Vacuum advance adjustment

[texasmike]

Thanks for all of your time and help, Richard. I'm not sure if I followed all of that but I'll give it a lot of thought and give it time to sink in.

 

[Ricko1966]

Thanks for all of your time and help, Richard. I'm not sure if I followed all of that but I'll give it a lot of thought and give it time to sink in.

What your looking for is a spot where you can accerate to 3/4ths or so of 2nd gear. Check your speed at that distance. start with 30 degrees total and start coming up 2 degrees each time until your speed starts to drop or you get ping. Then back it down 2 degreees. Then we know what your engine likes for total. Once we know that Then we can move on to figure out how fast we can bring g total In. Then we can figure the vacuum.

 

[75gmck25]

Vacuum advance is used to improve gas mileage at cruise, not improve acceleration. You are not losing any performance/acceleration with 9 degrees (vs.18 or 20) because it all goes away as soon as you hit the throttle and the vacuum drops. Racers looking for max performance on the track or strip don’t use any vacuum advance, since it does nothing to make the car go faster.

I have aluminum heads and about 9.2 compression, with a 213/217@.050 cam and it runs about 18-19” vacuum at idle. I use 15 degrees base timing, 20 mechanical by 2000 rpm, and a GM vacuum can that adds about 15. I don’t know whether it’s making max performance, but it works and does not cause pinging. The big difference from your truck is that I have a TH350 and 4.10 gears, so I’m turning about 3000 rpm on the highway. It provides decent acceleration, but gas mileage sucks.

 

[rich weyand]

You could get an AR23/VC1853 vacuum advance can. It will give you 15 degrees advance at closed throttle. That will allow a base timing of 16-18 and mechanical advance of 18 without going over 51 degrees advance at cruise.

But your numbers now look right to me. As long as you're not getting any knock, you're probably about where you want to be. And manifold vacuum is the right choice without pollution controls.

The only advantage of the AR23/VC1853 can for you would be that it is not adjustable and can't wander off. Since you now know what you need, the fixed can is a little more reliable in that way.

See this post for all the particulars on timing the GMC first-gen V-8s.
https://www.gmsquarebody.com/threads/ignition-timing-for-first-generation-gm-v-8-engines.14508/

 

[Ricko1966]

36 degrees total is not a written in stone number some engines like less some like more that was why I said sneak up 2 degrees at a time until trap speeds start to drop then go back 2 degrees. Ideally we would figure this out with trap speeds at the strip but for most people a pull through second gear is going to be our only tuning tool.

 

[texasmike]

You could get an AR23/VC1853 vacuum advance can. It will give you 15 degrees advance at closed throttle. That will allow a base timing of 16-18 and mechanical advance of 18 without going over 51 degrees advance at cruise.

But your numbers now look right to me. As long as you're not getting any knock, you're probably about where you want to be. And manifold vacuum is the right choice without pollution controls.

The only advantage of the AR23/VC1853 can for you would be that it is not adjustable and can't wander off. Since you now know what you need, the fixed can is a little more reliable in that way.

See this post for all the particulars on timing the GMC first-gen V-8s.
https://www.gmsquarebody.com/threads/ignition-timing-for-first-generation-gm-v-8-engines.14508/

Thanks for the reply Rich, and BTW, I have read your excellent article from 11-22-15 many times. Since I have the Allen screw in the can of this Cardone distributor turned out all the way, tight, I'm hoping the advance will stay fixed at 16º. I think I'll drive it this way for awhile and see if I get any mpg change. Then, I'll check all of the settings and decide if I should give Richards suggestion a try.

 

[Matt69olds]

Read this:

https://www.jegs.com/installationinstructions/200/270/270-99600-1.pdf

What you’re suppose to do is find the base/mechanical advance that the e gone likes, and thenn move on to the vacuum advance.

 

[Ricko1966]

Mike read what Matt put up for you it's the same thing I was trying to tell you in post 17 except I like to use best trap speed with no ping to determine my total. Other than that I'd say those are the best directions I've seen. Trap speed is a better indicator of what your engine wants,just because you don't have ping,doesn't mean you hit the sweet spot. If you light off too early even if you don't have ping your losing horsepower.

 

[Ricko1966]

@rich weyand @Matt69olds when I look at his numbers here's my thoughts,if he doesn't play with base timing or springs(which I think he should do) my opinion is he needs to add at least 5 more degrees vacuum advance and make sure he's got a can that takes high vacuum to be fully in. So his vacuum advance drops out before it overlaps with his centrifugal.As in let's say his can is all in at 8hg, but he's got 20hg at idle and let's say 10hg at 1/4 throttle,now his Vacuum advance is still all in, at 1/4 throttle.My opinion is he needs to give it more vacuum advance and make sure it drops out at light throttle. Right now his only got 41 degrees at 65mph cruise. I think even just 1 lighter spring, 2 more degrees initial and a couple degrees vacuum would be better as long as it doesn't ping. Those are just my thoughts.