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timing, long.
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Posted by Simon S (more from Simon S) on Thu, 7 Oct 2004 12:27:42 Share Post by Email
In Reply to: chg. timing to 20deg BTDC on C900, why?, Mike H, Wed, 6 Oct 2004 17:22:01
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basically, because the factory timing curve is set to accomodate low octane (fast exploding) gasolene, a smooth idle, and low emissions.

Making the spark earlier gives the high octane (slower burning) gas, time to explode, so the peak of the explosion and internal cylinder pressure is closer to the top of the stroke. If the explosion happens too late, when the piston is already in the fast part of it's downward stroke, you don't get any torque, just heat. Especially at low cylinder pressures just off idle, it's possible to set the spark off much sooner, because the flame travels slowly.

Because you're messing with the low end of the curve, you'll have to compensate in order for the timing to be late enough at higher load/boost. Modification of the vacuum diaphram by filing, will let you make it retard more on boost. The only way to tell how far you're changing things is to file a little, put it back together and put a timing light on it. People are selling adjustable diaphrams, or you can go with an MSD-BTM, and get much more control over your timing under dynamic situations.

I refer you to TwinSaabs, and our friend Kevin K's notes.



posted by 148.78.24...

http://www.twinsaabs.com/msd/msd_part2.html

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