Manifold Vacuum Advance what am I not getting?

[340inabbody]

Guys,
I know this subject has been beat to death. So if you don't want to partake in this discussion that's fine I don't blame you. All I ask is that you be respectful as I want to better understand this and would much rather do this in a pub over a bear with a pencil and paper. I want to know if this makes sense or if there is wrong logic etc. I am just trying to learn.
OK so here is my thinking. Manifold vs. Ported vacuum for vacuum advance distributors is engine and application specific. My engine/application is problematic.

The Car:

The engine wants roughly a base 42° of advance at idle with 15Hg of Vacuum. It cannot obviously handle that much without detonation under heavy acceleration/WOT conditions. 34° max is pushing it under load. But for the sake or argument let’s keep it at 34°.

Engine losses like 300 RPM min from P to in gear.

The engine is a high compression 12:1 TRW piston and will assume its roughly 12:1 not assuming anything about the deck or gasket height etc.

The cam is a solid mechanical lifter with a high lift and long duration aggressive design. Unfortunately I don’t have the specs.

The torque converter appears to be a lower flash tight unit.
Power brakes

J heads with solid lifter springs
Currently MSD box and a MSD non-vacuum distributor.

Banter:
So you can see I am trying to make this 4 speed drag/strip motor run in a heavy automatic car with a tight converter on 91 Octane in Arizona.........
I know that if I had money right now to throw at this I could rebuild the motor and lower the compression and put a better cam in it for street use and a higher stall converter but for now I want to see if I can tame (not detonate and not overly de-tune/retard the motor to much) this set up.
I already have slowed the distributor’s curve way down and that has helped a lot.

Given this set up it appears to me that a vacuum advance distributor set up with manifold vacuum would help me.

Here's my rationale:
Again my engine wants a higher base tune at 42° (I also assume at cruise) maybe more and 34° total under acceleration/load and WOT. If my vacuum advance gave me 8° then I would get 42° of advance at idle and 34° under acceleration/load and WOT. The benefit would be a better idle with more vacuum and a more efficient burn. More vacuum for my power brakes as well when I take my foot off the brake and vacuum builds. Ported will not do this since there is no vacuum at idle.

This is my rationale for saying it seems to me that manifold vacuum is a better choice for my set up than ported.

Does this make sense? Is there a reason that ported would be better? Am I missing something?

 

[Carnut68]

Make sure you get an adjustable VA. I would try MV first. I went VA on my brand X car and it helped a bunch, idles and drives much better.

 

[mbaird]

Post your cam specs so that we have an idea of what you consider a big cam .

 

[RustyRatRod]

First thing you need to do is run a compression test and post those numbers.

Vacuum advance cans generally pull in about 20 degrees. So time the thing at 20 initial at the distributor, limit the total mechanical plus initial to about 34. Then plug the vacuum can into manifold vacuum. This will give you about 40 degrees initial at idle and part throttle, but the extra the vacuum can pulls in will fall off on acceleration. As mentioned, get an adjustable vacuum can and turn the adjustment screw all the way clockwise to start with. Stop trying to frikkin understand and just do it. It'll either work for you or not. If it doesn't just use ported vacuum.

 

[340inabbody]

First thing you need to do is run a compression test and post those numbers.

Vacuum advance cans generally pull in about 20 degrees. So time the thing at 20 initial at the distributor, limit the total mechanical plus initial to about 34. Then plug the vacuum can into manifold vacuum. This will give you about 40 degrees initial at idle and part throttle, but the extra the vacuum can pulls in will fall off on acceleration. As mentioned, get an adjustable vacuum can and turn the adjustment screw all the way clockwise to start with. Stop trying to frikkin understand and just do it. It'll either work for you or not. If it doesn't just use ported vacuum.

I am waiting for Ray to send me one of his Distributors. I spoke to him yesterday. When you refer to 20° you mean max vacuum right? I asked for a variable can of course so I can adjust it.
I’m trying to get compression numbers but it’s been 120°F in my shop. And I’m still not feeling up to doing it right now but hopefully soon. then I’ll post them.
You didn’t say anything about manifold versus ported though, and I’m curious to know what you think about using Manifold for my set up.

 

[340inabbody]

I am waiting for Ray to send me one of his Distributors. I spoke to him yesterday. When you refer to 20° you mean max vacuum right? I asked for a variable can of course so I can adjust it.
I’m trying to get compression numbers but it’s been 120°F in my shop. And I’m still not feeling up to doing it right now but hopefully soon. then I’ll post them.
You didn’t say anything about manifold versus ported though, and I’m curious to know what you think about using Manifold for my set up.

