[Kevin Johnson]

Originally Posted by greasemonkee

The above is true about less windage and better ringseal, but the primary benefits are canceling the forces acting on the pistons themselves.

ON your typical 4 cylinder you'll have one piston continuously exposed to the atmosphere (or lack of), give or take given the intake valve duration, but for simplicity sake suppose the intake valve duration is 180* and omit scavenging, resonances, ect for a moment since they are of a minor influence to the subject.

Imagine each piston in the dead middle of its stroke, and firing order of 1,3,4,2



cyl# ->: ------- 1-----2-----3-----4



90* ----------- INT - COM - EXH - PWR

270*---------- COM - PWR - INT - EXH

450*---------- PWR - EXH - COM - INT

630*---------- EXH - INT - PWR - COM


This is what I sketched up quick. 2 cylinders cancel out the other 2 in the sealed crankcase and the absolute pressure remains the same down there (omitting blowby), BUT the force acting on the top side of the piston is a different story.

Put it to the test: Balance the ambient pressure in the crankcase (if it's vented to an ambient pressure source) by opening the throttle to 100%. Drive at certain speed and at specific landmark, cut the ignition then floor it (this is with a manual trans btw) and at another landmark a few seconds later check your speed. Repeat this again at the same landmarks and same speed, but with the throttle closed. I'll bet your final speed is quite a few mph slower.

So the same thing can be achieved by balancing the absolute pressure in the crankcase to the Intake manifold. The limit is going to be a safety limit at a debatable 10-15 in/hg. Thus, if you cruise at a 15 in/hg of intake manifold pressure, you can have 15 in/hg crankcase pressure.

The problem (with race engines) is the consistency of the vacuum. The genesis of the difficulty will vary depending on whether you have intake induced vacuum or exhaust induced vacuum. Either way will yield an expensive blown engine under the right circumstances or possibly just banned from the track for fogging everyone with oil. Driving at a constant speed and engine rpm is another matter.