Vacuum advance information and myths

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I would say that a mostly stock street able engines with plenty of vacuum at idle would benefit from MV and any combination of parts that creates vacuum issues, single plane, big cam ect will need to be set up custom to what makes them run their best as all the different parts affect the engine differently and at the modified engine point there is no ONE WAY to tune engines. Adj and accommodate as needed for best results.
 
Mattax said:
Not if the distributor's advance curve is set up right.
Get a strong idle at 500 rpm with the non-CAP set up as mentioned earlier.
Click to expand...
I see what your saying. Sort of. You are saying the correct distributor accommodates the timing mechanically at idle and does not benefit from MV? Some of the old mopars only have MV ports on the carbs. I still think tuning a performance street engine you tune the mech adv to the most power and response then run MVA and set the vacuum to below ping under load. This has always worked best for me. But I will experiment when it warms up since I can taylor both mech and V adv curves easily.
 
Dmopower said:
You are saying the correct distributor accommodates the timing mechanically at idle and does not benefit from MV?
Click to expand...
Basically yes.
The initial timing of a pre-emisions or non-CAP engine provides good power, and idles smooth and strong.
The same engine with a CAP package used much less initial advance. The mechanical advance in the distributor makes up for that above idle rpm.

Pick an engine in the mid-60s that had both a non-emissions and an emissions reduction package on it and compare the inital timing and rpm. Then look at the mechanical advance of in the distributor of each. I've posted a number of these in graph form. Here's one

upload_2020-12-12_18-37-53-png.png


Even more extreme change in initial was the CAP vs nonCAP 273 4 bbl here.

Dmopower said:
Some of the old mopars only have MV ports on the carbs.
Click to expand...
Love to see them.
Last time we thought we found one it turned out to be ported. That was a 1966.
 
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Mattax said:
Basically yes.
The initial timing of a pre-emisions or non-CAP engine provides good power, and idles smooth and strong.
The same engine with a CAP package used much less initial advance. The mechanical advance in the distributor makes up for that above idle rpm.

Pick an engine in the mid-60s that had both a non-emissions and an emissions reduction package on it and compare the inital timing and rpm. Then look at the mechanical advance of in the distributor of each. I've posted a number of these in graph form. Here's one

View attachment 1716030499

CAP vs nonCAP 273 4 bbl here.


Love to see them.
Last time we thought we found one it turned out to be ported. That was a 1966.
Click to expand...
I'll take a pic in a few and post it. The tag fell off years ago but I'll get pics of the numbers it has. I don't have the car anymore but still have the carb and converted it to elect choke. It was on my 66 300 440 tnt, dual point dist. Might have dist also but sold my dist machine 10 years ago so I won't be able to compare actual mech curves like I would like.
 
Clearly, there are two ways to run vacuum advance. The manufacturers ran MVA on some vehicles. The Big Block Chevelles were one example. All that's necessary is to adjust initial timing accordingly to whichever vacuum you are using. Whose to say one way will work better across the board for everything? How dumb does that sound. I know from my personal experience ported has worked best for me. Even with my high compression, big cam slant six, ported works better and yes I did try manifold vacuum. Ported is a much smoother transition and that's where I'm keeping it.
 
Dmopower said:
I see what your saying. Sort of. You are saying the correct distributor accommodates the timing mechanically at idle and does not benefit from MV? Some of the old mopars only have MV ports on the carbs. I still think tuning a performance street engine you tune the mech adv to the most power and response then run MVA and set the vacuum to below ping under load. This has always worked best for me. But I will experiment when it warms up since I can taylor both mech and V adv curves easily.
Click to expand...

How much timing at idle do you have? With MV hooked up.
 
Mattax said:
Basically yes.
The initial timing of a pre-emisions or non-CAP engine provides good power, and idles smooth and strong.
The same engine with a CAP package used much less initial advance. The mechanical advance in the distributor makes up for that above idle rpm.

Pick an engine in the mid-60s that had both a non-emissions and an emissions reduction package on it and compare the inital timing and rpm. Then look at the mechanical advance of in the distributor of each. I've posted a number of these in graph form. Here's one

View attachment 1716030499

Even more extreme change in initial was the CAP vs nonCAP 273 4 bbl here.


