I post earlier about issues with slight pinging on tip in throttle while driving - new summit HEI 12 initial 34 3000 and have vacuum advance to ported vacuum (left side port on edelbrock carb).
after doing research and following suggestions on forum, I have moved advanced to manifold vacuum (right side port on carb). 12 initial 34 total but when I hook up advance I get 30 degrees initial and it climbs to 50-55 degrees at 3000. I took it out for quick drive and runs great. I thought though that timing would not climb but drop once throttle opened when using manifold vacuum. Is this timing too high when only 34 with advance disconnected at 3000 but 50+ at 3000 w vacuum advance on manifold.
thanks for any help in understanding.

 

If the engine runs well that way, go with it. Nothing wrong with those timing numbers as long as there is no pinging. You can always limit the vacuum advance if needed.

 

I thought once throttle opened vacuum dropped and I expected manifold vacuum advance to fall off. Is it behave differently in park revving engine vs on the road driving.
thanks

 

I ran nearly 60 degrees on iron head, low compression Olds V8s. Those numbers are fine if it isn't pinging.

 

This was basically a stock rebuild 350 weeks ago mild person cam, 650 edelbrock, rpm intake, stock exhaust manifolds

 

It depends upon how much the throttle is opened. Wide open throttle results in a low vacuum signal, but partial throttle can generate a strong vacuum signal.

Yes, with no load on the engine (in park) the vacuum signal will be somewhat different.

 

You need to mechanically limit the vac advance to 10*

 

I have an adjustable vacuum can, I’ll back out adjuster and see how timing changes.
i need to take it on a longer drive and see how does, around the neighborhood with current timing it really felt good, i was just worried about 50 degree total timing if it would hurt engine
I haven’t dealt w vacuum advance in years - last few builds have been performance/racing and were either locked out timing or initial and mechanical
thanks for input

 

I had a 9 to 1 Olds 350 214/214 .472/.472 on a 110 LSA cam. I ran 22 base and 38 total. It had a 30 degree vacuum can on manifold vacuum. In the 40's at idle and high 50's part throttle. No pinging but had low speed bucking just off idle. The 20 degree vacuum can fixed it but was slightly less responsive at part throttle.

 

Limit the vacuum advance with a stop. All the distributors I ever tried either left me with part throttle ping or low power. I tried the adjustable cans, tried heavier distributor springs. I tried Buick and Cadillac vacuum cans with different spring rates than Olds vacuum cans.

The limiter was crude, but it worked. The hard part with that is the limiter can get rid of part throttle ping, but when you tweak (advance) the timing up to 34 or 38, I found my engines had a hard time starting, even with stout batteries, fresh starters and high-amp alternators.

The end of that road for me was Progression Ignitions HEI distributors where I can make my own timing advance maps. Done. Not cheap and they come with software risks, but I'm on that train across 2 cars and happy.

Your quick cheap fix is a homemade "vacuum stop" which limits advance at part throttle to stop the ping. Look here or on Chevy forums for more detail on how to make one.

The big problem here is our gas has gotten worse, now contains alcohol, and the manufacturers are not building as many varieties of distributors and parts as they did in the past.

The other, not very cheap solution is to use race gas or AV-gas if you can get it, but I'll bet the prices make your eyes water....

Chris