Distributor Vacuum Advance?

[MopaR&D]

RR,
MVA has it's benefits...........................PVA is useless & we got it because of emissions [ like lower comp ratios that made engines inefficient ]; engines that used PVA had MVA in the background to come to the rescue & MVA do so until the end of the carb era.
There are some combos that will not benefit from MVA......buuuuuuuuuuut there are very few.
MVA is not a crutch, no more so than using higher ratio rockers to increase HP. The engine needs a certain amount of ign timing advance at idle for best power & MVA is one method of achieving it. Another method would be a locked dist. However, this method may cause detonation because the timing is no longer load sensitive; it is load sensitive with MVA.
The problem with MVA is that there is no procedure [ that I have seen ] to dial it in.
The instructions that come with the adj VA units such as Crane, Accel, Mr. Gasket etc are written by people who are clueless to what MVA was designed to do.

D. Vizard reviewed the then new Crane Elec Dist in Nov 04 of PHR magazine; note that Crane went to the expense of adding VA curves, not just centri curves.....
" At idle & low speed operation, the amount of advance reqd to most efficiently utilize the air & fuel entering the engine can be as much as 50-55*. This is handled by the vac adv: a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen & listen up good. A functional VA is the single most effective camshaft tamer you can get...."

And closer to home, Mopar related, MM magazine Ovt 2015:
" The initial timing went from 15 to 26* while adding an inch of vacuum. Total timing didn't change, but a dramatic gain in low rpm throttle response was realized. If tweaked correctly, you'll see your idle vacuum creep up, as we did here."

These are the exact results I had from switching from ported to manifold VA. My engine already ran GREAT but I figured, let's hook up MVA and see what happens. With my cam it only pulls about 12" of vacuum at idle so with MVA the initial advance only went up maybe 4-6 degrees but that was enough to increase my vacuum another 0.5inHg or so. It's more responsive at small throttle openings like slowly pulling away from a stop. It literally runs exactly the same at medium-to-wide-open throttle as before but it runs better with light throttle. And there's no feeling of bog or hesitation whatsoever when the throttle is opened and the vacuum advance drops; there's still adequate mechanical advance to keep the combustion happening at the right point.

Biggest difference I noticed is when closing the throttle while coasting; before there was a bit of "roughness" when I felt it cross the threshold that shut off the PVA. Now since it's always open that threshold no longer exists. And at speed I don't need to give it as much throttle to maintain speed.

 

[Bewy]

R & D.
Glad to hear about your positive experience. Your experience of better tip-in response is very common & I have cured many off idle hesitations by adding MVA.
You have taken the time to properly set up MVA.....& now are reaping the rewards.

 

[Rat Bastid]

Rat Bastid.
Total nonsense by you.
An engine, any engine, requires a certain amount of timing advance at idle. It doesn't care how it gets it, as long as it gets it. MVA is one method of providing of it. MVA has the twofold advantage of providing more timing at idle plus providing additional timing at cruise.

So an engine that needs say 35 or so at idle is correctly built? That’s nonsense. Something is wrong with it if ANY engine wants that much timing at idle. Simple as that.

Just like beehive springs, this is your one trick pony. MVA is a crutch for a piss poor build.

EDIT: so is locking out a distributor. It’s a crutch. Fix the issues and the engine won’t need timing like that.

 

[Bewy]

Rat,
So now locking the dist is also a crutch in the Rat Bastid world. So what 'issues' would need to be fixed so that a locked dist is not needed??????????????
You have NO idea. No idea how combustion at idle is affected by extended valve timing events. Among other things.
I could sit here all day & provide examples from engine builders with far more knowledge than you who actually understand what is needed & give their engines the idle timing it likes.
As far as 'trick ponies', I have had more than you will EVER have....

 

[RustyRatRod]

Oh my GAWD. Bring a thread back from the dead just to argue some more. Who gives a ****? Just use the vacuum advance like you want to or not at all. I ain't gonna lose any sleep over it.

 

[92b]

Different applications have different priorities. The methods used to provide timing for engines in different applications vary based on those priorities and the comprises that have to be made. In some applications it will make sense to have a locked distributor with no mechanical or vacuum advance. In other applications it will make sense to have a host of sensors and a sophisticated computer to control the timing. There probably won't be one method that is perfect for every application. Your going to have to be more specific about the application if you want to debate what the best method might be. Even then there will likely be more than one option.

 

[Rat Bastid]

Oh my GAWD. Bring a thread back from the dead just to argue some more. Who gives a ****? Just use the vacuum advance like you want to or not at all. I ain't gonna lose any sleep over it.

I only replied (again) because the dude is just dead wrong. I don’t give 2 runny ***** what anyone does with their timing, but claiming that MVA is THE answer to all that ills the internal combustion engine is foolish. That and beehive springs is his answer to everything. It’s silly.

As I said, locking out the timing is the same type of crutch. And I used to lock out everything, always. Then I set aside my childish, preconceived notions and did some real study and I learned how wrong I was.

That’s all I can say. He is wrong and won’t learn. If an engine needs MVA to idle it has other issues. But by god, GM was using MVA back in the day so that’s how you do it. Not in my world or the real world.

 

[Bewy]

No RB, you are the one who is WRONG. While telling everybody that uses MVA that they are fools, you have yet to tell us your magic solution. Not one example from you.
As for beehive springs, the last engine I built which was finished 4 weeks ago did NOT use BH springs for reasons that might be too difficult for you to comprehend.
You have no clue.

 

[Bewy]

And just for the record, Rat Bastid, if could read you would see in posts 36 & 50 that I did NOT say MVA could/should be used in every case. But don't let the truth get in the way of a big LIE.......