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How much filter surface do I need?

Old 06-04-09, 03:38 PM
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How much filter surface do I need?

Trying to figure out if the filter I bought has enough surface area or will be a restriction.

The setup.... 13bNA streetport, Microtech, 3" exhaust, spends all its time at 6-9k RPM

From the TB I have a 3.5" AWR intake tube .... from there to the filter is what I am currently trying to work on.

The plan which has been started, not yet fully completed, but now is causing doubt... 4" tube to an aluminum box that sits in the headlight area. Inside the box an 8.875" x 8.875" K&N panel filter. The box will taper after the filter to the 4" tube. Air will get picked up, more like shoved in at 100+mph through the FTP and turn signal holes in the bumper. This is very similar to the complete AWR intake setup.

Will an 8.875" x 8.875" K&N panel filter provide enough airflow? That's about as big as I can go and still fit it in the headlight area. For reference, stock panel filter was 10.875" x 10.25", the filter proposed is used on alot of Mitsubishi 3.0L V6 cars.

Stock filter - http://www.knfilters.com/search/prod...x?Prod=33-2017

Filter I bought - http://www.knfilters.com/search/prod...x?Prod=33-2045

What does all the airflow experts think?
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Old 06-04-09, 04:58 PM
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I've been running a K&N cone filter in the RF headlight bucket with a 3" outlet, through 3" piping, on my turbo car without issue. That probably doesn't help you, really, but it sure sounds like the piping is overkill so I'd imagine 100in^2 of filter should be plenty. If somebody can guess at the area covered by a 3.5"->6" cone that's been sufficient to hit 21psi of boost on a stock port 13BT motor for me (stock turbo at the time).
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Old 06-04-09, 06:11 PM
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We had positive pressure on both sides of a filter about that big on Ed's car, Mark. I hooked up my magnahelic guage to the 'box' area behind the FTP and turn signal holes, in front of the filter. And then tested the area behind the filter. At speed(Daytona) we had a few inches of positive pressure both places with just a little lower number behind. Any filter will give you obstruction and it will show up more on the chassis dyno(no ram air) unless you place a squirrel cage blower right at your air intake holes.
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Old 06-04-09, 06:38 PM
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The 2c I can add to this is that I side by side tested ISC's tiny little ebay cone filter on a dyno with a huuuge K&N I ripped out of my S2000 and the car lost power with the big filter.

Damn rotaries make no sense to me.

with this reasoning honestly the best setup might be routing all of the intake air through a hollowed out bottle of Pantene Pro-V with some Gym shorts shoved inside... then letting it get to your intake.
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Old 06-05-09, 06:55 AM
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Thanks guys, I'm going to proceed with the box and the 8.875" square filter. The box will taper behind the filter to the tube. If the intake tube is 3.5" from the TB, is my idea of using 4" tube from the box and meeting up with the 3.5" a worthwhile idea? Will join with a silicone reducer coupler. I don't have any engineering or physics background but am basing this idea off the Venturi effect.

The ever popular 3" cone with the 3" pipe works, I did it before inside an intake box. Time for something better with the new build. Here's some pics of the old setup https://www.rx7club.com/race-car-tech-103/custom-cai-708263/
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Old 06-05-09, 07:35 AM
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Do some research on Helmholz resonance and you may want to test a few set ups behind the filter on the dyno. Pipe length and volume will effect the power band even though its before the throttle body.
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Old 07-22-09, 04:50 PM
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Air box is done, still need to do a little grinding and polishing. Went with the 8.875" square K&N panel filter and a 3.5" outlet as the pipe to the TB is also 3.5". Will be ready for the dyno in about 2 weeks.











And this is where it is going....

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Old 07-23-09, 05:19 AM
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Nicely done!
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