The ignorance on this site about MVA benefits is breath taking. Not sure when GM started using MVA, but it was used in the early 60s. GM was forced to switch to the useless PVA around 1968 because of stricter emissions, having used MVA prior to this. MVA to PVA, not the other way round.
MVA remained in use until the end of the carb era, used by Ferd, GM, AMC & Chrysler.
This is Chryslers version during the 1970s:
'When engine coolant temp at idle reaches 225F, the valve opens & APPLIES MANIFPLD VACUUM DIRECTLY TO THE DIST....This increases engine idle speed & provides additional engine cooling.'
Ford's version:
' If engine overheats at idle, increased vacuum will flow to the dist to INCREASE ENGINE SPEED. '
AMC:
' The coolant override switch incorporates a thermal switch which reacts to coolant temps to route either INTAKE MANIFOLD VACUUM or ported vacuum to the dist.'
GM:
' the coolant temp switch overrides the system & provides FULL MANIFOLD VACUUM to the advance unit, ADVANCING TIMING in turn lowering the operating temp.'
Anybody with half a brain should be able to see that if you change the ign timing at idle [ in this case advancing the timing via MVA ] & with no other changes, idle rpm increases, the engine is making more hp from the additional timing & it is a beneficial move.
The other obvious result is that because the engine does not have to work as hard [ it runs cooler with less load ], the A/F has burned more efficiently.Statistics: Posted by GTO Geoff — Tue Jun 22, 2021 8:45 am
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