Originally posted by GilesGuthrie
So, it costs us a fortune, but we get more bang per litre, I suppose.
Actually,
no. You get less bang. Anybody who tells you they get more performance just from running higher octane gas is lying to you.
Forget what Ts1 just said, for a minute. We'll come back to it.
All things being equal, higher octane gas actually has
less explosive force than lower octane gas. CC per CC, the lower-octane gas will deliver more power.
BUT all things are not equal.
Too-low an octane gas will cause the car to diesel or preignite, which causes that knocking or pinging sound when the engine is under load. Preignition is B-A-D, for a number of reasons. First, it means the explosion occurs as the pistion is still on the upstroke, rather than at Top Dead Center. This stresses the reciprocating parts, and means that power is being wasted. But worse than that, it also means that when the ignition spark
does arrive at the proper time, the cylinder is lean because some of the fuel has burned off already. This can drive cylinder temps through the roof, and in fact will slag the top of the piston if sustained long enough. Not good.
So really, what you want for maximum power is to run the lowest octane you can run
without getting preignition. This depends on a lot of things. I'll use my 2 Neons as an example. One is a SOHC, the other is a DOHC. Now, it so happens that one of the ways that Dodge got more power out of the DOHC was to optimize the spark timing for 92 octane and calibrate the engine controller accordingly. This means it will start at a baseline ignition timing, and it will listen to its knock sensor. This baseline of spark advance is set to make the most power. If the controller hears knock (from running cheaper gas), it will back off the spark advance until the knock goes away, sacrificing power for durability. It will always try to nudge the spark advance back up if it can, so if you switch to good fuel, it will work it's way back to full power across several tanks' worth. Yay!
The SOHC, on the other hand, is calibrated for 89 octane. Even if I run 94 octane,
it's never going to advance the spark past that point, so I'm just wasting money and power by running the safer, less volatile gas.
Which is, of course, why I got a Mopar Performance SOHC engine controller, which has tons o' spark advance, needs 92-94 octane, and which made a big difference in the power output. Not from the
gas itself, but from what the gas lets the engine do.
Which is why
TS1 does what he does. Forced induction or high compression are two ways of making big power, but they both totally depend on fuel that will not preignite. So the more stable, higher octanes are a must for boosted or built engines.
) tell you to use? 
