Compression Ratio and Octane Ratings: What You Need to Know

Compression Ratio and Knock Sensors

If we all had to calculate what our car's compression ratio is, well, plenty of us would be in big trouble. That's because the formula is so long and has so many variables that even those who are proficient in math would never dare to consider it [source: Nielsen]. Fortunately, most owners' manuals have the compression ratio already calculated, and there's really no reason to know it or understand how it's derived. "Drivers don't ever have to calculate this. Mechanics usually don't, either," says Mike Arman, who has written numerous technical manuals on airplane and motorcycle engines. "People who modify engines must calculate this to determine -- which usually means guess -- the octane ratings of the fuel required."

Sometimes, a car's knocking and pinging is audible to the driver. But in other cases you'll never even hear a thing because a knock sensor has already corrected the problem. According to Arman, knock sensors are so-called piezo-electric microphones bolted to the engine block. "The knock detector talks to the ECU [the car's computer], which retards the timing to stop the knock," he says.

Read on for the bottom line.