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Discussion Starter #1 (Edited by Moderator)
The "hot rod advance curve" used mostly on a 9:1 engine with a mild camshaft (duration less than 220 degrees at 0.050-inch of valve lift) is 10 to 12 degrees initial timing plus 22 to 24 degrees of additional advance from the mechanical advance mechanism. In most cases, full advance (32 to 36 degrees) will have occurred before 3,500 rpm. An engine with camshaft duration of above 240 degrees, but less than 260 degrees of duration @ 0.050-inch valve lift, will respond well to 18 degrees of initial timing; however, the total timing will remain the same (32 to 36 degrees). The ideal ignition timing for power occurs just before the point where detonation or pinging takes place. Correctly timed ignition will cause peak cylinder pressures to occur around 12 to 15 degrees after TDC. If the peak cylinder pressure is reached too early, power will be lost as the piston fights to compress the burning air/fuel mixture. Detonation may also occur, which can lead to engine failure. Conversely, if peak pressure is reached after the 12 to 15 degree range, energy is wasted and dispersed through the exhaust system as heat.

The Innovate Motorsports LM-1 Air/Fuel Ratio Meter is a valuable tool when it comes time to adjust carb jetting and ignition timing. The meter provides data on exactly how rich or lean an engine is running at any load. Its self-calibrating circuitry also compensates for changes in temperature, altitude, and sensor condition.

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. In conclusion, 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 12.5:1 requires less advance timing than a leaner air/fuel mixture of 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. According to Bishop's Tuning Shop, this additional 10 degrees of advance timing should occur with 10 inches of vacuum or more.
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