"The main purpose of the PCV (positive crankcase ventilation) valve is to recirculate
blow-by gases back from the crankcase area through the engine to consume unburned
hydrocarbons. Blowby is a mixture of air, gasoline and combustion gases forced past the
rings on the combustion stroke. The PCV system usually has a tube leading from the
crankcase to the carburetor or intake manifold. Vacuum within the engine intake manifold
pulls blowby gases out of the crankcase into the combustion chamber along with the
regular intake of air and fuel.
A valve can become clogged with sludge and varnish deposits and trap blowby gases in
the crankcase. This degrades the oil, promoting additional formation of deposit material.
If left uncorrected, the result is plugged oil rings, oil consumption, rapid ring wear due to
sludge buildup, ruptured gaskets and seals due to crankcase pressurization, oil thrown out
around the filler cap and consequent rough engine operation." Amsoil TSB: MO-2004-04-03
This sums up what I've always understood. I've also always been told that a it's not as important on a quarter mile car as long as 2 breathers are employed because it doesn't run for extended lengths of time at temperature but that on a driver, if you don't have good flow venting through the crankcase, you run the risk of your engine developing more sludge sooner. The suction on the PCV promotes that constant venting and flow.