Being between contracts (programmer) for the moment, I went out to address a situation with the air filter on my car.

Scenario: My latest hotside is tubular manifolds into a PT7675, 3” downpipe exiting to the passenger side of my 96 Z28, built by Jon(LT1boost.com). The configuration of my hotside leaves little room between the compressor housing and the frame rail. In haste to get it up and going, I got the shortest Spectre Air filter (4” to 5” installed?) and crammed it in there. Afterwards I got my 2 bar speed density tune AFR to mediate a 14.5 at both Idle and 75mph cruise.

Fast forward to yesterday when I decided (initially) to cast my own inlet tube using hardware cloth (essentially a sheet of wire mesh) and a fiberglass kit I bought at the local parts shop. I took an old Spectre air filter and cut off the filter portion, leaving only the metal base plate and its attendant rubber mounting flange. This would give me both the readymade seal onto the turbo compressor inlet and a flange to append the wire cloth. The intention was to cut a portion of the hardware cloth and shape it over the flange and trial fit it, modifying it until it would fit in the confines and be easily removed/installed. Afterwards, I would be able to mix my resin/catalyst and wrap the fiberglass cloth around the wire mesh to get the desired sealing and shape.

It was while I was under the front of the car, having removed the small Spectre filter and looking at the space available that I renewed my suspicion that my old ‘Cobra’ turbo inlet *might* fit in there and, with the proper 4” elbows/tubing, allow me to use my maxxy sized K&N airfilter. I fetched the cobra elbow and pushed it into place. After rotating it to a few angles and contemplating the consequential tubing routes, I settled on one that looked tight, but promising. I got out my array of 4” aluminum tubes/elbows (from Racepartsolutions.com) and my sawzall and, after some measuring, cut the tubes and mocked them up. Well, I’ll be. It all fits just fine and the filter is not in harm’s way. It actually gets some cool air too as it is in the passenger side front fender near the overflow bottle.

Since my last trip to the ¼” was disappointing in terms of MPH (126 whereas it had gone right near 130 with the old kit), I had been wondering where the performance loss might have been sourced. I found a couple of things suspicious. For one, the kit would get to 14 psi for a nanosecond, but then the wastegate (JGS) would open and the boost would stay at 12psi for the majority of the pass afterwards. Next, that tiny air filter. I had misgivings about its ability to pass enough air on a naturally aspirated 350, much less a 76mm turbo . I still have to work out what to do on the wastegate issues but here’s what I found immediately upon starting the car after the air filter change.
As mentioned above, my Speed density tunes AFR at idle and cruise with the small spectre was solid at 14.5 to 14.7. Now, with ONLY the filter change to the large K&N, the AFR was now 2 points higher (lean) across the board. Instead of Idle being 14.5, it was now 17.5. Cruise was the same, .. prior was 14.5 to 14.7, now 17.x to 18.x. Aha! That filter *had* been a restriction.

I got out my tuning software (Tunerpro) and set the injector constant down two notches, to richen up the fuel across the board, and took her for a test drive with datamaster. Hmm. Now it’s a tad rich, showing 13.x. I reset the injector constant back up by 1, load it and test drive. Perfect! Now if it will quit raining for a bit, I’ll take her out and see how she runs on a drag race style pass.

If you’re curious about the Cobra Turbo elbow inlet, I don’t know what to tell you. I found it online in 2007 or so and damned if I can find it anywhere. It’s perfect for tight confines where you need an immediate sharp 90 elbow. It’s made of a thick heavy rubber and has construction embedded in it so that it won’t collapse under high vacuum. I have a picture of it somewhere. I just have to find it. Now that the Elbow is on the car, I don’t want to have to pull it off just for a picture.

 

Why are you changing the injector constant instead of the VE table?

 

I went from 137mph to 143mph in the quarter just by taking my filter off. This was a big K&N 4" diameter inlet.

 

I had the VE dialed in from dozens of logging runs. What I needed was a fuel change across the board so I just changed the constant. No need to go thru all that rescaling. The Wideband says I'm good at 14.x under the conditions of idle and cruise.

