92b,
PVA is useless in the sense that MVA does the same as PVA...and more. It adds timing at idle, which helps the engine run cooler & increases idle fuel efficiency. A win-win.
MVA is still with us today, in electronic form, with EFI. The LS engines idle at about 22* & their efficient, high compression, chamber only requires about 27* at WOT. When you extrapolate the numbers on our old dinosaurs that need 35+* at WOT, suddenly 28-30* at idle doesn't sound excessive.
