Quote:
Originally Posted by Sun Tuned
Bill,
The boys here above, as usual have already nailed it down.
I would like to add youre correct, you have to advance the curve ahead and somewhat faster as the RPM goes up to “get ahead” somewhat of what’s happening. There is a fixed amount of time to complete one combustion cycle, as the rpm changes, theres less time to get that done as the speed goes higher.
However theres a law of diminishing returns with this. After a certain point advancing anymore just doesn’t produce the gains you get early on. In fact up top it’s advantageous to pull a couple degrees back out.
To illustrate a point. If you hook a gas analyzer up on say a 360 Hp 389 with an 068 cam…. And set the timing dead stock @ 6 btdc, and set the carb and get a hydrocarbon and CO reading,then you watch it and as you bring initial timing up towards 12 , youll actually see the HC go down and clean up a bit. Now obviously as you do that the rpm will come up and you’ll need to readjust carb just a bit and reset idle speed but it will clean that up from 6 degrees.
But as I said there’s a diminishing returns thing at play, if you go past 12, on towards 14,16 etc.. you’ll see the HC start climbing back up again…
So there’s a sweet spot for a particular setup, and you can get greedy.
Hope that helped.
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Just curious tuning question..
Do aftermarket digital ignition and/or FI systems optimize timing curves this way across the entire rpm range? I would expect so... but not sure it could happen in real time. In other words, can a fuel ratio sensor in an exhaust stream allow "real time" optimization as an engine is revving or is there a lag before the AFR stabilizes?
If there is a lag, which I expect, do these timing computers provide "learning" or "tuning" procedures where the optimum timing is measured by dwelling at rpm intervals to build a full "base curve" that is retained in memory and modified by load-sensitive changes (ie vacuum, temperatures, etc.)?
It would be interesting to see a graph of that type of "optimized" advance curve vs a stock mechanical curve for different levels of performance on a single engine platform.