i have vortec heads. i was wondering if the necked down stems actually help flow as much as they say? if not is ther an adantage to going to the manly valve over stock?

I've witnessed at least three different flow bench sessions with those necked-down valves. In all three cases the valves gave no improvement at all. The cylinder head developer said said something along the lines of "Of course they don't, there's no flow in there by the stem anyway." I think he's right.

Tom

Years ago when these valves came out I asked 3 different cyl head porters the same question...all of them had tested them....All said they didnt do much... one commented about the big advertizing claim of improvement in low lift flow was most likely they who ever did the test forgot to put the spark plug back in the cyl head !( That was ofcource just a guess ).

While the may not do anything for flow, they might weight less and that could be advantageous.