Nater Potater
Well-Known Member
- Oct 19, 2020
- 2,653
- Boat Info
- 1992 300DA Sundancer
- Engines
- Twin Merc Alpha I Gen II I/O's with 5.7 V8's
Interesting twist the thread has taken; 'right up my alley.
It's a common misconception that higher octane will make the combustion run hotter/cooler/slower, where, in fact, octane rating only denotes how resistant the mixture is to detonation. Believing you'll get lower temps when running higher octane fuel is more a function of low octane fuel leading to detonation which drives the combustion temps up. 'Not the desired situation...
During normal operation, the mixture in the combustion chamber doesn't explode, but actually burns at a controlled rate across the top of the piston. The hope is that it finishes burning at the same time that the piston reaches TDC, which is why the ignition has to fire the plug before TDC (thus, ignition advance). As it's burning, both pressure and heat rise. If either exceeds the auto-ignition point (the point where the mixture will spontaneously combust), you get a nasty pressure spike while the piston is still rising. That's detonation, and what you hear as a "knock". Run high enough octane rating that you don't get detonation. Running anything higher than that will not gain you anything but a lighter wallet.
So, after all of that, I discovered something interesting with my late-model turbo car; even though it's designed to run just fine one 87 octane fuel, by changing to 91 octane, not only does it run noticeably better, but it gets enough better mileage that it more than offsets the price difference. The assumption is that the cornputer was pulling the timing a lot more than I realized.
It's a common misconception that higher octane will make the combustion run hotter/cooler/slower, where, in fact, octane rating only denotes how resistant the mixture is to detonation. Believing you'll get lower temps when running higher octane fuel is more a function of low octane fuel leading to detonation which drives the combustion temps up. 'Not the desired situation...
During normal operation, the mixture in the combustion chamber doesn't explode, but actually burns at a controlled rate across the top of the piston. The hope is that it finishes burning at the same time that the piston reaches TDC, which is why the ignition has to fire the plug before TDC (thus, ignition advance). As it's burning, both pressure and heat rise. If either exceeds the auto-ignition point (the point where the mixture will spontaneously combust), you get a nasty pressure spike while the piston is still rising. That's detonation, and what you hear as a "knock". Run high enough octane rating that you don't get detonation. Running anything higher than that will not gain you anything but a lighter wallet.
So, after all of that, I discovered something interesting with my late-model turbo car; even though it's designed to run just fine one 87 octane fuel, by changing to 91 octane, not only does it run noticeably better, but it gets enough better mileage that it more than offsets the price difference. The assumption is that the cornputer was pulling the timing a lot more than I realized.