[cableguy0]

Locking out the mechanical advance will kill your performance and mileage. locking out a distributor is typically only done on race engine which have another form of ignition advance electronically. The amount of timing provided by the mechanical advance depends on the distributor and the springs and weights used. Generally on a sbc you want 36-38 degrees total timing(initial plus mechanical. Then the vacuum advance will come in on top of that. usually sbc's like between 12-18 degrees of initial initial timing. Mechanical will provide the rest to get to the 36-38 range. Theres a good article at the top of the engine and drivetrain forum on how to properly set the mechanical advance and curve the distributor.