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How does an '808 determine dwell time?
I've looked right through the $12P settings and can't find anything that seems likely.

Have you updated to the very latest version as it won't be in any older versions of 12p.

My version is maybe 18 months old. Is there a version $12Q or greater? What actually determines the dwell in $12P if it is not use settable?

http://pcmhacking.net/forums/viewtopic.php?f=27&t=356

dwell control was added in $12P v112. you will need to update.
in earlier versions and factory tunes the dwell is hard coded to a specific value.

Hey thanks for that Jayme. I'm using version 111, didn't look closely at the version numbers. I was thinking they went by letters.

Righteo, having a look at 12P V112 right now.
Those terms - first, second and third slope for high, mid and low rpm; I certainly understand what dwell is all about but can't decipher the terminology in the parameters. Can anyone set me straight?

They are used to calculate a 3 slope dwell, its not simple to calculate or change. It also changes for the cyl count as its based on time between reference pulses, not RPM. This is also referred to as the static dwell calc, dynamic dwell is added for AE to give increases when more energy is needed.

This is what the standard settings for static dwell look like vs RPM for a 8 cyl, where the pink line crosses you start dwell limiting to its value due to not enough charge time between firing...

not too bad! most of the time your below that point anyways for a streeter!