need something 4" inlet, 600fwhp absolute max and 400-500 more likely. Anyone got something they know works? I've seen some filters with "ratings" on the packaging and they seem like they may be optimistic for the size.

 

AFE power makes custom filters, use 7-layer best protection

Measure the pressure drop using 1-bar map sensor to find breathing capability
Dial pressure drop as needed to achieve successful pcv crankcase evacuation for clean oil and long oil lifespan

Ideal range 1.5" to 2.5" Hg for WOT in performance engines
Published papers for OEM engines typically 0.5" to 0.7" Hg for idle/cruise

 

Check out Anderson Ford Motorsport (afm) website. They have different lengths of 4" filters. What's nice about theirs is the filters have filter media on the end with and internal cone and radius lip on 4" side. They flow good cfm. Have been using them for years.

 

600hp is roughly 790cfm for reference...

 

I found this calculator:
https://strikeengine.com/air-filter-size-calculator-for-power-bhp/

My results are:

For a boosted 347ci engine with 1.1 volumetric efficiency and 500rwhp; I need a filter that is:
12" long
7" Diameter on big end
4" diameter on small end

This will give me an air filter area of 207 SQ/IN to reach 645CFM.

That's one big filter for a limited space.

 

I don't think that calculator is accounting for the pleats in the filter itself? That seems huge for 500hp.

Engine masters did a comparison on filters, basically they found as long as you weren't running ridiculous tininess it didn't make a ton of difference. On their 550 HP engine a 2 3/4 tall filter with a 6" base only lost 9hp. Their 6" tall by 5 3/8" base showed no loss. Their test method seemed reasonable.

On a street car going a little big probably isn't bad, as it gets clogged with garbage you have more area to get clogged up before you see a loss. I usually stick the biggest thing I can fit in there, because I am terrible about cleaning them.

My guess is a turbo car is less sensitive, since the turbo is going to draw harder than purely N/A. Could it effect spool? Maybe, if it was a significant restriction. Spool isn't happening at max airflow though.

 

I thought about the filter design as well but isn't CFM; CFM regardless of design?

 

Sure, but that calculator isn't telling you CFM, its like a generic surface area calculator... I don't really like that calculator at all.

Pleat design changes from manufacturer to manufacturer. There can be big changes in the pleat depth. If you have double the pleat depth you have almost double the surface area. How close are the pleats to one another? That will change pleat count, which... surface area.

It also doesn't account for filter media, there is a big difference in a paper/synthetic/cotton/foam filter.

Look at a factory square filter from a modern 5.0 mustang... they aren't that big, maybe 11" by 8", or 88 square inches (that's a guess from memory) but the pleats are deep. If you pull one apart its like 6' long, that's a lot of surface area, that's why they flow 500+hp fine, with a 12000-15000 mile service interval.

The filter size the calculator has the surface area right, it just doesn't account for anything in the filters design.

 

Ah; gotcha. Good points. I noticed that my AEM dry filter has deep pleats compared to K&N.

 

You can't go by expected flow rate in CFM for any air filter
As the filter ages and becomes dirty the CFM rate will change

The only thing you can do is measure the pressure drop behind the filter and adjust as needed to maintain some pressure drop.

pressure drop in front of turbocharger affects the compressor map moving you right and up, higher pressure ratio and more flow to reach the same power, since CFM is volumetric rate and not mass rate. In other words, as pressure drops, the same CFM rate is LESS mass rate of flow, so for example at 500hp with a pressure drop of 1" vs 3" is completely different CFM rates. The proper way to adjust is using flow rate or dynometer result by increasing boost until the turbo is flowing what you need it to flow to make said power, then adjust the pressure drop (pressure ratio) if needed. Assuming the intake manifold pressure is driving the wastegate the turbo impeller RPM will compensate all by itself to maintain some number of PSI in the intake manifold at whatever extra impeller RPM is needed.