STi intake pressure drop test - Subaru WRX Forum
 
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#1 Old 08-20-2005, 12:32 AM
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STi intake pressure drop test

I tested my intake system today on my 2005 Sti (stock) for pressure drops. Results were:

Stock airbox & filter, stock resonator, removed ram duct: 42" H20 (1.51 psi) vaccum
K&N typhoon short ram, without the heatshield: 22" H20 (0.79 psi) vacuum

I placed the vacuum connection on the wastegate bleed/intake pipe nipple, without the bleed hose connected of course.

The WRX STi intake box is by far the most restrictive intake that I've seen tested to date. Mostly due to the fact that an STi flows 300 h.p. The K&N typhoon is really effective at reducing intake pressure drops: they've eliminated the flex bellows, removed one unnecessary tube bend, made the intake pipe only 8" long, included a huge intake velocity bell, etc. Filter size is on the small side, around 80-83 in sq.

Which makes more power? The stock intake box produces MUCH more part throttle and midrange power than the K&N. The K&N makes a touch more topend, but barely. I think the K&N is throwing off the Maf somehow. It feels like part throttle fuel is really lean. I'm sticking with the stock airbox because it produces more torque under the curve, even though its 2 times the restriction of the K&N.
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#2 Old 08-20-2005, 10:55 PM
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interesting test with interesting results.

the tests that refer to the stock box making more low and mid power, where they done on a dyno? doesn't seem to take much for the stock box to make up a socalled pressure drop.
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#3 Old 08-21-2005, 10:43 AM
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Pressure drop wouldn't make much difference because the intake is not operating on atmospheric pressure like a N/A car. It is being sucked in by the turbo. Which is why intakes don't do anything...
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#4 Old 08-22-2005, 09:18 PM
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I tested the K&N drop in panel filter (P/N 33-2031-2). Cost $49.55 total from Pep Boys. Filter is about the same size as the stock Sti filter. Anyway the results were:

Stock airbox/stock resonator/stock STi filter: 42" H20 vacuum
Stock airbox/stock resonator/K&N 32-2031-2: 42" H20 vacuum

K&N panel filter actually fits very well into the stock airbox, and it looks really good too. However, zero difference in spoolup, zero difference in power, zero difference in noise levels, & zero difference in pressure drop.

Conclusion: Stock paper filters flow about the same as the K&N cotton filters of the same size.
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#5 Old 08-23-2005, 09:18 AM
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i can to the same conclusion with my MAFv testing with a paper OEM filter vs. KnN. It doesn't flow any more air for it to be relevant, however it DOES let in more dirt.
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#6 Old 08-23-2005, 12:45 PM
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Quote:
Originally Posted by hondaeater
i can to the same conclusion with my MAFv testing with a paper OEM filter vs. KnN. It doesn't flow any more air for it to be relevant, however it DOES let in more dirt.
I'm glad you also did your own testing, any interesting results? Yep, apparantly paper and cotton flow close enough to be a wash. Only thing that makes a big difference is filter area. I'm not crazy about foam filters either, my HKS was horrible at filtration. I'd love to find a supplier that offered paper cone filters like K&N does with their cotton ones.
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#7 Old 08-23-2005, 02:16 PM
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i guess it depends on what you thought you were going to find. I THOUGHT i would see some MAFv differences, albeit small ones. Truth is the differnce is so minute our MAF sensors cannot pick up a difference in MAF between a KnN (unoiled too!) and a paper style OEM filter. However i have found high levels of dirt in my oil my last two oil changes (were sent into analysis to blackstone labs), telling me they don't do much but just let the bad stuff into your engine.
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#8 Old 08-25-2005, 07:16 AM
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Here's another test:

Stock airfilter, upper airbox, removed lower airbox, stock ram duct: 31" H20 vacuum.

I yanked the lower airbox mount out and zip tied the stock airfilter to the upper box, then just used the stock ram duct to blast cold air onto the completed exposed airfilter itself. This kinda illustrates that the stock resonator and lower air box are very restrictive with a pressure drop of 11" H20. I think replacing the rubber bellows after the MAF with a Perrin silicon hose would yield another 3-5" H20 reduction. That would put this combo fairly close to a short ram aftermarket intake as far as intake restriction goes.

With this combo, the part throttle power was pretty flat, very much like the K&N typhoon. The intake roar was also the worst of any intake pipe combo, it sounded extremely terrible. This combo was the worst of the bunch by far, I couldn't wait to go back to stock.
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#9 Old 08-25-2005, 07:52 AM
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riddle me this, you keep using terms like "very restrictive" of the stock box, but yet you have admitted it makes more low-mid end power, are you not contradicting yourself? Are you explaining something i missing?
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#10 Old 08-25-2005, 02:14 PM
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Quote:
Originally Posted by hondaeater
riddle me this, you keep using terms like "very restrictive" of the stock box, but yet you have admitted it makes more low-mid end power, are you not contradicting yourself? Are you explaining something i missing?
Not at all. The car contradicted itself, I just observed and noted it Seriously though: An intake that breathes in hot engine air, an ECU that doesn't allow boost increases, and a MAF that gets wonked out by even the slightest of intake mods has no hope of producing more power than the stock intake.

Now, toss on and enginement management system with full boost control, and maybe a real MAF sensor, then I have no doubt a low restriction intake would provide some nice gains.
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#11 Old 08-25-2005, 05:46 PM
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you lost me. an ecu that doesn't allow boost increases? A crappy MAF sensor? the stock intake sucking in hot air?

My ECU (reflashed of course) runs way more than stock boost. Yes boost is controlled by the ECU and not some MBC, but our cars have EFI and advanced ECU's too.

THe MAF sensor on our cars is NO cheapie, denso's in general are high quality sensors. go try and price one.

And my stock intake sits all the way at the front of my engine bay out of the hot air, not IN the eninge bay like a short ram.
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