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Old 02-03-2014, 03:15 PM   #7
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Join Date: Sep 2003
Location: Ann Arbor, Michigan, USA/ Bloomington, IN, USA
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Originally Posted by nthfinity View Post
No, it doesn't "need" an engine out service... it just allows me to change a few more things more easily in the process.

I get concerned with leaning out too much, as there is always 1 cylinder who needs the extra "cooling"; but I definitely expect better economy after the tune/mods this summer.
The cooling effect of the fuel is mitigated somewhat by the fact that the WRX motor isn't direct-injected. In my car, the injectors actually do spray a fine mist of fuel that, in part, washes down the cylinder walls providing some evaporative cooling. Port-injected engines have more difficulty doing that, as the air/fuel mixture is already formed prior to the fuel entering the combustion chamber.

I'm not sure exactly what the specs on the WRX engine and OE tune are, and I suspect they're pretty dramatically different from what was on my car.

Stock, my car ran ~14.7:1 AFR at cruising speeds and under low load conditions. At WOT, the mixture tapered down to about 9:1 (which is pretty ridiculously rich) to avoid knocking, particularly with bad fuel.

Since the compression ratio of the MZR DISI is a relatively high (for a turbo motor) 9.5:1 and it runs 15.5psi of boost in OE conditions, there is a pretty high probability of knocking if you run poor quality fuel or in very hot conditions. That said, with the appropriate monitoring tool(s), and air-flow/cooling modifications, it's safe to run around 16.5:1 - 17:1 AFR in low load conditions, and cruising, and a somewhat more aggressive 11.4:1 - 11.8:1 AFR at WOT. There are noticeable performance gains to be had by doing so, both from a WHP/WTQ standpoint, and from an economy standpoint. In addition, I'm running about 3psi more boost than stock, and I have advanced my ignition timing by a few degrees as well.

I can also monitor a variety of other parameters that aid in determining whether combustion temperatures or EGT has risen to such an extent that there's a serious risk to the valvetrain, exhaust, turbo, or other components.

Anyway, you might well be able to run a bit more lean, with a bit more ignition timing, and a bit more boost, and still be well within the "safe" tolerances of the motor. Furthermore, since if memory serves, the exhaust system on the '09 WRX has equal-length exhaust headers, and intake runners, the likelihood of having a serious imbalance in the amount of air/fuel flow seems like it's probably going to be relatively small.
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