This post is intended to address the discussion of whether distributor vacuum advance should be used......
To optimize efficiency, a Streetcar must run vacuum advance. Optimal full cylinder pressure for Max torque is about 12 degrees after top dead center. Under Full Throttle and over about 2500 rpm the cylinders are full of fuel and air and the burning occurs quickly and we need about 36 degrees before top dead center to have maximum cylinder pressure at 12 degrees after top dead center. When cruising we are not filling the cylinder with fuel and air so the burning occurs much slower. So to maintain our max cylinder pressure at 12 degrees after top dead center we must ignite the mixture even earlier. Typically 12 to 20 degrees of vacuum advance facilitates this goal.
The allowed amount of vacuum advance is dictated by the initial timing setting. (A minimal initial timing setting and a slow, non performance mechanical advance curve allows 20 deg of vacuum advance. A more advance initial timing setting with fast, performance advance curve will reduce the allowed amount of vacuum advance to about 12-15deg to avoid misfiring at and around idle.
(NOTE: For best off-idle performance and response, initial timing must be maximized. 16 ish deg initial timing is about right. Then mechanical advance must be adjusted (limited) to 20 Engine deg to attain 36 ish deg total engine advance.) BUT: When running this much initial mechanical advance, Vacuum advance must be limited at idle and off idle to avoid erratic/rough idle (mis-firing due to early spark ignition at low speed).
The only real option is whether to use full vacuum at idle or not. Using full vacuum at idle allows the engine to have a smaller carburetor opening and use less fuel and will actually run cooler at idle and cruising speeds. Using ported vacuum eliminates the additional cooling at idle, but still allows for cooler engine temps while cruising.
This applies to all of our older School engines not just Ford.
To optimize efficiency, a Streetcar must run vacuum advance. Optimal full cylinder pressure for Max torque is about 12 degrees after top dead center. Under Full Throttle and over about 2500 rpm the cylinders are full of fuel and air and the burning occurs quickly and we need about 36 degrees before top dead center to have maximum cylinder pressure at 12 degrees after top dead center. When cruising we are not filling the cylinder with fuel and air so the burning occurs much slower. So to maintain our max cylinder pressure at 12 degrees after top dead center we must ignite the mixture even earlier. Typically 12 to 20 degrees of vacuum advance facilitates this goal.
The allowed amount of vacuum advance is dictated by the initial timing setting. (A minimal initial timing setting and a slow, non performance mechanical advance curve allows 20 deg of vacuum advance. A more advance initial timing setting with fast, performance advance curve will reduce the allowed amount of vacuum advance to about 12-15deg to avoid misfiring at and around idle.
(NOTE: For best off-idle performance and response, initial timing must be maximized. 16 ish deg initial timing is about right. Then mechanical advance must be adjusted (limited) to 20 Engine deg to attain 36 ish deg total engine advance.) BUT: When running this much initial mechanical advance, Vacuum advance must be limited at idle and off idle to avoid erratic/rough idle (mis-firing due to early spark ignition at low speed).
The only real option is whether to use full vacuum at idle or not. Using full vacuum at idle allows the engine to have a smaller carburetor opening and use less fuel and will actually run cooler at idle and cruising speeds. Using ported vacuum eliminates the additional cooling at idle, but still allows for cooler engine temps while cruising.
This applies to all of our older School engines not just Ford.
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