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Any suggestions on what the new curve should be? Use or plug the vac advance?

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Tech Question

bob santarsieri, schaumburg, IL, 71 plymouth road runner 440

I have a msd pro billet distributor and msd 6al box. What would be the best setting for the distributor? Right now it is set at 2 heavy silver springs and silver bushing per the instructions. It starts to advance at 1400 and goes to 5500rpm then levels off. What would you suggest. 440ci, 0.040" overbore, 0.474" purple shaft cam, Hedman headers and 3.23 gears. Just street driven. Thanks for your help.

Bob, assuming sufficient octane vs. C.R., you need a curve something close to this:

Under 1400RPM - no advance - set initial at 15 degrees BTDC
2000 - 10 dergees mech (25 total)
2500 - 20 degrees mech (35 total) - all in.

Plus, at LEAST 15 degrees of vacuum advance under light throttle (50 deg. total, 52-55 better).

If it currently doesn't go to full advance until 5500, you're giving up lots of power.

If it currently has less (or no) vacuum advance), you're giving up LOTS of fuel economy and fouling plugs.

Rick

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I Don't care about track. I'm not going racing.

Really turns on around 4000 rpm.

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I'm not going racing....

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440 big cam. Likes 19 20 degree's initial advance. Got a recurve kit. Dist is a MP electronic with vac advance. Any suggestions on what the new curve should be? Use or plug the vac advance? Street car. Red light to red light tire frier.

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The initial timing is to provide a clean idle and crisp throttle response. Based on the degree of the camshaft, 10- to 12-degrees of initial timing when the duration of the camshaft is less than 220-degrees @ 0.050” of valve lift; 14- to 16-degrees of initial timing with a camshaft duration of less than 240-degrees @ 0.050”; and 18- to 20-degrees of initial timing when the camshaft duration is less than 260-degrees @ 0.050” of valve lift.

As engine revs increase, it is necessary to advance the ignition timing. If not, the burning process in the combustion chamber would take longer than the speeding piston would permit, resulting in an incomplete burn. To give the burning process a head start, an ignition advance mechanism is provided within the distributor. It is activated by engine revs and by the centrifugal force generated through the weights and springs within the distributor. Consequently, as the engine revs increase so, too, does the advancement of the ignition. The ignition advance mechanism operates either as a mechanical/centrifugal advance only or as a mechanical/centrifugal advance with vacuum assistance. Mechanically operated distributors are usually associated with high-performance and racing engines; vacuum-assisted distributors are chosen for their improved fuel economy and exhaust emissions. Vacuum timing is controlled by the vacuum or depression in the intake manifold and flourishes under light loads. Under acceleration, however, the vacuum in the manifold weakens as the throttle plates of the carburetor open and the ignition advance reverts to the control of the mechanical/centrifugal mechanism. With either design, the timing advance mechanism should not begin advancing before 1,000 rpm and, usually, full advance will have occurred before 3,500 rpm. Too much advance while the engine speed is low can cause harmful pinging or detonation; too little advance as engine revs increase will demonstrate a lack of power. The rate at which the timing advance occurs is referred to as the advance curve. The advance curve used on most production engines and on aftermarket distributors is designed to allow an engine to operate under a wide range of conditions (flat terrain, hilly terrain, towing, fluctuations in the quality of fuel, etc). As a consequence, ignition advance curves tend to be conservative for general use.

POINTS TO REMEMBER
Retarded or insufficient timing advance will cause an engine to lack power. Conversely, excessive timing advance can also cause a lack of engine power but, worse, it can lead to overheating and costly damage from detonation. In many cases, the advance curve of a distributor may not be appropriate for today’s fuels or the engine combination that is being used. Many performance replacement distributors exhibit very slow advance curves; however, they’re usually supplied with an assortment of bushings and springs to facilitate a more rapid curve. The bushings regulate the amount of timing advance; the springs regulate the rate of timing advance. Whether the distributor is an original or a performance replacement unit, it’s prudent to have it checked to ensure its proper operation and its suitability for the vehicle. A rich air/fuel mixture that produces its maximum power at, say, 12.5:1 requires less advance timing than a leaner air/fuel mixture of, say, 14.0:1 (a typical hot rod AFR at cruising speeds). In the former example, extra power is realized by the richer air-fuel mixture; in the latter, economy is achieved by the action of the
vacuum advance mechanism. The reason an engine can exploit the additional spark advance provided by the vacuum advance mechanism is because the leaner air/fuel mixture, occurring at light throttle cruising speeds, takes longer to burn than a richer air/fuel power mixture. In most cases, when using a “hot rod mechanical advance curve,” the desired amount of additional advance generated by the vacuum advance mechanism is around 10-degrees. This additional 10-degrees of advance timing should occur with 10-inches of vacuum or more.