Compression ratio (CR) is one of the most fundamental numbers defining an engine's character. It's the ratio of the cylinder's total volume (piston at bottom dead center) to the clearance volume (piston at top dead center). A 10:1 CR means the air-fuel mixture gets squeezed into one-tenth of its original space before the spark fires.

Why it matters: Higher compression extracts more work from each combustion event. Thermodynamic efficiency scales roughly with CR using the Otto cycle formula:

Efficiency = 1 − (1 / CR^(γ−1)), where γ ≈ 1.3 for real combustion gases.

Running the numbers: at 9:1 CR, theoretical efficiency is about 37%. Bump to 13:1 and you reach roughly 42%. That five-point jump in CR buys you a meaningful gain in fuel economy and power — which is exactly why Mazda's Skyactiv-G engines pushed to 13:1 on regular pump gas, using clever tricks like 4-2-1 exhaust headers to reduce residual gas temperatures.

The knock problem: Compress the mixture too hard and it auto-ignites before the spark plug fires — that's detonation (knock). Knock hammers the piston crown with pressure spikes that can crack ring lands or burn holes through pistons within seconds at full load.

Octane rating measures a fuel's resistance to knock. Regular (87 AKI in the US) handles about 10:1 CR in a naturally aspirated engine. Premium (91–93 AKI) lets you safely run 11:1–12:1. Turbocharged engines have effective compression ratios well above their static CR because boost pressure pre-compresses the charge. A turbo motor with 9.5:1 static CR running 18 psi of boost has an effective CR around 16:1 — which is why most turbo cars specify premium fuel.

Rule of thumb: each full point of compression ratio gained is worth roughly 3–4% more torque, assuming you can control knock. Modern engines use knock sensors and adaptive spark maps to retard timing if they detect detonation, sacrificing some of that gain to protect the engine when running lower-octane fuel.

• Direct injection helps raise CR because fuel evaporation inside the cylinder cools the charge by 20–30°C, reducing knock tendency.
• Variable valve timing can implement an Atkinson-like cycle at part throttle by holding the intake valve open late, lowering effective CR for efficiency while retaining the high geometric CR for peak power.
• Ethanol (E85) has an effective octane rating around 105 AKI, which is why flex-fuel performance tunes can run significantly more timing advance and boost.

A practical example: the Toyota GR Corolla's G16E-GTS three-cylinder runs 10.5:1 CR with a turbo making 21 psi. It requires 93 octane. Put 87 in it and the ECU pulls timing so aggressively you'll lose 30+ horsepower, and sustained high-load driving still risks damage.