Manifold Vacuum Advance what am I not getting?

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I don't understand why this is such a controversial topic.Oddly I just went through this with my brother. I thought we were going to have fight like when we were kids.

I'm in the camp of manifold vacuum for advance is a useful crutch to help something with too much cam and not enough compression idle.

But I'm no expert.
 
Hank,
MVA is no more of a useful 'crutch' than ceramic coating your headers, using a phenolic spacer under the carb or making sure your brakes are not dragging. They all provide a benefit that was not there before.....
 
Mbaird,
I can guarantee that your engine will idle better & have more vacuum with more timing with that cam & 360 cubes. My guess is 35-45* at idle will be optimum. You are going to this by electronic means? It needs to reduce timing as the engine comes under load or you could get detonation. That is why the simplicity of a vac adv unit is so good: it does this automatically.
 
CFD244,
Replying to your question in post #142.
Centri curve #s. Amounts are in dist degrees, so double for crank deg.

Start: 0-2* @ 500 rpm
Intermediate: 10.5-12.5* @ 1000
Max: 13-15* @ 2200
 
Bewy said:
CFD244,
Replying to your question in post #142.
Centri curve #s. Amounts are in dist degrees, so double for crank deg.

Start: 0-2* @ 500 rpm
Intermediate: 10.5-12.5* @ 1000
Max: 13-15* @ 2200
Click to expand...
Dist RPM or crank RPM?
 
Bewy said:
Mbaird,
I can guarantee that your engine will idle better & have more vacuum with more timing with that cam & 360 cubes. My guess is 35-45* at idle will be optimum. You are going to this by electronic means? It needs to reduce timing as the engine comes under load or you could get detonation. That is why the simplicity of a vac adv unit is so good: it does this automatically.
Click to expand...
I use vacuum advance hooked to manifold port. Actually my carb has no VA port . Why I don’t know since I told Patrick (Pro Systems ) this would be mainly a street car. So I plumbed one into the back of the plenum on the Victor .
The mechanical advance is digitally controlled.
The engine has 9” of vacuum at idle and I adjusted my VA to start at 10-11”s.
 
Bewy said:
CFD244,
Replying to your question in post #142.
Centri curve #s. Amounts are in dist degrees, so double for crank deg.

Start: 0-2* @ 500 rpm
Intermediate: 10.5-12.5* @ 1000
Max: 13-15* @ 2200
Click to expand...
Some motors won't like the advance curve to start below or at idle speed. They will hold a little advance when first returned to idle from a higher rpm causing the idle speed to be high. The unstable motor then slows slightly causing some centrifugal timing to drop out which in turn causes the motor to dip in rpm causing the motor to drop a little more centrifugal timing out causing the motor to drop in rpm.... Sometimes to the point of the engine dying. if you rev the motor again the whole cycle starts over. This can also be triggered by an ac clutch kicking in or an auto car when placed in gear. A motor with a good strong idle doesn't usually have this problem but some do. Sometimes it's best to start the mechanical/centrifugal advance above idle speed.
This same phenomenon can be complicated by manifold vacuum with engines with low vacuum and a vacuum canister that has a spring that is too stiff. If the vac can is not fully applied at idle and the unstable idle dips a little in rpm the vac will drop causing the timing to drop a little causing the rpm to drop a little causing the vac to drop a little causing the timing to drop a little causing the rpm to drop a little causing the vac to drop a little.... Some vac advance cans have an adjustment for the preload on the spring in the canister. These cans usually have a hex on the front by the vac port. It is adjusted with an allen wrench through the vacuum port.
I'm not saying you shouldn't use an advance curve that starts at or below idle speed or that you shouldn't use manifold vacuum on the advance can. Use what ever makes the most sense to you for your application. I'm just making you aware of what you may run into. Manifold, ported, centrifugal and programmable electronic are all just a means to an end not right or wrong. In some applications simplicity reigns as king. If that means welding the timing in a locked position then so be it. I admire people who are willing to put the time and effort into tuning their motors.

Step 1. Verify top dead center.

Edit: to make this post longer to guarantee no one will read it.
 
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Unless it is a stock engine, MVA shoulds ALWAYS use an adjustable VA unit. There are adj units [ Crane, Accel, Mr. Gasket ] that have a spacer inside the spring. Have only seen these on GM units, it is a most stupid idea.......because you can lock the thing solid when adjusting the Allen Key. With these, I prise open the can & remove the spacer.....
Here in Oz, it is almost as cheap to buy a Chinese dist [ comes with adj VA unit ] than it is to buy the VA unit on it's own.
 
Bewy said:
Hank,
MVA is no more of a useful 'crutch' than ceramic coating your headers, using a phenolic spacer under the carb or making sure your brakes are not dragging. They all provide a benefit that was not there before.....
Click to expand...
I will happily capitulate. You clearly have more knowledge on the subject than I do.
But this is the case in which I personally have used manifold vacuum for the advance as a benefit.
On the dirt track cars I played with the "wisdom" was to lock the distributor solid.
Personally I thought this was dumb and left power and throttle response on the table but I was young and the old heads weren't listening to a punk fresh out of trade school.
So they stayed locked.
 
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HankRearden said:
I will happily capitalize. You clearly have more knowledge on the subject than I do.
But this is the case in which I personally have used manifold vacuum for the advance as a benefit.
On the dirt track cars I played with the "wisdom" was to lock the distributor solid.
Personally I thought this was dumb and left power and throttle response on the table but I was young and the old heads weren't listening to a punk fresh out of trade school.
So they stayed locked.
Click to expand...
They shouldda listened.
 
RustyRatRod said:
They shouldda listened.
Click to expand...
Yeah. I believe that is the conventional wisdom now.
I don't suppose I would respond well to a wet behind the ears teenager telling me I'm doing it wrong at thos stage of my life. So I don't blame them.
But when everyone else was boozing and getting laid In high school I was reading everything automotive theory related that I could find. I was obsessed. I had big ambitions.
Now I'm in my late 40's and can't remember much of it. Knowledge and skills are the same use em or lose em.
 
4spdragtop said:
Sorry but I gotta ask, why are we talking distributor RPM?
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Advance numbers posted are in dist degrees (double it for crank degrees). Since those were posted in dist degrees, I thought maybe they were readings from a distributor machine RPM scale. :)
 
I was wondering why dist degrees, i figgered a dizzy machine was involved lol.
CFD244 said:
Advance numbers posted are in dist degrees (double it for crank degrees). Since those were posted in dist degrees, I thought maybe they were readings from a distributor machine RPM scale. :)
Click to expand...
 
The cam gear is twice the size of the crank gear so the cam travels at 1/2 the speed of the crank . 1 cam revolution performs the intake function the 2nd performs the exhaust .

#1 = Suck and squeeze
#2 = Bang and Blow

I still havent decided which cycle is my favorite ! Lol
 
mbaird said:
The cam gear is twice the size of the crank gear so the cam travels at 1/2 the speed of the crank . 1 cam revolution performs the intake function the 2nd performs the exhaust .

#1 = Suck and squeeze
#2 = Bang and Blow

I still havent decided which cycle is my favorite ! Lol
Click to expand...
There's 4 cycles. lol
 
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