MVA is manifold vac advance.
PVA is ported vac advance.
With MVA, at idle, the vac advance kicks in and is part of the idle timing. To make this work consistently, you often have to 'pedal' the car just as it starts (when cold) to get the vac advance to help 'kick in' and stabilize.
With PVA, there's no vac at the port at idle, so the vac can doesn't add any advance at idle. Only at part throttle cruise.
My understanding is that PVA helps keep the exhaust hot to help cats burn better and sooner. MVA works good except with bigger cams and lower idle vacuum. The low vacuum issue can be overcome by 'blipping' the engine after start to help get the idle timing up and 'on the can' quicker.
In my experience, this all holds pretty true but there are several considerations. I have a true 11:1 ford FE with dual quads and a pretty moderate cam. I've run it with 8 degrees of idle timing, and as much as 26 - while keeping the 'all in' around 38, with and without the vacuum canister. Lower idle timing equated to super hot exhaust pipes, and ever-rising coolant temps when idling. Was fine at cruise though. The lower idle timing also warmed the engine faster after starting. The exhaust had a definite odor, which sucked. There was also a lack of power, even with super soft advance springs because I was starting so far 'down in the hole' with the timing curve. After 3k it's all the same, but under 3k the lower timing was a dog. Sounded mean though.
With the super-high idle timing (16 deg static initial, 10 degrees vac, 750 rpm) the engine takes forever to warm up. I can drive for near 20 minute before the thermostat opens. The exhaust pipes are much cooler, less heat in the engine bay, engine is a bit snappier off-idle but can be tougher to feather the clutch. The issue I have is that the throttle gets cracked, the vac drops, the timing drops, and then I load the engine with the clutch and it wants to stumble and die. Once warm, it's a non-issue, but it can take upwards of 30 mins for the engine to warm up completely. The engine also sounds super tame, and much quieter at idle. The difference between 8 degrees @ idle, and 26 @ idle was the difference between setting off car alarms idling down the road vs being able to converse with my passenger. It was that significant.
With my dart, I've seen similar but not the same. I run an Fitech EFI with timing control. The engine is a 408 with around ~250 duration @.050, and 3.91 rear gears with a 4spd. I've tried everything from 12 degrees idle timing to 45 degrees. The higher the idle timing, the more the clutch wants to kill the engine. Too low though, and it stinks to high heaven and gets way too hot too fast. With the Fitech I can set idle timing, and then timing at 1100 rpm for both high and low vac situations. I've found the 'best' to be an idle timing of around 25 degrees with a WOT 1100 rpm setting of around 16-18. This keeps the engine from trying to stumble when creeping on the clutch, or when you let the clutch out a little too quick (even with the 3.91s). I've not noticed the same impact to temperatures with the dart though vs the FE. I'm fairly certain it's because my dart uses a clutch fan, and the fan/shroud combo sucks - if I open the hood all is well, but with it closed my air recirculates out from under the car back into the radiator. Driving down my gravel road, I actually blow a dust cloud forward (yes, the fan turns the right direction). Above 15mph, no issue.
