SB Ignition Timing with a vacuum advance

[Cobra 29]

My goal was to incorporate a vacuum advance module for quicker ignition advance / engine response from off-idle to mid-throttle, easier starting and some gas mileage improvement. I believe that I have accomplished this goal with the advice from many on ClubCobra. Thanks in advance.

The engine is a FMS 351W which uses a 8478 MSD vacuum advance distributor and Holley 650 DP. After a lot of study, I realized that I needed to limit the total advance to 32 BTDC. More sounded better but more BTDC advance is not better given modern fuels that burn quicker, a high output ignition system and aluminum heads!

At first I disconnected the vacuum advance module and tried different total advance degrees until I found a setting that provided strong power, no pre-ignition and avoided the mid-RPM miss. The engine ran great with 7 degrees initial and 25 degrees centrifugal in the distributor. But 7 degrees at idle is not enough advance for a clean burn at lower engine speeds with the cam duration I have. So, I kept the initial advance at 7 degrees, changed to the distributor to the 21 degree bushing and added an Accel vacuum advance module. I adjusted the advance degrees from the Accel vacuum module to a maximum of 4 degrees. The vacuum advance modules from MSD (original with distributor) and an “adjustable” unit from Crane cannot be easily limited to a few degrees. Both added 12 to 15 degrees which is way too much. The MSD module was not adjustable and Crane unit only permits you to adjust the amount of vacuum that operates the unit but not the amount of advance (meaning travel of the arm connecting the vacuum diaphragm to the magnetic pickup plate).

The engine is limited to 32 degrees (7 initial plus 21 from the distributor and 4 from the vacuum module). When starting, the engine is at 7 degrees which is easy for the starter and flywheel teeth. Once started, the ignition advances immediately to 11 degrees because the module is connected to manifold vacuum. Idling at 11 degrees provides more advance for a cleaner / more complete fuel burn at slower engine speeds. Because the Accel unit only requires less than 10 inches of vacuum to deliver 4 degrees, anytime the engine is running, it is likely the module is adding all 4 degrees. I am going to try some WOT runs and monitor the inches of mercury in the vacuum line connecting the manifold and vacuum module to see if it drops below 5 inches or so. If it does, the vacuum module may reduce its contribution of degrees (retard) which is desired at high engine RPMs.

Presently, I am using the two blue springs that allow the total centrifugal advance to occur at 3200 RPM. I plan to exchange one blue spring for a light sliver spring to accomplish the maximum centrifugal advance at 2700 RPM. I want to see if the engine will become even more responsive.

After I finalize the springs (centrifugal advance arte), I will attempt to add more vacuum advance from 4 degrees to 6 to 8 to improve the idle timing (cleaner burn) and potential for better gas mileage at part throttle but I must avoid the mid-RPM miss and pre-ignition etc. The good news is this adjustment is accomplished by an allen wrench!

I think the above is a good approach assuming you are not attempting to set a very high initial advance, like 16 to 18 degrees and limit your centrifugal advance to 18 degrees or so. My mini-torque starter never liked any initial advance over 13 degrees or so.