Subaru Performance Handbook Blog

Hi Fellow Enthusiasts,

Depending on what model Subaru you own, it may not always be a good idea to change the air intake in a bid for chasing that elusive little bit of extra horsepower. That’s right, you may be surprised to find out that particularly some models such as the MY99 and MY00 STI and WRXs actually suffer quite badly when a poor quality cold-air intake or pod-style filter is fitted.

I’m not saying that you shouldn’t direct cold air to the factory air box , but what you should seriously consider is avoiding the opportunity to replace that ugly, black plastic box with the flat panel in it for a [flash] pod-style filter or some other jazzy type of air intake. The particular model on that car suffers quite badly from poor air flow through the air flow meter sensor because the air flow meter sensor which measures the amount of air going into the engine is very, very sensitive and unique in that model year of ’99 to 2000.

And if you fit a cold air intake, the air is disturbed going through the air flow meter sensor assembly and sends a bad signal to the ECU, which results in either poor fuel mixture or a lean fuel mixture which then can also result in bad ignition timing and ultimately a risky environment for your engine to run.

Ninety-nine percent of the cases that we have tested and found cold air intakes fitted to clients’ cars have always been badly affecting the ECU signal, and by removing the incorrect air filter and fitting either a accurately machined, good quality pod-style filter with an adapter for the correct diameter of the air flow meter sensor, or in fact going back to the original factory air box, resulted in better fuel economy, more reliable power, and obviously and ultimately a more reliable engine.

Let me go into a little bit more detail. When the air passes through the air intake going into the air flow meter sensor, the air is measured through the sensor that sits in the center of the pipe, or the housing for the air flow meter sensor. Now, it is assumed by the ECU and the sensor that that sample of air that the sensor is measuring is similar for the whole volume of air going through that pipe.

The problem is that if the air is disturbed by poor quality design air filter or the wrong diameter adapter or something like that, it gets a wrong signal, so ultimately what happens is you end up with a badly running car. What you need to make sure is that the air is what they call laminar, meaning the air is traveling in a nice, constant, straight path directly down through the pipe so the sensor, when it gets that signal gets an accurate signal, which means your ECUs got an accurate information on what’s going on with your engine and ultimately you’ll end up with a happier running engine.

Some examples of what we have seen are people removing the air filter box and unbolting the air flow meter assembly because on the ’99, 2000 model, the actual air flow meter housing, which is a three to four-inch long plastic pipe, is actually bolted to the air box with four, 10-mil-headed bolts. They then connect an air-filter-style pod, whether it’s a big diameter filter or a small diameter filter, but obviously you need something to adapt the pod to the air flow meter sensor pipe, and typically that is used with a short piece of pipe connected to a flange that is then bolted on the end of the housing.

Now if that adapter itself is the wrong diameter, which in a lot of a cases it commonly is, you are effectively introducing a restriction to the air intake because the I.D. of the adapter is smaller than the I.D. of the air flow meter housing and straight away you’re behind the eight ball in getting good performance. Of course yourself or people you may know as the owner of these vehicles may think it looks impressive because you’ve replaced it and the car sounds a bit better, and in some cases ironically, the car actually may go better. But I’ll go in to that in a bit more detail later on in this section.

So the air is coming in through the nice, snazzy air filter assembly, through the adapter, and into the air flow meter housing, through the air flow meter sensor, and into the turbines of the engine. Of course as the air goes through the adapter and then changes diameter back out again to the inner diameter of the air flow meter sensor, of course it’s all disturbed and turbulent and not flowing nicely, so the sample of air that the actual sensor is reading is not accurate.

The reason why you can end up having a car that feels like it’s running better is because the signal going to the ECU is not accurate or indicative of the actual correct amount of air going into the engine, the ECU in some cases can actually run less fuel, more ignition timing and things like that, resulting in a leaner mixture and a higher ignition map, which obviously gives the feeling of more power. The down side is, you are actually taking a risk with the reliability of the engine.

Because the ECU is not getting the correct sample, you end up with a car that’s running too lean, or too much ignition timing, or a whole cocktail of horrible settings out of the ECU that ultimately can result in catastrophic engine failure or long-term detonation damage to the crown of the pistons, or cracking of the piston ring lands through other types of detonation, lean fuel mixtures, and so on.

You might ask, “What’s the solution?” Unfortunately, I’m going to tell you something you probably don’t want to know. The best solution is to stick with the factory air box with a nice, neat, cold-air intake coming through from the inner guard down behind the fog light and up into the air box, while the big diameter, which will flow a hell of a lot of air and a nice, neat, good-quality, flat-panel filter in the factory air box.

The factory air box will actually flow a hell of a lot of air and as an example, our Group A rally car actually runs the factory air box to ensure that we run the accurate signal through the air flow meter housing, of course because we run the factory ECU using the EcuTeK tuning software to adjust it to suit our Group A engine. So if our Group A car can run with a factory air box, so can yours.

You might say to yourself, “But I want something to look good on the engine bay or I want a bit of extra power and I really don’t like that air box.” The only solution, which is still a bit of a compromise and not the best solution, but if you ultimately must go to an after-market style replacement for your factory air box, MRT Performance supply an accurately machined adapter for the air flow meter housing so there’s no disturbance or restriction of the air going into the sensor and on the end of that, you can fit a pod-style filter and such.

It’s not the best preferred option in 100% of the times, but if it’s combined with a tune of the ECU using EcuTeK software, you certainly will get a good result. So there you have it, an air filter replacement with a pod or a ram pod or after-market pod-style filter is not always the best. Don’t believe everything that you see!  Just because it looks good doesn’t mean it’s going to go better.

Happy Driving