[glhs#116]

First of all, apologies. I stole this blatantly from some post on a Chevy board somewhere. But I've never seen it here on the Pontiac forum. It's a simple idea, really.

The problem being that the vac cans often give you more advance than you want. Often guys will weld the slot shorter but two problems for some of us are:
1. The need to be able to weld
2. It's not easy to try different settings

The adjustable cans often come with a cam to allow limiting but it limits at the wrong end if installed as per directions. Instead of limiting the travel it "pre-loads" the spring. This means that when you only want to limit travel you get stuck adding extra spring tension you didn't want and raising the amount of vacuum required to move the advance.

The solution takes just a drill, a spare small screw and a few minutes.

Here's a stock can with the modification


Here's my adjustable can installed with the modification


Again, I didn't come up with this but I found it very helpful so just boosting the signal. This is how you should do your vac advance limiting..

Sam

[Cliff R]

That works fine but you MUST make sure the screw doesn't protrude thru too far and lock up the unit when you bolt it in place and tighten the mounting screws down.......Cliff

[glhs#116]

Originally Posted by Cliff R

That works fine but you MUST make sure the screw doesn't protrude thru too far and lock up the unit when you bolt it in place and tighten the mounting screws down.......Cliff

Cliff, absolutely correct! I should have noted that. Although you will notice it soon enough when you take the vac hose off and the timing doesn't budge..

Sam

[Schurkey]

Or MIG, TIG, braze a tiny bit of sheet steel into position. File or grind as needed to achieve desired advance.

[lust4speed]

Guess I'm cheaper than the other posters, and simply use a stiff piece of coat hanger wire. You don't want to use the thinner hangers, and have to raid the wife's closet for a thicker hanger to cut up. The vacuum advance movement is way weaker that the wire and I've never had one move once bent and set. Basically make a loop for the screw to go through at the far end of the advance assembly and bend a J-hook that limits the movement towards the pot. A pair of long nose pliers can reshape the bend either tighter or looser while the distributor is on the machine or in the car. It's free and takes less than two minutes once you've done a few.

[RocktimusPryme]

Just to piggyback onto this, what is ideal for added total timing of vacuum advance on the crank? With the canister unplugged I go from about 12* initial to 35-37* on the crank. Which is about where my engine made the most power on the dyno. But what should I be looking for optimally with the canister connected? Another 20* on the crank? Less? Its been a while since i checked but I suspected mine added too much.

[glhs#116]

Originally Posted by RocktimusPryme

Just to piggyback onto this, what is ideal for added total timing of vacuum advance on the crank? With the canister unplugged I go from about 12* initial to 35-37* on the crank. Which is about where my engine made the most power on the dyno. But what should I be looking for optimally with the canister connected? Another 20* on the crank? Less? Its been a while since i checked but I suspected mine added too much.

I don't think there really is a "too much" for vacuum advance. It's never there under full throttle so it has zero to do with full power or full throttle ping, rather it's for helping the sparse slow-burning light throttle mixtures. As such you can pretty much add it until the part throttle and lift throttle ping starts to bug you and then back it off a little. The more you run the cleaner the car will run and the better economy you will see on trips.

Sam

[RocktimusPryme]

Originally Posted by glhs#116

I don't think there really is a "too much" for vacuum advance. It's never there under full throttle so it has zero to do with full power or full throttle ping, rather it's for helping the sparse slow-burning light throttle mixtures. As such you can pretty much add it until the part throttle and lift throttle ping starts to bug you and then back it off a little. The more you run the cleaner the car will run and the better economy you will see on trips.

Sam

Makes sense, I had been experiencing some part throttle, from a stop, ping lately. But I suspect that is related to my fuel issue documented in a previous post. Once I get my fuel system back in I may investigate limiting it a little.

[Formulajones]

In almost all cases I find that you only need about 10-12 degrees of additional timing added with the vacuum advance if the engine is pretty efficient, has some compression, and running pump gas.

If you add it all together, ideally you want somewhere around 44-48 degrees all in at light throttle cruising conditions. This seems to be a safe area for most engines.

