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Old 10-02-2014, 07:06 AM   #18
djmarchand's Avatar
City: Litchfield, Ct/Punta Gorda, Fl
Country: USA
Vessel Model: Atlas Pompano 23- outboard
Join Date: Aug 2012
Posts: 4,250
Originally Posted by High Wire View Post
How does one measure blowby?
Blowby is exhaust gas that gets by worn rings and valve guides and into the crankcase. There is a vent for it. Some people have installed an Air Sep in the vent to catch the oil entrained and recycle it. A better idea is to use Sbmar's Envirovent device.

Cummins has published a procedure for accurately measuring blowby on their engines and gives new and worn specs. It consists of installing a temporary orifice in the vent tube and measuring the pressure drop across the orifice. More pressure drop equals more cfm of blowby. If anyone wants a copy, send me a PM with your email address and I will send you the procedure.

You can do a quick and dirty test by opening the oil fill cap, running the engine up under load and put your hand over the opening. If you feel a rush of pulsing gas that pushes your hand up, you have significant blowby.

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