ScarabEpic22
Member
+1 on that , took my car to a tuning shop to ask some questions about octane , buddy hooked my car up to his lap top . long story short , my 2016 rs has 2 fuel maps . one for low octane and one for high octane . my car is stock , i wonder why gm would waste time and money giving the ecu 2 different maps for octane if the car is going to perform the same on 87 or 94 .opcorn:
All GM vehicles since at least 1997 with the introduction of the LS1 PCM have 2 spark maps, High and Low. (I dont know if earlier ECMs do or not, I'm an OBDII tuning guy and dont deal with anything OBDI or TBI.)
GM does this so if you get a bad tank of gas or run low octane fuel, the ECM can accurately compensate for it and not knock like crazy. Plus, everything is a blended percentage between the high and low tables based on the Octane Scaler value in the ECM. Example time, stock 13 RS tune: 3000 rpms, 0.44 g/cyl, 16* spark in high and -0.5 in low. That's quite a gap, 16.5* of timing! (After a real tune, you'd probably want to keep this about 6-8*, especially if you know you're always going to run 91-92 octane.) But, after refilling your gas tank with 5+ gal of fuel the Octane Scaler value resets to 100%, or using the value in the high octane table with 0% blending with the low octane table. You drive along for a bit, and the ECM detects a bit of knock at 16*, so it decrements the Octane Scaler from 100% to let's say 75%. Now, it's going to look at the delta between the high and low octane tables, (16.5*) and apply the new Octane Scaler % (75%) to it, resulting in ~12.375* or 12-12.5* of timing. If it drops to 40%, spark timing will be 6.6 or 6.5-7*. If the Octane Scaler decreases to 0%, first you're running crap fuel and second, the car is going to be a dog and run -0.5* of timing.
The Octane Scaler is a global value, so it will apply to all spark values not just the specific one I called out above to keep the example more focused.
Do you guys now see HOW the ECM actually CAN take advantage of running premium fuel IF it is detecting knock on lower octane fuel? If the Octane Scaler is decreasing, then the ECM is taking power away in the form of spark timing. Run a higher octane of fuel, and the ECM wont decrease the Octane Scaler thus allowing more timing to run before knock and increasing power.
Remember, this is just the spark component of how the E78 ECM determines power delivery; airflow, boost, VVT, AC, pedal demand all factor in as well (plus a few others).
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