Air filter breather box

[kena]

Has anyone seen or fabricated an aftermarket air filter box like a K&N setup that works for a 914?

[campbellcj]

Sure...ya mean like this?



On a "plain old 914/4" with D-jet I don't see how a K&N type cone filter setup could possibly work: you need a closed-loop system for the FI to operate. There would be no way to run all the vacuum lines to an open cone filter. With carbs or possibly certain aftermarket FI systems, it's obviously not a problem and most guys are running foam or gauze filters on their Dell's or Webers.

You could always just try a drop-in K&N element.

Chris C.

[This message has been edited by campbellcj (edited 08-06-2001).]

[Dave at Pelican Parts]

Uhhh, Chris... I think you're a bit confused here.

"Closed loop" refers to the control system of the FI, not the vacuum system. It means that the control system has some measurement of its output and uses that signal to modify the gains in the control system. In an engine's case, the FI has an O2 sensor in the tailpipe, and uses the signal from that to tweak the mixture.

The K&N thing has been done. I've seen a 2.0 with a K&N cone directly on top of the throttle body, sticking straight up. An aluminum rain shield sat above it to keep most of the water out of the motor--needed, since there was no room for the rain tray.

Jeff Shyu also had a homemade intake with a K&N filter. He used some PVC pipe to plumb from the throttle body to the filter. I forget if he plumbed the pressure-side vacuum lines into the pipe, or if he had smaller filters that he just stuck on the ends of those lines. Either approach should work with D-jet.

If you have L-jet, then you have to plumb the lines that are in between the air flow meter and the throttle body into something that is in between the air flow meter and the throttle body.

--DD

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Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling

[campbellcj]

Dave - I guess I am confused I just didn't see how a wide-open K&N cone sitting on the throttle body could work with an induction system that normally operates with a number of vacuum hoses connected to a sealed air cleaner box. (There would be nowhere to connect the hoses.) I suppose if you duct it to a remote location via a PVC pipe (like Jeff's) then that solves that problem, as you said you run the hoses to the ducting.

With that cleared up, is this a worthwhile mod in the first place? I'm sure there are some small gains but without other mods to pump more air thru the engine (heads, exhaust) I don't see picking up a lot of HP.

Cheers,
Chris

[This message has been edited by campbellcj (edited 08-07-2001).]

[Dave at Pelican Parts]

Oh, one more thing on the "closed loop" topic--none of the 914 stock EFIs uses an O2 sensor.

If you look at the stock 2.0 air cleaner setup, you can see how that might be some restriction to incoming air. It has to go into the snorkel, then make a 90-deg turn up into the main "box", then through the filter,
then a 90-deg turn to go out toward the throttle body, then a third 90-deg turn to go down into the throttle body. A cone filter right on top of the TB would make for zero turns--probably less restriction.

As for the other hoses, none of them have vacuum under normal operating conditions. The one from the AAR gets vacuum when the engine is cold, and the one from the decel valve has vacuum under "overrun" conditions.

Now consider what happens if those are open to the air (but filtered air, of course!). The air gets sucked in, which raises the air pressure in the manifold. (Or lowers the vacuum, same thing.) The MPS detects this change in pressure, and adds fuel. The result? More air and more fuel--a reasonably correct mixture, and an engine that is turning more RPMs than it would otherwise. (Mostly noticeable at idle.)

Which is exactly what you want to happen with the AAR and DV.

The L-jet system has to have those things fed by filtered air that has been measured by the Air Flow Meter. So in L-jet's case, you have to tap into something between the AFM and the TB.

--DD

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Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling

[kena]

Interesting conversation guys. I really just hate struggling with those stupid clips on the 2.0 airbox. Opening up my engine lid will never be the same again though after looking at the picture Chris posted. Wow!!

[bowlsby]

Okay Daveman,

Are you advocating a cone filter replacement for the stock airbox for performance enhancement? I would think that the vacuum lines could be attached to the system onto an extension tube under the cone filter.

I don't have an opinion, just curious.

[Dave at Pelican Parts]

I'm not advocating it. I am saying that I can see that it might be an improvement. But the restriction through the throttle body itself might be the dominating factor, so it may not matter.

Sounds like an interesting test to make, though!

I've seen one time-trial car set up this way, but I have no idea if it really works or not. I've seen one or two other autoXers/time trialers with nothing at all over the throttle body except some "chicken wire".

--DD

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Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling

[campbellcj]

A 914 racer most of us know (let's call him "B") says that he's seen a measureable improvement by running either a K&N drop-in or running without the filter entirely, with the stock airbox. Obviously I've never dyno tested this myself, but I ran my final timed laps sans-filter at Buttonwillow a few weeks ago, and dropped a full 2 seconds compared to my previous best lap of the day. I can't attribute that all to filter restriction (duh), but it certainly didn't hurt! This was an OE filter with < 1500 miles on it.

Chris