Single Turbo RX-7's Questions about all aspects of single turbo setups.

air filter size effect on power

Old 12-19-08, 11:37 AM
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air filter size effect on power

For turbos with 4" inlets, has anyone done any dyno testing of no cone filter vs a filter? Or ow about a smallish filter (b/c of fitment issues) versus larger ones in applications with more space? I'm talking about filters placed directly on the turbo inlet or with a short inlet duct, no "cold air" setups.
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Old 12-19-08, 01:01 PM
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I did a test last year on my brothers car with the GT35 and it made no difference.

I was shocked as i was expecting 5~10hp..
We were using a k/n air filter...
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Old 12-19-08, 01:05 PM
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Originally Posted by arghx View Post
For turbos with 4" inlets, has anyone done any dyno testing of no cone filter vs a filter? Or ow about a smallish filter (b/c of fitment issues) versus larger ones in applications with more space? I'm talking about filters placed directly on the turbo inlet or with a short inlet duct, no "cold air" setups.
I saw about 10-15max whp difference in my setup between having it and not having it. I have a 45Deg 4"alum elbow on the turbo's intake with a very small air filter at the end, no cold air. These changes where noted at 16psi over 6-7 runs. If you consider that you get variability between runs of 5-7whp then I guess it is safe to say that at least 7-8whp where due to the filter being off. highest we got that day with the filter on was 403.xx and with it off it was 414.xx. The 15whp difference was between the lowest with filter and the highest without.

This was with a T04S, .84 A/R P Trim on a Renesis.

I'd like to specify that the filter I use is tiny(about 3-4" long) due to space constraints.

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Old 12-21-08, 12:25 PM
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My K&N was small too but no bend just straight off the turbo.. We did one run and immediately did another with the filter off back to back and netted no difference at all. no changes were made. Im sure the quality and size of the filter makes a difference the k&N's must flow good.
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Old 12-21-08, 01:17 PM
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A small filter will not significantly affect power unless it is so small that it becomes a restriction. If a filter is not a restriction to flow, increasing the size to a larger filter will have zero effect on power, but it will not get dirty as fast as the smaller filter.

As for any horsepower gain, it is a percentage and not a fixed number. For example, were I a slick businessman trying to sell my filters, I would test one on a 3,000hp engine and advertise that my filters can gain up to 40 hp! Of course, it would only add about 2 hp to an otherwise stock 13B, but I did advertise "up to" 40 hp. Also, turbocharged engines see more of a gain with high-flow filters because turbos multiply pressure.

For K&N brand filters, I derived the stupid Evil Aviator K&N filter rule of thumb, which hopefully isn't any more stupid than the other stupid internet guidelines:

Horsepower Potential = Filter Media Area square inches * 4.53

If in doubt, just go to the next larger filter size as it will not hurt anything. If you have limited space, then just get the filter with the most filtration area possible.
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Old 12-22-08, 12:33 AM
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I remember reading a thread a long time ago..

ReTed had a 20B they were tuning

by removing the filter the A/F ratio went leaner by a whole point!

so I guess it depends on the application and filter itself as well

I ran no filters on my stock twins the other day.. I swear it sounded like a T51-R!
so mean!
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Old 12-22-08, 04:41 PM
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nice to see you still on here aviator
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Old 02-17-10, 03:19 PM
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I know this is an old thread, but I finally got around to doing this...

I am surprised this little air filter isn't making a greater impact on power, hell the air filter makes more power at lower rpms..
Attached Thumbnails air filter size effect on power-davedyno.jpg   air filter size effect on power-100_4844.jpg  
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Old 02-18-10, 05:20 AM
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Originally Posted by 1Revvin7 View Post
I know this is an old thread, but I finally got around to doing this...

I am surprised this little air filter isn't making a greater impact on power, hell the air filter makes more power at lower rpms..
The dyno looks fairly typical, showing that the filter is doing its job of straightening out the air and increasing the torque, but the impact of the pressure restriction is seen at the high rpm end. Although we are dealing a bit with voodoo math here, it looks like the filter is a bit small for the application.
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