You don't have to have a high-compression engine to run high-octane fuel, however it is one of the situations where you may have to.
"Octane" is simply the measure of a fuel's resistance to burning. In other words 89 octane burns faster, and in some cases more uncontrollaby, than 93 octane. Cylinder temperature dictates how gasoline will burn- if it is too hot for the octane rating of the fuel the gas will flash burn in an uncontrolled explosion that results in detonation, aka spark knock or "pinging." High compression engines have higher cylinder pressures and thus higher cylinder temps, so they usually need higher octane fuels. Lower compression engines are the opposite and can usually get by with lower-octane fuels.
Another way cylinder temps can rise is when the air-fuel ratio is lean, as in the case of most new H-D's. In this case high-octane fuel can be used to mitigate detonation as it resists uncontrolled burning better than lower octane fuels.

"Dieseling" occurs when something in the combustion chamber (like a piece of carbon) is hot enough to promote ignition in an engine that is turned off. Hence the term "dieseling" in that it needs no spark from the spark plug to ignite the fuel. This was more common in older engines because when the engine was turned off the carburetor butterflies stayed open a little as they remained in the idle position. If a hot spot in a cylinder could keep igniting fuel & air after the ignition was turned off the engine would continue to draw air & fuel into the engine and fire on that one cylinder, resulting in dieseling.
Modern carbs get around this problem with an anti-dieseling solenoid. When the ignition is turned off the butterflies are completely closed. When the ignition key is turned on an electric solenoid engages a plunger that contacts the throttle linkage and opens the carb butterflies a little to allow air & fuel to pass for idling and normal engine operation. When the ignition is turned off the solenoid retracts the plunger which allows the butterflies to completely close, and no fuel & air can pass. So even if a cylinder has a hot spot dieseling can't occur as there is no fuel mix to burn.
If dieseling occurs in a modern engine it's usually an indication that the anti-dieseling solenoid has failed rather than indicating the engine needs high-octane fuel.
Higher summer temps, which lead to higher operating temperatures and higher cylinder temps, would be a good reason to run high-octane fuel if detonation becomes a problem.

 

I've got TERRIBLE gas.........do you think they will help me?......status post-Taco Bell![&:] .......bbbbuuuuuuuurppp!!!....faaarrrrrtt!....ooop s, I just shat![sm=escape.gif]

 

So, if I understand correctly anything under 9 to 1 compression should be able to run on 87 Octane? I have a vette with twin turbos that has 8.5 to 1 compression, and the engine builder (Callaway) highly recommends 93 octane only. Just curious, because at 40-50 cents a gallon difference, that would add up to a significant savings over the year for me.

 

8.5:1 is at idle/0% boost. What's the compression ratio at max psi?

 

YEA I TRIED THE GAS DROPS,,,,, MY EYES ARE STILL BURNIN!!!

 

Yes and no, ignition timing has a lot to do with it also. Put in what is recommended.

 

You can run the 87 if you take off your turbos. When you are running boost you are going to have a higher pressure in your cylinders.

 

Yep. The blowers increase the pressure. That's the 8.5 part of the 8.5:1.

Many people build motors with a lower compression ratio than stock if they're going to add blowers.

 

You are not wrong. You nailed it. High octane is basically for engines with 9-1 or higher compression.

 

high octane fuel burns at a slower & more controlled rate....high compression benefits from this...low compression does not....boost or any other power adder will require higher octane to control the detonation bc of cylinder pressure...cp is more of a determiner than hi comp....most miss that....

the drops are SNAKE OIL ... they tout the octane boost bc of the toulene or benzene the pills are made of....the Texas State Attorney Gen shut em down for false advertisement....they were tellin folks it'd get em better gas mileage....you'd do just as well performance wise by puttin a turd in your tank