB is at part throttle, max flow doesn't matter diddly!

Matt

 

Just for giggles, the K&N kit for a 2.0 litre Mitsubishi EVO (Turbo) includes a K&N RP-4600 conical filter, which has a base of 7.5" a top of 4.5" and is 4" high. If my calculations are correct, that is only approximately 28 cubic inches in air filter volume.

 

andy's measurements of the pressure drops in the intake system are also crucially important in this whole air filter / ags / intake analysis.

my recollection is that at wot with a slightly modified car he got ~.14 psi across the STOCK (~60 sq. in) filter and ~.24 psi through most of the convoluted stock system. (these compare favorably with pressure drops measured on other cars, i.e., they are much less than 1 psi.)

so ANY claimed air intake / filter performance improvements should use these as a first order engineering sanity check. as andy correctly points out, at best you're going to get a small % improvment equivalent to a few horsepower with a stock or slightly modified car (i'm not talking about these 400 hp twin charged monsters). it doesn't surprise me that the hai shows a few hp gain, since it eliminates many lossy bends. however, i don't believe any of the intake/filter claims of 12 hp improvements with a stock or slightly modified car.

i suspect the advantage of the larger filters is more from a service life perspective.

lost in all this debate over relatively small intake and filter pressure drops is a concern over the quality of the filter media.