Quote:
Originally Posted by SSLance
I don't think I want to add any vacuum advance in at idle, I think I want it to kick in just off idle...so here is my idea for a Boost Retard Curve...
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I would not make sharp transitions like that, it could cause the engine to surge, oscillate between two points or do something funny. Sharp transitions in control systems are never a good idea.
Why the aversion to running so much advance at idle under no load? It may actually let the engine run cooler. As soon as you put a load into it the advance will drop out. People run locked out distributors at full advance on race cars, I don't think your idle speed is going to run away or anything funny. Just try it out and see how it behaves, you can always plug off the vacuum port and disconnect the MAP sensor until you can change it back if it does something you do not like. Disconnecting the MAP sensor will trick it into thinking you have no manifold vacuum and it will pull out all the timing. If you are worried try a smaller amount to begin with, like 7-10 degrees. Just match the amount of advance you put into the initial timing with the amount you take out with the "Boost Retard Curve".
It doesn't matter much if you start at 4" or 7", it just changes the amount of advance a little. Starting at 7" should be fine since that is where your MAP sensor is telling you the engine runs. You can also cheat the end point on the right a little. When you disconnect the vacuum from the MAP does it read 14.75"? I would want the full amount of spark advance pulled out before that point, maybe set it to 14" even to have some margin
The difference between 5" and 7" could be the MAP sensor calibration or your vacuum gauge, either or both could be a little off. 20 sounds like a lot to me for an engine that is not stock, maybe your gauge is a little generous or non-linear. It does not matter much, it is just a reference point, use the MAP sensor since it will be controlling the spark.