Vacuum advance

[dartfreak75]

Im getting ready to start working on my distributor and get my mechanical advance limiter setup. Right now It looks like I need about 14 degrees of mechanical advance to get to a total of 34* i have a ton of questions but im gonna start with this one. I have read and watched alot videos about manifold vs ported vacuum, the book i have (fbo tuning tricks) says to use manifold this is where im lost. So if im using a manifold vacuum im getting about 18 uom at idle let's say I have 15* in the can and im at 15 initial that's 30 degrees at idle. Why not just lock the dissy at 34* and roll with it? Then on the other hand if you use ported the vacuum don't come in until partial throttle. So if you have a total of 34 plus 15 in the can your up to 50* at cruising speed isn't that asking for detonation?

 

[RustyRatRod]

Just use ported. You have a very mild engine and it will benefit greatly from ported vacuum on the advance can. It's a street car, remember?

 

[dartfreak75]

Just use ported. You have a very mild engine and it will benefit greatly from ported vacuum on the advance can. It's a street car, remember?

So do I still with my original timing tune? Right now I have it at 20* initial and im shooting for 34-36 total with my fbo plate. At 36 total plus the can im gonna be in the 50s

 

[dartfreak75]

I understand how it works i guess I just don't understand why it's needed. When you hammer down on the throttle vacuum drops and the advance drops as well, so it's gonna drop from 50* back down to the 36* total what I don't understand is why do you need that extra advance under low load? Is it a fuel economy "full burn" thing?

 

[RustyRatRod]

So do I still with my original timing tune? Right now I have it at 20* initial and im shooting for 34-36 total with my fbo plate. At 36 total plus the can im gonna be in the 50s

Yes. That's correct. It will only have that much at part throttle, which is as designed. Once you step on the gas, vacuum drops out and you have no advance from the can.

 

[RustyRatRod]

I understand how it works i guess I just don't understand why it's needed. When you hammer down on the throttle vacuum drops and the advance drops as well, so it's gonna drop from 50* back down to the 36* total what I don't understand is why do you need that extra advance under low load? Is it a fuel economy "full burn" thing?

Yup, it cleans up part throttle combustion to help keep the plugs from fouling, helps fuel economy a good bit. That's why I always recommend running the advance if it's a street car. The vacuum signal on mine is 6 at idle and only 14 at cruise, but I still run the advance can.

 

[dartfreak75]

Thanks Rob much appreciated! Hopefully get it all setup and start dialing it in this weekend.

 

[RustyRatRod]

The MP engine book talks about timing as high as 60 degrees or a bit more. Perfectly normal at cruise with the vacuum advance on a ported inlet.

 

[RustyRatRod]

Thanks Rob much appreciated! Hopefully get it all setup and start dialing it in this weekend.

Sho nuff.

 

[dartfreak75]

Next question. Is the vacuum canister adjustable for the amount of advance? I know that its adjustable for when the vacuum kicks in but does that also adjust the amount? Like if I bring it in at a higher vacuum does that decrease the amount of advance?

 

[Mattax]

the book i have (fbo tuning tricks) says to use manifold this is where im lost

The simple version is that he's wrong, in general. The more accurate version is that when doing it everything his way it can work out OK.

So if im using a manifold vacuum im getting about 18 uom at idle let's say I have 15* in the can and im at 15 initial that's 30 degrees at idle. Why not just lock the dissy at 34* and roll with it?

So here's why it can work. Lets say you have a distributor with a long advance curve, one that the factory intended for 5*BTDC at 750 rpm.
You dont care about emissions anymore and want pre-smog initial timing without reworking the slots and springs. One way to do that is connect the vacuum advance to manifold vacuum. That can be a way to get the intial up to 10 or 15* BTDC and will be reasonably stable if the manifold vacuum at idle is strong and steady.

Then on the other hand if you use ported the vacuum don't come in until partial throttle. So if you have a total of 34 plus 15 in the can your up to 50* at cruising speed isn't that asking for detonation?

RRR answered this.
Chrysler illustrated his answer this way.

