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

Photography Credit: Komorebi Photo

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

Andy Hollis
9/30/24 1:52 p.m.

Surprised no mention of dynamic compression vs static. The former is what matters, not the latter.

Big cams with more overlap will reduce the dynamic compression, and therefore the octane needs.

I've experienced this personally during my Honda K series journey. Just upping the compression ratio from stock to 12.5:1 caused some knock issues at MBT ignition settings with stock cams...especially on warmer days.

Same static compression ratio but with some bigger cams had no such issues...and also made more power. Win-win.

Pete. (l33t FS) GRM+ Memberand MegaDork
9/30/24 2:05 p.m.

In reply to Andy Hollis :

Kind of the opposite... big cams with high overlap will INcrease dynamic compression, compared to the same duration with low overlap. Intake valve closing does a lot of the determination of compression, and closing up the lobe centers to increase the overlap will make the intake valve close sooner.

That's generally why higher overlap gets you more mid range torque, all other things considered.

Ranger50 MegaDork
9/30/24 3:59 p.m.

I'm sure my 12:1 DI/PI coyote can work on 87... but always ran it on 93 because they ain't cheap to fix and now on corn.... I like the idea of 10:1 30+psi boosted on E... and on a drive can just program the gates to be open with the ECU or just not turn on the co2.