What is the correct amount of vacumn advance?

2shelbys said
Here's an easy way to determine the corrrect initial advance for your particular setup:

Start the car and let it warm to operating temperature then shut it off.

Advance the timing 2° and let it sit for 3 -5 minutes to allow the temperature to spike.

Fire it back up. Did it pull the starter down like the battery was weak? If not, repeat the process until it does. Back the timing off 2° and lock it down. This is the maximum advance that your setup will handle and that is what you want.

That is not the best way to determine initial advance. Warming the engine up is correct but you need a vacuum gauge connected to manifold vacuum. Initial timing is a function of the duration of the cam and the design of the chamber. Assuming the the heads don't change then it's just a function of the cam. Cams with more duration than stock will require more initial advance for good idle and off idle throttle response.

Increase the timing 2 degrees. If the manifold vacuum increased reset the idle speed and increase the timing 2 more degrees. Continue this until there is no longer an increase in manifold vacuum.

Now if your engine kicks back against the starter (it shouldn't until your initial is 20 or more degrees BTDC) you will either need to turn it back until it doesn't (now no longer ideal for the engine) or if you are using something like an MSD add a timing retard for starting.

The total mechanical advance is a function of the design of the chambers and intake ports. Without dyno work the typical recomended numbers you here for sb 35-36 or bb 36-38 are the best you can do. NOTE: magnum heads becuase of their efficient chamber and port design like less, 32-33 degrees BTDC.

Once the intial an total mechanical is dialed in you go after the vacuum advance. You want as much vacuum advance as you can get at part throttle cruise that does not cause part throttle tip in detonation. The sum of total mechanical and vacuum should add up to around 50-52 degrees BTDC.