rpengr wrote:
kd4pbs wrote:
...whether by plugging in a 3D timing map to an ECM or twisting the distributor.
I see the word "or" there in your quote. That is the issue here... I don't think you fine tune the timing by twisting the distributor if it has an ECM. You only set base timing (per the procedure).
In changing base timing, one is also changing total timing. The ECM does not know the position of the crankshaft relative to the distributor on our EFI engines if the distributor is "twistable", so it will merrily command the spark timing to give the same total computer derived amount of advance over "normal base timing" given the same RPM and MAP regardless of where the distributor is twisted. As such, twisting the distributor changes total timing throughout the whole RPM range, from zero to the rev limiter. This is why I mentioned to make sure that it starts okay without the telltale "gronk, gronk, gronk" of a too-far-advanced engine while trying to start as one of the "you've gone too far" conditions of optimally timing the engine. Think of it this way: If one sets the distributor to give 12° BTDC ignition timing at 800RPM idle, and the ECM commands a total of 20° advance over "base" timing at 5000 RPM, that gives a total timing of 32° BTDC at 5000 RPM. If one comes along and sets the distributor to give 16° BTDC at 800RPM idle, the ECM will still command 20° advance at 5000RPM, giving a total timing advance of 36° BTDC at 5000 RPM. This is because our EFI engines with twistable distributors have no crankshaft or camshaft position sensors. On engines without a distributor, indeed the timing is set by the ECM via crankshaft and camshaft position sensors and is, for all intents and purposes, impossible to change without changing the timing map in the ECM. I've never laid hands upon a MPI marine engine, so I'm not sure if they have a distributor or not.
Of course, if the timing is optimized at WOT for maximum power output and minimal detonation, but one gets a "gronk, gronk, gronk" start condition, then the only remedies would be to live with the gronk to have optimal WOT power, live with sub-optimal WOT power to get rid of the gronk, or change the spark table in the EPROM so that one has no gronk and has optimal timing for optimal WOT power. Since most people don't have the ability to change spark maps, the method I posted would be the acceptable routine to set optimal timing on either an EFI engine with a twistable distributor, carbeureted engine with electronic ignition, or a breaker point distributor.
Why do I recommend setting the timing this way? Mainly due to the ethanol in most of the fuel nowadays requiring a different total timing amount for achieving max power. Even if one were to run high octane fuel all the time, a few more HP could be squeezed out of our engines due to being able to add in more ignition advance. Mercruiser, VP, OMC, Yamaha, et. al. set timing specifications for a certain grade of fuel and atmospheric conditions, and sometimes for a certain targeted power output. If you want to wring an extra 10-20 HP, I imagine it can be done by optimizing your timing amount at WOT. One might even be able to squeeze a few more feet out of each gallon of fuel at cruising speed by using this method. Every little bit helps.
I hope this makes sense...
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