Love to see them.
Last time we thought we found one it turned out to be ported. That was a 1966.
Click to expand...
I stand corrected. As you said it is ported not manifold. 4640 sa.
 
Mattax. Post #57. I am well aware that Chrys was not the only one using MVA as I described earlier. The major auto makers all used it. I specifically mentioned Chrys because there are some who think Chrys never used MVA.
 
A lot of BS creeping in here. Post #58 claiming the switch to PVA had nothing to do with emissions is BS. It had everything to do with emissions. GM, who used MVA, could not meet the stricter HC emissions standards coming into effect in the late 60s & had to switch to PVA. Switching to PVA retarded idle timing.
From the Holley book: ' the retarded spark helps to reduce HC emissions by raising exh temps....'...&....'The use of retarded spark requires richer jetting in the idle & main systems to get off idle performance & drivability'....& ...'Because retarding the spark hurts efficiency, the throttle plates must be opened further at idle to get enough mixture in for the engine to continue running.'
 
RB,
Post #56. Using MVA at idle is one way to give the engine the idle timing it WANTS. There are other ways such as locked timing. The problem with locked timing is that the timing.....is locked. Nowhere to go if you get detonation. MVA is load sensitive & can be adjusted to keep out of detonation. That is the beauty of it & why smart people use it. And, no, you don't just plug the VA into a man vac port & walk away. This is where the tuning begins & forget if you are NOT using an adj vac adv unit. Idle timing is set up first, then the centri/dist curve. Not the other way around.
 
re: post #85
Go and look at actual examples and documents to determine the facts just like the other gentleman did.
The change in initial timing to reduce CO and HC to meet California and then Federal targets was not accomplished by changing the vacuum advance source. That's a myth.
 
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re. post 85
"GM, who used MVA, could not meet the stricter HC emissions standards coming into effect in the late 60s & had to switch to PVA."

So its been claimed.
But evidence shows that Chevy had used a timed port before needing to meet 1968 emissions requirements.


l78_9_4cb76915662fccb81e408f6ad568bf5e10224fb9-jpg.jpg

^This is a 1965 Cheverolet L-78 with a 4150 list 3124.^

Next
-an-original-holley-2818-choke-housing-2818rh3-jpg.jpg

^This appears to be an original 4150 Holley List 2818^

As shown in this diagram.
b-engine-ign-05-vacuum-advance-lines-gif.gif
 
Bewy said:
RB,
Post #56. Using MVA at idle is one way to give the engine the idle timing it WANTS. There are other ways such as locked timing. The problem with locked timing is that the timing.....is locked. Nowhere to go if you get detonation. MVA is load sensitive & can be adjusted to keep out of detonation. That is the beauty of it & why smart people use it. And, no, you don't just plug the VA into a man vac port & walk away. This is where the tuning begins & forget if you are NOT using an adj vac adv unit. Idle timing is set up first, then the centri/dist curve. Not the other way around.
Click to expand...

I say smart people build engines that don’t need 30, 40, 50 degrees of timing at idle to run.
 
Bewy said:
RB,
Post #56. Using MVA at idle is one way to give the engine the idle timing it WANTS. There are other ways such as locked timing. The problem with locked timing is that the timing.....is locked. Nowhere to go if you get detonation. MVA is load sensitive & can be adjusted to keep out of detonation. That is the beauty of it & why smart people use it. And, no, you don't just plug the VA into a man vac port & walk away. This is where the tuning begins & forget if you are NOT using an adj vac adv unit. Idle timing is set up first, then the centri/dist curve. Not the other way around.
Click to expand...
"The problem with locked timing is that the timing is locked and nowhere to go" but to detonate and no compensation. This is where i'm at. If you tune it the way you described instead of adding timing under acceleration and load where fuel quality decides if it pings you have the vacuum advance which actually reduces timing and eliminating the chance of ping as the timing goes away under load. My cars have never stumbled using this method. AND they run cooler this way. I also get beter mileage than the other guys IF they can keep their hot rods running.
 
