[Dave Ray]

NOPE, there are 437 different GM OEM large HEI mechanical curves, and just at 20 stock vacuum advances to choose from, and, NO FACTORY SPECS to view.

A couple of things to avoid, if some super tech tells you to weld up the slots, jam a screw in the plate, run, as fast as you can away, they are not the right ways to do it. Also, those after market "curved kits" are NOT FOR STREET USE. They severely stop down the mechanical degrees of advance, and jack the initial to the moon, while removing the vacuum advance, STRICTLY FOR DRAG RACING. The kit spring sets are the only parts kn those kits that are worth having, do not use the weights, center, bushings.

A good combo would be to find a pair of 41 weights, 375 center, use the kit springs, from a Crane 99600-1 adjustable vacuum advance, mount the degrees stop plate the way I outline, NOT the way Crane does it, and go from there.

If you want the FREE, NO ADS info and pictures package on mounting the Crane stop plate, ask for them @:

[email protected]

 

[ricko1966]

Wasn't going to modify the stops wasn't interested in using any after market crap and definitely wasn't wanting to go full mechanical.Was hoping to find listings on O.E distributors and select what I wanted or order O.E. pieces to build what I want. Thank you for letting me know you haven't found distributor specs.either.If it helps you I did find a chart on O.E. vac cans. Hg to open and total advance in Distributor degrees.

 

[Dave Ray]

I have them for the vacuum advances. Best thing in the vacuum advance issue is to go with a Crane full kit, all the 20 different stock settings, and everything in between them.

Until the ZZ crate engines came about, the various divisions did their own weights, center, start/stop, curve settings by varying the weights and center combinations. When we designed and developed the large HEI, we made the basic parts large enough to use a lot of different mechanical advance curves, with ONE basic floor plan, so it would be easy to swap weights/center/springs to match the various emissions curves for all GM engines of the time. We even designed them to be placed one face up for right hand rotation distributors, the other face up, to work with left hand rotation distributors. They were even modified to have no mechanical/vacuum advances for EFI distributors.

There were a very few late 1970's/early 1980's Chevrolet small and big blocks in things like trucks, Suburbans that used the 41/375 combo, that is basically where the ZZ curves came from in the first place.

Some have taken it upon themselves to plot and record curves for various combinations, but the amount barely breaks the surface of what is out there.

The most popular is the ZZ combo, weights 41, center 375, but the ZZ is sprung so tight that the curve doesn't start until about 1,400/1,500 rpms, so, change the springs to get start near 800 or so, limit at or near 2,900 to 3,200 rpms. With that combo, you will attain 22 crankshaft degrees of mechanical advance, and if added to say, 12 initial, total will be 34, 14 initial, total would be 36.

Now, some might have combos recorded to use less mechanical advance, as the various kits give, but wider applications, and they can be beneficial, but don't get too much initial over mechanical, supplement a more conservative initial with a correctly set vacuum advance degrees, and the engine will run great.

ZZ vacuum advance is marked 69120, good for moderate vacuum levels for stock to fairly moderate engine vacuums, only needs the Crane plate mounted on the one side of the vacuum advance pull pin I recommend placing it, not the Crane method. There are a few people here that have done the mods, I am sure they would confirm good results. The vacuum advance modifications allow you to use full manifold vacuum to supplement the initial timing, bringing the idle timing to between 22 and 24 deg/BTDC, just where the engine likes it.

So, yes, I do have the vacuum advance info, thank you for offering it to me.

 

[69-CHVL]

I have them for the vacuum advances. Best thing in the vacuum advance issue is to go with a Crane full kit, all the 20 different stock settings, and everything in between them.

Until the ZZ crate engines came about, the various divisions did their own weights, center, start/stop, curve settings by varying the weights and center combinations. When we designed and developed the large HEI, we made the basic parts large enough to use a lot of different mechanical advance curves, with ONE basic floor plan, so it would be easy to swap weights/center/springs to match the various emissions curves for all GM engines of the time. We even designed them to be placed one face up for right hand rotation distributors, the other face up, to work with left hand rotation distributors. They were even modified to have no mechanical/vacuum advances for EFI distributors.

