Is an unmodified stock airbox restrictive?

[kenikh]

Does anybody have experience with the stock airbox choking off airflow? Wondering at what power levels it needs to be modified or replaced.

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[OldSpool87]

I haven’t see any data on stock box being restrictive. Primary reason to switch is to clean up engine bay and create ease of access. I think Brian at Rarelyl8 has field tested this to some extent.

I have heard of aftermarket plastic units deteriorating and breaking into pieces.

[Rich76_911s]

Obviously it's restrictive. How else could fabspeed get 21 WHEEL hp out of the conversion???

[patina]

Originally Posted by Rich76_911s

Obviously it's restrictive. How else could fabspeed get 21 WHEEL hp out of the conversion???

$295 vs $45 for the K&N unit that works just as well!

[kenikh]

I’d really like to know some CFM and or HP numbers. I’ll do some math for determining how much air can flow through that hole.

Given the size of the stock air cleaner, it just seems that that’s the choke point.


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[TZ 197]

I've spent countless dyno hours working out velocity stacks and filter area on motorcycle applications. Biggest gains were not in overall filter diameter/area but providing air directly over center of stack draw. Also the further the filter top away from the opening the happier. Funnel shaped aids inside closed filter ends helped when it wasn't possible to use open tops. Just to note this was using oiled K&N's.

When talking airboxes, they typically improve driveability by slowing the air pulses in the mixing area, but might not be as relevant in a porsche aplication with injectors downstream. Possibly a oiled foam filter panel retrofitted over the metering plate could improve airbox performance and enhance metering area draw.

If building a race car I would experiment with a velocity stack on the metering unit and run it through the deck and seal. Then make a still airbox out of the cargo deck area and pressurize with external ducting.

Food for thought, now get out your sawzall!

[john p]

I raced a Kremer 930 in the late 80's and did a lot of dyno work during that time. We found that the original air box made more H P than all of the options available. The Kremer cars came with the original air box so i asume they also did the research and arrived at the same conclusion. Kremers told me they believed it had to do with the cone on the air box above the intake
Had this discussion with Chris from Turbo Kraft ( who i consider the guru on turbo engines bar none in the world ) and he made an interesting comment. He was told that was true but over a period the heat soak on the steel air box affected the intake temp . Good point. This conversation started because I wanted to run the plastic air box with the cone cleaner as to give more space in the engine bay on my Kremer road car ( which I will now do )
Thanks Chris, I hope have not offended you by disclosing our discussion, but he is definitely the go to person

[TZ 197]

Would be interesting to catch some clean air and pressurize the airbox opening hole. Should help heat soak and have ram effect at speed. The original design is quite sophisticated in the use of a large plenum then decreasing size for velocity w cone aid in tight mix area. Also cover and hole helps control resonance pulses. Source some high pressure area air and you have a very modern think intake tract.

[patina]

Originally Posted by john p

I raced a Kremer 930 in the late 80's and did a lot of dyno work during that time. We found that the original air box made more H P than all of the options available. The Kremer cars came with the original air box so i asume they also did the research and arrived at the same conclusion. Kremers told me they believed it had to do with the cone on the air box above the intake
Had this discussion with Chris from Turbo Kraft ( who i consider the guru on turbo engines bar none in the world ) and he made an interesting comment. He was told that was true but over a period the heat soak on the steel air box affected the intake temp . Good point. This conversation started because I wanted to run the plastic air box with the cone cleaner as to give more space in the engine bay on my Kremer road car ( which I will now do )
Thanks Chris, I hope have not offended you by disclosing our discussion, but he is definitely the go to person

Chris is the man!

[TurboKraft]

Originally Posted by john p

I raced a Kremer 930 in the late 80's and did a lot of dyno work during that time. We found that the original air box made more H P than all of the options available. The Kremer cars came with the original air box so i asume they also did the research and arrived at the same conclusion. Kremers told me they believed it had to do with the cone on the air box above the intake
Had this discussion with Chris from Turbo Kraft ( who i consider the guru on turbo engines bar none in the world ) and he made an interesting comment. He was told that was true but over a period the heat soak on the steel air box affected the intake temp . Good point. This conversation started because I wanted to run the plastic air box with the cone cleaner as to give more space in the engine bay on my Kremer road car ( which I will now do )
Thanks Chris, I hope have not offended you by disclosing our discussion, but he is definitely the go to person

All good!

Yes, a colleague of mine -- whose opinions I respect -- believes that the claimed performance gains from the plastic ("Powerflow"?) air filter housing are only because it does not heat soak.

If this is the case, then we should be able to install one on a CIS 930 next time we have one on the dyno and measure a performance gain, right? Because a stationary chassis dyno is hardly an optimal test condition, with the drivetrain doing 125+mph but the car is stationary so not benefiting from 125mph of high pressure cooling air. The stock filter housing should heat soak and cost us HP while a plastic one will not, right?

Heck, we could also test the engine with a standard filter housing to establish a baseline, and then heat the filter housing with a map-gas torch and see if we start to lose power.

