Pro Flo 4 Timing

This may be useful information from the edebrock EFI forum by on of the Edelbrock tech guys:

"You're close but there are a couple other details to keep in mind. First, the vacuum advance is applied across the full vacuum scale. This means that vacuum advance is minimum at lowest vacuum (like when near atmospheric pressure) and maximum at highest vacuum (-30 inHg). If your vacuum advance is set to 5° and you're idling at -10 inHg of vacuum, your actual vacuum advance is only going to be 1.67°. The point of vacuum advance is to apply more ignition timing in low load cruise where you're going to have a lot more vacuum than just sitting at idle. The idle control timing is for timing control at idle - see next paragraph.

If you're checking your timing while in idle, you're going to see significant changes in ignition timing because the ECU is using a corrective timing trim to help control idle engine speed. It's much quicker to control engine speed using ignition timing than it is to try and move the idle air control plunger in and out to control air flow. You would need exit idle (apply a little throttle to increase engine speed) to see what the timing is doing without the idle ignition trim being active. One step you should have done was to lock the timing in the app to 12° in order to set your base timing. By locking the timing, you disable the idle and vacuum advance trims and run at a fixed 12° advance. You can check it again in Advanced Tuning>Base Timing. Any variance you see in timing at the crank with a timing light with the timing locked is due to cam and distributor drive slop."

I hope is useful.

So basically if you are above your start time increase threshold and the ECU is not reading zero the vacuum advance is being partially added to your base timing. This becomes very important if the MAP is not reading zero at full throttle, because it will be adding some vacuum advance portion to your highest timing setting even if under hard acceleration.

At least the vacuum advance seems to be linear, so you can make a pretty easy calculation on how both should combine to not screw up and have your desired top atiming advance at full throttle.

Take care,