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+--- Thread: what is blow-by? (/showthread.php?tid=3358)
what is blow-by? - Purplecomputer - 02-14-2012
This term comes up every once in a while in different forums and I dont know what it means...
RE: what is blow-by? - aaa - 02-14-2012
Gases slipping out of the combustion chamber past the piston rings. A little of it is unavoidable, hence the crankcase ventilation hose on top of the valve cover. And the more that slips past, the less power you get from your engine (less compression).
RE: what is blow-by? - larsalan - 02-14-2012
Gas from inside the combustion chamber sneaking past the rings into the crankcase. Where the gas is actually blowing by I dunno. Where did that term come from?
RE: what is blow-by? - Purplecomputer - 02-14-2012
Oh...I see. So how doe people check for blow-by?
RE: what is blow-by? - larsalan - 02-14-2012
Take the oil filler cap off and watch the gas fly by. It looks like smoke. Mine varies depending on the operating tempo and the amount of boost.
RE: what is blow-by? - w123love - 02-15-2012
A good check is to unscrew the oil filler cap, and let the cap just sit while the engine is idling. Inevitably gasses will pass by the cap, and make it move a little bit. If it bounces the cap off the valve cover, then you have some SERIOUS blow-by. If it chills, you can chill.
RE: what is blow-by? - sassparilla_kid - 02-15-2012
How does it make its way to the valve cover, through oil passages and stuff?
RE: what is blow-by? - dieselboy - 02-15-2012
Blowby is also water steaming off and other chemicals being evaporated from hot oil.
RE: what is blow-by? - Captain America - 02-20-2012
Compression/Boost is lost past the rings on the compression stroke. It goes into the crankcase and raises up through the oil drain back passages to escape out of the valve cover hole. It is easiest if you leave the oil fill cap and take off the breather tube. While the engine is running you can cap the hole for a few seconds and when you release you will hear a whoooosh of air pressure.