Hi I just bought an 03 WRX - I pick it up in a couple of days. When I test drove it (first time driving a WRX) I was amazed at how fast it was. THe car has an AEM CAI kit which is very noisy and makes a blow-off valve type of noise on shifts??? I read on this forum many different views of these systems - mostly negative.

I test drove this car with the CAI and also another 02 without - the difference was HUGE - the car with the CAI was MUCH quicker.

I'm curious about why it sounds like there is a blow-off valve? Is this typical of AEM CAI's or does the dealer not know what he's talking about and there's actually an aftermarket blow-off valve in there somewhere?

THanks a lot!!! Can't wait to drive this baby!

 

First off the wrx as with all turbo cars does have a blow off valve, with the stock intake and resonater it tends to silence the noise.2nd I find it very hard to believe that the car with the cai is noticeabley faster, if you buy it put the stock intake back on. What a cai does is create an angle that the maf isn't used to seeing, causing it to run dangerousley lean.Thus impeding performance and causing detonation. If you like the sound get the short ram or the silencer delete elbow.

 

Did it "sound" faster or have some other mods that you don't know about?

And it is true that the CAI tends to give you more engine, turbo and bov/bpv noise. (Maybe that's the only thing that it can give you...)

 

It was much faster than the 02 I drove. I didn't hear any rumbling noises typical of detonation. I would think if the car was running dangerously lean I'd be seeing a check engine light?

Thanks for the replies.

 

You will not hear the BOV, there will be a minimal increase in turbo noise if you remove the intake silencer and I dont think a short ram is better than a CAI since the problem is with the MAF sensor.

Here is a good explanation by BAN SUVS

--------------------------------------------------------------------------------

Most of you are missing the point about why there is an argument against using a cold air intake. It really has nothing to do with whether or not the temperature of the intake air (not "charge", that refers to the intake cycle within the combustion chamber) is hot or cold, but rather the MAF sensor. Let's start at the beginning-

The factory ECU relies on the MAF signal to determine how much fuel to deliver during each combustion event (or series of events in some cases). It is basically saying "the MAF says there will be X amount of air in the CC, so it needs Y amount of fuel". This is the basis of crude engine management such as S-AFCs as well as a feature of more sophisticated systems like the Xede and stand-alones; it warps this signal to fool the ECU into adding or reducing the injector pulse time. I doubt that is news to any of you however.

What may be news is the fact that the actual MAF sensor is tiny, and is calibrated to read the exact flow for that exact diameter pipe in that exact configuration. If, for example, 300 cfm of air is flowing through, then it will see one specific voltage from the sensor, and it won't see that signal value for any other amount of air flow. The problem with changing that configuration causes the signal to be different for that 300 cfm of air- the ECU thinks it is seeing more or less air than it is really getting and calculates fuel accordingly. This is why some intakes can make horsepower on the dyno- they lean the car out in a very crude and dangerous manner, which almost always results in more power, but is far less reliable and safe than using tuning methods to do this.

So how is it possible for 300 cfm to give a 250 or 350 cfm signal? Simple. As a gas passes through a tube at a given pressure (atmospheric pressure in all cases, as we are talking about the intake air prior to reaching the compressor) its velocity is dependent on the diameter of said tube. Increase diameter, decrease velocity, and voila- incorrect readings.
But it gets better. There are also currents in the intake tubing that come into play. Notice how the factory intake tubing is all bent up and corrugated and convoluted? That causes little eddies and cross currents that affect the way the MAF reads the signal of X cfm of air passing through it. While these make theoretical computation of the exact airflow over any given point within the intake tract impossible, they are stable and repeatable. Thus, the factory ECU can be calibrated to rely on them.

Then there are those who say the factory intake is "restrictive". Bull****. WRC cars make well in excess of 300 horsepower, and they are breathing through a restrictor that is 32mm in diameter. That's barely an inch and a quarter! So what is restrictive about the 2-1/2" tubing of the factory intake? Granted, it isn't the most efficient shape possible, but it works fine. This inefficient shape is often why aftermarket tuners look at the engine and say "gee whiz, I think I can make a better one of those!", and so they do. Only not really. Because by changing the air flow/signal relationship, they screw with fueling. You can indeed flow more air through an aftermarket intake, but that's just one small part of making lots of safe, reliable power. The tricky part is making it burn efficiently.

By now, many of you are putting 2+2 together and thinking that a more efficient intake plus good engine management able to properly calibrate the signal will increase horsepower. Well... no. Funny thing about turbocharged engines- they don't seem to really care what's happening upstream of the compressor, up to a point (slap that WRC restrictor in there and you'll see what I mean). This is why the WRX needs absolutely no recalibration to go from Death Valley to Mammoth Lakes, an 8,000 foot vertical journey. The MAF signal allows it to automatically adust fuel for the amount of air being ingested by the motor. This is relevant because the lower atmospheric pressure at high altitude has a similar effect to inefficient tubing.

