Choosing an air filter
06-02-2008, 04:32 PM
#21
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Or ~ 195 cu. in for a 1.6 liter.
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06-02-2008, 04:39 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.
Regardless, the formula I used (provided from K&N) is simply relating volume and rpm to minimum surface area.
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06-02-2008, 05:06 PM
#23
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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.
Regardless, the formula I used (provided from K&N) is simply relating volume and rpm to minimum surface area.
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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06-02-2008, 05:25 PM
#24
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.
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.
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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06-02-2008, 09:22 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.
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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06-03-2008, 12:09 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.
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06-03-2008, 12:23 AM
#27
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
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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06-03-2008, 09:00 AM
#28
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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