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Old 06-02-2008, 05:32 PM   #21
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Quote:
Originally Posted by Braineack View Post
97.7 x 7000 / 20,839 = 32.8 sq in. required for minimum surface area.

RU-5111, 4.5" x 5.75", slight taper to 3.5" at tip.

4.5 x 3.14 x 5.75 = 81.24 -.75 = 80.49 sq in.

this is not factoring the small taper, but I doubt it will lower 48 sq in of area.
Brain I don't agree with your # for displacement. I believe this formula is intended to calculate a maximum acceptable volumetric air flow rate per unit area (flux?) based on displacement, RPM, and an assumed volumetric efficiency for a naturally aspirated engine. With a turbo, say running at 15 psi, the flow rate through the filter would roughly double for the same displacement and RPM. So, for a 1.8 liter, I think the displacement should be ~ 110 cu. in. x 2 = 220 cu. in.
Or ~ 195 cu. in for a 1.6 liter.
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Old 06-02-2008, 05:39 PM   #22
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The same volume of airflow is drawn in regardless of intake manifold pressure. The air is compressed; your cylinders don't act like balloons. Your displacement NEVER increases, cap bold italic. If the VE increases, use a multiplier of 10 - 20% or so.


Regardless, the formula I used (provided from K&N) is simply relating volume and rpm to minimum surface area.
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Old 06-02-2008, 06:06 PM   #23
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Originally Posted by Braineack View Post
The same volume of airflow is drawn in regardless of intake manifold pressure. The air is compressed; your cylinders don't act like balloons. Your displacement NEVER increases, cap bold italic. If the VE increases, use a multiplier of 10 - 20% or so.


Regardless, the formula I used (provided from K&N) is simply relating volume and rpm to minimum surface area.
Dude, I KNOW the displacement never increases BUT, the flow through the filter does increase for a turbo versus naturally aspirated. No question. The pressure drop across the air filter is a function of flow rate. So if the flow rate increases, the pressure drop across the filter increases. No question. The point of the K&N formula is to help keep this pressure drop within reason by specifying a minimum size filter element area for a given airflow (determined by engine displacement and RPM) based on a NA engine.

Since the formula does not allow one to input flow rate or peak engine power directly, one has to compensate for the increase (turbo) in airflow somehow. I estimated this increase by doubling the displacement in the formula. One could just as easily multiply the whole thing by the ratio of boost to ambient pressure. Hence I used 2X for 15 psi of boost (atmospheric at sea level is roughly 14.7 psi.). This is conservative, that is, 15 psi boost is not going to exactly double the flow, but it is a worst-case estimate. It is certainly more than a 10-20% increase in VE.

If you do not compensate for the increase in flow from the turbo, the filter selected based on the formula is going to be too small per K&N's guidelines. Seems obvious to me, maybe I am not explaining this correctly. Hopefully this explanation makes more sense.
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Old 06-02-2008, 06:25 PM   #24
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Dude, I KNOW the displacement never increases BUT, the flow through the filter does increase for a turbo versus naturally aspirated. No question. The pressure drop across the air filter is a function of flow rate. So if the flow rate increases, the pressure drop across the filter increases. No question. The point of the K&N formula is to help keep this pressure drop within reason by specifying a minimum size filter element area for a given airflow (determined by engine displacement and RPM) based on a NA engine.

Since the formula does not allow one to input flow rate or peak engine power directly, one has to compensate for the increase (turbo) in airflow somehow. I estimated this increase by doubling the displacement in the formula. One could just as easily multiply the whole thing by the ratio of boost to ambient pressure. Hence I used 2X for 15 psi of boost (atmospheric at sea level is roughly 14.7 psi.). This is conservative, that is, 15 psi boost is not going to exactly double the flow, but it is a worst-case estimate. It is certainly more than a 10-20% increase in VE.

If you do not compensate for the increase in flow from the turbo, the filter selected based on the formula is going to be too small per K&N's guidelines. Seems obvious to me, maybe I am not explaining this correctly. Hopefully this explanation makes more sense.
You are absolutely correct. Sometime in the past year, K&N dumbed down their sizing formulas. From what I recall of the older equations, which I have here circuitously scribed into an excel macro that I stupidly provided no documentation, it could be approximated just as you describe.

I used to use a cylindrical paper filter for a late 80s Prelude. While the area was about triple what K&N suggests as the minimum size by their current N/A-based calculation (granted a K&N obviously is more free-flowing than paper, but regardless), and still satisfactory using their old equations, it definitely didn't flow enough up top, or in vacuum. Switched to an appropriately sized K&N, and off-boost response is better, and top-end isn't joked (at least on the intake side) anymore.
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Old 06-02-2008, 10:22 PM   #25
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The pleated K&N filter material will flow 6.03 cfm of air per square inch, says them.

if this is the case, the RU-5111 I posted above should flow something like 450CFM.

the volume air flow at 7000RPM on a 1.6L at 2PR would be 395CFM give or take.


I was incorrect before, im silly like that. however, on the dyno, I've seen a car with a 2" intake tube and a tiny little motorcycle filter make 190rwhp at 10psi or so. As a test I had him remove the intake tubing all together and run it open at the turbo, as i thought it might be choking him off up top...there was no difference in power output.
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Old 06-03-2008, 01:09 AM   #26
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however, on the dyno, I've seen a car with a 2" intake tube and a tiny little motorcycle filter make 190rwhp at 10psi or so. As a test I had him remove the intake tubing all together and run it open at the turbo, as i thought it might be choking him off up top...there was no difference in power output.
Interesting... Always good to hear real-world test data like that. This would seem to indicate the K&N formula is conservative.
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Old 06-03-2008, 01:23 AM   #27
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Ok so realllllllly sorry for the ignorance, but ever since i can remember formulas/equations and me just dont mix...I just cant seem to ever get the same answer twice...

This is what I'm using...
http://www.knfilters.com/search/prod...?Prod=RC-70040

Am I choking my chicken?? Eggh I mean turbo
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Old 06-03-2008, 10:00 AM   #28
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Quote:
Originally Posted by Braineack View Post
on the dyno, I've seen a car with a 2" intake tube and a tiny little motorcycle filter make 190rwhp at 10psi or so. As a test I had him remove the intake tubing all together and run it open at the turbo, as i thought it might be choking him off up top...there was no difference in power output.
Did you happen to see both AFR logs for comparison? I'd be curious to see if he went leaner up top without the intake.
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Old 06-03-2008, 10:05 AM   #29
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IIRC it did go leaner....here's the two runs, and yes we corrected the lean mid-range.
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Old 06-03-2008, 11:05 AM   #30
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Bryce why dont you try this. I found my old filter set up.
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