Kings of timing – what do you think of this plan?

[Cobra 29]

I have a FMS 351W Crate engine. I want total advance (initial plus mechanical) to be 34 to 36 degrees at 2800 RPMs. Some sources say that I should have between 14 to 16 degrees of initial timing to get a better burn at idle given my cam which specs at 236 degrees @ .050” intake and 246 degrees @ .050” exhaust.

With my MSD distributor, I have tried 14 degrees initial and 21 degrees in the distributor to get a total of 35, all in by 2800 RPM. The engine does like 14 initial versus 10 or so. It runs smoother and the exhaust fumes are much less, I guess this indicates a more complete burn at slow engine speeds which is good.

My problem is that my mini FMS starter does not like this much initial advance especially on hot summer days in Florida after the first run. It does a better job than the old heavy original type starter but some cranks are slow and I want to avoid this stress (mechanical and emotional!). I have good Optima battery in the trunk, good cables and grounds. For even more direct current, the positive battery cable connects directly to the starter. When the key is turned to start, the starter is tripped by a smaller 12v feed from the starter terminal on the starter solenoid.

Opportunity: I want to use my vacuum advance which came with my MSD small diameter pro-billet distributor for better gas mileage, crisper part throttle response and to help my occasional slow starting situation on hot days. Before you tell me, I know that I should not be concerned about the mileage but I do find the engine revs quicker with the vacuum advance. If I can get quicker response and gas mileage too, I’ll take both!

Here’s the plan. I have physically limited the MSD vacuum advance module to only permit 8 degrees of advance (by installing a bushing on the vertical part of module arm immediately under the mag pick-up plate inside the distributor). I plan to connect the vacuum advance module to manifold vacuum so it will provide 8 additional degrees of advance at idle and less toward WOT. I will then set the initial timing at 10 degrees and keep the 25 degree bushing in the distributor for a total of 35 degrees. Since I will not develop much vacuum when starting, the engine will have only 10 degrees of advance when cranking but get approximately 18 as soon as it starts (the engine pulls 15 inches of vacuum at idle, the module only takes 5 inches to provide all 8 degrees of movement). The 18 degrees at idle will provide the advance for a cleaner burn.

I also understand that during quick acceleration and WOT moments the vacuum module (connected to manifold vacuum) will provide none or less than 8 degrees of advance which is desirable under these conditions. At part throttle, when the manifold vacuum returns, it will provide advance up to 8 degrees for better gas mileage and crisp part-throttle response.

One concern I have is the source of vacuum. Previously, I have had the vacuum attached to ported vacuum (side of carb) and that did provide the crisp engine response when accelerating. I am not sure I will feel the same when the vacuum source is manifold. When connected to ported vacuum, I also believe I achieved too much advance in total at higher RPMs under partal throttle which was due advance from initial and centrifugal and then vacuum a the wrong time!).

Before I re-do all my timing again, what do the timing experts think?

BTW, the vacuum advance modules from Crane and Accel work slightly differently. The Crane adjustment only affects the amount of vacuum needed to pull the entire movement (or advance). You must physically limit the advance by blocking the module arm or blocking the pick-up plate like what is done in some GM distributors. This is not easy to do in a MSD distributor. The adjustment on the Accel module affects both the required vacuum to move the arm and how far the arm will travel. It does not need a physical limiting device to limit the arm (advance). I wish I knew this before I spent the time machining a bushing to limit the MSD module!!!