[rbenotti]

Originally Posted by Dragonman

Certainly.

While I'm not gonna pretend to know what's an optimal detonation point in the compression stage, I imagine it's at that point where the ratio achieves its designed 11.5:1, I do know that's what adjusting the timing does. Not sure if the LFX has variable valve timing or not but if the intake valves open to soon or remain open to long can also contribute to "knock", which is detonation of fuel/air outside the point it's supposed to be exploding and is not good for the motor. Pistons and valves are in the wrong place when it happens. The lower the octane rating of a fuel usually means it can be fired simply by compressing the mix, think diesel engines, the higher it is requires external stimuli to achieve explosion.

So, the car is designed to give it's full power within a single cylinder with a air fuel mix containing 93 octane when the piston has compressed it all to that 11.5:1 ratio. Putting 87 in it causes pockets of the A/F mix to detonate before or after the ratio is reached. When the computer senses it happening it adjusts the timing which will change the compression ratio the A/F mix gets to when fired. Either before, at or after the piston reaches top dead center (TDC). I don't know which way it adjusts but I'm sure someone here can tell us.

In my experience, however limited, retarding timing makes the engine output less power while advancing the timing has the opposite effect, up to a certain point. If the timing is advanced to far the spark will not hit the A/F mix during any amount of ignitable compression and the car won't run.

Just thinking about it, I wouldn't think you'd want the explosion to hit while the piston is traveling up. TDC probably isn't really all that good for the rods or crank either, but, it can't be very far past TDC on the down stroke or you lose the compression ratio needed.

Alright, I've probably rambled ignorantly on long enough.

Still about 4 1/2 months away from the end of my 6 month 93 octane study.

No rambling at all! I always thought advancing the timing would "increase" horsepower to a point.

11.5 to 1 is a high ratio, so by this logic alone - it should probably use higher octane.