Quote Originally Posted by 34_40 View Post
...first at wide open throttle (or under heavy acceleration) there is little to no vacuum! That vacuum chamber does nothing but retards timing in that situation so it prevents ping. And 2, when tuning centrifugal only or dual advance systems, you MUST KNOW the rpm range you want to work in and what the available vacuum will be in that range. Being at wot is really immaterial as the full timing should be "all in" before say 3K rpm.
Yeah Mike, we're saying the same thing but from different directions. You're considering running at cruise and going to WOT, where the vacuum drops out and it "retards" back to the mechanical curve; while I was looking at accelerating at full throttle, and backing off to cruise, where the vacuum recovers and pulls in more timing. I peeked at my spark map, and indeed it goes "all in" to 35.5 degrees (pistons say not to exceed 36) at 3000 rpm and above. But it also throws in more timing above 3000rpm but with MAP at zero, 10" and 12", going all the way up to 40" advance. I had a hard time getting my head around that until I realized that the only way to get to that condition was to be coasting down the mountain above 3000 rpm, off the throttle. One of the guys at Edelbrock helped me understand that area of the map.

Quote Originally Posted by jerry clayton View Post
I'm sorry boys, but the distributer in the pic that Roger referanced has a vacume advance unit on it!!!!!!!!!!!!
Indeed it does, Jerry, indeed it does. So you're saying that the old mechanical advance only distributors actually have a different body, where they use the same shaft but everything sits higher since they don't have the vacuum canister boss & linkage?