Removing SC airbox

Brett · Brett's Garage

Can somebody tell me whether it is possible to remove the 911SC ('78) air box and replace it with a performance cone filter?

Thanks.

SpeedracerIndy · 12-29-2004, 05:40 AM

No. As in, no you can't put a cone filter on there. Don't know why you would other than looks either, as there is no performance benefit to it. There are a few air cleaner covers that are available that look a little better than the stock one, but that's it.

sammyg2 · 12-29-2004, 07:05 AM

Air box? Are you talkng about the intake manifold plenum that is often called the air box, or are you talking about the air filter housing?

No, you can't replace the plenum air box unless you replace the fuel injection system with a different design.

Yes, you theoretically could remove part of and modify the filter housing to fit a cone filter but it would look like crap, would not perform as good as stock, and be a real PITA.
No reason to even contemplate this type of modification unless you are gonig to rip out the entire CIS and install motronic.

Even modifying the air filter cover on a CIS engine is a complete waste of time and is only cosmetic and IMO borders on the poser ricer category.
No performance improvement will be gained unless you run without the cover completely, and even then the improvement will be so small it would be hard to tell.

Andras Nagy · 12-29-2004, 07:12 AM

I will be placing my recently rebuilt 3.0L 911SC engine on a dyno soon, and will do two runs - one with everything (CIS) where it's supposed to be, and one without the filter cover, exposing the filter.

My prediction (hope it's not self-fulfilling) is that there will be no gain to having more air going in through the filter. The venturi plate is supposed to allow all the air that the CIS needs.

All those "new" filter covers you see are cosmetic only (they do look "cool", but I doubt that they enjoy any pefromance gains).

If anyone has done any such testing, let us hear from you.

Brett · Brett's Garage

Thanks guys . .

CarreraS2 · 12-29-2004, 07:56 AM

Quote:

Originally posted by Andras Nagy

If anyone has done any such testing, let us hear from you.

Bruce Anderson has done it with a dyno and the CIS system (stock filter, KN, no filter at all) - got the expected results, as I recall reading in Excellence a few years back.

You could probably send him an e-mail.

Andras Nagy · 12-29-2004, 08:31 AM

Yep, Bruce is an old friend, and I also remember that he has maintained that the stock filter cover lets in as much air as is necessary by the CIS.

Has anyone wondered why that filter cover has a venturi shape pointing to the left? Might it be that the engineers at Weissach know what they are doing, and have tested such things? Might the "venturi" shape channel the air, and also compensate for its speed throughout the rev range? Do any one of us have more information or empirical testing that might be a "better" way? I would love to hear from those who have tested these things in a controlled "scientific" manner, not just seat of the pants.

CarreraS2 · 12-29-2004, 08:51 AM

Info on the venturi shape would be interesting.

They changed the shape over the years - I think the early CIS cars had a much longer, narrower opening.

Andras Nagy · 12-29-2004, 09:05 AM

Right, and our SC's have a shorter and fatter one. Possibly having to do with the "ram air" effect and the speed with which the air enters, is compressed, thus speeds up, etc. And that's why we say venturi-like.

This is quite analogous to variable intake systems that Porsche, among others, are using in the intake traks. Note that all of these, including VarioRam, are all designed to take advantage of air requirements at different air speeds, and engine speeds. Note also that certain "velocity stacks" of old time hot rods varied their heights, depending on what the RPMs were required to get specific torque.

The long and short of it (no pun intended, though acknowledged) is that these are complicated issues asnf complicated physical properties, and engineers at Porsche, BMW, Mercedes, and all other manufacturers spend lots of research time optimizing these variables.

Isn't it fatuous of us amatures to think we can "do it better" than the results of rigorous, analytical engineering (nay, even scientific) testing?

Show me the data!!!!!!!