[track73]

I'm trying to tune my .030 over 455 vacuum advance. I have a "Mity Vac" that can put vacuum into the Vac. advance can and see what it's doing while idling and watching the timing light. I figure that for 14 degree more I can add about .83 inch to the 36 mark on my balancer and check the timing with a buddy in the car reving the engine to about 3000. Or do I need that? I can also see when it starts to come in. When should it? I have a Crane advance unit.

[SD421]

Your vacuum line should be plugged and not be advancing when trying to get your 36 degrees of total mechanical advance at 3000 RPM.

Once you have your 36 total timing at 3000RPM' THEN hook up your vacuum advance.

If it pings on light throttle then 14 degrees is too much and you need to limit the amount of vacuum (12 or 10) of advance until it doesnt ping.
You could use the adjustable can or you could put create a bracket to limit the amount as well.
Don

[Joe's Garage]

I've always used 2 1/8" as the point to mark for ~36 degrees or mechanical advance, so the .83" is correct for 14 degrees.

But you shouldn't have to remark the balancer, if you don't want to.

Just line up your 36* mark with 14* on the timing scale on the timing cover.

Is your Crane unit adjustable to make it react to less vacuum or more vacuum? Once you figure out much vacuum advance makes your engine happy, you may want to adjust that tension to speed up or slow down how quickly the vacuum advance happens (sensitivity to vacuum changes).

And, once you do that, you may want to re-set the amount of advance it pulls in to optimize.

It's a never-ending battle. Because our cars can ALWAYS run better, right?

On street engines, I use 36* mechanical with 10* of vacuum and have usually been satisfied.

Good luck!

[Sun Tuned]

Go get ya a delco 1973516 and be done with the stress and erector set theatrics.

They are still available.
Fwiw

[Firebob]

ST, is that a vac can?

[Kenth]

http://www.rockauto.com/en/moreinfo....=45471&jsn=252

[Shiny]

What vac advance can was original on a 1112008 distributor (70 350 auto)?

How much did the vac advance add at idle?

Is the part number 1973516 mentioned below (which I assume adds about 10 degrees max) good for a points distributor or only for HEI? If not, is there a recommendation?

I'm also curious if there are "curves" for these cans (ie advance vs vacuum or rate of change in advance vs rate of vacuum change? Are these characteristics published somewhere?

Mine has an adjustable unit on it now and rather than guess at a stop, I figured it might be easier to just get the factory can back on it and move on. The adjustment is tedious and does not affect the stop, so getting it adjusted sounds like more trouble than it will be worth on a near-stock 350.

I don't know that it's a problem, but the adjustable is adding a lot at idle... maybe 20 to 25 degrees and all the threads on this site recommend about 10.

[Shiny]

A search on RockAuto said they sell an AC Delco C1382 equivalent to original 19110657 that is correct for the 1112008 distributor:

https://www.rockauto.com/en/moreinfo...256992&jsn=461

Where can I learn about the characteristics for this part (lowest vacuum to get any advance, max limit)?

[Kenth]

1970 Pontiac (GM) did not use vacuum advance at idle speeds due to the TCS system.

1112008 uses a 1115365 vacuum advance unit, starts @8-10 in/hg max 20° @ crank (10° @distributor) @15-17 in/hg.
Last three digits of the part-# and the amount of crank advance is stamped on the bracket on original units.

The 19110657 (stamped B1) vacuum advance is close but only 16° advance, starts@8-11 in/hg max @16-18 in/hg.


I would use the TCS-system or a ported vacuum source for the original vacuum advance and forget it.

[70GS455]

Originally Posted by Shiny

I'm also curious if there are "curves" for these cans (ie advance vs vacuum or rate of change in advance vs rate of vacuum change? Are these characteristics published somewhere?

Yes:

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

Sent from my SM-T817V using Tapatalk

[Shiny]

Thanks to you both.

I stumbled on that article last night and it was very helpful.

As to the TCS, seems counter to what the article said. I don't see any practical benefit to keeping that system, but adding advance at idle sounded like a good thing for cooling. It may have been needed for emissions in 1970, but there's nothing unique about my engine that would force me to keep it.

I also don't want to deal with verifying the rest of the TCS is working if there is no benefit. There is a switch on the transmission and a valve on the back of the engine. Both are probably 48 years old and I got enough uncertainty.

Unless I find motivation to keep the TCS, I'll just buy the VC1802 as recommended in the article and try it with manifold vacuum. This seems in line with general wisdom for idle timing and is maxed out at a low enough vacuum to avoid hunting.

The car is not a driver yet, so just trying to get it idling well enough to move it around on its own power for now. For this purpose, I will probably just leave the vacuum off until I get the carb straightened out, but may install the vac can if it is obvious I need it.

[Kenth]

If your engine can´t handle the original vacuum advance at idle speeds, go TCS or ported source for it.
Using a "lighter" vacuum advance is asking for light part throttle ping.
This and "all in" mech advance at 2500 rpm´s are the worst advice available.
May have been good in the 60´s, not with today fuels though.
At least that is what i found some twenty years ago when i got my .030" 455 finished.
JMHO

[Shiny]

Kenth, I'm not sure I follow you. I have never had the engine running well, and the original can is long gone. So it's not about handling the "original" vacuum advance. I'm just trying to establish a reasonable starting point as I try to get it driveable and think the adjustable unit on there now would take more effort to set up as a starting point than just buying a fixed unit with conservative specs.

