[dataway]

I was just looking through my parts and I have a choice of five difference VA units I can use. (stock 68 YS 400, auto, 2.93 gear, AC, Q-jet, 068 cam, 10:1 comp, 12 degrees initial)

GM unit - 412 20A-10R (68 dual port) starts at 12" vac. (OEM correct unit)

GM/NORS 20A-10R (aftermarket dual port) starts at 7"

GM 364 20 starts at 6.5"

GM 201 15 starts at 12.5"

Accel adjustable ... "says" up to 10 degrees, start point moves with total adv.


My engine runs at very high vacuum ... 20+ at idle. My thinking is I'd like the VA to drop out as soon as possible when I accelerate to avoid any part throttle pinging. I assume the VA units that start advancing at higher vacuum levels will drop the advance sooner under acceleration.

I'm leaning towards either the 412 20A (currently installed) and modify to limit total advance (correct dual port for my year (the VR doesn't function))

Or ... the 201 15A and maybe just leave it alone at 15 degrees (possibly drop initial to 9 degrees)

My MA comes in at about 1100 rpm. All in by 3200 (adds 24 degrees)

If I ran 9 initial, 15 VA, 24 MA that gives me 48 total which sounds about right.

Any thoughts? I'd just like to get in the ball park before engine is in, hood is on ... I hate working on Pontiac distributors in the car.

[steve25]

Your plan sounds good, but you need to see what the motor can take in regards to how fast your vacuum level actually falls off.

The rate of vacuum decrease takes place faster with a spread Bore Carb then a square Bore type and this is a big part of the timing puzzle!

This in conjunction with the car's rear gears and how fast the rpm comes up will be the ultimate factor in regards to how much vacuum advance the motor can tolerate, and how fast the advance needs to receed.

Whatever you do be sure that during your on the road timing test that the motor is fully warmed up!

[455rebel]

I like the adjustable unit. It takes some trial and error to get it right. Never found a bolt on that was Just Right!

[track73]

A few years ago Sun-tuned gave the part number of a vacuum can that he recommended. I think it maxed out at 10 degrees. It was a Delco unit that was stock on a big block Chevy. I bought a Delco and and a Standard clone. The Delco is on the car. It seems to work fine.

[dataway]

Thanks for reminding me ... I think I have an email from Sun-Tuned that mentioned that VA unit. Wish my notes were organized better.

If someone made an aftermarket unit with an adjustable vacuum start point, AND independently adjustable amount of advance that would be ideal.

[Schurkey]

Originally Posted by dataway

If someone made an aftermarket unit with an adjustable vacuum start point, AND independently adjustable amount of advance that would be ideal.

Welcome to the Crane vacuum advance. Install the limiter per Dave Ray and not the Crane instructions. (Limiter on forward/canister side of rod, not on rear side of rod.)

https://www.summitracing.com/parts/crn-99600-1 (HEI)
https://www.summitracing.com/parts/crn-99601-1 (Points)

[dataway]

My problem with the adjustable units is that if you adjust them to kick in at around 9-10" (the highest they go) then they only advance about 3 degrees at that point. That setup would work for me if I could have it start at 12" or so, and then limit to 10 degrees.

They lose stroke as you adjust them to require higher vacuum.

I was watching the unit I have installed while the engine was in operation, it starts moving at 5" of vacuum .... when I blip the throttle there is a noticeable lag before it starts removing advance because it's waiting for vacuum to come down below 5".

Tonight I'm going to install a similar unit that I tested on the bench at a 12.5" start point and see what that does.

Since I'm running basically a stock engine, maintains a lot of vacuum ... the adjustable units seem geared toward engines with larger cams running less vacuum.

[Cliff R]

The good news is that any VA is fully adjustable for how much it adds and there are many ways to modify them to do so.

I use the MIG, takes about 2-3 seconds to add the needed material, they a round chainsaw file to fine tune it and precision numbered drill bits more measuring.

The bad news is that many of the better VA (lower starting points and lower all-in) units are NLA. We're down to a few for the points distributors and half a dozen decent cans for the HEI's. I have a couple that I use here and modify them if/as needed to add the amount of timing we're looking for.

I really see no need for a VA that has a really "low" starting point. All of these engines make excellent vacuum for "normal" driving so any VA used will work pretty well. Really no need to have one come in much below 10" vacuum, unless of course you have a big cam, low vacuum at idle, and you are trying to add a bunch of timing with the VA via manifold vacuum to make things happy.

