Adjustable vacuum advance

[midnight sun]

Anyone have a preferred way to adjust an adjustable vacuum advance?  Ive read most of the different manufactures instructions which quite frankly run the gamut from starting at full clockwise and backing off if detonating, or start at full counterclockwise and go up one or two turns with the allen wrench until detonation and then back off a turn,  or start right in the middle and go up or down as needed.  I understand that you want as much vacuum as possible without the engine detonating but Ive tried all methods and cant seem to find much difference.  At full clockwise it runs great and I dont hear any detonation but that seems a bit much to be at the top end of the vacuum.  I performed these different ways of adjustment at acceleration up to about 3500rpm and not WOT.  This is on a GM HEI distributor.

Thanks in Advance.

[WZ JUNK]

I set timing for total advance which is a combination of vacuum and mechanical.  So what you are setting is the vacuum part of the total advance.  I suppose if you had the vehicle on a dyno, you could play with different settings on the vacuum and mechanical and see which worked best.  I would try different variations of the vacuum but set the timing using total advance.  33 degrees total is a good reference for me.

That is all I know.

John

[midnight sun]

I set timing for total advance which is a combination of vacuum and mechanical.  So what you are setting is the vacuum part of the total advance.  I suppose if you had the vehicle on a dyno, you could play with different settings on the vacuum and mechanical and see which worked best.  I would try different variations of the vacuum but set the timing using total advance.  33 degrees total is a good reference for me.

That is all I know.

John

Thanks John.

Yeah Im at 34 total right now. Guess Ill just have to mess around with the adjustment.  Wondering if I should just hook up a vacuum gauge and see where im at in inches?  Just a thought.

[Mikej]

What is your total timing with the vacuum unhooked? When you are at WOW there shouldn't be vacuum advance.  You may need to limit your mechanical adv. or you initial adv.

[midnight sun]

What is your total timing with the vacuum unhooked? When you are at WOW there shouldn't be vacuum advance.  You may need to limit your mechanical adv. or you initial adv.

Total timing being initial (idle) plus mechanical is 34 degrees.

[Mikej]

If you hook up the vacuum, how much does it increase? You should be able to see this number change as you screw the adjustment screw in and out. Just for reference I  am at 28 deg and than about 10 deg on vacuum. When I bought the car I have it was at 38 and 20= 58. What a pinging turd. Limit the initial and mechanical is what got it to stop pinging. 91 octane and 10.9 compression. Vacuum for idle and cruise.

[Arnold]

Find out how much advance it will tolerate easily on starting. Sbc's..around 15 or so with a warm engine and about 9:1 c/r.
Have all your advance in by about 3000. Either have the dist done or do it yourself. Weights,springs specs etc.Timing tape whatever.
Vac advance. Find the factory can that has the stiffest spring,comes on the latest and has the least advance.This is the safest way to start.

I wouldn't be trusting self adjustabe vac advances unless I had some SURE way of making SURE there was no high speed detonation.

You cannot always hear higher speed detonation. Sometimes by the time you feel it the engine is toast.

Propane sbc's would tolerate 22 initial and a really steep curve!
Ya..too bad that it was firing so early that they took out rod bearings.
ZERO noise detonation wise.Ya..a BIT less and you could torch pistons silently too.

You can use a thermo couple and an egt probe in the ex man right at the port.

I used to like to use a Floscan mpg computer when I played around with advance. WITH vac advance..AT the maximum mpg there was no need for more advance.

My .02