A Problem with CAI/SRIs and an Alternative (yes, finally)

Yes. Just for you all, i delved into the deeps of Nasioc (x.x) to bring you some tech goodness.
http://forums.nasioc.com/forums/showthread.php?t=1471819


OK, after a month of promising to write this up properly, I've given up on it because I just don't care about it anymore. I've posted what I had done so at least you guys will see some results.

Also, I'm high as a kite on allergy meds due to an encounter with a fluffy cat earlier today. If I've accidentally written a treatise on Sino-Argentine relations in a Hegelian wold-view, let me know.



Who does this report apply to?
This report applies to any MAF-based naturally-aspirated GD/GG H4 Subaru. Most of the people who will be interested in this are going to be 05 2.5RS owners and 06-07 2.5i owners. It is very likely that this also applies to MAF-based NA Forresters and Legacys as well but I have not done personal testing with those cars.

Does this apply to MAP-based NA H4 Subies
No, the MAP sensor would not be affected by intake system resonances.

Does this apply to MAF-based forced induction Subarus?
No, limited testing with a WRX strongly suggests that the presence of the turbo keeps the MAF sensor from being exposed to the resonant mode.

What's the basic problem
The MAF sensor is vulnerable to a nasty resonance in the intake tract between 1,000RM about about 1,800RPM at WOT. Inside the passenger's side fender is a large Helmholtz resonator, the so-called snorkus, tuned to damp out this resonance. Removing the resonator (snorkus) either as a snorkus-delete or as a consequence of putting in a CAI/SRI causes the MAF sensor to massively over-estimate the air entering the engine and cause the car to run very, very rich.

Wait, that's something I never do, does this even matter to me
You'll have to make that decision yourself, but I think it does. First of all, I end up in the problem area when I get too sideways in a rallycross. Also, imagine having the car in a low gear at idle in stop and go traffic. Suddenly, you see a chance to get into the faster moving lane next to you so you punch it. Right into the trouble spot. Basically, it's not that likely to end up in the trouble spot, but if you do get there, it'll be a situation where you REALLY don't want to have a problem.

Why not just take it out with tuning
While the MAF scaling can be adjusted to tune out errors in the estimated amount of air entering the engine, this requires that the MAF sensor voltages (MAFv's) being adjusted occur only at one point. If they occur at two points, fixing one area will screw up another. In this case, the MAFv's that occur in error below 2,000 RPM occur legitimately between 2,500 RPM and 4,000 RPM. Tuning out the error below 2,000 RPM would therefor cause a problem higher in the rev range.

Why isn't everyone who has an intake complaining about this problem?
I believe there are several important reasons this problem isn't more widely noticed. A lot of people just aren't that in-touch with their cars. Some people just float through life in a fog and wouldn't notice their engine exploding. Second, the stock fuel map targets stoich in this area and remains in closed loop throughout the trouble spot. This creates two layers of protection against the problem. First, assuming the error causes the ECU to add 90% more fuel than necessary, targeting a 14.7 AFR means that the resulting AFR would be crazy rich, but not rich enough to make the car stall, buck, or belch obvious amounts of black smoke. Second, when the car is in Closed Loop, the front O2 sensor is used to monitor the actual AFR and make corrections to fueling to return the engine to stoich if it runs rich or lean. The ECU has the authority to make a 20% correction to fueling instantaneously based on the front O2 sensor reading.

UPDATE:I have now been able to test and 05, 06, and 07 with snorkus-deletes or full intakes and the stock tune. As expected, the MAFv jumps WAY too high, AF Correction #1 hits the negative limit, and the car runs richer than the ECU intends it to, but not so rich as to produce the severe drivability problems a car with no CL->OL Transition Delay and a richer AFR target would see. In other words, what I suspected was happening was completely confirmed.

Additionally, the ability to flash-tune a NA Subaru is very new. I strongly suspect that as we see more and more cars get tuned, we'll see more and more problems with intakes.

What are the different styles of intakes evaluated
The first intake style is stock. This consists, running from the outside world to the engine, the plastic intake snorkel, the Helmholtz resonator in the fender (snokrus), the airbox, the MAF section, the accordion pipe, and the so-called torque box. This will hereafter be called "stock"

The second intake style is the snorkus delete. This is identical to the stock configuration with the exception of the deletion of the plastic intake snorkel and the Helmholtz resonator in the fender (snorkus). However, there is also a second version of this which also eliminates the stock airbox, replacing it with a cone filter. For all purposes other than looks, these two designs can be considered identical.

