[Diy_efi] SPARK 101

[TKramer@Snipepod.com]

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I have a $8d TPI unit in a 76 blazer, automatic tranny. 

I have a stock 90-91 F-body, with auto tranny .bin file to study.

I live in western Montana, and constantly drive mountain passes. On the
downhill side, I frequently hit high vacuum, (20KPA and lower),  in the
1800-2800 rpm range. It makes sense to me that I would want the spark
advance to be gradually increased  in these areas, since the engine is under
light load, and basiclly coasting. For example, as I go from 40 to 30 to 20
KPA at a constant 2400 rpm, shouldn't the advance get progressively more?
Yet on the stock F-body, it is suddenly backs off the advance as the vacuum
gets towards max (20KPA and lower). 
A good friend explained this to me like this....  "The timing is "retarded"
so you get engine braking. If the timing is held high the engine "wants" to
keep running stronger than if the timing is lowered some in the coast down
MAP range. It also makes the engine a little richer there for snappier
throttle response when you get back on the gas suddenly. Late timing leaves
some residual fuel since it doesnt have time to completely finish
combustion. So it finishes burning in the exhaust. It also builds heat which
helps to lower hydrocarbon emissions but only to a point. Too little timing
does just the opposite."
That makes sense, he also told me about an old document from the DIY EFI
site on the basics of spark advance. Kinda a "Programming Basics 101
rambling", that talks about these conceptual issues.  Does anyone have such
a document, or any other spark curve theory that they could share with me?
Terry K

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