Anyway, the point is that thermodynamics dictates that thermal efficiency goes up with compression ratio, as you can see by that plot and equation above. And that means more horsepower, better fuel economy, heavier wallets and bigger smiles. Drive any sluggish, wheezing, gas-sucking, old low-compression American V8 and you’ll know what I mean.


Compression ratio is also what makes engines like Mazda’s Skyactiv-G engine so efficient. The first of a wave of new high-compression and variable-compression engines from Mazda, Nissan/Infiniti and Toyota, the Mazda has the highest compression ratio in the business right now, at 14:1, which is why it can manage high fuel economy and power figures even without a turbocharger.

Why Higher Compression Means You Need Higher Octane

Why doesn’t everyone just use high compression ratios? Well, high compression is why a lot of performance engines need premium fuel, or high-octane gasoline. Octane ratings are, as this How Stuff Works article points out, a measurement of the gasoline’s ability to resist detonation.


Compared to gas with a high octane rating, gasoline that has a low octane rating is more likely to auto-ignite due to high air-charge temperatures and pressures. Basically, you want the gas that ignites when you want it to, not the kind that ignites when you don’t want it to. That kind of uncontrolled combustion is called knocking. Knocking is bad; it reduces torque and can cause irreparable damage to your engine.

High compression increases your risk of knocking, which is why very high compression engines run high octane race gas or (more commonly now) E85. Gases tend to heat up when they’re compressed, so the increased heat density could lead to the fuel prematurely combusting before the spark plug ignites it. To reiterate: That’s bad.


Mazda had to do a lot of work to its piston and exhaust design to mitigate knocking on its 14:1 engine running on pump gas. The pistons in a Skyactiv-X engine, for instance, have a cavity in the middle, to allow for Mazda to shoot a burst of rich fuel around the igniting spark plug in an otherwise lean mixture and, yeah, there’s a reason why this wasn’t an easy technology to develop.

What’s also interesting is that you can’t just make an engine with as high a compression ratio as you want. I reached out to John Hoyenga, an owner at the performance exhaust and rally shop Nameless Performance, to chat about risks and benefits of high compression.


John is building a Nissan 240SX rally car into which he’s swapping a SR20VE four-cylinder, currently making about 250 horsepower at the wheels from just 2.0 liters. This is, surprisingly, with no turbo. All John has to thank is its very high 14.5:1 compression ratio. “There’s more work done by compression,” he explained, “so the more power [an engine] will make without boost.”

That being said, because this is a race engine, he’s running it with race gas or extremely high octane E85. John said that anything over 14.5:1 compression ratio would run the risk of auto-ignition, and it could shoot out a rod or spin a bearing. This is what’s casually referred to as “blowing up.”


There’s A Limit To How High You Can Go

I asked if this is why we don’t see people aren’t running around with engines that have significantly higher compression ratios than anything we see today. Obscenely high ratios, like 60:1. John laughed. He explained that metal simply cannot withstand such high levels of stress, and a compression ratio like that would run things so hot that it’d blow up any current engine.


Of course, not all of us are building race cars with race engines, so altering compression ratios isn’t something we ever have to worry about. But we are casual car owners and quasi-engine enthusiasts, so this was an explanation for what compression ratio means and why it matters. You don’t have to fake it anymore, you now know what it is.

Now, go and find that trapeze artist and tell him how you feel!