Does anybody have a resource for compression ratio and octane requirements? All to often I hear an argument over what fuels are "better" and which are not. Example, someone has a motor with 11.1 compression and uses 100+ racing fuel. I say that it's a waste and doesn't do much of anything but help you pass emissions. For some reason, higher octane fuel is a placebo for folks that think they're running faster with 100+ when it's not needed. For that motor to run good, I think 89 would suffice just fine. 12.1? How about 93? 9.1? Stick with 87.

Is there some kind of a chart out there with compression and octane requirements. Something that says 8-9 use such and such? 10-11 use such and such...?

 

If someone made such a chart, it wouldn't be useful. There are a number of variables besides CR which determine octane requirement. Among the more obvious are head and piston design and materials and the cam used. Valve timing (determined by the cam design), along with static CR, determines the so-called dynamic CR, which relfects the actual amount of cylinder pressure that the fuel "sees" at relatively low rpm. At higher rpm, volumetric efficiency becomes important in determining cylinder pressures. The higher the cylinder pressure, the more octane is needed.

If you had a lot of information about your motor, there are probably computer programs that could model combustion and predict octane requirement, but they would be something OEM's have. I'll be even OEM's use empirical methods though. IOW, they design a motor to work with a certain octane fuel based on prior experience and simulation. But then they have to test it and fine tune the design to get the desired characteristics.

Luckily, there is a lot of practical knowledge out there about a motor like the LT1. So you can utilize that knowledge when selecting components, etc.

Rich Krause

 

Then there's the other things that alter octane requirements including altitude (you wont' find a lot of 93 in Denver - you don't need it), coolant temp, inlet air temp, air/fuel ratio, humidity and combustion chamber deposits. Some of these have been quantified, to give rough guides of the impact of each...

absolute humidity - adding 10grains/# air reduces octane requirements by 1/3 octane unit

altitude - going 1,000' higher in altitude reduces octane requirements by 1.5 octane units

inlet air temp - increasing IAT 10degF increases octane requirement by 1/2 octane unit

coolant temp - increasing coolant temp 10degF increases octane requirement by one octane unit.

A/F ratio - leaning A/F ratio by 1 unit (e.g. 12:1 -> 13:1) increases octane requirement by 2 octane units.

Too many variables.........

Source: "High-Performance Automotive Fuels & Fluids", Jeff Hartman

 

Interesting Fred, looks like I got some calculating to do.