Ported Vac or Manifold Vac
Aslant,
MVA is a method of increasing the ign timing at idle. Chrys missed the boat on MVA when it was most needed, but was forced to use it in the 70s when the emissions junk made the engines run hotter.
MVA, when properly set up, gives these advantages over the generic 6-12* BTDC that Chrys used:
- smoother idle
- cooler running
- more idle vacuum
- better economy
- better tip-in response.
Note people running locked timing are getting the same benefit at idle as using MVA.
Stock or near stock engines will only benefit with a few more degrees if idle timing. Plugging the VA in on a stock engine with a VA unit that adds 25* will likely result in poor running, with MVA getting blamed [ didn't work on my car.....] when just adding 8-15* of VA by limiting the total amount of VA.....would have made a world of improvement.
GM used MVA on their cars, idling at 24-26*.
The factors that will require more idle timing for best idle are: cam duration/overlap & CR. More cam needs more idle timing, & so do low compression engines.
You can determine very quickly & easily if MVA will benefit your engine: warm up the engine, chock the wheels, & put in gear if auto. Loosen dist clamp & SLOWLY advance the timing. If rpm increases, it needs more timing & this can be done with MVA.
Aslant,
MVA is a method of increasing the ign timing at idle. Chrys missed the boat on MVA when it was most needed, but was forced to use it in the 70s when the emissions junk made the engines run hotter.
MVA, when properly set up, gives these advantages over the generic 6-12* BTDC that Chrys used:
- smoother idle
- cooler running
- more idle vacuum
- better economy
- better tip-in response.
Note people running locked timing are getting the same benefit at idle as using MVA.
Stock or near stock engines will only benefit with a few more degrees if idle timing. Plugging the VA in on a stock engine with a VA unit that adds 25* will likely result in poor running, with MVA getting blamed [ didn't work on my car.....] when just adding 8-15* of VA by limiting the total amount of VA.....would have made a world of improvement.
GM used MVA on their cars, idling at 24-26*.
The factors that will require more idle timing for best idle are: cam duration/overlap & CR. More cam needs more idle timing, & so do low compression engines.
You can determine very quickly & easily if MVA will benefit your engine: warm up the engine, chock the wheels, & put in gear if auto. Loosen dist clamp & SLOWLY advance the timing. If rpm increases, it needs more timing & this can be done with MVA.