My theory is that it may make a very slight difference on the dyno, but is won't make a flip bit of difference on a Turbo driving down the road. The engine is flooded with high pressure ambient air that I think will overcome any effects of heat soak. You can get considerable changes in engine output (and response) with variances in exhaust and intercooler temperatures, and my bet is that these dwarf any effects from a hot filter housing.

At that point, it's up to the competing designs of the filter housings to generate a measurable performance gain and in past testing I've seen no measurable benefit from the plastic "Powerflow" filter housing. Not on MODE's engine dyno, and not on back-to-back tests on a Dynapack chassis test (average of 3 tests before, 3 tests after). I know the manufacturer made some big claims and sold a lot of them, but the testing was so biased that *anything* would have shown gains. The baseline runs were performed with a tiny Type-1 VW sized filter that was restrictive.

Final thoughts on modified vs. stock filter housing: the stock filter housing has a ~3in air inlet, and a 3" inlet will easily support 600whp. The filter element has surface area comparable to cone filters we've used on engines making over 550whp. I don't see either of these features of the factory filter housing as limitations a CIS 930 engine.

[RarlyL8]

I've dyno tested various filters just to eliminate them as a variant to exhaust system baseline. There is no difference. None. As expected, if you want to make power with a turbo the air filter isn't the place to do it.
I never run the stock mailbox for 2 reasons: it's metal and it's huge.

[turbo owner]

The stock air box is designed to increase air velocity as it moves from the open plenum behind the filter to the metering plate

[ficke]

Originally Posted by john p

I raced a Kremer 930 in the late 80's and did a lot of dyno work during that time. We found that the original air box made more H P than all of the options available. The Kremer cars came with the original air box so i asume they also did the research and arrived at the same conclusion. Kremers told me they believed it had to do with the cone on the air box above the intake
Had this discussion with Chris from Turbo Kraft ( who i consider the guru on turbo engines bar none in the world ) and he made an interesting comment. He was told that was true but over a period the heat soak on the steel air box affected the intake temp . Good point. This conversation started because I wanted to run the plastic air box with the cone cleaner as to give more space in the engine bay on my Kremer road car ( which I will now do )
Thanks Chris, I hope have not offended you by disclosing our discussion, but he is definitely the go to person

Good information, thanks.
My Ruf modified 930 also uses a modified by Ruf ( clearance for the inter cooler)stock air box. I would imagine if it had some detriment to power it would have been much cheaper and easier for Ruf to to use something like a K&N, but they did not, I am sure for a good reasons.

[voitureltd]

Had a blue fibreglass air box ( forgot the brand ) with a K & N on a for street use modded 3.3 79 930 on the dyno@ C S Motorsports. They asked if I had a stock one as they thought the Porsche designed one better for the build and mentioned the cone above the flap in the stock one as one reason. Indeed just by putting the stock one that came with the car back on was way better. Probably still have the dyno sheets but memory serves it went from around 340 to 352 WHP and better torque.

[RarlyL8]

Man that must have been one horrifically bad filter box design!

[kenikh]

Probably not worse than what I’m currently using.




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[voitureltd]

Originally Posted by RarlyL8

Man that must have been one horrifically bad filter box design!

Was a major brand ( upgrade) sold around 15 years ago. Was on the car when I bought it. Don't think the engine builder drove it both ways just bought the claimed improvement PR about 15 years ago they were fed. Car was stock before so @ around 340 must have felt faster than before when @ around 260 so all were OK with it. Not seen any for quite awhile soooo? I put it on ebay and was sold as it was a known brand. Should have just junked it. A example that some of the original development was not so bad.

[ficke]

Originally Posted by kenikh

Probably not worse than what I’m currently using.




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Less surface area of the filter compared to the stock unite, so yeah, you are probably losing some flow there.

[mark houghton]

Here's how I handled this conundrum years ago....the only picture I could find. Hacked up a perfectly good mailbox, got rid of some heat soaking metal and shortened the travel path while keeping the cone over the intake. For what it's worth. Didn't accomplish anything more than giving better access to the right side of the motor, and using a huge cleanable K&N. Probably gave me another 100 horses (yuk yuk :-)

[RarlyL8]

I used the small carburetor air filter many years ago and found it would support 350whp, so how well the filter works has a lot to do with the performance level of the engine. I don't imagine it makes any difference at all what filter you use on a stock engine. Also dyno data is a static test, the car isn't running down the road with air rushing through the engine bay. You're not going to accurately detect a 5hp gain or loss due to the effects of heat on turbocharged performance. I made a deflector off the back of the IC to direct ambient air to the filter area. A temp sensor told me that that area stayed close to ambient temp whenever the car was moving. The amount of air moving through the engine bay is surprising. Testing in the "winter" was a huge eye opener, there could be a 10% difference in HP when testing in July versus January.
Bottom line, don't worry too much about the air cleaner, keeping the intercooler supplied with ducted cool air will give much better returns.