Guys, how do you degree in a cam? I always thought that the timing was pre set at TDC when you make the dots line up on the chain or the gears. And can someone explain the distributor timing, mechanical and electrical = total and how you get there. I am going to do my distributor over ( in previous thread ) but total timing was mentioned and I really do not understand the concept. I understand 0 degrees is TDC and I always would set at about 8 - 10 degrees BTDC. Please explain...... thanks,
Roland in NY

here's one link...

http://www.hotrodder.com/kwkride/degree.html

david

Degreeing the cam is the process of checking that the cam, when installed, acutally does initiate the valve events at the time desired.

It allows you to verify that the cam is manufactured and marked correctly.

It is also done when you want to advance or retard it to adjust performance, more or less, it will shift the power higher or lower in the rpm band to some degree.

For a stock rebuild, I do not always do it. For my hotrod motors, mostly yes.

Ignition timing typically has 3 components, base timing, mechanical advance and vacuum advance.

base timing is where you set the distributor, at idle, with neither mech or vac effective.

Mechanical or centrifugal advance has weights that advance the timing purely based on engine/distributor rpm and centrifugal force.

Vacuum advance works in response to engine load via venturi vacuum through the carb.
Higher airflow at higher speeds gives more vacuum and more advance. It is typically not manifold vacuum, that varies opposite to what you want for advance.

A typical setup would be 10 deg base, 20 deg mech, all in by 2000rpm, and perhaps 10 more deg of vacuum. Total timing would be 40 deg. This is more of a street car cruise scenario. In many cases you will not see full vacuum at normal cruise.

Most cars with distributors work per the above. However, the timing can be done totally by electronics, doing away with vac, mech or base adjustments, and lately, getting rid of distributors....

Vacuum advance works in response to engine load via venturi vacuum through the carb.
Higher airflow at higher speeds gives more vacuum and more advance. It is typically not manifold vacuum, that varies opposite to what you want for advance.

Vacuum advance does not work on venturi vacuum, that's a signal reserved for the vacuum secondary pots on the Holley carbs. Vacuum advance senses manifold vacuum and adjusts the timing accordingly. Some systems use a port above the throttle blade that doesn't present manifold vacuum to the VA can until the throttle blades are partially open, and this is called 'ported' vacuum.