During my research to find the cause of failure for the many Greddy kits, including my own, I got many differing opinions from various "experts". The one thing most people were on agreement about was that the sponge type filter that is included in the Greddy kit is not very good. So I decided to replace it with a K&N.

I wanted to do a direct replacement of the Airinx with a K&N instead of extending it through the bumper because I didn't want to have to deal with removing the bumper everytime I wanted to clean it. After a little research though, I found K&N does not make a filter that will fit in the standard Airinx location and still has sufficient flow capacity. I calculated the filter will need to flow 530cfm at 8 pounds of boost. The largest filter I could find to fit in the airinx location only flowed 340cfm. Almost enough for a stock engine, but definitely not enough if you're boosted. So I ended up extending the filter through the engine bay and using a K&N RU-3130. Here are some pics of the final product.

Should also give a few HP since it'll be breathing much colder air.

where did you get the tubing?

I bought it from ebay. I got an 18" straight peice, a 90 degree bend and a 45 degree bend, all 3" mandrel bent. The cost was $15 per pipe, and then shipping, which unfortunately was around another $15 per pipe. So $90 for all the pipe, but that's the cheapest I could find.

Any noticable difference? Good job though!

Any noticable difference? Good job though!

Haven't got the turbo back on yet. I just got it back last Thursday from being rebuilt. Now I'm waiting on the exhaust manifold and downpipe. I sent them out to Jet hot coat for coating. As always, its taking longer than they originally quoted me.

good luck on it..

Nice, bout time someone did this. I've always disliked the greddy intake setup.

Does the bottom tray for the stock intake still fit or not?

Yes it still fits. It just wasn't in. But, I'm thinking about making a replacement one out of aluminum though, just for appearance sake.

Looks great! I was going to try the same thing in a week or so

Where did you find the flow rates for the K&N filters? I checked their website and didn't see them.

I don't like that sponge mushroom either. Are your "experts" saying it can't flow enough, either? Or, do they just think its a piece of crap in general?

I have a smaller K&N right now. But, you've piqued my interest. I'm going to do a run tonight with something besides my K&N to see if that improves the ability to hold boost in the higher RPM.

The flow rates are not listed on their site. I don't know why, they should be. Here's the forumals to compute the flow rates based on the filter dimensions. At 10psi, you'll need an air filter capable of 560cfm, if yours is smaller than myne than it is likely undersized.

For a round filter:
cfm=3.14 X OD X h X6

For a tapered filter:
cfm=(BD+TD)/2*h*3.14*6

The "experts" were mainly complaining about the filtering capability of the Greddy sponge. Not so much about the flow.

Okay. So questions about these formulae:
1) units for the inputs? inches or feet?
2) It looks like its just the surface area times 6. Is the flow rate really that insensitive to the material the filter is made of?


EDIT: I just kinda answered my own questions by calling K&N.

The units for the inputs are inches, and give cfm.
I asked for the rated flow for the filter I'm using. I'm not sure if the number they quoted was a flow bench test result or not, but it was damn close to what the formula gives.

If 560cfm is correct, I don't think there is a filter that exists that will flow that much. In fact, I wonder about the ability of the 3 inch pipe to flow that much to the turbo. Dual intakes anyone?

It will flow 570cfm at 9k rpms, 95% VE, 70 degrees ambient temp, 7 psi boost at redline, and some other standard conditions.

I have a great spreadsheet that I made for doing flow calculations. If you want a copy give me your email and I can send it to you. You can play with the numbers and come up with your own flow calculation.

My filter flows 530 cfm. There are definitely larger filters as well.

Any idea what the sponge mushroom will allow? I'm thinking of trying it tonight to see what, if any, effects there are on the ability to maintain boost.

Well, I used to be optimistic that you had found the source of my boost droop, Ryan. But, not any more. I just got back from some test runs. I ran my 4x5x4 K&N tapered oval first in third gear and logged and photographed the boost curve. Then, repeated the procedure for the sponge mushroom on the same stretch of road. There was absolutely no difference in the boost curves. So, I decided to play Russian Roulette and go with NO filter at all. Even with no filter at all, the boost curve was EXACTLY the same. I'm not going to bother posting the photos.