I avoid that scenario all the way around with my engines using higher compression and well chosen camshafts so they don't want or need a butt-ton of timing at idle speed to be happy.....Cliff

[dataway]

I just machined up a little jig for measuring the rod movement on various VA units.

The GM units were fairly consistent ... but in a nonsensical way.

GM 412 20A 10R rod moved .2155"

GM 364 20A rod moved .2150

GM 201 15A rod moved .2235 (? 15A moved farther than 20A)

Aftermarket 20A 10R rod moved .2470


The two GM 20A units moved pretty much the same .2150 ... but the GM 15A moved farther ... should have been the reverse. Aftermarket unit was the farthest off with .2470

Went right up to about 23" of vacuum to record the full movement.

Several of the units had noticeably different rest position of the rod as measured from the mounting holes.

So this makes me wonder, is there something else at play here? I thought the advance spec like 20A, 15A etc. would indicate how far the rod strokes at maximum.

The GM 412 20A 10R had the most "curve" .. meaning the rate of advance climbed sharply ... just barely moving at 7" and then moving at a higher rate as vacuum climbed and by 12" the movement started being very visible by eye.

[dataway]

What size timing tape should I buy for an OEM 68 Pontiac Damper?

[Sun Tuned]

That may have been a GM 1973516.

Not available from GM anymore but the aftermarket I believe still offers it. I haven't bought any vac advances in some time now, as I'm still running off the load I bought 10 years ago. The problem I was worried about was what happens when they start consolidating? After thinking about that for a few moments, it did not take me long to shift inventory into high gear. In retrospect, I'm damn glad I did.

Even some of the Delco stuff you can still buy today, which isn't very much, seems to be "close" or "somewhat in the relative ballpark". 10 years ago this stuff was dead nuts on the mark. Period.

It was quite refreshing to buy these things time after time and have peace of mind knowing they were perfect on specs every time. Some of the stuff today is rather hit or miss. And generally speaking the advance numbers are pretty close but the start and finish vac numbers are about 2-3 numbers off, high or low, I guess just whatever they got handy at the time. Best bet is to check whatever you use for clarity.

I use to utilize 5 different HEI canisters for the various applications. Stock, street/strip, race, towing, big cam with low compression, big cam with 11:1 and up. The point is we used to have lots of choices and a wide latitude to work from. About all you get today is 2 choices. Unless you have a stockpile, and I assure you that got stupid expensive quickly.

Still viable units are still found along with a little creative vac hose connection,one can make this work well.

[Cliff R]

"Some of the stuff today is rather hit or miss. And generally speaking the advance numbers are pretty close but the start and finish vac numbers are about 2-3 numbers off, high or low, I guess just whatever they got handy at the time. Best bet is to check whatever you use for clarity."

+2

The ever popular B-1 can for example will typically have a pretty stiff spring in it, and most have a "Mexico" sticker on them, but it is still readily available.

Most of the better points cans are NLA and have been for quite some time. A few of the better HEI cans are NLA, but show up frequently on Ebay usually Highgrade Standard, but I've found a few others under different labels and grabbed them up for cheap.

Right now there are two really good HEI cans still in the system. I'm not in the shop at the moment but the one I use the most adds 14 degrees and starts around 9-10". I can play with the 14 degree number and put it wherever I want in minutes, but it's proven to be a good all-around unit and so far no shortage of them.

Several years ago Rocky sent me the list from Lars with the NAPA part numbers. I went thru most of that list and picked out the good cans and tried to get them....no luck with any of that.......FWIW......Cliff

[dataway]

I was looking at this article for some VA unit specs.

http://www.corvette-restoration.com/...c_Adv_Spec.pdf

Seems like some of the max advance figures are off? Using dist. degrees as shown in the chart some of those units advance 40 crankshaft degrees at max ... can that be right?

I was under the impression that the markings on a GM VA unit indicated crankshaft degrees ... for example the dual port 20A 10R ... no way that is dist. degrees.

I'm looking for at least 10" HG start, and max about 15 degrees at the crank. At present ... the OEM unit for my 68 as tested coming in about 10" would be my best bet if I limit the advance travel.

[Sun Tuned]

Does it have to start at 10?

Try this, VC-24A.

Give that a whirl and see what that does..

The deal on these vac advances is simply that they are a load compensating device and not onething more. So what does this mean? It means simply the engineers devised a way to keep economy better, and plugs just a tad cleaner, and throttle response a shake crisper by incorporating these on the distributor. How do they work? Simple...
If you got a heavy right hoof,they don't do too much, if you drive normally ,like a cop is behind you ,then they work fantastically well.

