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Looking at the factory fuel and spark maps is a good starting point. It also helps to understand the emission control strategies and not copy them. All the OEM's target the EPA emission and mileage test for obvious reasons and sometimes they get pretty sneaky retarding timing at certain points, even using different maps when the A/C is on. The reason they are using 40 BTDC at 15-20% load is fuel economy at cruise. An engine under low load can handle a leaner mixture, this requires much more ignition advance because of the burn time, and it is not a problem at low load. This was the concept behind vaccum advance, add 5-10 degrees of additional timing under high vaccum, low load cruise and take it away as soon as the throttle is opened. You can also learn a lot by looking at the distributor spec on pre EFI motors, but with Porsche after 1968, timing was determined by emissions more than optimum performance. The idea of setting idle timing to 12 BTDC came from having to live with a 20 degree mechanical advance and wanting 32 BTDC at full advance. The later 5 BTDC setting was all about getting to TDC with vaccum retard to lower HC emissions for the curb idle test. With a programmable system these are not issues.
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Paul
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