airbox and intake

[JonathanBee]

Hello all - brand new to this forum. I decided to sell my 997.2 and get myself a well-loved 911SC.

I have my eyes on this 1982 and was a bit puzzled by the state of the air intake cover(airbox). Is there any reason one would drill holes in it like this?



Looking forward to making new friends here

Jonathan from Berlin (the original one, in Germany)

[Geronimo '74]

holes allow more air directly to the filter.
More air = more power, so they say. Gains are marginal, it's is not a miracle fix.
It does make the car sound a lot deeper, throatier.

Maybe you have to be careful that no rain gets in the filterhousing (during heavy downpour), but I wouldn't worry about it too much.

[Geronimo '74]

They also make aftermarket airbox lids, with larger intake openings.

same idea.

[Geronimo '74]

[SkiVT]

Does this really add air into the engine? Seems it would circulate more air volume over the top of the air filter. Not as easy to understand how it then would get more air past the airflow sensor plate. It doesnt seem the same as the old “ram air” feature in muscle cars. Would you have to increase fuel as well to keep afr at correct levels?

[T77911S]

you don't want air to get past the sensor plate.

they put holes I it to bypass the "horn" opening on the left, that's all.

I would doubt anyone can tell it makes a difference just by their seat of the pants driving.

[Geronimo '74]

if the initial opening of the filter lid is restrictive, than I would say opening the lid up, will get more air to the filter.
I doubt air has the time to circulate in that region as the engine is actively sucking in air, creating a low pressure area in front of the filter.

This does result in a few HP gains in some (most?) cases. (Steve Wong researched this a bit, IIRC)
Maybe a slightly faster throttle response.
Porsche also did this in their Cup race cars, so it can't be all bad.
For us mortals, it mostly helps the sound and butt dyno.




This is what the cup airfilter lid looks like.
I did the same in my 3.2.
Like I said, sounds deeper than stock, sounds faster, but real world gains are not that big.

[Lyle O]

I would sure like to see some real data on this. The stock configuration inlet is essentially a velocity stack, which should accelerate the incoming air and therefore result in increased volume/density. Drilling the holes negates this effect (no velocity beyond engine draw). This doesn't even address laminar flow, etc. I wish someone smarter than me would model these two configurations so we would know for sure...please tell me if I am full of it...

[Geronimo '74]

http://forums.pelicanparts.com/porsche-911-technical-forum/539332-3-2-air-box-mods.html

Interesting reading on the subject.

[SkiVT]

Can water soak the air filter, for those that drive in the rain, and then possibly drip into the airflow sensor plate to engine. The holes on the side seem less problematic but across the top possibly more so. The trunk grill isn't water proof. The tradeoffs seem slim on this one for the 3.0 CIS engines but maybe I am overthinking it.

[Geronimo '74]

The 3.0l airbox is shaped very differently than the 3.2l version.
And it is situated much closer to the grill. (3.2l has the airfilter much to the right and well clear from the grill. )
Instinct says that makes it more susceptible to water ingress through a modified air filter cover.

Maybe you can take a hose and spray water on the grill, simulating heavy rain.
Open the lid and see if the airbox lid is wet.
If it is, I would not make holes in it.
If it is dry and you feel confident, go for it.
Maybe others have real world experience and will chime in.

[Flojo]

rule number one (or was it two?): don't drill holes in the airbox cover.

it is useless for whatever one might say it could do.

and the airfilter ist not protected from water/rain anymore.

[Geronimo '74]

I’ve never heard of anyone having problems with rain or water going in the airfilter because of a modified air filter lid in the 17 years that I am a member here.
Not saying it is impossible, but I’d like to hear from those people.
I’ve been driving like this for years, zero issues.

[T77911S]

when I had my 2.7 I put the throttle body on it off of a 3.0 because it was bigger.
unfortunately my 2.7 wasn't running right when I put it on but then later I found a restricted injector line so I really did not have a fair comparison of before and after.
in my case I was not going to see a difference except at WOT, I would imagine the same with this mod.
you would really need a dyno to know if either did any good.

[cmcfaul]

I believe the air box inlets are positioned to pull cold air at high pressure points. You dont want to suck hot air and compete with the engine fan.

Chris
89 Carrera

[Geronimo '74]

I think the air inside the engine cabine, while driving is not much higher than ambient temperature outside the car. The fan keeps new air coming in at a good pace.

[Jack Olsen]

Yes. For all practical purposes, there is no hot air in an air-cooled engine compartment.

[JonathanBee]

Originally Posted by Geronimo '74

Goodness - it's a disease! It's spreading.

Thanks all for your inputs - and here's a red one.

[MichaelSJackson]

Much ado about nutin?

If my memory serves, and it often doesn't anymore, I read a post way back on Rennlist where some one dyno'd with the CIS cover and filter completely removed vs. in place. The results didn't show any significant difference.

If you think about the Rube Goldberg path the CIS air flow takes, it makes sense. Down thru the filter, then up around and past the metering plate, through the rubber bridge and down again into the throttle body, into the airbox (which is a fluid dynamics nightmare) and finally into the long runners to the heads.

Changes to the filter cover will effect sound and aesthetics, both worthy goals, but the improvement to the butt dyno is the same one I get from washing and waxing my car.

[GH85Carrera]

Originally Posted by MichaelSJackson

Much ado about nutin?

If my memory serves, and it often doesn't anymore, I read a post way back on Rennlist where some one dyno'd with the CIS cover and filter completely removed vs. in place. The results didn't show any significant difference.

If you think about the Rube Goldberg path the CIS air flow takes, it makes sense. Down thru the filter, then up around and past the metering plate, through the rubber bridge and down again into the throttle body, into the airbox (which is a fluid dynamics nightmare) and finally into the long runners to the heads.

Changes to the filter cover will effect sound and aesthetics, both worthy goals, but the improvement to the butt dyno is the same one I get from washing and waxing my car.

This. It will indeed change the sound of the intake, but any power increase is just wishful thinking placebo effect.