 

Holy cow! That's amazing Mike.

 

Atomic saw pretty substantial gains on the dyno from taking the filter off as well.

 

i always try and fit the largest filter possible, even if its alittle over kill.

this next setup i'm going to try and figure out a inlet tube basically to route the filter on the outside of the frame rails

 

these filters allowed Camaro Andreas's car to go 6.3 at over 220mph. Says he tried without the filters and it made no difference.

So I'd find it very hard...in fact impossible to believe changing an air filter caused 2 full AFR points difference at idle, considering how relatively little air the engine would be consuming at idle.

Unless previously you had the most ridiculous piece of **** air filter possible. Pictures would speak a thousand words as to what you had before and have now though

By all means use the biggest and best quality filter you can, that's just common sense. But unless it's very bad to start with, I cant see it making a huge difference. And if there was a difference, it would mostly only be when the restriction is there. ie at high loads.

 

I was pretty surprised, this was on back to back runs. I've seen 50-75rwhp gains on F1A and F1C cars.

 

Stevie,
Here's the air filter I 'was' using because space was limited.


The one I'm now using is a K&N and is at least as big as the tall red Spectre in the back of that same image.

As for as being impossible, no other changes were made other than changing the filter and the location. I shut it off and my wide band was saying 14.5/14.7 at both idle and cruise. It had been showing that for the past month and I've been daily driving it. Every week I drive to Rochester from Albany and back (3hr 40min drive each way) and it says 14.5/14.7. I change ONLY the filter and location and when I start it up, get it up to temp and drive it for an hour and it's showing 17.x at idle and cruise. What do you think this is? Couple that with the prior turbo (TC76) and Log manifold hotside, running with cracked (1/4" crack) manifold, this same large K&N I just put on it and it traps at 129. Now with tube manifolds, a PTE7675, this small Spectre air filter and it only traps at 126?

 

What filter is that Stevie?

 

No idea, but my point is that it is not huge, and Andreas said on YB that removing it made no difference to performance

I'm sure K&N or other brands will offer similar.

 

Yeah, I totally understood your point, i'm like the OP with clearance issues. Just looking for another brand to look at.

 

That said, Ive seen small filters like the above on turbocharged Hyabusa's making 5-600hp !

K&N are bound to do something, their range is massive

http://www.knfilters.com/search/universal.aspx

 

photobucket-45771-1353885652230.jpg?t=1353885653

Your turbo is reverse of my style?

 

Yes. If you click on my link in my sig, you'll see the config I have now. Exits on the passenger side, so the turbo inlet is pretty dang close to the drivers side fender wall. What benefit I get from exhaust clearance is somewhat offset by the limited room for an air filter.

 

Came across this while searching for collapse using certain filters.

We test these filters on the big demand diesels routinely. Look at the NW Dyno Circuit for examples.

KN conical filters flow 68-72 CFM per sq./ft. at .5" w.g.
Spectre filters flow 170-180 CFM per sq./ft. at .5" w.g.

Both cause problems on big demand diesel engines.

R2C Black air filters flow 480-495 CFM per sq./ft. at .5" w.g.

These are what we use on the big power applications. These are dry filters.

 

I too have read that removing the smaller filter does little to nothing. I run two (TT) small 4.5" K&N filters and cant notice a difference when they are on and off. As far as the cobra elbow goes, I think this is what you are talking about.

http://www.intakehoses.com/mm5/merch...ct_Code=90CB40

 

Thanks for the tip, just ordered one. (needed new filter anyway) Looks like the typical 5" inlet filters are usually $99. But you can get them for $79 through LMPerformance.com. They sell them as 350Z filters. Black Hex is a 10$ up charge.

http://www.lmperformance.com/26012

On a side note I noticed no difference between this cheapo blue APC china filter and the "nicer" AFE filter on the right. Tested both around 20lbs (S475) on a 5.3 with no noticeable changes in trap speed. I've always been too chicken to take the filter off completly.

 

Presumably this test was on same size etc of filter ? And how well did they actually filter ?

It's easy to make something flow a lot of air, maybe less so flow and filter.