[Jack21]

Here's the "secret". You Duraspark fans ever notice that to do anything to the advance mechanism, even clean it, you have to take the distributor out of the engine, then take the distributor apart before you can do anything. Great idea, huh?

What MSD did was to take a Delco distributor (aka Chevrolet), and make a Ford sized stalk to put it on. Now, the advance mechanism is tight on top where you can work on it. MSD, for reasons known only to MSD, 1) does not make an adjustable vacuum advance unit, 2) will not use or even recommend a competitors unit (Crane, Accel). The MSD vacuum advance is fixed at 16 degreees which is too much with the centrifugal advance dialed in where you need it. Other than that, the vacuum unit is a '57 - '73 Delco (Chevrolet) vacuum advance. Crane and Accel (and possibly Moroso) sell replacement adjustable vacuum advance units that let you crank the vacuum advance down to 8 - 12 degrees.

Vacuum advance on a street driven car is there to optimize part throttle cruise fuel economy because a leaner fuel mixture takes longer to burn, and requires more ignition lead. Obviously, on a race car, there is no such thing as part throttle cruise economy and vacuum advance is not necessary. Todays' EFI cars use a computer to dial all this in, let you run in the 11's or 12's at the strip, and get 22 MPG on the street. Us carb guys running on the street can choose 10-12 MPG without the vacuum advance, or 16-18 MPG with.