Sorry, I just read the last part of your post. You did say try manifold first then ported.

 

[RustyRatRod]

I am waiting for Ray to send me one of his Distributors. I spoke to him yesterday. When you refer to 20° you mean max vacuum right? I asked for a variable can of course so I can adjust it.
I’m trying to get compression numbers but it’s been 120°F in my shop. And I’m still not feeling up to doing it right now but hopefully soon. then I’ll post them.
You didn’t say anything about manifold versus ported though, and I’m curious to know what you think about using Manifold for my set up.

You cannot adjust the amount of advance in the can. Only the "when" you get the advance. I've told you multiple times now. This is combo dependent. ******* TRY IT AND SEE.

 

[340inabbody]

You cannot adjust the amount of advance in the can. Only the "when" you get the advance. I've told you multiple times now. This is combo dependent. ******* TRY IT AND SEE.

Gotcha the adjustment is for the level of vacuum that actuates the unit and it’s a set movement/advance.

 

[RustyRatRod]

Gotcha the adjustment is for the level of vacuum that actuates the unit and it’s a set movement/advance.

Correct. That's why you START with the screw turned all the way clockwise. If it spark knocks then you turn it counter clockwise ( I go one turn at the time) until it stops.

 

[Bewy]

340,
I have been using MVA since the late 70s. I realised the benefits back then. I had Chrys cars until 1995, when I bought a GM car. Chrys did NOT use MVA & missed the bus.....
GM did use MVA until emissions forced the useless PVA on us in 1968. In the 70s, I could not get adj VA units in this country.......& it was before you could get hi torque starters that would crank the engine no matter what the timing was. I was rigging up solenoids on the side of the dist so that engine had Chrysler's [ C ] favourite 12* BTDC initial timing, & once running the sol added more timing. Night & day difference to running.

Your engine.
I am not sure about all C dists, but some I have seen with adj VA units only provide about 22* of VA. In your case, I would get a HEI dist for your engine. You do NOT need an expensive dist, you are only paying for the name. I can buy a Chinese HEI here for $100, comes with polished alum body, all new, well made; I have used heaps of these, never had a problem. The benefit is that the adj VA unit that they come with has about 30* of travel.
I shake my head sometimes....people worried about what what their timing is at 6000 rpm, when they are only there for a second......compared to idle & cruise where the engine spends a LOT of time.

[1] If the engine really is 12:1, then it is going to be tough to use a lot of timing & not get detonation. Nature of the beast.
[2] With a tight c'ter, you will always have a large drop in rpm going into gear with a big cam. Big cams shift tq from the low end to the top end. With that tq gone/lower, the engine struggles with the c'ter load, hence the big rpm drop.
[3] Using the above dist, turn the Allen Key [ AK ] fully CW. Aiming for 40* at idle.
[4] Try 18* init. That means VA adds 22*. If you get det, try 12* + 28*.
[5] To limit VA plunger travel, you have to make stop out of scrap metal; mount the stop under the VA mounting screw nearest the vac pod; measure the length of the plunger movement, it represents about 30*. By proportion, calculate 22* of movement. After mounting the stop, make sure nothing binds. I put a bend in this stop & I can fine tune the reqd movement by flattening out...or increasing the bend.
[6] Centri curve. These dists have too much travel. On the T bar where the rotor mounts, there is a hole inboard on each side. I use a small nut/bolt in these holes to limit the travel of the centri mechanism. Nut goes on top; very accurate positive stop method. You will likely need to file or grind the nut to limit travel. Be careful grinding/filing near the pick up; the magnet will attract ferrous particles which will short out the air gap...& no spark. On the rotor, where the mounting screws are, there is a boss; that sometimes hits the nut. You will need to grind the plastic around screw boss so that the rotor sits square on the t bar without interference.


TUNING.
Ready to fire up & test.
If you get det:
- turn AK 2 turns CCW & check timing. Keep going 2 turns CCW & check timing. When the timing drops, or becomes unsteady, go 3 turns CW.
Done.

 

[CFD244]

Guys,
I know this subject has been beat to death. So if you don't want to partake in this discussion that's fine I don't blame you. All I ask is that you be respectful as I want to better understand this and would much rather do this in a pub over a bear with a pencil and paper. I want to know if this makes sense or if there is wrong logic etc. I am just trying to learn.
OK so here is my thinking. Manifold vs. Ported vacuum for vacuum advance distributors is engine and application specific. My engine/application is problematic.

The Car:

The engine wants roughly a base 42° of advance at idle with 15Hg of Vacuum. It cannot obviously handle that much without detonation under heavy acceleration/WOT conditions. 34° max is pushing it under load. But for the sake or argument let’s keep it at 34°.

