So, ive always heard that the biggest you can fit is best. Its a functional rule of thumb. But....
So, how do you properly size an airfilter without a dyno/dragstrip/etc? Is there a math thing? Does it change for na vs forced induction? Change based on distance from throttle body?
Im sure that this is really complex, but i think theres a lot of us that can benifit from learning more.
Air speed is a good indicator if you dont want to setup a manometer to measure pressure drop.
CFM / area filtered in square feet = fpm
RossD said:Air speed is a good indicator if you dont want to setup a manometer to measure pressure drop.
CFM / area filtered in square feet = fpm
but what's a good fpm to shoot for?
I don't know if there is a useful math thing around given the variables like filter media.
Does it change for na vs forced induction? Forced pulls more air so very likely.
Change based on distance from throttle body? Might depend on na vs forced and bends in the tubing. The math would be basic air flow through tubing numbers.
OEM filters are often bigger than they need to be, especially in trucks. There are a lot of things on cars that are made bigger than they need to be so that they can degrade further before causing a noticeable symptom. In other words, building in a lot of margin of error.
Air filters do change from naturally aspirated to forced induction, but probably not the way you think. The airflow requirement correlates almost perfectly with how much hp the engine makes, so whether it's turbo or na it would need the same size of air filter. The thing about turbos is that their shaft speed is variable. If you have a restrictive filter creating a pressure drop to the turbo inlet, the turbo will basically just operate at a higher pressure ratio and spin faster to make the same amount of boost, which will mostly mask the problem. As long as you have 'headroom' in your compressor map and aren't running hot enough to be on the edge of detonation, the turbo will just spin faster and you won't know that your filter sucks until spool times become noticeably worse or the added heat from the higher PR turns into detonation problems.
So basically a bad filter would be LESS noticeable on most turbo setups. They would affect a supercharger with a fixed drive ratio that couldn't spin faster to compensate.
Honestly, best way to figure out if your filter is a problem is to try to detect pressure drop behind it.
In reply to AngryCorvair :
I would need some specs on auto filters, but you could backwards math it. Take a high performance with a square filter, take redline, guess at volumetric efficiency, and math to get CFM.
Looking at Camfil for some panel filters for HVAC, taking in consideration of the pleats, anywhere from 88 fpm to 200 fpm.
You'll need to log in to post.