The third intake style is the traditional Short Ram Intake or Cold Air Intake. This intake style deletes the plastic intake snorkel, the Helmholtz resonator in the fender (snorkus), the stock airbox, possibly the stock MAF section (depending on brand), the accordion pipe, and the so-called torque box. For the purposes of the this discussion, a CAI and SRI are identical, and all brands/models are identical. Something else I'll mention here is that a SRI is crazy bat**** insane loud. Do not put one on if you value the company of a woman. The exact SRI tested is pictured below:




The fourth intake style is a hybrid design I thought up in response to the problems with a CAI/SRI. I later found out I wasn't the first person to have this idea though. The plastic intake snorkel, the Helmholtz resonator in the fender (snorkus), airbox, and MAF section are retained while the accordion pipe and torque box are replaced with a metal pipe. The other thing I'll mention here is that the hybrid is almost as quiet as stock. It just has a little more growl. Very wife-acceptible. The tested hybid is shown below:


What if I use a non-OEM filter?
Doesn't matter. The resonance doesn't care what kind of filter is in the box, all it cares about is wither or not you have the Helmholtz resonator installed. That being said, use an OEM paper filter or reusable nanoscale polyester filter (AMSOil EaA, AEM Dryflow, Donaldson). Those provide the best balance between flow and protecting your engine. Other filters (K&N, Perrin, foam, oiled cotton, etc) give you a tiiiiiny bit more airflow but let in quite a bit more dirt.

OK, I'm a geek, hit me with the technical backstory
The MAF sensor measures the voltage required to keep a small metal element at a specific temperature as air flows past it. More airflow creates more cooling and thus higher MAF sensor voltages. Turning it around, higher MAFv's mean more airflow. In other words, the car is relying on the assumption that the cooling to the MAF sensor is in direct relation to the actual airflow. The important things to walk away with here are that the car can't measure airflow directly and that any air moving past the MAF sensor will cause cooling and therefore change the MAFv.


The intake tract of a car is like any wind instrument: blow air through it and noise will result. In the case of the car, we also have the intake valves acting like the lips of a person blowing a raspberry into a tuba. This sets up a resonance, a "note" if you will, in the intake tract. Resonance means that there's air moving back and forth in addition to the general flow of air through the intake tract. If you remember, the ECU can't measure the airflow directly and the MAF sensor will be cooled by air flowing past it in any direction. This leads to each "chunk" of air being cooling the MAF multiple times as it vibrates back and forth before migrating downstream and into the engine. The ECU determines the amount of fuel necessary by estimating
the actual airflow from the cooling to the MAF. Since MAF is cooling more than it should, the ECU is mis-estimating the amount of air entering the engine and therefore putting in more fuel than it should.

The most-basic display of this is to look at MAFv itself. In the plot below, you can see a graph of Wide Open Throttle (WOT) pulls from idle to redline. As you can see, the Stock and 3" Hybrid lines are quite similar on the low end. However, the SRI line "blows up", indicating to the ECU that the MAF is measuring a tremendous amount of air. Obviously, this isn't happening, so this is the beginning of our problems:


The ECU calculates engine loading using RPM and MAFv. Engine loading is the amount of air in a cylinder each time it fills. Loading ends up looking almost exactly like torque if the load numbers are correct. Here, you can see that the Stock and Hybrid load lines are very similar at the low end, while the SRI line is clearly in error:


Now, let's think logically about this. The Stock and Hybrid intakes are similar only in the parts before the MAF sensor (plastic intake snorkel, Helmholtz resonator, and airbox) and they have the same load on the low end. The Hybrid and SRI intakes are dissimilar only in the parts before the MAF sensor (SRI deletes the stock parts) yet they behave very differently in the trouble area. From this, we can conclude that changing the pre-MAF components of the intake causes the problem.

So, now we know to leave the front half of the intake alone, but what about the back half? Is there any point to changing the back half of the intake if we don't change the front half? Yes! Examine the Engine Load graph again, focusing on 4,000 RPM to redline. The stock intake system just hits a ceiling at about 4,000RPM while the SRI and Hybrid continue to build. From 5,000 to 6,000 RPM, you can see that the SRI did flow slightly more air than the Hybrid, but that both are far more free-flowing than stock.

How do I run the test myself?
1) Log RPM, MAFv, AFL#1, AFC#1, throttle opening angle %, CL/OL status
2) Get the car rolling and get it into 3rd gear
3) Take your foot off the gas with the car still in gear, let it slow down until it starts bouncing along at idle (THIS IS IMPORTANT).
4) Stomp the gas to the floor and hold it there until about 4kRPM
5) Post the log to this thread


Conclusion
In the end, you're going to have to make up your own mind. For me, it was worth the small and un-feelable loss in peak load between the full SRI and the Hybrid to make sure I didn't have a problem down load. You might have a different opinion.