Engine losses like 300 RPM min from P to in gear.

The engine is a high compression 12:1 TRW piston and will assume its roughly 12:1 not assuming anything about the deck or gasket height etc.

The cam is a solid mechanical lifter with a high lift and long duration aggressive design. Unfortunately I don’t have the specs.

The torque converter appears to be a lower flash tight unit.
Power brakes

J heads with solid lifter springs
Currently MSD box and a MSD non-vacuum distributor.

Banter:
So you can see I am trying to make this 4 speed drag/strip motor run in a heavy automatic car with a tight converter on 91 Octane in Arizona.........
I know that if I had money right now to throw at this I could rebuild the motor and lower the compression and put a better cam in it for street use and a higher stall converter but for now I want to see if I can tame (not detonate and not overly de-tune/retard the motor to much) this set up.
I already have slowed the distributor’s curve way down and that has helped a lot.

Given this set up it appears to me that a vacuum advance distributor set up with manifold vacuum would help me.

Here's my rationale:
Again my engine wants a higher base tune at 42° (I also assume at cruise) maybe more and 34° total under acceleration/load and WOT. If my vacuum advance gave me 8° then I would get 42° of advance at idle and 34° under acceleration/load and WOT. The benefit would be a better idle with more vacuum and a more efficient burn. More vacuum for my power brakes as well when I take my foot off the brake and vacuum builds. Ported will not do this since there is no vacuum at idle.

This is my rationale for saying it seems to me that manifold vacuum is a better choice for my set up than ported.

Does this make sense? Is there a reason that ported would be better? Am I missing something?

Kudos for trying to learn and understand. I have always found that understanding operating principles paramount in diagnostics.

 

[oldkimmer]

I must be missing something. U say the motor wants the base timing at 42*. So what do u have when the mechanical comes in, then when the va comes in? My guess is around 70-80*. Get yourself a good timing light. And maybe someone that knows how to use it. Are u timing on number one for sure. Maybe the damper has slipped. Or your terminology is wrong. Base timing is just that. Add in your mech at 15-20 then your va at 15-20 so your around 80*. Can’t work no way no how. Kim

 

[RustyRatRod]

I don't mind responding and helping folks, but when they ask the same questions over and over on several different threads, it gets confusing and it's just mind boggling. Keep it confined to one thread and it'll be easier to find the information and easier for people who are trying to help YOU keep up with the subject. I've done my best to help you on other threads you've posted, but it seems like when "you can't get it", you just post another thread. I try to help lots of folks on here and I, nor anyone else is getting paid squat, so when people make it more difficult, I just move on.

 

[RustyRatRod]

I must be missing something. U say the motor wants the base timing at 42*. So what do u have when the mechanical comes in, then when the va comes in? My guess is around 70-80*. Get yourself a good timing light. And maybe someone that knows how to use it. Are u timing on number one for sure. Maybe the damper has slipped. Or your terminology is wrong. Base timing is just that. Add in your mech at 15-20 then your va at 15-20 so your around 80*. Can’t work no way no how. Kim

Yeah that's no joke. One problem is, lots of these younger guys SLAP REFUSE to buy what we used to call "Bibles". The Mopar Engine and suspension books. ......and how to super tune timing is spelled out so a third grader could understand it and it's RIGHT THERE in black and white so you can refer to it at any time you want. .....although I don't believe they discuss using manifold vacuum on the vacuum advance, it's not that tough to figure out from what they do discuss. They do discuss how it's not uncommon to have advance as high as sixty degrees at part throttle just cruising down the road. They also discuss how to tune the advance taping a vacuum gauge to the windshield. But then you have guys say "I don't have a vacuum gauge" and they're not interested in getting one. So how can THEY be helped? Sometimes the only way you can really understand something is just go open the hood and do it.

 

[RustyRatRod]

FWIW, I have my hei problems sorted now, set the initial to 10 btdc with no vac adv. I connected it to manifold vac this morning. Restarting was immediate and improved, dropped the idle back down to where it was without mva, no stall putting it in gear. Combo is low compression, mild cam 318. The adv can I have installed on the reman distributor is dual port and not adjustable. If it is too much on the test drive, I will just swap to an adjustable advance to tune it. Good enough for me.

Manifold vacuum can work for some combos, but I always say be prepared to swap it back to ported, because it doesn't work for everything. Very simple to swap back and forth.

