Also, I see plenums attached with silicone hose and hose clamps. Is the plenum really the weak point?
And those routinely blow off if they are not properly restrained with straps across the hose to keep the joint from spreading.
To answer your direct question, the issue is more of shape than anything else.
A round tube is inherently strong at resisting internal pressure but square and angular shapes get a real workout from pressure changes "oil canning" the flat surfaces, and often fail at the corners.
If you get a good healthy backfire in a NA engine you can get high enough pressures to split a plenum, especially if the situation causes fuel mist to get blown into the intake plenum then that fuel air mixture gets ignited by a second backfire.
Even at relatively low over pressures like 3-4 psi, if you add up the surface area of the flat surfaces and multiply by that small over pressure the resulting numbers are pretty impressive if the plenum is any size at all.
Suppose the top of a square plenum is 6 inches wide by 8 inches long, that is 48 square inches. If your backfire pressure is 4 psi, than there is 180 pounds of force trying to rip the top off the plenum. Is your plenum strong enough for a good sized man to jump up and down on it with no failure?
If a pre-existing crack is developing it can unzip the corners and launch that flat plate with surprising force (and velocity).
If a spin or some other situation causes fuel air mixture to get pushed back into the intake plenum for some reason and then it ignites you have a small fuel air bomb.
Larry
