Weapon R intake

[Legucygt]

I own a 97 legacy GT. I've had a weapon R intake for quite some time and I was wondering if it was really okay to put on the car. I've heard from some people  it'll make the check engine light come on because the car is running lean. Others have said you shouldn't really tamper with your stock intake just let it be. I'd like to hear others opinions. Let me know guys thanks!

[doublechaz]

TLDR;  IT depends and is really hard to describe and probably can't be tuned out with the factory ECU.

 

One good sign is you didn't say your check engine light is on.

My 95 uses MAF for air charge sensing.  That can be a problem for some setups where the intake is modified.  I'm not sure that 97 GT is MAF only or even MAP only.  If it is MAP only then your intake mod is very unlikely to cause the problem I'm about to describe since it uses manifold absolute pressure and carefully calibrated RPM dependant volumetric efficiency for air charge sensing.  If yours is MAF+MAP then it would come down to the details of the software strategy used to combine the two sensors and arrive at a fuel control strategy.

In the second gen Subarus the MAF is the heated element flow rate sensor.  The more air flows over the element, the more electricity it takes to keep the element at the preset sensing temperature.  The computer uses the temperature and current flow to calculate how much air passed over the element, and therefore how much fuel to start with.  (later the O2 sensor is used to make a fine adjustment table called fuel trim from that starting point)  This works great, automatically corrects for density altitude, and can be very accurate as long as you don't get the sensor dirty with the oil from your K&N wet filter or some such.  The down side to this method is that there is a great deal of testing and calibration done at the factory to understand that relationship between current needed and actual air flow.  Some intake mods can change that relationship.  If the change is small and even the trim table will correct for it and everything will be fine.  If the change is not even then the trim table will have more difficulty correcting things.  The biggest way this can trip you up is that the factory intake is very carefully designed to not resonate in the RPM range the engine can run through.  You may have noticed a weird bottle thing hanging off the side of the intake if yours had it (it is engine and year specific if you have such a chamber or two and what size and shape they are).  That bottle is designed to cancel a resonant spot in the RPM range.  The problem is that the sensor can't tell which direction the air goes past it, so if the intake resonates at say 3600 RPM you have nice smooth flow into the engine below that RPM and the sensor reads correctly, and you have nice smooth flow above that RPM and the sensor reads correctly, but from say 3550 to 3650 the air is actually going back and forth really fast and the sensor reads way high.  This makes the computer deliver way too much fuel in that range.  Worse, the range is too narrow for the cells in the trim table to trim it out so you start out fine, get too rich at 3350, way too rich at 3600, and are still too rich until above 3850.  I suppose in theory if your intake resonated right at the RPM you drive on the highway for long stretches it would eventually trim it out and then when you went back to side street driving it would be too lean in that range until it trimmed again.  Hard to say without knowing the short and long term correction rate in your version and revision of ECU.

This kind of thing can be measured easily on a dyno, and can be corrected in a stand alone ECU.  This kind of thing is why dyno tuners make the big bucks.  On a stock ECU you probably can't tune it out, so you would have to measure and test to be sure your intake setup does not resonate.

If you know enough about tuning, have a wideband O2 and can datalog all your relevant sensors you can do the needed testing on the street, but you ought to have a helper to deal with the logging while you only drive the test drive patterns called for.

 

All that said, I run an intake I built from parts without issue, but that is only after I went through the anxiety you are now in, going back to stock, learning more about it and getting a wideband, and then building my way back in again.

Edited February 22, 2023 by doublechaz

[Legucygt]

Right! I'm finally going to dive back into doing some research myself. Thanks for the great insight brother! Much appreciated. I had to take my turbo off causing a lot of issues when I first purchased the car and when I had the motor rebuilt and never hooked it back up because of the issues it was presenting to my car, with that said I've been running stock intake for a while and albeit it's not as quick to the go, it's actually enough for me as it is, since I'm not out fast and furious stylin her hahaha. That's where that weapon R dragon intake comes in. They look sleek and make the car sound a lil more aggressive, but I'm going to do more research as you did before I commit.