[DanEC]

I left a few things out of my post. To get the best advantage of vacuum advance it has to be hooked up to a full time vacuum source below the throttle plate or directly from the manifold. This improves the idle quality and lowers the engine tempreature when sitting in slow speed traffic. Using a ported vacuum source defeats the purpose of using vacuum advance.

There is a problem that can arrive from an over-advanced situation at cruising speed - this occurred on my Corvette. I have a special vacuum can that works at low vacuum due to the old-school cam in my car (only pulls 8 inch max vacuum at idle). On the highway I had a constant light miss - it almost felt like a surge. This was from too much total vacuum and firing the engine too far from TDC. I solved the problem by configuring a stop on the vacuum can to limit the total amount of additional advance from vacuum to 8 degrees. That amount still helps my motor idle better and seems to work at keeping my engine temperature down. With 36 degrees mechanical and initial advance in the car - I have a potential max of 44 now at cruise if all the mech advance is in.

But your right - sometimes the vacuum unit can bring in too much total advance in cruising conditions. Not a good thing but since vacuum falls off as soon as you lean on the throttle I would think there is little danger to the engine.