So do I still with my original timing tune? Right now I have it at 20* initial and im shooting for 34-36 total with my fbo plate. At 36 total plus the can im gonna be in the 50s

You need to plot out the timing curve. I'd prefer to see you do that without the FBO plate.
I mentioned plotting out the curve in your previously here Too much timing?

Let me borrow from a post I made yesterday to illustrate why you need to measuring timing every 200 or 250 rpm or so.

Circles represent the timing we measured for a 1967 440 with the correct distributor set at 12.5* BTC @ 650 rpm.

That distributor is going to work fine on that engine when the vacuum advance is connected.

But if we accidentally had been given the distributor intended for the '67 440 with Clean Air Package, and set it to 12.5* at 650 rpm, we'ld get a plot like this.

If we limited the advance to 38* BTDC, it would be OK at the drag strip.
It may or may not be OK on a long trip with this much timing.
When vacuum advance is connected this engine will be very unhappy on the road.

Knowing how the timing between 1400 and 3000 rpm advances is critical in deciding how to modify the distributor.
The FBO plate is not helpful for street performance when dealing with a situation as shown above. (It would be OK for drag strip as long as the vacuum advance was disconnected). In other situation the FBO plate can work.
Take the timing readings from as slow as you can get the engine to run, and then in regular increments of increasing rpm.

 

[Mattax]

Next question. Is the vacuum canister adjustable for the amount of advance?

It can be modified to limit the maximum advance added. There are two methods illustrated in Part 1a of the article linked below.

I know that its adjustable for when the vacuum kicks in but does that also adjust the amount? Like if I bring it in at a higher vacuum does that decrease the amount of advance?

Basically adjusting when the vacuum moves the advance does not effect maximum.
See part 1b of the same article

How To Limit and Adjust Chrysler Vacuum Advance Cans

 

[RustyRatRod]

Next question. Is the vacuum canister adjustable for the amount of advance? I know that its adjustable for when the vacuum kicks in but does that also adjust the amount? Like if I bring it in at a higher vacuum does that decrease the amount of advance?

If the vacuum can has a hex on it where the nipple for the hose is, yes, it's adjustable. And no, it's not adjustable for the amount of advance. It's adjustable for the "when" the advance comes in.

 

[dartfreak75]

The simple version is that he's wrong, in general. The more accurate version is that when doing it everything his way it can work out OK.

So here's why it can work. Lets say you have a distributor with a long advance curve, one that the factory intended for 5*BTDC at 750 rpm.
You dont care about emissions anymore and want pre-smog initial timing without reworking the slots and springs. One way to do that is connect the vacuum advance to manifold vacuum. That can be a way to get the intial up to 10 or 15* BTDC and will be reasonably stable if the manifold vacuum at idle is strong and steady.


RRR answered this.
Chrysler illustrated his answer this way.
View attachment 1715759140


You need to plot out the timing curve. I'd prefer to see you do that without the FBO plate.
I mentioned plotting out the curve in your previously here Too much timing?

Let me borrow from a post I made yesterday to illustrate why you need to measuring timing every 200 or 250 rpm or so.

Circles represent the timing we measured for a 1967 440 with the correct distributor set at 12.5* BTC @ 650 rpm.
View attachment 1715759141
That distributor is going to work fine on that engine when the vacuum advance is connected.

But if we accidentally had been given the distributor intended for the '67 440 with Clean Air Package, and set it to 12.5* at 650 rpm, we'ld get a plot like this.
View attachment 1715759142

If we limited the advance to 38* BTDC, it would be OK at the drag strip.
It may or may not be OK on a long trip with this much timing.
When vacuum advance is connected this engine will be very unhappy on the road.

Knowing how the timing between 1400 and 3000 rpm advances is critical in deciding how to modify the distributor.
The FBO plate is not helpful for street performance when dealing with a situation as shown above. (It would be OK for drag strip as long as the vacuum advance was disconnected). In other situation the FBO plate can work.
Take the timing readings from as slow as you can get the engine to run, and then in regular increments of increasing rpm.

Thanks mattax! That's alot of good info! Much appreciated!

