Here's What 'Compression Ratio' Actually Means And Why It Matters

Have you heard the term compression ratio before, but do you know what it means? Now let's talk about compression ratio, and why every carmaker is obsessed with it like it's the Holy Grail.

As it turns out, compression ratios are more complicated than they seem. It doesn't help that it's a term that gets tossed around at car meets and in press releases without much explanation. Most people pretend to understand it while trying to impress that trapeze artist at the circus last weekend.

It is good to have high compression, and bad to have low compression. Mazda's new Skyactiv-X "Holy Grail" engine is high compression, like Infiniti's "diesel slayer" and Toyota's "Dynamic Force" models, which all claim to be more powerful while being more efficient.

We live in an age when engineers can't just make an engine bigger to make it more powerful. Changing an engine's compression ratio is becoming the norm.

(By the way, if you’re reading this and snorting because you already know what compression ratio is, good for you! Not everyone else does.)

Compression Ratios Are Simple To Understand

As the name suggests, a compression ratio involves compressing a maximum cylinder volume into a minimum cylinder volume. This is the volume of a cylinder when a piston is all the way down compared to all the way up. The ratio is written out. For example, for an engine with a 9:1 compression ratio, you’d say that it’s “nine to one.”

Think of your head as a cylinder. Imagine the piston moving inside. It is called Bottom Dead Center when the piston is at its lowest point. At that point, the cylinder volume is the largest. Top Dead Center is where the piston is at the highest point within the cylinder, and this is where the cylinder volume is smallest. This is where the ratio comes from.

I made this GIF to show how a four-stroke engine works if you are a visual learner like me. Can you see how the piston moves up during the compression stroke? That's all the air and fuel being compressed in the cylinder. With a high compression ratio, a given volume of air and fuel in the cylinder is squeezed into a smaller space than with a low compression ratio.

And now for an example with simple math, my favorite kind.

When the piston is at bottom dead center of the cylinder, the cylinder and combustion chamber volume is 10 cc. A single cubic centimeter of air and fuel is squeezed into an intake valve after the intake valve closes and the piston rises during the compression stroke. This engine has a 10:1 compression ratio.

There you go! You now know what compression ratio is. Compiling swept volume plus compressed volume (including the volume of the cylinder head and everything above where the piston sweeps) into compressed volume alone.

Why It’s Better Is Complicated

But it's less important to know what compression ratio is than to know why we care about it, or why high compression is so desirable.

Fortunately, my coworker and engineer David Tracy was able to provide me with the best explanation of this, who then sought the help of other engineers and professors. One of the best answers came from Dr. Andy Randolph, Technical Director at ECR Engines. He explains NASCAR's powertrain research in simple terms:

"From a layman's perspective, engine power is created when combustion exerts a force on the piston and pushes it down the cylinder.

The higher the piston is in the bore when combustion begins, the more force is exerted.

As compression ratio increases, the piston moves higher in the bore at top dead center, hence there is additional force for the expansion stroke (additional force for the same amount of fuel equals higher efficiency)."

Combustion exerts a force on the piston and pushes it down the cylinder during the expansion stroke.

The higher the piston is positioned in the bore, the more force is exerted when combustion begins.

The piston moves higher in the bore at top dead center as the compression ratio increases, therefore there is additional force for the expansion stroke (more force for the same amount of fuel equals higher efficiency).

As you can see by the plot and equation above, thermodynamics dictates that thermal efficiency increases with compression ratio. This means more horsepower, better fuel economy, fatter wallets, and bigger smiles. Drive any sluggish, wheezing, gas-sucking, old low-compression American V8 and you'll understand.

In addition to the compression ratio, Mazda's Skyactiv-G engine is highly efficient due to its high compression ratio. As the first of a new wave of high-compression and variable-compression engines by Mazda, Nissan/Infiniti, and Toyota, the Mazda sports the highest compression ratio in the business 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

Can't everyone just use high compression ratios? Many performance engines require premium fuel, or high-octane gasoline, because of their high compression. According to this How Stuff Works article, octane ratings indicate how well gasoline resists detonation.

Because of high temperatures and pressures associated with air-charge, gasoline with a low octane rating is more likely to auto-ignite than gasoline with a high octane rating. In essence, you want the gas that ignites when you want it to, not the kind that ignites when you don't want it to. It's called knocking. The act of knocking reduces torque and can damage your engine irreparably.

This is why very high compression engines run high octane race gas or (more commonly now) E85 to reduce knocking. The increased heat density of compressed gases may result in the fuel combusting prematurely before the spark plug ignites it. To reiterate: That’s bad.

As a result of the 14:1 engine running on pump gas, Mazda had to modify its piston and exhaust design a lot to mitigate knocking. For example, Skyactiv-X pistons have a cavity in the middle that allows Mazda to fire a burst of rich fuel around the igniting spark plug in an otherwise lean mixture, and yeah, it wasn't easy to develop.

In addition, you can't just make an engine with as high of a compression ratio as you want. To discuss the risks and benefits of high compression, I reached out to John Hoyenga, owner of the performance exhaust and rally shop Nameless Performance.

John is building a Nissan 240SX rally car, into which he's swapping a SR20VE four-cylinder, which currently produces about 250 horsepower from just 2.0 liters. There is no turbocharger. John only has to thank its high compression ratio of 14.5:1. “There is more work done by compression,” he explained, “so an engine will produce more power without boosting.”

In any case, because this is a race engine, he's running it on race gas or extremely high octane E85. Any compression ratio over 14.5:1 could auto-ignite, causing a rod to shatter or a bearing to spin. This is commonly referred to as "blowing up."

There’s A Limit To How High You Can Go

I asked if this was the reason we don't see people running around with engines with significantly higher compression ratios than we see today. Compression ratios as high as 60:1. It made John laugh. According to him, metal cannot withstand such high levels of stress, and a compression ratio like that would run things so hot that it would blow up any engine.

Since we aren't all building race cars with race engines, we won't have to worry about modifying compression ratios. Since we are casual car owners and quasi-engine enthusiasts, this was an explanation of what compression ratio means and why it is important. Now that you know what it is, you don't have to fake it anymore.