Interesting findings with exhaust mods and tuning
 

Very interesting findings after tuning (ProECU from ECUTEK) my GT86 and
a friend's of mine at the same day at the same dyno.

List of Mods
============

Mine: K&N Filter and TRD High Response Exhaust Muffler. Rest stock

Friend's: K&N Filter, HKS Resonated - Decat Front Pipe and Legamax Axleback. Rest Stock.

Tuning
======

Mine: Dyno test prior tuning showed 166 whp. After tuning we were at 173
whp and an increase of torque on an average of 0.7-0.8 kg within all RPM
range. Tuning not finished yet. With some further changes we expect to
gain 2-3 whp more.

Friend's: Dyno test prior tuning showed 155 whp (only)!! AFR was lean
within all RPM range (3% from target low, 14% from target mid, 23% from
target high). After MAF Rescaling, Fuel Trim optimisation (both closed and
opened loop), AFR Remapping, avanche, various other details that only my
tuner knows and without Touching VVT (yet..), the car went 169 whp and
gained 1.3 kg torque within all RPM band!! Basically, this is another car!!
That was correction of the system and not tuning!! Tuning though is not
finished yet. Expecting to get 5 whp more with some further changes as
my tuner advised.

Imortant detail: Both cars never tuned in the past. This was the first time!!

CONCLUSIONS
===========

This car's air and exhaust system is fully optimised from factory. The
replacement of front pipe with resonated de-cat HKS and axleback
Legamax most probably increased the vacuum in the cylinders (exhausts
flowing freely), therefore more air coming inside..air that could not be
handled by the stock tuning, leading to lean AFRs. The percentage
differences from target noticed high are so big (23% !! ) that someone
could even say is dangerous, in case you drive the car always on the
limit!

Well of course it's optimized from the factory. It's optimized for the factory components. When you start changing things rip optimization. How can the subaru and toyota engineers possibly come up with a tune that predicts what specific intake and exhaust modifications that you in particular might or might not choose? I thought it was common knowledge that you should strongly consider a tune of some sort when doing anything more than a catback and that it is a must when replacing headers, but I guess it's nice to confirm it

Totally agree with you. The thing is that we never expected such big differences without changing headers.

If you both have the stock header, the catback won't make any difference and won't create vacuum in the cylinders (?) and the 2 cars should be almost identical. I've heard of people with only a gutted FP losing some torque, but down in power by 11HP.... there's something else going on. Your friend has a hardware issue with his car- maybe a poorly installed catback and damage to the exhaust/o2 sensor or in the intake side.

The tuner is just tuning out a hardware issue which is bad- your friend should tear that system apart and find the issue THEN tune it. Then he'll see some minor gains.

Thanks! We will check it!

O2 Sensor was confirmed with external one. No any issues there.
I do not think we have any issues with hardware though. System checked for any leakages, with no any findings whatsoever. I believe that is just it. For some reason the system was un-calibrated. MAF Calibration and fuel trim optimisation brought the car to 166 whp, same as mine.

Did the car recently had its battery disconnected? Maybe it wasn't done calibrating after a reset. Were the cars using the same petrol quality?

Tuning with a header on our cars is in no way a must. I ran my catless uel header, catless front pipe, and 3in catback on the stock tune before I tuned with my OFT and with data logging there was absolutely nothing wrong. The AFR's were exactly the same as before, there was no knock correction, FLKC, and Advance Multiplier was at 1.0 right where it should be.

On an NA car there is little that will change from exhaust changes (definitely not without changing the header), especially on a 2.0 liter 4 cylinder. The reason other subaru's must have a tune when replacing exhaust more than a catback is because of the turbo. Turbocharged cars can gain way more power by freeing up exhaust flow than any NA car will.

Tell that to the people who did the same thing: catless UEL without tune and they get put into limp mode within weeks if not days. Doing that is well known to be risky. Good for you that nothing happened, but that doesn't mean your experience is normal

A check engine light is not limp mode. The CEL is for CAT inefficienc, nothing that is bad just obviously there is no cat anymore. I have not heard of a single person getting limp mode from a catless header.

Check engine light is just indicator that may lit up due various CELs, each of own reason. Yes, unless cat efficiency checks are disabled in tune, there is high probability of CEL thrown being regarding cat efficiency, and all this particular CEL does on twins - disables cruise control, but it also can be other reasons, that may in turn cause limp mode. OBD reader & check what exactly CEL thrown is about. For example, what if changed header leaks through connections?

This is generally the case with cars in the +100 hp/L range. Due to factory emissions requirements, the header is usually the first place to look for power, along with a tune that optimizes the variable cam timing for performance over emissions.

3" catback on a NA 2.0L car? That will kill torque right there. Now that I reread the OP, Deleting the FP cat without a tune may have killed some power. Still, it's odd to me that it performed so much worse.

As for the CEL P0420, the reason it's bad is that the car may not run some self diagnostics when it sees a CEL. You can in theory have hidden issues come up but the ECU isn't running those tests. I'm not sure exactly what tests that'd be but there are techs familiar with techstream in this forum who might know

Compare the maf scales from the cars before and after tuning. If your still on stock intake they should be fairly close if one cars maf scale is 20% different at mid to high rpm there is likely some hardware issue, that you just compensated for with tuning.


Ive seem maf sensor vary by 5 to 8% but over 20% sound like hardware issue if intake components stock




The MAF sensor is scaled for the size intake tube is in, if you increase flow by changing exhaust or cam timing , this extra flow is metered by maf and ecu add fuel based on maf flow (I gather your not using speed density on an NA car).