Ignition Timing

 

Ignition Timing

 

 

As you probably can guess, the timing advance curve for older Chevy V8’s was set up by the factory as a compromise. The distributor timing curve and initial advance settings were designed to accommodate a wide range of operating conditions, such as driving habits, weather, altitude, maintenance schedules, and gasoline quality.  Also, different vehicle weights, gearing, etc., enter into the equation.
Starting in 1969, government emissions standards started affecting the factory settings as well.  Of course, we Chevelle owners tend to narrow the range of these conditions. To that end, our engines will usually run more efficiently (read MORE POWER without sacrificing fuel economy) with a more aggressive timing advance curve.  Idle quality and drivability also improve.

As an example, the ’69 Chevrolet Chassis Service Manual specifies for most V8’s total (mech plus initial, not including vacuum advance) advance of 30 to 36 degrees, with initial settings ranging from 0 to 4 degrees.  The maximum mechanical (centrifugal) advances are specified at from 3800 to 5000 rpm.

Vacuum advance settings were designed for maximums of 15 degrees.  The ideal timing curve will vary some with factors such as vehicle weight, overall gearing, modifications to the engine, etc.

Generally, most Chevy V8’s, both small and big block, operate most efficiently with timing curves of 36 to 40 degrees advance total timing (mech plus initial), with initial advance (set with a timing light with vacuum advance disconnected) in the 10-14 degrees for a street use engine.

The rpm where maximum mech advance is reached should be in the 2000 to 3500 range. This setting is affected most by vehicle weight and gearing.  Vacuum advance, actuated by full manifold vacuum as pre-’73 Chevys were, should be limited to 10-12 degrees with the more aggressive total timing setups.  Initial+ mech+vacuum advance should not be over about 54 degrees in any case.

Distributors can be “recurved” by changing mechanical advance weights, springs and bushings in order to tailor the advance curve to a specific engine and vehicle.

Vacuum advances can be replaced with different stock type versions or a Crane adjustable unit.  Limiters can also be fabricated to limit vacuum advance to the desired amount.

It’s possible to recurve a distributor with it in the engine using a timing light and timing tape (see last issue) or it can be done on a distributor machine.

It is important to not have too much timing for a specific operating condition, i.e. rpm, load, gear, temp, gas octane, etc., or pinging (detonation) can result.

This can severely damage an engine, so caution is advised when revising timing curves.  Changes must be made or gas octane increased if pinging is encountered while driving.