I'm currently using a cheapo old filter and among the parts I am stockpiling over winter to freshen up the car and get it ready for the next stage of performance is a proper filter to breathe through.

The filter I have found is the perfect size/shape for the location it needs to go, but realistically, it's pretty small:
Air Filter Top Measurement 3.500 in. L x 2.500 in. W
Air Filter Bottom Measurement 4.500 in. L x 3.750 in. W
K&N RU-2780 - K&N Universal Performance Air Filters - Overview - SummitRacing.com

Now, I know a lot of the intakes companies sell have filters that are far far larger than they actually need to be - the whole, bigger must be better thing and all. And I've seen boosted cars with far more displacement/power using just regularish sized cone filters...

I want all of the components I use to be good for 300 whp so that I have some headroom and am not straining anything too much. Is this filter an acceptable size? Are there any rules of thumb when it comes to filters?

Thanks
-Ryan

 

I just went through this same dilemma for my application, and from what I recall the big issue is intake velocity vs surface area. You want to use a big enough air filter otherwise you'll end up with a very high flow rate over a small area and not only can that reduce HP but also you will suck the oil out of the filter as the air is moving too fast for the size and reduce overall filtration. I called K&N to get their flow numbers in CFM and a small filter can indeed be insufficient.

There is also a calculator online but I lost the link, basically it allows you to calculate surface area vs. pleats vs pleat depth and filter dimensions. When I did the math I ended up having to upsize to assure it was as freeflowing as possible. I was using a 5" diameter x 4" length straight K&N filter that I since upgraded to a 5" x 8" length tapered.

I would use the biggest you can fit, however for real world application on the last dyno day in DE turbotim did a air filter on/air filter off using a very small k&n, having the air filter off was good for a 5 hp gain. At 300whp it sounds like you need a far larger air filter.

mike

 

There's a whole article on Garrett's website about air filters and how to make sure you are getting one in the proper size, but it seems their website is down at the moment, here's the link: TurboByGarrett.com - Turbo System Optimization

 

Im not a fan of the K&N even with the oil they let to much crap through, check out these filters, Ive been running them for a while and love them. AMSOIL Ea Air Induction Filters (EaAU)

 

that's exactly the link I was referring to, thanks.

 

K&N filters suck **** for sand filtration. You want an Amsoil filter, they also seem to have more stuff that fits in my air box.

 

K&N has some formulas on their site for selecting minimum filtration area.

K&N Air Filter Facts You Should Know

When I went through the numbers some time ago I came up with needing more filter area than the Filter that FM uses I multiplied displacement by number of atmospheres of boost to get an estimated equivalent displacement for there formula.

I think this is the one I ended up with.
RX-3990-1 - K&N Universal Air Filters, Universal X-Stream Clamp-on

I got pretty creative and modified how my headlight motor was oriented to make a nice spot for it behind the headlight. Also made a nice cold air box fed by the TSI intake.

Bob

 

The amsoils on the webiste are 3" inner diameter inlet flange and larger.. no 2.5's...

 

2.5 is dang small I run a 4" inlet on my holset. when I had my greddy I used a 3" inlet filter, I had a silicon adapter from the turbo to the tube and then the filter just clamped right to the tube, but I dont know how you routing is.

 

I've been a fan of the RU-5111 for years!




and filtration is overrated, limits flow! if you're worried about sand, get an extra foam wrap and/or prefilter:

 

The Amsoil filter is far better filter for the money, and Im not just saying that because Im a dealer, I wont make any money off of you if you buy one unless its from me. Ive used many different brand cone filters and find that the cheep china knock off filters seem to filter better than the K&N. Amsoil filters filter better with out restriction. Get one.

 

dirt > $$$

 

What about...............

Automotive Air Filters

?
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