 

[RustyRatRod]

Oh and one thing I've noticed while messin around with mine is worth sharing. These vacuum caps that these parts stores sell now don't last ANY time before they crack, split and LEAK, so keep an eye out for that! I don't know where a good source for vacuum caps is. What a stupid problem to have!

 

[Mopar Sam]

This is NOT my picture, but I saved it from another thread on here a while back. Easy way to limit the amount of vac advance. I plan on doing this to mine soon.

 

[Newbomb Turk]

Manifold vacuum can work for some combos, but I always say be prepared to swap it back to ported, because it doesn't work for everything. Very simple to swap back and forth.

I agree. Using MV is a crutch for a piss poor build.

That’s what this guy has. A piss poor build. If it wasn’t, there is no way in HELL it would need or want that much timing at idle.

I don’t know what heads the OP has but if they are P car heads he ain’t nowhere near 12:1. It might be 11 but again, who knows.

He can do a cranking compression test but all he will know is he didn’t have enough so it’s jack a bunch of timing it it to make it move.

I’ll say it publicly and I don’t care what people think

If you build engines AND your **** constantly needs outrageous initial timing YOU ******* SUCK AS AN ENGINE BUILDER and you need to shag your sorry *** to the nearest fast food joint and see if they will let you work the drive through.

It is beyond incompetent to build **** like that.

It’s evident that the OP has way too much cam and not nearly enough compression.

So my advice to the OP is two fold.

1. Fix the thing correctly. That means swapping out the can to what the engine needs.

Then fix your tuneup and it won’t require MV to idle.

2. Hook it to MV and live with it.

2 is cheaper but it ain’t the right way to do it.

And for those about to come along and tell me you’ve been doing MVA since Moses was fighting the Egyptians then refer to point 1 above. Be a that is YOU.

 

[340inabbody]

Getting back to basics here. Lots of emotional feelings about my post. Is this quick graph I drew wrong?
A. Idle to tip-in
B. Acceleration/load
C. Vacuum Advance comes in at tip-out
D. Cruise

 

[Newbomb Turk]

Getting back to basics here. Lots of emotional feelings about my post. Is this quick graph I drew wrong?
A. Idle to tip-in
B. Acceleration/load
C. Vacuum Advance comes in at tip-out
D. Cruise

View attachment 1716281838

Right. You end up with an inverted curve.

I just thought of the most simple fix that will work.

Get yourself a Progression distributor and use that.

IIRC, you can put any digital curve in it you want.

That’s what I would do in your case. You can change the tune up with your phone.

 

[CFD244]

The way I read this.......

22* Base timing
12* Mechanical timing (restricted to)
20* Manifold Vacuum

Cranks at 22* and allows to idle at his preferred 42*
Step on it and the timing drops back as the vac is lost hopefully no ping (or that's the idea)
As rpm goes up, timing advances to 34*
Lay off the accelerator and the 20* MV comes back to cruise.

Of course, curve rate would have to be tailored for the application.

Just my 2 cents.

 

[AJ/FormS]

Just so you know,
I built a 367 @11.3 and 8.8Dcr pumping over 180 psi; and it idled at 5*Advance. This was with the Mopar 292/292/108 cam; and with OOTB Edelbrock aluminum heads. I ran it on Ported vacuum. But I modded the can by filing the stops until the diaphragm bottomed inside the can, and got 22 degrees. Using the same trick, Trailbeast got 24* . Any Mopar VA can be so modded. Not every combo will like 22 degrees. Well, any VA in my collection.
This engine was for a Manual-trans car, and at 5*, it had just enough power at 550 rpm, to pull itself around the parking lot with 3.55s.
It's "normal" timing was 28* at 2800, going to 34* at 3400 This gave me 12>14 at idle. I ran this, exclusively, on 87E10.
Initially,. with a regular 4-speed, it liked 56* at 65mph, but the more timing I ran, the leaner I was able to cruise at.
Just saying, don't shoot the messenger.