And that's it. No more no less.

Now with that said, and I don't remember for sure the exact number, I can count em up for you , but I think there were once like 64 some odd points advances and somewhere in the neighborhood of 76 or so HEI advances. Let me assure you Delco had every conceivable scenario covered. I spent close to 7-8 hours pouring over the advance unit specs in the Delco book to arrive at what I used regularly on the ones I send out. In that 7-8 hours I came up with 4-5 for the HEI alone. A like period of time was spent on the conventional points distributors, except I already knew what I was trying to accomplish. The other thing on that was by that time , there were fewer choices that could be made simply because by 1984 ish, Delco had cut the number still available for those down fairly good by then.

The main thing you want to do is be sure you select a vac unit that is out of the picture as the vac drops. Well out of the picture. The vac units used on the 64-65 gto's was still active at a fairly low vac reading and it also allowed a fairly good amount of advance some 20 crank degrees if I remember correctly. Early on Frank Rediker advised disconnecting it and leaving it inactive when a bobcat kit was installed.

This was due to how slowly it went away when the throttle was mashed home. It went away slowly enough that it caused momentary pinging that they didn't want. At that time they couldn't just select a different one because the "good different" ones didn't come along until about 1971 or so, so it wasn't an option then, they just unhooked it and let it be.

Not the best option but about all they could do unless they fooled around with it for longer than they would have wanted to.

In a normal street application with a reasonable size cam, look for one that starts in the 7" range and is all maxed out by 12". This is a nice street setup as there is a nice fat 5" range for it to operate in. There are some that went from nothing to all out in 3", too fast in my opinion. However the way the engineers had them installed they may have worked fine, but not in a Perf application.

Now let's say you have a big cam and the idle speed is only drawing say 9-10" of vac at idle. And let's say you're trying to hook this up in a manifold vac source.
This scenario is why Jim butler advocates not using a vac advance. Because in this particular instance the thing don't want to work right because there is just short of a couple inches of needed vac to make it work fully advanced. Now it's constantly trying to kick in and out and it's gonna run funny. No way around that and the only way to fix it, is to remove that vac unit and change it to a different spec unit. In that deal you want to pick an advance that starts earlier and is max advanced earlier. For example starts at say 4" and in by 8". That way it's not kicking in and out and you can get it to idle right.

The only problem with that is there are not as many advance units as there once were to choose from so your gonna have to do some work figuring things out.

But it can be done. And when you get it right it works quite well.

[Cliff R]

If you got a heavy right hoof,they don't do too much, if you drive normally ,like a cop is behind you ,then they work fantastically well.

[Sun Tuned]

Lemme add here. On the points advance subject.
For those of you who can remember back that far. The old H.O. Racing advance, it was a good one.

The last Delco made version was listed as a D1383.
No longer available as of close to 2006 ish
That superseded to a Standard ignition VC-24A.

The last one of those I had my hands on was 10-12 weeks ago. Good but the vac numbers were higher than the Delco specs by 1.5". HG lower than spec. No big deal with that. I can live with an inch and a half. Total advance was spot on.

Well as I've said before I do some freelance work for a company up the road from me. Now I'm still parked on a good number of these things (Delco originals), that stash stays in its cabinet for basically the folks on this forum that send stuff in and need one, I'm not huntin these things anymore. Done got too old for that game. Anyhow the guy there says can you fix this? I said sure, but that advance unit has a hole in it needs replacing. He says gimme a number and I'll get one from oreilleys... after I quit laughing, I reeled off VC-24A. I figure if they want to have at it.

He comes up with 8.97 each. That's cheaper than what they cost me currently, so I'm curious, I tell him go ahead. He's a young guy and all computer savvy and such. He's smiling, says it'll be there in two days....

It gets there and it's some no name knockoff box, but inside looks like a Standard VC-24A allright. Way off the mark!!! 22 degree canister, and must have had a rear coil spring from under a car for a diaghpram spring in it. Started at 12" done by 18"

Moral is it was cheap. Likely some kinda rock auto thing as he said that's where it was ordered from . It looked good, but fell way short on specs. This business of opening a box and installing and plugging a hose to it and going on your way ain't gonna cut it. Better check it!

I'm saying be cautious as to what you source, and always verify what you get. Needless to say we haven't bought anymore of those. And that's a shame cause I was thinking ... just maybe...if we get lucky here.... I'll buy a bunch of em.

No deal this time fellas. I'll keep looking though..