[footjoy]

I want to know what people are using for a vacuum advance on their lightly Modified Pontiac 400 engines.
I am trying to find a vacuum advance that gives me 12 timing degrees (crank) at idle. I have tried an adjustable can but it seems to not be consistant. I do have a HEI distributor and it is hard to find a good listing. The best I have found is Lars
paper (it is a great explanation) on chevy.

When I am in overdrive and cruising at 2200 rpm 39 degrees advance. I don't need that much.

If any body has any productive ideas I would like to hear them.

Thanks

Greg

[Scott65]

Crane used to make an adjustable unit. I had one for a points distributor. It worked pretty well, I believe I was able to limit to 10 degrees with it. Is your car overheating, or detonating with the current setting? 39 at a light cruise isn't super excessive, as long as it isn't causing other issues.

[ta man]

Weld a limiter on it

[Cliff R]

Most of the VA units in the Lars articles are long discontinued.

We have one that we use here that has light spring tension (in early). We modify it to add the amount of timing desired. Ten degrees is a pretty short throw on an HEI advance, just under 1/8" or travel.......Cliff

[Rocky Rotella]

Greg,

I see you’re in Kansas City. Will you be attending the car show in Olathe next Saturday? I’d be glad to visit with you and try to help out.

[Formulajones]

Most of the good ones are discontinued.

I use the adjustable units now and modify them.

I set the spring tension for lightest setting and then weld the slot to limit the amount of travel to roughly 10 degrees.

[many birds]

How do you figure out where 10 degrees is?

[67GTONUT]

There used to be a website on the internet that listed all the specs on vacuum advance cans for different models...... But all my links to those pages are now dead....

I think I am using a can from a 70's Buick spec that has like only 12deg of advance and works perfect...... I will keep looking for that page.....

[abefromen]

http://www.crankshaftcoalition.com/w...ance_Specs.pdf

[Scott65]

Originally Posted by many birds

How do you figure out where 10 degrees is?

I had to do it with a timing light. If you have access to a distributor testing machine that would be easier.

[phil400]

About .125" of travel will give around 10*. I measured with a vernier and made a little plate held down by the screw.

[Formulajones]

Originally Posted by many birds

How do you figure out where 10 degrees is?

Depends on the distributor. Some breaker plates require a little more movement than others.

I weld the slot and start with about .125" of movement and increase from there as necessary. Some distributors require up to about .200" of movement. Shooting for 10-12 degrees of total timing is all you need from it.

The adjustables work like this. Tighten the spring it limits it's travel at the same time. So you can limit them to 10 degrees by tightening the screw but then they don't even start to work until they get hit with about 12-15 inches of vacuum and they aren't all done till 20 inches of vacuum or more, so that's not going to do it.
If you loosen the screw they'll work with about 5-7 inches of vacuum but then they travel up to 20 degrees of timing, and that's too much, so that doesn't work either.

So, I set them for the light vacuum setting where they start to work around 5 inches and they are all in by 8 inches, then I just weld the slot and limit the movement. So you get the best of both worlds. Works excellent in engines that have a lot of camshaft in them, and even mild engines, and also works well when you're a mile up in elevation and the engine isn't making as much vacuum.

Here's one I just set up for an engine I just built. This distributor was originally set up by someone on the forum, but they used plastic bushings to limit everything. When I took it apart the plastic bushings were hardened from heat and split, and falling out. So I weld them here as a more permanent measure.
This particular unit needed about .200" of travel on the vacuum advance which gave me 11 degrees at the crank.

I also set up the distributor advance the same way, welding the slot until I have about 10 degrees, which is 20 at the crank.

[Formulajones]

Here's the vacuum advance

[footjoy]

Originally Posted by Rocky Rotella

Greg,

I see you’re in Kansas City. Will you be attending the car show in Olathe next Saturday? I’d be glad to visit with you and try to help out.

Yes I will be at the show. Looking forward to visting with you.

[footjoy]

I really appreciate all of the information I have gotten. I have always wondered how you determine where to start. This thread has been very helpful.

Thank you.

Greg

[glhs#116]

That's weird. I have the Pertronix (which seems to be a rebranded Crane) and the instructions clearly state that full clockwise gives you the most advance. Unwinding it progressively limits the advance. I just put on high compression heads and I have it cranked back to like 5 degrees added advance. I got there by unwinding the spring like the instructions say. Are there different kinds? Because my unit looks like the one you show..

Sam

[Cliff R]

The Crane vacuum units come with a cam that goes under one of the screws to change how much they add. Adjusting the screw inside the unit is for spring tension.

I do not like or use adjustable vacuum advance units. I prefer to buy a stock replacement type that has the spring tension and cut-in point required, then modify the slot so it adds the amount we are looking for.

Problem is that they have discontinued most of the good ones, especially for OEM points distributors. When it's all said and done the only one to be available will be the B-1 can for points....IMHO. One by one over the last 15 years or so the others have became NLA. Some of the better units are still available on Ebay if you know what you are looking for, just bought half a dozen of them recently and we've just about used all them up.

At one point I purchased all the really good vacuum advance units (for points distributors) when they became "special order" only that were still in NAPA's inventory but have finally used all of them up too.

It's not really a big deal for me as I very seldom use manifold vacuum for the advance unless we're working on an engine where poor choices have been made in the CID/compression ratio/camshaft selection area have been made. IF you find yourself having to use a vacuum advance with about 5-7" all in and adding tons of timing at idle to make the engine happy......you just didn't choose your parts well during the engine building process.

An well thought out engine will be fine with about 8-12 degrees initial timing and not really want, care for, or respond well to adding another 15-30 degrees at idle speed......FWIW.......Cliff

[glhs#116]

Cliff,

I spent a little quality time on this in the work parking garage and pretty much figured out the same thing. That little limiter cam dealy.. I have to find that thing. I have seen it but I didn't have it installed. Unscrewing the allen key is basically adding tension to the can. That's why at a certain point it also acts as a vacuum limit.

For now I stuck a crusty old factory vacuum unit on that gives 14 degrees and has a reasonably stiff spring. It's steady at idle vacuum levels. But it will ping no load between like 1800-2600rpm (not with load, like against the torque converter..). I'm guessing it's going to be part throttle sensitive on the way home.

This is a particular problem I have had forever on this engine. I'm using all factory mechanical advance stuff with the strongest springs I can find but I still get all my timing in faster than the engine has ever wanted (new heads or old heads, new cam or old cam). Just not sure how to slow it down more. It's not too much total, it comes in too quickly with RPM. 394 centre plate, 139 weights, numbers down (if I remember correctly) which I believe is correct for Pontiac rotation direction.. Aw heck, I don't want to derail the vac advance talk. I'll start a thread..

[pastry_chef]

Vacuum advance is not mandatory if too complicated to dial in for now.

After you have moved back, something to consider.
http://www.daytona-sensors.com/tcs-1...ol-system.html

[footjoy]

An well thought out engine will be fine with about 8-12 degrees initial timing and not really want, care for, or respond well to adding another 15-30 degrees at idle speed......FWIW.......Cliff


Somewhere I went wrong.

Engine specs

400 .040
flat top pistons
#15 heads small valve
crower 60240 cam (is cam to big for heads)
8.75 compression
guessing about 300-325 horse power

we have to keep the timing at 8 degrees to keep it from surging at idle. vacuum advance adds about 25 degrees at idle.
I have 16 degrees mechanical at 2800 rpm

I would like to get the initial up to 12-14 with less vacuum advance.

As you can see I am lost in all of this.

Thanks

Greg