351W crankcase evacuation

[Jeff Chambers]

Crankcase evacuation is a good way to make extra power. By pulling vacuum in the crankcase, you promote better sealing of the rings (creating less blow-by), better oil control (we all know that is better), and stronger signal to the venturis if you're a carb'd car. There's a power gain due to just the differential pressure that the piston sees when the crankcase is evacuated. Imagine that under normal config the top side of the piston sees a combustion chamber pressure of 200 psig and the crankcase is under say 5 psig positive pressure. The net pressure on the top of the piston is 195 psig. Now if you suck 15in-Hg (~7.5 psi vacuum) vacuum on the crankcase you now have a net pressure on the piston of 207.5 psi. That's a difference of 12.5 psi. On a 4" bore piston, that's an extra 157# of force pushing the piston down. When in motion, that's HP.

I run a crankcase evacuation system using the air pump from an F-150, spinning it backwards. I pull from the valve cover and exhaust to a breathered cannister. I pull about 12 in-Hg at 3/4 track when the engine's under its heaviest load. It pays off. On the engine dyno, a good vacuum system can be worth as much as 3-5% extra HP/TQ.