So for instance if your engine is making best power on the dyno at 36 degrees of total timing (no vacuum advance) Then adding another 10-12 degrees with the vacuum advance for your light throttle cruising conditions would work just fine and should be more than enough.

On most of these cans and breaker plate combos I find that limiting the travel to about .200" will give you 10-12 degrees at the crank. However that might change ever so slightly depending on the combination of parts you have. But that distance will get you in the ball park.

Another helpful tip. The rated working vacuum of these cans all vary quite a bit, and since the good vacuum cans that are more sensitive for guys running decent camshafts are drying up, I prefer to use the adjustable units and set the spring tension very light so it starts to work around 6-8 inches and is all done by 10-12 inches, just like the factory performance cans worked. Then limit the travel. They work excellent in this configuration once the mods are done.

I prefer to weld them here.

[Schurkey]

Originally Posted by glhs#116

I don't think there really is a "too much" for vacuum advance.

You surely can get too much vacuum advance. And it's not just "ping" you need to worry about. Too much advance--vacuum or centrifugal or electronic (the engine doesn't know the difference)--can cause surging and other driveability problems including REDUCED horsepower even if there's no detonation involved.

Yeah, 10--15 degrees of vacuum advance is a good starting point to tune from.

[70GS455]

Another benefit of putting the limiter on the pull side is that changes to how much is limited does not result in changes to base (vac hose off) timing

Sent from my SM-T817V using Tapatalk

[HWYSTR455]

Originally Posted by Formulajones

In almost all cases I find that you only need about 10-12 degrees of additional timing added with the vacuum advance if the engine is pretty efficient, has some compression, and running pump gas.

If you add it all together, ideally you want somewhere around 44-48 degrees all in at light throttle cruising conditions. This seems to be a safe area for most engines.

So for instance if your engine is making best power on the dyno at 36 degrees of total timing (no vacuum advance) Then adding another 10-12 degrees with the vacuum advance for your light throttle cruising conditions would work just fine and should be more than enough.

On most of these cans and breaker plate combos I find that limiting the travel to about .200" will give you 10-12 degrees at the crank. However that might change ever so slightly depending on the combination of parts you have. But that distance will get you in the ball park.

Another helpful tip. The rated working vacuum of these cans all vary quite a bit, and since the good vacuum cans that are more sensitive for guys running decent camshafts are drying up, I prefer to use the adjustable units and set the spring tension very light so it starts to work around 6-8 inches and is all done by 10-12 inches, just like the factory performance cans worked. Then limit the travel. They work excellent in this configuration once the mods are done.

I prefer to weld them here.

This is basically what I used to use, about 8-12 vac. But, what I did to change the vac that impacts the advance is to put the adjustment/limiting cam on the hold down screw at the end of the can, and doctor the cam to fit. (on my small body the cam was limited by hitting the outside of the body).

I never had luck running more than 10 degrees vac, and my total was in the 32-34 range. Round port e-heads @10.7 SCR. I know many say they need more lead, but in my situation that wasn't the case.


.

[RocktimusPryme]

Originally Posted by HWYSTR455

I never had luck running more than 10 degrees vac, and my total was in the 32-34 range. Round port e-heads @10.7 SCR. I know many say they need more lead, but in my situation that wasn't the case.


.

I had been contemplating something similar. I have the early bathrub E heads which are said to require sometimes 38*. Mine made peak power at 35-36*. So I was wondering if the target for vacuum added timing would scale with your peak power total mechanical? IE if a car that wanted 32* of total, needs 10* of vacuum canister, would a car needed 36* total still want 10* more, or would it be more/less?

[Formulajones]

Originally Posted by HWYSTR455

This is basically what I used to use, about 8-12 vac. But, what I did to change the vac that impacts the advance is to put the adjustment/limiting cam on the hold down screw at the end of the can, and doctor the cam to fit. (on my small body the cam was limited by hitting the outside of the body).

I never had luck running more than 10 degrees vac, and my total was in the 32-34 range. Round port e-heads @10.7 SCR. I know many say they need more lead, but in my situation that wasn't the case.


.

I used to do it from the other end, but the issue with that is that method also moves the breaker plate and increases your initial timing setting.