 

[dartfreak75]

I went back and read your posts on the other thread again @Mattax what curve should I be looking for? I have a spring set i can play with im gonna start over and check my timing again starting at 600 rpm and go up in 200-250 increments documenting it along the way. I will give you the numbers so you can make your neat chart graphs lol

 

[RustyRatRod]

I went back and read your posts on the other thread again @Mattax what curve should I be looking for? I have a spring set i can play with im gonna start over and check my timing again starting at 600 rpm and go up in 200-250 increments documenting it along the way. I will give you the numbers so you can make your neat chart graphs lol

If you take a compression test, you can get pretty close with the ignition cure the first time. It's just something you're gonna have to experiment with. Did you say you were running it 20 initial? I think you said you were going for 14 mechanical. What gear do you have? Maybe try it all in by 2500 and see. If you don't get spark knock, I'd call it good.

 

[dartfreak75]

If you take a compression test, you can get pretty close with the ignition cure the first time. It's just something you're gonna have to experiment with. Did you say you were running it 20 initial? I think you said you were going for 14 mechanical. What gear do you have? Maybe try it all in by 2500 and see. If you don't get spark knock, I'd call it good.

On the compression test I got 150 psi with the throttle closed. At the moment its set at 20 initial. I have 3.73 gears im at around 2900rpm highway speed (60mph)

 

[512Stroker]

I understand how it works i guess I just don't understand why it's needed. When you hammer down on the throttle vacuum drops and the advance drops as well, so it's gonna drop from 50* back down to the 36* total what I don't understand is why do you need that extra advance under low load? Is it a fuel economy "full burn" thing?

Yes it is a fuel economy thing.
At light load your engine can handle 50* which will increase the efficiency of the burn thus increasing fuel economy.
Unless you have a custom distributor I doubt you will get to 50*.
My distributor with vacuum advance was curved by FBO and it max's out at 44*

 

[512Stroker]

On the compression test I got 150 psi with the throttle closed. At the moment its set at 20 initial. I have 3.73 gears im at around 2900rpm highway speed (60mph)

Auto trans or Manual ?

 

[dartfreak75]

Auto trans or Manual ?

Auto

 

[Mattax]

@Mattax what curve should I be looking for?

In the end, probably a curve similar to the one in the Chrysler built MP distributors.
Going from your cam and engine, initial will probably be strongest somewhere between 14 to 18*.
So something in the gray shaded areas would be a good place to end up.

What I think you'll find is another matter. LOL. That's why I'm pushing you to measure it. I've got a couple of guesses but no way I'm risking your machine with my guesses.

Sure I'll plot the results as long as you give me the rpm for each measurement.

 

[RustyRatRod]

On the compression test I got 150 psi with the throttle closed. At the moment its set at 20 initial. I have 3.73 gears im at around 2900rpm highway speed (60mph)

Ok, you might be able to have it all in by 2000. Try 2500 first and see how it runs.

 

[mbaird]

@Mattax explain why you dont like using the FBO plate on the street.

 

[Bewy]

To Dartfreak 75.

The only difference between PVA & MVA is that MVA is operating at idle, PVA is not.
I suspect your engine will benefit from MVA. Easy to tell. Hook it up! If idle rpm increases & idle is smoother, then the engine likes it. It may not want all of the available advance that the VA can can deliver, so a stop would need to be fabricated to limit the total VA.
The difference between a locked dist providing, say, 34* at idle & MVA giving 34* is that the engine is always going to see 34* with a locked dist, which might produce pinging. MVA is load sensitive, so the 34* will reduce to a lower number under load.

You would be surprised how much extra idling timing even stock engines like.
Example: Pontiacs idled with 26*, 6* init + 20* MVA. These were high comp engines [ 10.75 ] with mild cams. Lower CR, more cam duration, needs more idle/cruise timing.
What is different between a Pontiac & a Mopar? They are made of cast iron & ingest fuel & air.

 

[MoparLeo]

Here is an old article with a lot of info. Keep it for reference and read the whole thing.
Cal Tech
Advance To Go