AAA study concludes "top tier" gasoline is worth the extra cost

[mailpup]

Just out of curiosity, does anybody remember having to set engine timing with a strobe light, instead of the ECM doing it for you via a crank position sensor?

I still have two 80s vintage cars that I have to manually check or set the ignition timing from time to time after certain maintenance or repairs.

 

[noc81]

In fact, given an engine set up to run on regular gasoline, too much octane could conceivably cause you to lose a tiny bit of power, as opposed to gaining power. That's because you could have the maximum fuel explosion pressure after TDC, when the piston is already traveling downward. <Which would reduce the effective compression ration dramatically!

This is very, very wrong.. The spark will still occur where it is intended, regardless of octane. We aren't talking about diesel, right?

 

[captaincranky]

This is very, very wrong.. The spark will still occur where it is intended, regardless of octane. We aren't talking about diesel, right?

No, not wrong, just not severe and a bit theoretical.

High octane fuel "explodes slower" than regular gas. The reason for that is to enable high compression and early timing settings without detonation Thus, given an engine setup and timed for regular gas, ideally the piston is at exactly TCD when maximum mixture expansion occurs. However, if we have the slower expansion rate of higher octane fuel, and the timing set where it was for regular, the piston will have passed TDC when maximum expansion occurs. If the piston has passed TDC and on the way down, the combustion chamber volume will be larger, thereby dropping the effective compression ratio, (at the time of full detonation). That's why I gave the example of retarded timing.

I'm not a math or physics major, so I can't compute the extent to which the effect might become detectable. At that point, the question mutates to, "how many degrees of crankshaft rotation equals 1 octane point @ "xxxx" RPM.

Again theoretically, you should be able to run a bit more advance with the same regular gas engine, with high octane fuel being used.

Keep in mind you could also do dumb experiments by retarding the timing in a high compression engine, fueled with regular, to allow the pistons to pass TDC when ignition occurs, eliminating detonation, but killing a ton of power in the process.

Detonation, is the "snap, crackle, & pop", an engine makes, when the pistons are being forced back down the cylinder. The motor is basically trying to run backwards, but crank inertia, prevents that from fully happening. Since the explosion is still ongoing, the pistons do manage to get past TDC eventually.