Vacuum Advance Adjustment Question

[68 P.O.S.]

Hey all. I’m in the process of setting up the ignition curve properly on my 72’s 350 and the vac advance is my last step. I’m installing the Crane 96600-1 adjustable vacuum advance kit and my question is, how many turns should I adjust the vac can since I have 20” of vac at idle? I’ve done a ton of research and found a lot of great info on timing, but not so much for this vac can. The directions don’t give much info either; other than insert the allen wrench, turn fully clockwise, test drive and adjust counter-clockwise for ping...rinse, lather, repeat. I guess I’m looking for a ballpark number so I get close, then fine tune from there. Anyone care to share? Thanks in advance!

[toolboxchev]

Simply put its different for every motor.


Adjust to get the max out of your motor without any pinging. Then turn back in one or two turns till it stops.

I get way more power out of my motor this way. Its not like making it into a rocket though. More advance with todays fuel means a more complete burn in the chambers.

[68 P.O.S.]

I knew “it’s different for every motor” was gonna be the first response Like I said, I’m only looking for a ballpark number. I know I’m not the only one on this forum who has the Crane vac can and an engine that produces 20” vac at idle. I guess my question really is how many inches of vac should the vac can start pulling at with the amount of vac I have at idle.

Your rocket comment made me laugh because I already changed out the garage door springs in the dizzy for the silver and blue ones from the Crane kit. What a difference it made. The truck wants to “go” now, like a rocket

[geezer#99]

The vac can will always pull 20 hg of vacuum at idle. The adjustment is for how far the diaphragm in the can moves. It has a spring in it. The vacuum overcomes the spring pressure. All the way cw is likely full vac advance. Turning it ccw reduces the amount of advance. Generally when you’re cruising in high gear above 25 mph and you squeeze the throttle gently, if it pings, you adjust the can until it doesn’t.

What did you set your initial timing at?

[68 P.O.S.]

Timing is set to 14 initial, 36 total. I'll limit the vac can to the 10-12* neighborhood when I install it this weekend. The engine is stock, as far as I know, with the exception of an Edelbrock intake and carb, so not tuning for anything crazy.

Right, I follow you. Bottomed out is high tension on the spring, allowing the advance to come in at higher vacuum levels.

[BossHogg69]

If you have 20" at idle, I'm pretty sure it's going to pull no matter where the spring is at. I honestly don't understand why you'd need the adjustable can if you have all that vacuum. I've used the Crane adj vac can kit in a couple of my rides, but for situations where the motor only pulls like 8"-9", then you adjust the spring so it starts pulling vacuum at 5" HG. I really only like the kit for the mech. adv springs and limiter plate after messing with their vac can (I don't care for it much). The easy thing to do is if you have a MityVac or similar vacuum hand pump (I had one I used for bleeding brakes & hyd clutches) you can mess with the adjuster spring, then pull vacuum with the mity vac on it to see when the diaphragm arm starts moving. Also, I use the limiter plate differently than the directions advise. I put the plate on the opposite side of where they recommend, and limit the total travel of the arm from 0. The way the directions have it is backwards to me, since it basically stops the arm in the middle of it's travel instead of it starting at 0".

There are many different cans you can use that start at different inches of hg, and add more advance than others. The can in my hand below is a 626-10 I had in my parts stash - the 10 is the degrees adv is adds at the DIST (this will be 20* @ crank). I've got the limiter plate set to 10* adv at the crank. I just divided the max adv by the total travel and set the plate at the amount of adv I wanted, then installed and put the timing light on it to double check. This unit is on my '65 Bel Air w/350 sbc.

I've got a similar setup on my '61 Bel Air, it's running a ZZ454 bb.

[geezer#99]

Originally Posted by 68 P.O.S.

Right, I follow you. Bottomed out is high tension on the spring, allowing the advance to come in at higher vacuum levels.

Not always so!
I’ve seen Them both ways. Some screwed in all the way give max advance but some give minimum advance. Depends who makes them.
Easy to check with your timing light.
A word of caution.
I had one that when I adjusted it too far trying to get less advance, something came apart inside the can. Kinda like a spring popping. Wouldn’t adjust after that. Had zero advance.

[BossHogg69]

Originally Posted by geezer#99

Not always so!
I’ve seen Them both ways. Some screwed in all the way give max advance but some give minimum advance. Depends who makes them.
Easy to check with your timing light.
A word of caution.
I had one that when I adjusted it too far trying to get less advance, something came apart inside the can. Kinda like a spring popping. Wouldn’t adjust after that. Had zero advance.

