289 Timing...What determines the Differences?

[Chad Johnson]

Ren man,

The cam card deals with valve timing. I was under the impression you want to talk about ignition timing. The cam would be degreed
during the building process; once the engine is buttoned up, that is a done deal.

Ignition timing is generally accomplished by distributor adjustment.

Initial ignition advance is the number of degrees BTDC
that #1 cylinder fires; 10 - 20 degrees is common and is in direct proportion to camshaft duration (the more duration the more initial timing that may be desired). Based upon your cam card 8-10 degrees should be fine. This is set using a timing light by rotating the distributor clockwise or counterclock wise with the engine at idle (vacuum advance if applicable should be plugged) until the pointer is on the desired degrees on the harmonic balancer. Too much initial advance will sometimes cause an engine to kick back or struggle to turn over (like a dead battery). Therefore some racers who lock their distributors at the desired total advance (see prior post regarding compression ratio) have a box to retard the timing when starting the engine.

Mechanical advance is a function of weights and springs attached to the advance assembly. The amount of mechanical advance is established by a limiter that prohibits advance assembly rotation. On MSD's there is several different "stop bushings" that can be installed to limit the amount of advance (standard are 28, 25, 21 and 18 degrees).

Total advance is the combination of initial advance and mechanical advance. In my case I run 14 degrees of initial advance with a 21 degree stop bushing so that I have total advance of 35 degrees (again refer to prior post on compression ratios)

Finally, for peak performance your distributor should be curved or tuned to provide total advance at the optimum RPM. Many high performance engines like full advance at between 2500 and 3000 RPMs. This "curving" is accomplished by changing the mechanical weights and springs. Lighter springs will allow the advance to come in sooner.

Do some searches on the web and you should find some additional information.