spark plug heat range questions.

You switched from manifold to ported v-advance didnt you?

I think your original issue was mainly due to too much vacuum advance at idle (vacuum retard under high load). Running off manifold, you may should have dialed back a little less total vac advance.

I think now your vacuum advance comes in off idle so it likes the curve behavior better.

My understanding is that ported vacuum advance still goes away under high load as well as it resides just above the throttle valve , below the venturi, to eliminate it during idle. Once the flaps are open a little they allow the port to be exposed to vacuum, the vac advance should be in full.

Unless the port is inside the venturi of the carb then vac-advance would increase as velocity thru the venturi increases. Which I dont think is the case.

I run vac-advance off the manifold on a 440 w no issues, runs quite great, but it is low comp so.

Here is a link that has good dialogue regarding burn rates, older vs modern and ported vs manifold. I am not trying to stir the pot. I just think it is applicable to your optimization.

Hopefully it doesn't upset folks that he talks about chevy. It sounds like one of the fellows is very keen on burn rate behavior.

http://www.hotrodders.com/forum/ported-vs-manifold-vacuum-advance-scenario-151264.html

Another link that I think clarifies the behavior and necessities of vacuum advance.

http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

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