At least in my case, there is something else causing the boost to fall with RPM. If I could just find the cause, I'd be smoking that '06 STi, instead of just running wheel-to-wheel with it! :)

I didn't think you'd see a difference with your boost. I changed over to the K&N over the Greddy for reliability reasons, not so much performance. If you're K&N is undersized though, I believe you will find a few more HP with a bigger filter as the turbo will be working less hard to suck air.

That is awfull ballsy doing runs with no air filter! You're braver than I am!

All of us, except Jeff for some reason, have the same boost problem. I would love to find a way to fix it. I have been talking to a local Mitsubishi shop about his problem. Since its a Mitsubishi turbo I figured they'd have the most experience with it. They told me all evo's have this problem and its related to the wastegate. They are very convinced that it is not a turbo sizing problem, as they have used 18g's on much higher hp cars running 20+psi. The owner is confident he can fix the problem by applying a few "tricks". Once I get my car back together we'll see.

What we really need is an intake air temp sensor to sensor post turbo to see if the turbo efficiency is plumeting or not. That would say for sure if its an issue with the turbo being unable to pump enough air or not.

So, did he tell you what the 'tricks' are? You aren't going to hold out on us Sapphonica-style, ;) are you?

If I knew, I'd share. I didn't get the impression that its any one particular thing. He said he needed to take a look at the setup before he could suggest how he would fix it.

We could confirm this if somehow you could watch the wastegate while boosting less than your preset amount (<10psi). Maybe if you jacked up your car and loaded it with the brakes and did a pull. Have somone watch the rod. With a boost controller it shouldn't budge until 10psi.

Well, I'm pretty sure it doesn't budge under 10psi - until the teriary intake runner open. That's when the boost starts to go down like a prom date....

Jeff is scheduled to come through town tomorrow night. Maybe I'll get him to watch while I toast my rotors.

I just recently welded a new wastegate onto the setup... Haven't driven the car hard yet... I'll let you know if it solves the holding boost issue. One thing I know it did fix when driving was that I don't have any boost creep problems anymore.

awesome. I was already looking at an external wastegate as an option. What boost are you running? What did it bleed down to?

please keep us posted!

Some pics would be nice too. When do you plan on testing it out?

One other thought. If you're impatient for results, like me, wire tying your wastegate flap closed would also tell us if this is the problem. Just be careful modulating the boost with the throttle to not exceed your tuning limits.

I have a 0.5 bar spring in a Tial 46MM wastegate. Yup, it's a gigantic wastegate and took quite a bit of work to reroute the piping and fit everything as we all know the spacing is VERY VERY tight already as is.

I can post pics if need be. Pretty impressive work.

I haven't taken the car up to high RPM yet so I haven't seen whether boost is holding or not.

I'm going to be installing a small wealth of other goodies soon too.... should be fun.

If you can get the Revi filter to work - it is HUGE

That looks nice, Hybrid. BTW I have my WB02 in exactly the same place. What are you waitig for? Take it to redline and lets see what you've got!

Ryan, I'm impatient as you are! I'm probably going to tie it shut this week some time.

The WG looks good except the angle where it integrates the exhaust back into the down pipe. That's not going to flow to well and will contribute to exhaust back pressure at the turbine outlet.

The wastegate is so big... it won't have any problems for flow... and that's just a dump pipe into the exhaust... I just didn't want to dump into the atmosphere and deal with all the noise!


I'm going to be installing a few more toys and tuning... so I'll take it redline then. That and there is snow on the ground here still in Toronto!! :Eyecrazy:

But what I'm syaing is that there will be a high pressure area where the wastegate feeds back into the exhaust pipe. This will decrease the pressure delta across the turbine resulting in less energy to spool the turbo. It should blend in the exhaust at as shallow of an angle as possible to minimize flow disturbance.

But, that is just my opinion from looking at it. Definitely let us know how it feels. I may have to upgrade to an external wastegate in the future. I had my internal wastegated ported, but I don't know for sure if that will be enough to bring my boost back down.

rkostolni.. I know what your saying.... nonetheless I feel that it would make minimal if any difference at all. I'll also be putting on 3 inch catless mid pipe which should help for flow.

I'll let you guys know how it goes when I install my new fuel management and get it tuned.