So I just prefer to modify the breaker plate for total timing and the curve I want and leave that alone, then modify the vacuum advance from the other end to limit it's travel, that way the rod stays in it's normal location at rest and doesn't monkey with the other timing variables.

[Formulajones]

Originally Posted by RocktimusPryme

I had been contemplating something similar. I have the early bathrub E heads which are said to require sometimes 38*. Mine made peak power at 35-36*. So I was wondering if the target for vacuum added timing would scale with your peak power total mechanical? IE if a car that wanted 32* of total, needs 10* of vacuum canister, would a car needed 36* total still want 10* more, or would it be more/less?

Dad runs the bathtub E-heads. Had them on 2 engines and both times they made best power at 34 degrees on the dyno. So on his engine I have the vacuum advance setup for 12 degrees max, that starts coming in at 6 inches of vacuum and is all done by 10 inches.

I run his from manifold vacuum, and the car idles around 12 inches of vacuum up here at 5,000 feet. So most of the time the added timing is there at idle, until he touches the throttle and vacuum dips then it will go down and come back up gradually as the engine load lightens.

To answer your question, even at 36 total, another 10-12 from the vacuum advance is fine. Some engines love to have as much as 50 degrees at light throttle cruise applications. If you look at factory timing tables for an LS engine for example, the factory has as much as 48 degrees in some light load light throttle areas of the timing table, and those engines are very efficient.

But on our engines, with pump gas, and the high compression ratios we build our engines with, I keep things a pinch conservative. If you end up with anything around 44-46 degrees all in with vacuum, you're probably okay.

My Z is 11:1 with iron heads, I run 36 total on that engine with an additional 12 from the vacuum can. Cam has a quite a bit of overlap too so that helps. The engine loves the extra timing at light cruise.

Each engine is a little different.

[HWYSTR455]

Originally Posted by Formulajones

I used to do it from the other end, but the issue with that is that method also moves the breaker plate and increases your initial timing setting.
.

Yeah, but you just compensate for it by turning the dizzy.

I have 2nd design 87cc round ports, and although I've heard many needing more advance than I ran, I found it ran all around better, and safer than the 36-38 many run. So I experienced the same as you did/do.

Just to say, rear gear/final drive does make a diff on how sensitive your engine will run on vacuum. (and of course cam selection too)


.

[RocktimusPryme]

Originally Posted by Formulajones

Dad runs the bathtub E-heads. Had them on 2 engines and both times they made best power at 34 degrees on the dyno. So on his engine I have the vacuum advance setup for 12 degrees max, that starts coming in at 6 inches of vacuum and is all done by 10 inches.

I run his from manifold vacuum, and the car idles around 12 inches of vacuum up here at 5,000 feet. So most of the time the added timing is there at idle, until he touches the throttle and vacuum dips then it will go down and come back up gradually as the engine load lightens.

To answer your question, even at 36 total, another 10-12 from the vacuum advance is fine. Some engines love to have as much as 50 degrees at light throttle cruise applications. If you look at factory timing tables for an LS engine for example, the factory has as much as 48 degrees in some light load light throttle areas of the timing table, and those engines are very efficient.

But on our engines, with pump gas, and the high compression ratios we build our engines with, I keep things a pinch conservative. If you end up with anything around 44-46 degrees all in with vacuum, you're probably okay.

My Z is 11:1 with iron heads, I run 36 total on that engine with an additional 12 from the vacuum can. Cam has a quite a bit of overlap too so that helps. The engine loves the extra timing at light cruise.

Each engine is a little different.

Its been a while, but at certain points I have had every gauge known to man on that motor. I believe mine pulls like 11" at idle. My first get me over carb before I had one built was an early vac secondary holley that ONLY had manifold vacuum. And I think it was pulling 50+ at the top which I didn't like. So I need to do this limit thing we are talking about here.

When my car runs again, Im going to experiment and try to get the vacuum dialed in.

[Kennybill]

Thanks for starting this tread. I'll be "stealing" some of the ideas. "Honor among thieves." Btw, that looks like a Cutlass louvered hood in the first picture. Thanks again.