full manifold vacuum advance help

[LONGHAIR]

There is one small detail about Indyduke's post that I disagree with though...
You will almost never get all of that advance (the "theoretical" 52 degrees) because of the lack of vacuum at 3000RPM on the street. The vacuum advance unit only adds to the advance while it is receiving vacuum....so at road speed, with the load of pushing the truck, your vacuum drops...and the vacuum advance drops too.
About the only time you are ever going to see all of that advence is when you lift off of the throttle while cruzing above 300RPM....you don't really need much advance then anyway...

[Indyuke]

Yes, that's true. On the highway I really only see about 45deg or so because manifold vacuum is something like 17-18Hg. Anything approaching 60deg advance and the motor starts to surge and choke on itself. Then at that point I'm just shooting myself in the foot, so I always try to absolutely limit total advance to 52-54.

Good call.

[myraley]

I wanted to add to this thread too. I have read everything I could find - Lars article (the one in the FAQ), High performance pontiac articles, 4X4 Power articles, the instructions by MSD and ACCEL, and Edelbrock, and then even e-mailed Lars Grimsrud (he responded right away, which was nice, but wasnt very patient with me being a slow learner and still having questions). I have a BBC and am currently at 12 initial, 24 mechanical, and ported vac adding another 20 (which was supposed to only be 14, Pertronix!) Which based on everything I have read is too much at cruise, so I am going to install the adjustable vac advance from ACCEL, put it at 12, and try that. My point in adding to the thread, is dont rely on what the company (Pertronix!) says the advance is, check it for yourself. I'm just giving Pertronix a hard time, but its true for every company. When I talked to the Tech at MSD, he said their Vac advances were supposed to stop at 10 degrees, but he has ones that have gone to 14 or 16. Probably not a problem, but if you're right on the edge of detonation, it could be. I also note that even among those recommending manifold vac advance, they sometimes disagree on what those settings should be.