Why compression ratios don't dictate octane requirements | Fuel Tips

Why can some high-compression engines get away with unleaded fuel while others can’t? It can come down to that engine’s technology, often centering on engine management and materials.

First, what describes a high-compression engine? For the sake of argument, let’s use a 12:1 compression ratio as the line in the sand–of course, that’s more of a guideline than a rule. 

If that 12:1 compression ratio is found inside a carbureted, all-iron, race-tuned V8 running a lot of ignition timing advance, then Zachary J. Santner, manager of Product Engineering and Business Optimization at Sunoco Performance Products, recommends a leaded, 110-octane fuel. “It’s the basic race gas,” he says, noting that it’s not legal for the street nor compatible with catalytic converters. The reason for recommending octane that might be a little on the high side? Extra protection from pre-detonation–also known as knock. 

As more technology is added to the setup–or that setup gets milder–Santner sees lower octane requirements. 

Aluminum heads? Okay, now the engine can likely better shed heat, reducing the need for octane. “More heat means more detonation,” he explains. 

Fuel injection? A more precise fuel delivery setup also reduces the need for octane. Less ignition advance also reduces the need for octane.

Keep following these technological advances, and that’s why a modern, bone-stock Miata can run 93-octane fuel along with a 13:1 compression ratio–and why, on the track, today’s GT3 and GT4 race cars only need 100-octane unleaded, today’s other race fuel commodity.

Comments

Displaying 1-2 of 2 commentsView all comments on the GRM forums

You'll need to log in to post.