Maybe somebody has run into this problem and can give me some help. I have done some slight mods to the stock air box on my 2001 Impreza 2.5 RS. I removed the maze that was included from the factory, leaving only the filter housing (which now holds the K&N). I then attached a 3" flexible pipe which I routed to one of the openings in the inner fender (to get some colder air). I now have a noticable power drop (about 10 hp) between 4000 and 5000 rpm. This only seems to occur when I pass through those rpm's at full throttle. Is my computer not able to supply enough fuel for the increased air, or what? Any Ideas?

 

did you reset the ecu after installing it?
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VAPORMATIC VAPORIZER

 

I disconnected the battery for about 4 hours.

 

we tried the same thing on my buddy's '04 wrx with pretty much the same results. that crazy silencer does something more than silence the intake system i guess. any other ideas on the matter. cause i've searched for more info on this mod and haven't found anyone with answers. just people who say it works and those who say it doesn't.

 

The modification definately works. Between the air box mod, the K&N, and a Flowmaster, I think I have probably gained 15 horsepower. The strange thing is that it seems to return to the stock horsepower between 4000 & 5000 rpm's, so that the more power I gain, the more noticable the dip in power becomes. I don't notice the dip nearly as much if I don't put the pedal to the floor until after 5000 rpm's, but I may not be getting my full power if I don't.

 

maybe get some engine management (utec, accessport). thats a FANTASTIC first mod for a wrx. as for the rs maybe a sAFC-II. (an a/f controller)
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V12 Engine

 

yeah management is cool and everything if you have money to be throwing into dyno time. you could just use the maps out there that someone else has created but who knows what kind of driving they were written for. just cause it's a 91octane map for CA doesn't mean that it was written for daily driving and economy or for that matter that it was written for maximum power. i agree with the idea of management being a great first mod but make sure to get the one that's right for you. and if you only use your car for one thing and one thing only (hopefully the track. j/k) then just get a reflash. as for the intake mod i'm still waiting for a legitimate answer as to whether or not it works. saying that it works, and then saying it has a power loss between 4k and 5k means it doesn't work. still looking for answers.

 

If you still have a MAF, take an compressed air can, (you know the kind that are used to clean keyboards and stuff), take off your MAF, and blow off the little wire element in the MAF, sometimes the oil from the K&N filters coat the element and it can't read the air properly. See if that works.....if not than your only out a couple bucks for the air.

 

Thanks, I'll try that.

 

I finally found the problem. It turned out to be the O2 sensors. They were jumping from one voltage reading to the next, instead of changing smoothly like they are supposed to. As far as the computer was concerned, they were still working, so it was difficult to see the problem. The solution was brought to my attention by a guy from Magnum Tuning in the UK. They sell O2 sensors, but to my surprise he advised me not to buy his product! He said I would not need his O2 sensors with my mild mods. Nice guy, huh? Once the sensors were providing smooth voltage changes, I was getting nice gains through my entire rpm range. I guess probably 15 hp at peak power.

So the answer is, yes, cold air + direct flow does gain a few ponies. Admittedly, not a big horsepower gain, but the air duct I bought at OSH didn't break the bank either. If I could find the same diameter hose with smooth insides, it would probably be even better.