The numbers you provided for the original 1115365 can: starting 8-10, total 20 @ 15-17.

The VC1802 I am planning on using (recommended in article) is: starting 7-9, total 16 @ 14-16.

Stock baseline was 9, so at idle, the TCS kept it at 9. I can start here.

I do not know the curve for the original 1112008 springs and weights. If you do, I can try to match it.

I set mine up to avoid any mechanical before about 1300 to avoid having the mechanical jumping around at idle. I limited the max to 22, but I don't yet know the rpm it takes to get there. I doubt it will be below 3000.

For now, I'm just trying to get it to run before dealing with trying to optimize.

My biggest issue is still the carb, which you gave me some advice on, thank you!

[Kenth]

The VC1802 (stamped B22) will do fine for a replacement, although i would prefer the VC680 (stamped B1) for Pontiac V8´s.

1112008 starts with 1-5° @ 1300 rpm, intermed. 10-14° @2000 rpm and max 24-28° @4800 rpm´s.

I always verify engine cylinder compression, intake manifold vacuum and function/ parts of the ignition before working on the carb.

[Shiny]

Much appreciated, Kenth!

I can file the stop later if I think I need more vac advance, but will replace the adjustable with the B22 for now.

Thanks for the mechanical timing info. I will try to check mine, but I know it isn't kicking in at idle.

As to getting the basics first, that's where I am.

Compression was 125 to 130 on all 8, base timing looks steady. I have it at 12 now, will see if it likes a different number better.

As to vacuum, I've seen 18" at idle, but now has dropped as it is ragged again.. I sprayed carb cleaner around the carb, got no response. I blocked the brake and tranny ports, no change. Beyond that, how would I detect a vacuum leak?

Original 11 heads, stock 71 intake, seems unlikely I'm not sealing there, but how would I know?

[Shiny]

I am revisiting this topic after going through my distributor to address historical erratic idle timing (another thread). Engine is yet to run so just trying to make good guesses before I try again.


After this discussion in 2018, I somehow ended up with a unit marked B31. I cannot find any specs. Anyone know what this is and whether it makes sense?

Or do I need to replace it??



1970 350, CR likely 7.9? with 11 heads, Summit 2800, QJ, vacuum was 18" last time it ran. Will run ported vac.



Anyone know the specs for this AC Delco PN recommended by Sun Tuned?

Originally Posted by Sun Tuned

Go get ya a delco 1973516 and be done with the stress and erector set theatrics.....

After going through the available charts, I think my original unit was equivalent to a "B12" based on the specs Kenth provided. I don't think this is readily available, nor do I know that it it's a good choice.

Here's a comparison of specs from the chart that seem "close" to the original.



The original provided 20 degrees max, other options go to 16 max.

Any of these are probably OK for now but thought I'd ask again, given the B31 is a mystery..

Mike

[Shiny]

Found eBay listing for an old Warner advance unit with the "original" PN 1115365. It is indeed labeled B12.

Link to eBay listing

[Kenth]

1112008 distributor uses 1115364 vacuum advance from factory, starts at 6-8 in/hg and have max 20° at 13-14.75 in/hg.

1973516 is a 1975-78 Chevy/GMC Truck V8 HEI vacuum advance, starts at 7-9 in/hg and have max 10° at 12-14 in/hg.

B31 would be a Crane adjustable unit.

Dana VC1650 stamped B12 starts at 8-10 in/hg and have 20° at 15-17 in/hg, same as 1115365.

[Sun Tuned]

Gonna stick my nose in the middle here for just a brief minute.

Everyone needs a good operational, accurate vacuum gauge. It’s an almost irreplaceable tuning tool none of us should be without. We’re going to need it to “check” these vac advance canisters regularly these days.

I guess two years ago Standard Ignition bought pretty much all the remaining competitors that were on the field selling ignition parts. As such they pretty much “rule the roost” now so to speak.

The factory info on all these vac advance canisters is historical fact and can easily be found. It used to be that the aftermarket manufactured their widgets awful close to the original part numbers/specifications they were offering replacements for. This is absolutely no longer the case.

GM/Delco still sells some vac advance units but here in the last 6 months , they have not been adhering to the original design specs as they once did. Theirs aren’t far off but they used to be so dependable about this one never really had to question it. The aftermarket??? Hell I don’t know what they’re doing anymore. It’s almost like selling spark plugs anymore where one plug number covers 6 heat range applications. How the hell does this work?????

Use the factory specs as a concrete guide but be advised one had damn well best be in the habit of checking with a vacuum gauge and timing equipment , what these idiots, and I use that term loosely are putting in these boxes.

It may be a direct cross to a factory equivalent number, but the smart money is to check and verify this.

Due to Covid, large scale takeovers, or what I don’t know… but folks please check this stuff these days. It’s a wild world out there these days.

That is all.

[mgarblik]

Sun Tuned is right on here, and he is talking about a relatively simple mechanical device. I would only add that EVERY auto part you buy needs to be checked and not assumed to be good, no matter what brand is on the box.