It will advance under vacuum and retard without vacuum. That's why its vacuum advance.

X 2

The higher the vacuum, the more it will advance up to it's rated max advance. This is for light throttle, high intake vacuum (ie: low load) situations for better fuel mileage, better emissions, more complete burn of a low air/fuel filled combustion chamber.

Higher throttle, low intake vacuum (:ie: high load) has a combustion chamber greatly filled with air/fuel and needs/wants less advance. Thus why the canister goes back to a non-advance setting.


The canister is referenced to atmospheric, same as fuel psi regulators on the opposite side of the canister/regulator diaphragm than the vacuum is hooked up to.

This is all done mechanically.....now, on the electronic boxes that change advance, they cannot move the relationship of the rotor vs the distribur cap like the mechanical ones with weights as we are discussing, therefore the rotor is set way advanced of the the cap posts because the eletronic boxes or modules can only retard. The more advance it gives means that it is retarding less. The less advance it gives, the more it retards. The end result is the same, but it has to factor in time which the mechanical advance distributor doersn't. I guess this is a subject for another thread.

Personally i wouldnt get too tripped up over voodo science of the hei , but i would be wondering if some cylinder pressure is absent that wouod cause you problems at 50 degrees. Either way , the hei units take a little extra power at the b+ terminal. good wiring with lots of dielectric grease is a good idea if you have to keep the crate distdibutors on that app. I seem to remember the typical hei centrifigal adv adding around 10° or so. But to be in the 50's total off the dyno sounds odd. Might be stump puller builds with 475 hp each and they rolled the timing out trying to squeak another mph out of it

I use the HEI's all the time, run them locked sometimes, but always with the vac advance a couple company's make a clean plastic block off to remove the canister all together

imo,a vac advance dist has no business installed in a boat,espically a heavy one.their is no such thing as lite load cruising in a boat.i will never understand why some guys want to install a vac advance dist from a 1980 chevy on their boat engine.

this is incorrect.the vac advance dist is made to advance timing in high vac times.

How the advance works depends on the application. Some vehicles run full manifold vacuum to the advance. In this app. while accelerating vacuum decreases along with timing. Some run ported vacuum which increases under acceleration advancing timing.

sorry, that was without vac advance...

have you verified that your balancer/timing marks are correct?if not,,your just spinning your wheels.

[ATTACH=CONFIG]545659[/ATTACH]I'm running 18 initial and 38 total timing on my zz572s. With aftermarket Mallory distributors with all mechanical advance[ATTACH=CONFIG]545661[/ATTACH]