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Blow By

What is Blow By?

While an engine is running, there is a compressed air and fuel mixture inside the combustion chamber that is ignited and that forces the piston down. But some of that ignited mixture will pass through the gap of the cylinder wall and piston (the piston rings are there to seal the gap between them). This is called Blow By.

The amount of blow by depends on many variables, including design, components used and age (or wear) of the engine. The older and more worn the rings and cylinder walls of an engines gets the more likely it will allow greater Blow By.

Blow By is detrimental for several reasons.  

As Blow By increases, horsepower decreases because you are loosing some of the high pressure mixture pushing on the piston. 
Contaminates from incomplete combustion are not kept inside the combustion chamber they make their way into the oil inside the crank case. This will cause oil contamination and dilution.
Blow By can "blow" lubrication off the cylinder wall causing high wear.
As Blow By increases significantly, it can actually allow oil to leak into the combustion chamber (by the rings) and reduce the octane rating of the mixture.  This can cause Detonation and cause severe destruction of the piston and chamber.
As Blow By increases significantly, the leakage of hot gasses by the edge of the piston can actually "torch out the piston" (like a cutting torch, cut the edge off the piston).
Blow By is also a pollutant.  In older vehicles Blow By was taken care of by simply allowing the pressure generated inside the crankcase created by Blow By to be vented into the atmosphere by way of the crankcase vent tube. But eventually (in the 60ís) the EPA mandated that these vented gasses where detrimental to the quality of the air in the U.S. That is when the PCV (Positive Crankcase Ventilation) valve came into play.

Typically, the higher the torque an engine makes or the larger the displacement, the higher the potential for Blow By.  Blow By typically does not increase drastically with RPM, but is highest at the engine's torque peak.

Performance Trends manufactures a Blow By Sensor so you can document how much Blow By your engine is allowing.

And we now have a Blow By Recorder that is a Micro DataMite that allows you to get the Blow By readings without having to purchase one of our more expensive DataMite Data Logger systems.

And Engine Analyzer Pro v3.9 has the ability to simulate how much Blow By an engine might experience.