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Correct and the answer is in the theory of hydraulics. Let us assume you hold your thumb over the valve cover vent pipe sealing the crankcase and the pressure fails to move the pump shutoff diaphragm cutting the fuel. Let us also assume that the pressure rises to 10 psi.(as it would quickly with high blow-by). The area of the breather pipe is approx 1/2 square inch – i.e. a force of 5 lbs on your thumb. The area of the VC is approx 25in x 6in = 150 in² x 10 psi. A vertical force of 1,500lbs on this 3mm aluminum casting held down with 4 screws and this same pressure is pushing horizontally on the sides of the cover. We have the same story down below on the crankcase.
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I appreciate the explanation. Here is another version of the math for the VC.
Shear strength of cast aluminum alloy: (a
low aproximation) 29,000 psi. AND, this is not considering the shape ot the VC, only treating it as a flat surface undergoing a force on its weakest spot.
I estimated a surface area of (18"x 4" x 3 sides)+(4"x 5" x 2 sides) = 256 sqr in total Area
force = pressure x Area
29kp= (x psi) ( 256 sqr.inch)
you get 113 psi positve crankcase pressure under the cover, in the worst case scenario, with a weak alloy.
While I agree that this achievable by some mechanical means, you will not be able to blow apart the VC with your hands. The PCV opening is less than a square inch, prob .85 sqrinch.
that still leaves your hand trying to hold down 96 pounds. OSHA considers 30 psi dangerous for air entering through your skin and poisoning you.
Other areas will fail before the pressure is built to accomplish this.
11-14-2008, 11:52 AM
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