BTW,
The 292 cam was real hard on gas cruising 65=2900. I slowed down to 55=2470; but it was not much better. I pulled it out and sold it to a racer friend.
After running two other cams in this engine, I figured out why it was hard on gas.
1) In at 104, it only had 102* of power extraction, so there was a ton of energy still in the exhaust, which could have gone into the flyeheel, and
2) this cam had 76 degrees of overlap, and actually didn't much like running at 2470. It liked 2900 just fine, but now the rpm was burning up the gas.
If you know about overlap, you know that the primary purpose of running headers is to pull on the intake in the overlap cycle, to get the Fuel charge moving towards the chamber, before the piston begins it's downward march with the Exhaust-Valve closed. Well at low rpm with so much overlap, the Headers were doing a bang-up job, pulling fuel charge straight across the piston and out into the pipes. Fat lot of good that did for me, Like I said, I pulled that cam, towards the close of it's first summer.
Thinking I had learned a good lesson, my next cam went the other way; 270/276/110
In at 109, this cam had 111* of power extraction. The Dcr went up to over 9.2, the pressure to over 195psi, and this thing ran like the proverbial scalded cat.
The overlap came down to 53*(from 76), and End of reversion moved down below cruising speed.
I hung a Mopar A833overdrive on the back, and a GV overdrive behind that, and geared it for 65=1600/85=2100 And fuel mileage jumped to over 30mpgs. 'Course it required a new timing curve, which for me is not that big a deal, cuz I have a dash-mounted, electronic, dial-back timing module with a range of 15 degrees.
This was my most awesome combo. I split the overdrive gears, with the GVod, and got SEVEN useable speeds.
BTW-2
At idle, the engine only needs enough timing so it doesn't stall, and doesn't bang the trans when you put it into gear. The first time the engine really cares about timing is from Stall-rpm to ~3600, at WOT. Everything else is optional. and there's no perfect numbers until you go with a computer.
The more Idle-timing you try to run, the greater will be the bang/rpm drop going into gear, and the further away will go the Transfer slot Sync, which usually creates a Tip-in Sag/hesitation, which when severe, ends in a bog or stall..

Long post I know, but that's the way I roll.
The point is, 12.5 Scr with iron J-heads, is probably not gonna fly, no matter what timing you throw at it. I see a compression ratio downgrade in your future

 

[RustyRatRod]

I agree. Using MV is a crutch for a piss poor build.

Not always. But probably most times.

 

[Bewy]

Nope, MVA is not a crutch for a poor build. MVA is but one method of giving the engine the timing it NEEDS.

OP has found that HIS engine has best/smoothest idle with 42*.
THAT IS NOT BECAUSE IT WAS BUILT POORLY OR THE ENGINE BUILDER DIDN'T KNOW WHAT HE WAS DOING. IT WANTED THAT TIMING BECAUSE IT IS MOST EFFICIENT WITH THAT MUCH TIMING.

What about racers who lock their dist? They are giving the engine the timing it NEEDS, does the same as MVA. Are they ALL 'piss poor' engine builders...... Yes those ones, winning races with locked timing????? Of course not.

People who say or think otherwise do not understand the dynamics of combustion.

Stock LS engines have about 27-28* @ WOT. They have a very efficient chamber that does not need a lot of timing. In stock form, they STILL idle at 22*. Not 5* or 0*.
Because that is where the most efficient use of the fuel is made.
Extrapolate the #s for our old bangers that need 36-38* @ WOT & SUDDENLY 30* at idle for stock or near stock doesn't sound so bad....
Engine might need 45* at cruise. So what's the difference between idle timing & cruise timing ? Uhhhh, nothing.......
In both instances, the air is throttled. The air is less dense & needs more time to burn, hence more timing needed.

 

[Newbomb Turk]

Nope, MVA is not a crutch for a poor build. MVA is but one method of giving the engine the timing it NEEDS.

OP has found that HIS engine has best/smoothest idle with 42*.
THAT IS NOT BECAUSE IT WAS BUILT POORLY OR THE ENGINE BUILDER DIDN'T KNOW WHAT HE WAS DOING. IT WANTED THAT TIMING BECAUSE IT IS MOST EFFICIENT WITH THAT MUCH TIMING.

What about racers who lock their dist? They are giving the engine the timing it NEEDS, does the same as MVA. Are they ALL 'piss poor' engine builders...... Yes those ones, winning races with locked timing????? Of course not.

People who say or think otherwise do not understand the dynamics of combustion.

Stock LS engines have about 27-28* @ WOT. They have a very efficient chamber that does not need a lot of timing. In stock form, they STILL idle at 22*. Not 5* or 0*.
Because that is where the most efficient use of the fuel is made.
Extrapolate the #s for our old bangers that need 36-38* @ WOT & SUDDENLY 30* at idle for stock or near stock doesn't sound so bad....
Engine might need 45* at cruise. So what's the difference between idle timing & cruise timing ? Uhhhh, nothing.......
In both instances, the air is throttled. The air is less dense & needs more time to burn, hence more timing needed.

I’d answer this but it would be redundant.

If you have to resort to racers locking out their timing and that the engine “needs” it says it all.

You think that a build that requires MVA will make more power and drive better than if the engine didn’t need MVA?

This is the EXACT scenario I always bring up. Nowhere near enough compression and a monster cam.

It’s a pig.