As for Shiv not tuning intakes- he does, but only on the dyno. He won't allow off-the-shelf maps to be run on cars with intakes because the fuel tables will be wrong. If you bring it to him to custom tune, then he can work around it. And yes, he's tested many many intakes on many many WRXs, and none of them have shown significant power gains (significant meaning 5+ repeatable, consistent horsepower) over the stock intake, and some even lose power. Particularly the short ram intakes. This is why you can't buy a Vishnu intake- we could certainly produce one, and certainly Shiv could make off-the-shelf maps for it, but it's pointless because no intakes make more power than stock. We could sell it, and people would buy it, but it wouldn't make anybody's car faster.
__________________
Jon Stewart is my president...

BAN SUVS
View Public Profile
Send a private message to BAN SUVS
Send email to BAN SUVS
Find More Posts by BAN SUVS
Add BAN SUVS to Your Buddy List

11-08-2003, 04:43 PM

 

Congrats on your new purchase and welcome to i-club.

Let me start off by saying that I dont know everything about Subaru's and the things I do know about this subject are from what I've read. With that being said I doubt an AEM CAI alone could make the car much faster unless it was makeing the car run very lean. For all you know the car may have been tuned and if it was it might not be a good idea to remove the CAI. Another possibility is that the car without the CAI may have had something wrong with it that made it slower. If I were you I would have someone hook up a wideband A/F sensor to check if its running lean.

Just so you know the AEM is one of the most unpredictable intakes on the market according to Cobb tuneing.

Here is some info from Cobb's site.

Q. I have XYZ intake, do you have a map for it?

A. We can do mapping for quite a few intakes that are predictable. We have maps for the original(black) APS, Injen, and most of the short ram hot air intakes. The Blitz is about the only short ram that is not predictable. The car may not be as smooth or might miss out on some power since we have to tune even more conservatively with most of the intakes, but it will be safe, and run better than the factory ECU. Also, let us know what specific intake you have. We have had people order the map for the APS intake when in reality they had other intakes. Needless to say the tune was far from optimal. So far the AEM is about the only intake we have seen that is not predictable. We could make a map for it that would make sure that the car runs rich rather than lean, but the benefits would be far less than that if the customer just removed the intake and used the factory unit. We even have to do that on the dyno for the AEM intake. Also, even if the intake is advertised as not needing correction, causing no errors, or factory specifications, let us know. This is not the case on the intakes that we have tuned that make this claim, they do require correction. Just because the piping that the sensor sits in is the exact factory spec does not mean that there is no turbulence to throw the sensor off. In the end if you want the smoothest running car use the factory air box. If you want the noise and slightly better spool-up, remove the silencer in the fenderwell and toss a panel filter in the stock air-box. On the stock turbo you will make the same power as you will with an aftermarket intake. If you have any questions or want to get the best map, your best bet is to give us the most information you can and let us choose the proper map for your combo.

 

when a car runs lean it loses power - it doens't gain power!

 

"It was much faster than the 02 I drove. I didn't hear any rumbling noises typical of detonation."
It's hard to see 5-7 hp making something much faster. I just had my R6 DYNOED and picked up 11whp , and I barely noticed it, and that only weighs 370 lbs.
"rumbleing"?

"You will not hear the BOV, there will be a minimal increase in turbo noise if you remove the intake silencer and I dont think a short ram is better than a CAI since the problem is with the MAF sensor."
You WILL hear the bov, not like a full atmospheric but you will hear it, trust me it's the first mod I did. The problem isn't with the maf or the air temp , it's with the bend right before the maf that it's not used to seeing.

If you don't like the answer don't ask the question
If you're gonna spend 20k on a car you should at least know what is on it and how it will affect it.

 

All I'm saying is that the 03 with the CAI felt a lot faster than the other WRX's I drove. I really don't think this was in my head. Perhaps there are other mods I'm not aware of. I got a very good deal on the car which is why I didn't research this stuff heavily beforehand... plus - from waht I hear I could probably easily replace the cai with the stock airbox.

 

of corse you can replace the CAI with the stock air box, it's the fuggen stock air box. CAI is a useless mod on a WRX untill you have a big turbo and big boost. It will cause you to run lean, wich will pick up power but be harmfull to your engine in the long run.

 

uhh yeha i know you can put it back on... i said "easily" i.e. not a big pita. RUnning lean should NOT pick up power - lean condition is a power-robber typically - I don't see why a wrx would be any different. Maybe somebody could explain that... and what does "harmful to your engine in the long run" mean. Detonation can cause damage to the pistons and the valves which is obviously bad... but like I said I'd be surprised if you could have detonation without a code getting thrown.

 

i bought a 04 wrx new, and the first mod was cai. my car did get a little faster w/ the cai, but the most noticible change is the intake and bov noise. my cars only slightly modded, so far for the bolt-on that gives u noticible power increase, i would say the down pipe. plus u will be able to hear the turbo

 

for me, the cai never give me any problem. i drove in heavy rain before, and my car was fine(though the air filter became real dirty). knocking? never happened. ur maf measures both volume and temp, thus density. plus wrx runs really rich under boost, about 10:1 AFR, the best ratio for power would be 12:1, and look at the hondas, they even run 16:1

 

it's not a honda though, on't treat it like one of those weed whackin' pieces of ****

 

thanks for the replies everybody!