That's why I don't care for those adj units, I've had one come apart inside exactly as you described. No issue for me to round up a few cans, test out what pulls what @ how much hg, and go from there. You can always buy a few from the local parts store, and return the ones you don't end up using.

All the ones I have were out of old distributors, and they work just fine.

[Gromit]

I too have read about there being two versions of adjustable cans, so be careful.

Wish I had photos of the guts of both types but just as a helpful mental picture here are photos of the Mallory type. (ignore they arm on the can in the photo it isn't from a 67-72 Chevy).

edit: just occurred to me I wonder if it could be so simple as just left hand threads on that allen bolt on the other ones.. (maybe to get around a patent?)

[Wrenchbender Ret]

For a stock engine I usually set them to start advancing at 5-7 in. & all in about 14. You really need a hand vacuum pump to test these things & adjust them.
George

[CST10]

I usually set them for starting around 6-7 inches vacuum, all in 14-15 inches vacuum. The top end really depends on how much vacuum the engine is providing. Different cams provide more or less vacuum.

Max degrees the vacuum advance supplies depends on maximum all in at XXXX rpms. I use 3000 rpms as my all in with no more timing being pulled in mechanically.

Example if all in is 36* at 3000 rpms with vacuum advance disconnected. I would shoot for 12* added by vacuum advance. Total would be 48* when cruising and WOT would be 36*. Another example would be 32* at 3000 rpms with vacuum advance disconnected. I would set the can to pull an additional 16*. Total again is 48*for cruising and WOT is 32*.

Never go over 50-52* total with vacuum advance. That’s the start of the danger zone and engine damage.

Others have different settings they may use. but this is how I set mine. Never had an issue in 34 years of doing it this way. I don’t use the ping method, one ping to many and you’ve got a burnt piston or some other damage.

[Steeveedee]

Originally Posted by CST10

I usually set them for starting around 6-7 inches vacuum, all in 14-15 inches vacuum. The top end really depends on how much vacuum the engine is providing. Different cams provide more or less vacuum.

Max degrees the vacuum advance supplies depends on maximum all in at XXXX rpms. I use 3000 rpms as my all in with no more timing being pulled in mechanically.

Example if all in is 36* at 3000 rpms with vacuum advance disconnected. I would shoot for 12* added by vacuum advance. Total would be 48* when cruising and WOT would be 36*. Another example would be 32* at 3000 rpms with vacuum advance disconnected. I would set the can to pull an additional 16*. Total again is 48*for cruising and WOT is 32*.

Never go over 50-52* total with vacuum advance. That’s the start of the danger zone and engine damage.

Others have different settings they may use. but this is how I set mine. Never had an issue in 34 years of doing it this way. I don’t use the ping method, one ping to many and you’ve got a burnt piston or some other damage.

My bolding. This. If you can find out what your engine corresponds to in terms of a stock vehicle, and you can find a helpful parts counterman, you can go to the back of the paper parts catalog and find the correct one for what you are running now, without experimentation. All the vacuum requirements and amount of advance are in there for the taking.

[68 P.O.S.]

Thank you to everyone for the replies, much appreciated! No weekend update on this tho. Spent most of the weekend spending time with my lady. Then on Sunday, while looking under the hood shaking my head at the wiring, I decided to tackle installing the AAW wiring harness I've had for a bit. So hopefully I’ll be done with the harness and the vac can this coming weekend.

BossHogg: Ahhh, a MityVac, brilliant idea! I have one and feel stupid for not even thinking about it I’ll definitely be doing that. I’ve also read about placing the stop on the opposite side from where Crane recommends; it makes a lot more sense so I will be doing that as well.

Geezer: Ok, I see what you’re saying now. I’ll check with my MityVac and see what bottomed out ends up being. The Accel vac can that died on me had reversed threads and adjusted the opposite way. The instructions say 4 turns out is approx. 12 degrees advance @ 7” of vac.

Gromit: Thanks for the heads up, I’ve heard of these things coming apart too. Gonna be mindful of that when working with it. Thanks for the pics, I wondered what they look like inside.

Wrenchbender: Perfect, those are the numbers I’m looking for. Thank you!

CST10: Thank you for the numbers and timing tips. Again, just what I’m looking for.

Steeveedee: Good advice, I think that’s a good idea. I already have the Crane can so going to stick with that tho. Thank you.