Did a test on an idea I had implemented on some previous Subarus I've owned (and one I still have). The basis of this 'trick' is to get more air flow to your engine. It is not recommended so do it at your own risk. I'm just sharing my observation for your entertainment.

Here is what I done. I removed the metal tubing that is connected to the intake, that is between the maf sensor and the throttle body. I believe this tubing is used to route the crankcase air back into the intake. Here is the location of the metal tubing.


I did this to allow more air into the engine. This will cause the engine to run leaner due to this extra air entering the engine not being metered by the maf sensor (Not recommended).

I tested this several times, using an Android Torque Pro Software. Test was done in two directions. Air temperature was 89F. Intake temperature was between 95F and 102F (From sitting idle on the side of the road). Here is what I noticed.

1) No knock was detected. Torque Pro has a setting detect knock while driving at wide open throttle or any other way. I recorded several times and no knock was detected.

2) Maf g/s (Grams Per Second) did not seem to go higher than it did before. Before this mod, the top maf g/s was around 105g/s. I recorded the same during these runs although the air temp was about 10 degrees higher during this run.

3) My 0-60mph time of 8.5 was the lowest ever recorded (In both directions). All of my Android runs before were in the 8.6-8.9 range, no matter what intake I had installed or how light I made the vehicle.

4) **My main reason for doing this** I did this modification to bring up my air/fuel ratio. My current full throttle air/fuel ratios have been at 11.6:1 - 11.7:1. This modification has leaned out my air/fuel ratio to around 12.2:1.

5) No drivability issues (Yet). The car drives around with like nothing has been changed. No check engine light, nothing.

I am ecstatic about this. I typically like my air/fuel ratio to be tuned to around 13.0:1 (Even 13.5:1) for best horsepower. So seeing that there was a slight bump in my air/fuel ratio to lean it out a little, I'm happy.

Yes, I have recorded runs with the data shown on the screen but I'd only post up 1/4 mile runs at the drag strip. I do not condone this activity.



Here is an example of the result of doing this modification. I previously had a Subaru SVX that I was doing dyno runs with testing an intake I had made up. I did this same modification to a stock intake just to see the result and this is what happened.


There was a slight bump in horsepower due to the air/fuel ratios leaning out a little.

I had a small filter that came with an HPS intake I ordered for testing. I installed this small filter on where the hard intake tubing goes in, just as an extra safety measure to prevent that little hole from sucking any debris in.



Currently, I'm running this stock intake with this modification and the 'ram-air' splash guard removed (The splash guard underneath the ram-air piece). I'm awaiting an HKS panel filter to arrive to allow slightly more air through and hopefully another small bump in leaner air/fuel ratios.


That's all for now!

 

Isn't this basically just creating a vacuum leak?

Anyone using an Ultragauge EM+ on a GK Fit? I ask this for comparison purposes since I have the UG EM+ on my GE Fit (it reads close to 1 lambda for AFR, and sometimes as low as .937). With the EM+, it has a reading supposedly of what the Fit ECU is "commanding" for AFR in the same range. And 0.937 translates to roughly 13.8:1.

 

Yes, this is creates a vacuum leak, the engine is running leaner. Any air that doesn't go through the MAF is called "Un-monitored Air". If the car was tuned with this mod: you would have a hp drop.

Do not disconnect the PCV!

It will trap moister inside the crank case which breaks down the oil and eventually destroys your engine.

 

Well, in OP's defense, the post did say twice that doing what was done was not recommended. As always, we can do whatever we want to our cars. Some things are awesome and should be done, some things are controversial, and other things should absolutely not be done.

 

It's never stated in the original post what the risks are. A proper DIY has all the proper and correct information; benefits, risks and warnings.This isn't controversial, it's a bad idea. Esp for a street car.

Even race cars have PCVing. The engine has to vent the crank case due to pressure build up.

 

My original peak hp before this was around 105-106whp (Per Android Pro Torque). After removing the metal tubing and adding an air filter, hp went up to 108whp.

I tested this again last night. This time I removed the small filter and did 4 runs, two runs in both directions. For the first time, I was seeing 112whp. Also, with the small filter removed, air/fuel ratio bumped up slightly to 12.5:1. 0-60mph hit 7.9 seconds twice for the first time.

Outside air temperature was 66F. Air intake temperature was between 75-80F. Also, I removed the rear seats again, along with the jack and spare tire. Stock 22lb battery was replaced with a 2.3lb battery. Even still, I was never able to achieve the whp and 0-60mph numbers in similar conditions before.

Either way, I'll be testing to see if this is all a fluke or real numbers Tuesday and Wednesday nights at the strip.

 

This mod is not acceptable. It is allowing un-metered air to go into the motor between the throttlebody and MAF sensor. This in the long run will eventually lead to random limp modes and the computer yanking out ignition timing in favor of attempting to increase reliability.


Your O2 sensor is doing its job of correcting the AFRs, so you are not actually running lean. That is the torque app seeing the fuel trim jump around and it calculates an AFR, though it is never accurate.

 

13Fit, I never did ask you so my apologies for replying to my own thread so late.
What do you attribute the quicker 0-60 and 1/4 mile runs as well as the boost in hp/tq too after the removal of the metal intake pipe?

If it did not lean out the air/fuel ratio, why is the car running faster than before the removal of the piping? I'm currently not running this way as it was only for a test. I have one of two ideal intakes in mind and I just ordered a piece moments ago to make one of the ones I want to complete. This was only a test. Thanks for any input.



Thanks for stating the risks to everyone. What is stopping the crankcase from venting with my setup? Why would pressure build up? There's nothing blocking the piping for the crankcase gases to come out.

 

While I agree with 95% Up to the part about Torque. Torque pulls the sensor data stream. So you get real AFR and voltage from the o2's

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On a sidenote Messing with metered air on a subaru will F your boxer right up.
Lets not play with systems we don't know about.