There were a very few late 1970's/early 1980's Chevrolet small and big blocks in things like trucks, Suburbans that used the 41/375 combo, that is basically where the ZZ curves came from in the first place.

Some have taken it upon themselves to plot and record curves for various combinations, but the amount barely breaks the surface of what is out there.

The most popular is the ZZ combo, weights 41, center 375, but the ZZ is sprung so tight that the curve doesn't start until about 1,400/1,500 rpms, so, change the springs to get start near 800 or so, limit at or near 2,900 to 3,200 rpms. With that combo, you will attain 22 crankshaft degrees of mechanical advance, and if added to say, 12 initial, total will be 34, 14 initial, total would be 36.

Now, some might have combos recorded to use less mechanical advance, as the various kits give, but wider applications, and they can be beneficial, but don't get too much initial over mechanical, supplement a more conservative initial with a correctly set vacuum advance degrees, and the engine will run great.

ZZ vacuum advance is marked 69120, good for moderate vacuum levels for stock to fairly moderate engine vacuums, only needs the Crane plate mounted on the one side of the vacuum advance pull pin I recommend placing it, not the Crane method. There are a few people here that have done the mods, I am sure they would confirm good results. The vacuum advance modifications allow you to use full manifold vacuum to supplement the initial timing, bringing the idle timing to between 22 and 24 deg/BTDC, just where the engine likes it.

So, yes, I do have the vacuum advance info, thank you for offering it to me.

I would think Having an advance curve start at 800 rpm isn’t a good idea considering most engines here idle at 900 or better. Am I off here?

 

[ricko1966]

Thank you I have the info I need now, now I just need to dig through my box of HEIs and see what cams and centers I have and what I'm already running.The ZZsetup. With 12 initial and 14 in the can ought to work. I need to go back and reread what you said but I was thinking 34 total 50 at cruise.

 

[suprdave1]

I have a Big Block with HEI also. I was playing with the timing and set initial at 12° and with vacuum I had 16°. When I increased the rpm's it went all the way to 51°. I am ordering a crane kit and have emailed a request for the information.
Thanks

 

[Dave Ray]

Please, the vacuum advance should be run on FULL MANIFOLD VACUUM SOURING ONLY, NOT PORTED VACUUM.

Then, with initial set to 14 degrees, and vacuum advance set to 10 degrees, IDLE degrees should be 24.

With a 14 initial, and the 41/375 weight/center combo of 22, the total would end up 36 degrees total.

At acceleration, the vacuum advance degrees would go away, then come back in as the engine goes to no load cruise. Total of 36, vacuum advance on no load only, 10, no load cruise 46.

800 is a very high idle rpm.

 

[Georgia69]

I would think Having an advance curve start at 800 rpm isn’t a good idea considering most engines here idle at 900 or better. Am I off here?

355 in my Impala with a 219/227 @ .050 roller cam - I reworked my distributor to Dave's ZZ specs, both mechanical advance and vacuum advance. I was able to idle it down to about 750 in neutral/550 in gear, with a stock converter. VERY smooth idle.


Agree with you though that many/most are running bigger cams now than mine, and they don't do the work to get their distributors right, so most are idling a lot higher, which you can get away with if using a looser converter. I personally just hate loose converters and high/choppy idle speed.

 

[Dave Ray]

Not only distributor curves, but carb setup and tune as well. Then, there are those people that insist on trying to set the carb up before the ignition curves are set correctly, some people, you just can't reach.

 

[Reelysalty]

Not only distributor curves, but carb setup and tune as well. Then, there are those people that insist on trying to set the carb up before the ignition curves are set correctly, some people, you just can't reach.

Most carb problems can be fixed with good distributor set up !

 

[Dave Ray]

As I have alluded to above, and many, many times previous to this specific topic.