Thoughts on air cleaners...

[Infidel!]

I wrote this email to a friend of mine who asked for my opinion (big mistake). I thought I'd share it here and get all y'all's opinions.

My thoughts on air cleaners:

An engine is made up of finite dimensions (bore, stroke, etc.) and as such there is a mathematical limit on how much air the engine actually needs at a particular operating range. We all know that the stock intake is severely restricted, but how much? You can calculate the CFM required by an engine using the equation cu. in. X rpm x VE / 3456 = CFM. So, for our engines:

103 x 6300 (MAX RPM) x 1 (assumed VE of 100%) / 3456 = 188 CFM

So, our engines need 188 CFM at WOT. Now how much do the aftermarket air filters flow? I found these flow bench numbers a while ago, and while the models may be outdated it’s still useful to demonstrate my point.

HD stock - 171CFM
Stage 1 Big Sucker - 200CFM
Stage 2 Big Sucker - 250CFM
Kuryakyn Twin Velocity - 270CFM
E-3014 = 248 CFM RK-3009 kit
E-3037 = 301 CFM RK-3910 kit
HD-0800 = 231 CFM SE replacement
Doherty Powerpacc - 287CFM
SE Air Cleaner - 287 CFM
Zippers - 321 CFM
Woods Carbs - 4 models 235CFM upto 340CFM
BC Overflow - 800+CFM

So the stock filter element almost has enough to cover the 103 running at 100% efficiency, and the regular old SE air cleaner flows 68% more than stock. The BC Overflow at 800+ CFM is just overkill. Even at the maximum 127% VE you’ll only require 238 CFM at wide open throttle. Now the stock airbox, which effectively seals the element is where most of the restriction comes from. I couldn’t find any numbers on that. Exhaust is another factor, but that’s a whole ‘nother tirade.

Bottom line is that any air cleaner you can find will flow way more air than our 103 engines will ever require.

Regarding open vs covered, my feelings are that an element covered by the football can only suck air that has been heated by the engine because the football has all other routes blocked. The air has to come from behind the football, and has already been heated by the cylinder fins. We all know that cooler air makes better power, so I think on a dyno an open element will make better numbers. However, in real world situations, that may not always be true. An open element has to deal with wind sheer (created by the movement of the bike). The air being rammed in through the exposed element has to make a sudden right turn to enter the throttle body. Air does not take kindly to bending, so it’s possible with enough sheer the air will continue out the other side of the element, thus actually creating a small amount of vacuum in the throttle body. I say small because the movement of the pistons and opening of the valves will be enough to suck some air in allowing the engine to continue running. This may not be as big a problem on our fully-faired bikes as it would on say, a softail. And of course, you have the rain and the bug problems to deal with. So, how much power do you lose by covering up? I don’t know, and I doubt a dyno can recreate an 80 mile an hour ride across west Texas on a 95 degree day. Who cares, anyway? 6 months out of the year you’ll have 1 or 2 more HP and 3 lbs more torque, and 6 months you won’t. I don’t know about you, but I doubt I’ll feel that.

So, bottom line of all this? Buy whatever you like the looks of. My bike made decent numbers with the SE paper filter element, so I’ll stick with that until I see something that looks really cool. On my Dyna I run the Kuryakyn Hypercharger, and it seems really happy with that, although it has been dogged by so-called experts because of it’s flow qualities. I just like the flapping butterfly on the front of the scoop.

That’s my story and I’m sticking to it.