Doug,

 

I'm not sure how much benefit in reduced windage losses you would get from applying a vacuum to the crank case. The biggest areas for gain here are in oil pan design, windage trays and crank scrapers that work to screen the oil droplets from the air down into the sump and protect the oil surface from the 'wind' in the crankcase. Yes, this can make a substantial difference in a race engine, but as Paul said at 2700 rpm it's questionable just how much power could be recovered from windage reduction.

 

There are two benefits from crank case vacuum that you haven't mentioned. The first is  reduced dilution of the fuel air charge in the cylinder and its effect on power. During the intake stroke, cylinder pressure is well below atmospheric (naturally aspirated) so ring leakage flows into the cylinder. With crank case vacuum, the ring leakage flow is reduced and at times even reversed. There are three effects as a result: First, the area between the top ring land and the cylinder is full of exhaust products, partially burned fuel and raw fuel all of which are detrimental to the power stroke. During the intake and compression stroke, some of these products are swept into the combustion chamber where they reduce power. Crank case vacuum helps to remove them from the cylinder. Second, the air leaking from the crankase is fuel lean and its addition to the cylinder is also detrimental to power. Third 

ring leakage into the cylinder carries with it a small amoint of oil. This also displaces fuel air mixture and reduces power, but more importantly oil has a very low octane rating and contributes to detonation.

 

The second benefit is improved oil life. Unburned fuel and exhaust gasses are continually leaking from the cylinder during the combustion, exhaust and compression strokes and contaminating the oil. The oil contains various additives to prevent damage to the engine as a result of contamination with these chemicals. As contamination increases, the additives are expended and protection is lost. Crankase vacuum helps to volitize some of the compounds and draw them out of the oil and into the exhaust. This was a beneifcial consequence of the positive crankcase ventilation (PCV) system on cars in the early '70's. While the vacuum generated in the crankcase was slight, the system scavenged vapors from the crankcase and greatly reduced contamination of the oil. This resulted in extended oil changes intervals from 3 thousand miles in the '60's to 4 or 5 thousand in the '70's. Of course today, thousands of people throw perfectly good oil away every 3000 miles anyway. Old habits die hard.

 

Rob