Dmopower said:
"The problem with locked timing is that the timing is locked and nowhere to go" but to detonate and no compensation. This is where i'm at. If you tune it the way you described instead of adding timing under acceleration and load where fuel quality decides if it pings you have the vacuum advance which actually reduces timing and eliminating the chance of ping as the timing goes away under load. My cars have never stumbled using this method. AND they run cooler this way. I also get beter mileage than the other guys IF they can keep their hot rods running.
Click to expand...

I asked before but how much idle timing with MVVA are you running?
 
Mattax said:
re: post #85
Go and look at actual examples and documents to determine the facts just like the other gentleman did.
The change in initial timing to reduce CO and HC to meet California and then Federal targets was not accomplished by changing the vacuum advance source. That's a myth.
Click to expand...
:thumbsup:
 
Mattax.

Post #88, fact not myth. Do not know why the L-78 used PVA, but millions & millions of GM cars used MVA. The changeover year was 1967-68.
My GTO was driven off the showroom floor, idling at 26*. 6* initial + 20* MVA. By 1968, the idle timing was 6*; the vac advance was now ported.
Same engine, same cam, same comp ratio. Going to PVA was a backwards step because it reduced mileage & added no advantage to the overall running of the car. Why would GM do that unless there was some other reason?
The switch to PVA was for emissions, as I detailed in post #86.
 
RB,
Post #90.

Smart people give their engine the idle timing it wants, which can be as much as 50*.
You only have to read this forum & others where people have advanced their idle timing & found the engine idled higher, smoother etc. It idled higher because the engine was making more HP from the extra timing; the smoother idle should be self explanatory....
I have just given you the production specs for my GTO in post #94, for a production engine, 10.75: 1 CR, intake cam duration under 200* @ 050.
Engines with longer duration cams produce less compression pressure at idle & have more exh gas dilution. The lower compression takes longer to burn & the exh gas dilution also slows the burn rate. Both require more ign timing at idle. Not rocket science.
The 26* of factory idle timing that my GTO had is a conservative number, as manufacturers have to cater for the worst case operating conditions.
 
Bewy said:
RB,
Post #90.

Smart people give their engine the idle timing it wants, which can be as much as 50*.
You only have to read this forum & others where people have advanced their idle timing & found the engine idled higher, smoother etc. It idled higher because the engine was making more HP from the extra timing; the smoother idle should be self explanatory....
I have just given you the production specs for my GTO in post #94, for a production engine, 10.75: 1 CR, intake cam duration under 200* @ 050.
Engines with longer duration cams produce less compression pressure at idle & have more exh gas dilution. The lower compression takes longer to burn & the exh gas dilution also slows the burn rate. Both require more ign timing at idle. Not rocket science.
The 26* of factory idle timing that my GTO had is a conservative number, as manufacturers have to cater for the worst case operating conditions.
Click to expand...

Smart people don’t build engines that NEED that much initial timing.


If an engine NEEDS 50* of timing at idle (and that’s a big IF) the engine is garbage.

The cam is too long and the compression ratio is way too low for that cam timing.

Thats a CRUTCH. Not the proper way to build an engine.
 
So what is your formula for cam timing/compression ratio so that we can all build that magical engine that needs.....how much idle timing?
 
Bewy said:
So what is your formula for cam timing/compression ratio so that we can all build that magical engine that needs.....how much idle timing?
Click to expand...

No magic to it. It’s called MATH.

Give it a whirl sometime.
 
Post #88 & the reason for the change from MVA to PVA.
It was for emission purposes, as I stated in a prior post.
You will find the info HERE: www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
You may have to type in the full address. I believe the article was written by GM engineer Lars Grimsrud.
Here is the relevant comment: ' Now to the widely misunderstood manifold v ported vacuum aberration. After 30-40 yrs of controlling vacuum advance with full manifold vacuum, along came emissions requirements.........One of these band aids was "ported spark". This meant the vac adv was inoperative at idle [ retarding spark timing from its optimum value ]....
 
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