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Discussion Starter #1
Okay, I already posted this in the Engine Management Forum by accident when I realized that it should probably be here instead.

First of all, yes, I did a search, and yes I know that a CAI is evil and that COBB and all the other tuners out there don't recommend it with any of their various mods/stages without custom tuning, but why? What exactly does a CAI do to that causes the car to run lean? Does it screw with the way the ECU sees airflow or something, cuz otherwise wouldn't the ECU add more fuel to compensate for the increased airflow? I can't imagine how an intake could increase airflow so much that the car is incapable of matching it with appropriate levels of fuel.

I'm asking cuz a buddy of mine was thinking of getting an AccessPort and going to stage 2, but then decided against it when he realized he'd hafta go through all the trouble of removing his CAI and putting the stocker back in again. Nevertheless, I'm worried that even if he sticks with a stock ECU, that his CAI might given him trouble down the road. Are my fears unfounded?
 

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the easiest way that makes sense to me is that with the stock intake the subbie is taking in plenty of air and is getting a good mixture in other cars CAI is a good thing because the stock intake barely allows for airflow in the engine, if the stock intake is already giving a perfect amount of airflow then trying an after market intake will be giving too much air to the engine and the ecu can not compensate for the extra airflow coming into the piston and that causes the car to run lean...makes sense to me..
 

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kind of..

of all the research i've seen,read, and talked to people about. it's not that the ecu is seeing more air it's that it's seeing less air.

If the maf sensor is calibrated for a certain diameter and you change that diameter (bigger or smaller) the amount of air coming through the pipe won't be read correctly. therefore you can run lean or rich depending on the changes. if the pipe is bigger, more air will be running past the sensor than it knows about. so it sees X amount of air and adds X fuel, but in reality there is X+10 air coming through, therefore you run lean.

also there is something about funny angles and bends before the maf that can screw it up.
 

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Discussion Starter #4 (Edited)
Hmmmmm, so basically it's the MAF sensor's fault then, not the ECU, right? If that's the case, then CAI intakes are going to be bad even on a completely stock setup then. Is my friend asking for trouble by keeping the CAI even with a stock setup?
 

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Yeah, we have a fickle MAF that dosent like too much air.

Its not bad, I kinda think it helps b/c our car runs rich anyway. PM HBPUNK for more info on this. There are other reasons not to get a CAI. Like my friend had one on his car (not a wrx) and when he went to change the filter, he had pebbles and other crap in it. They can also suck up water and in that case, you are screwed. I would probably go with a SUS, Blitz or GodSpeed..
 

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From what I understand reading about this the diameter may be diffrent (wider) on many aftermarked intakes. This means the engine gets more air than the ecu know of and you run lean.

Try serch! You'll find many pages about this subject. We don't want another CAI post where we are just going to talk about the same things we have done before..
 

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you can get your ECU tuned for a particular intake, but why??? the stock intake flows enough air for about 300-350 HP. It also flows enough air for any of the VF series turbos. why blow $200 on an intake and more money on tuning it (which won't get you any performance....just louder motor) when you can spend it on an uppipe of uspension mods or something that will actually DO something. Some tuners offer off the shelf tunes for some intakes (I think Cobb offers one for AEM for stage 1 and 2), but you don't get any more power...you just don't lose any.

WRX's run rich on purpose. The extra fuel is used to cool the motor, so anyone who says you can run more air because it runs rich anyway is on crack...it does that deliberately, not by accident.

regardless of how much you THINK you gain with a CAI, it nothing compared to getting an uppipe, or some better brake pads and steel lines, stiffer sway bar, or underframe brace. With these mods there's no question that you installed a new performance part.
 

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blarg said:
(I think Cobb offers one for AEM for stage 1 and 2),
Its Injen that Cobb offers a map for....and i dont know if the car running lean shows a noticable difference or not, but I've had my Injen intake on for a year now and it runs just as good as it did before i put it on
 

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You *can* tune your car to account for the different flow, but it's a lot harder than you think.

The normal thinking would say "the area increased by X%, therefore my airflow increased by X%"... and that works for a few CAIs out there that were well designed. Not many.

What usually happens is they have a bend in them, before and after the sensor. AEM is like this I know. What happens is the flow through the tube actually varies with the speed.... damn, this is hard to explain. But... ok. First, understand that the MAF sensor is reading the fastest part of the air in the straight tube, which is in the center. The center is the fastest, the sides of the pipe are not moving at all (it's called the zero slip boundary condition). So, we only want to measure the fastest air and base everything else off of that. That's just simple calibration done by the nice people at Subaru.

What happens through these bends is the highest velocity air might not hit the center of the pipe where the sensor is. Further, it will vary with engine speed. Like... take some bendy roads and drive them kinda slow. You might end up with one line, and that line isn't likely to have you in the middle of the road. Now go faster, and not only will you still not be in the middle of the road, but you might be in a different place than going slow. If you just imagine the air having momentum and visualize it it might make sense.

Since the stock pipe is pretty straight and has that annoying corrogated crap, that effect is pretty small.

Cliff notes: Most CAIs don't just change the AFR, they change the AFR varying amounts for every RPM.
 
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