Vacuum advance information and myths
The initial timing of a pre-emisions or non-CAP engine provides good power, and idles smooth and strong.
The same engine with a CAP package used much less initial advance. The mechanical advance in the distributor makes up for that above idle rpm.
Pick an engine in the mid-60s that had both a non-emissions and an emissions reduction package on it and compare the inital timing and rpm. Then look at the mechanical advance of in the distributor of each. I've posted a number of these in graph form. Here's one
Even more extreme change in initial was the CAP vs nonCAP 273 4 bbl here.
Last time we thought we found one it turned out to be ported. That was a 1966.
Basically yes.
The initial timing of a pre-emisions or non-CAP engine provides good power, and idles smooth and strong.
The same engine with a CAP package used much less initial advance. The mechanical advance in the distributor makes up for that above idle rpm.
Pick an engine in the mid-60s that had both a non-emissions and an emissions reduction package on it and compare the inital timing and rpm. Then look at the mechanical advance of in the distributor of each. I've posted a number of these in graph form. Here's one
Even more extreme change in initial was the CAP vs nonCAP 273 4 bbl here.
Love to see them.
Last time we thought we found one it turned out to be ported. That was a 1966.
