What Does Blowby Mean?

"Blow-by" is a fairly common term across all types of engines—diesel, gas, etc. For diesels, it's when compressed air and fuel in the cylinder bore is greater than pressure in the oil pan, and gas leaks past piston rings and down into the crankcase.

Is Blowby bad?

Blow-by that makes it into the cylinder can lower the effective octane rating of the air-fuel mixture. If the octane rating of the air-fuel mixture drops enough, it can cause knock (also known as pre-ignition), where the fuel mixture ignites before the spark plug fires, causing very high cylinder pressures.

What does a Blowby do?

The unburned fuel dilutes the lubricity and viscosity of the engine oil, attacking engine bearings, the valve train, and cylinder walls. When equipped with an engine brake, higher-than-normal blowby will be induced when the system is engaged. ... Blowby tears the oil from the piston and rings.