Deceleration Tunning


Active Member

I am trying to straighten out my street tune on my CLASSIC FAST for when I lift the throttle and then mash the gas again (tune PIA engaged lol). Anyway, the problem I am working on is the engine is going to 9.80AFR and lower when I lift the gas and the throttle blade closes. So much that the engine stumbles a little. If I try to press the gas again too quickly, it hesitates and stumbles real bad. The issue as I see it, is the computer uses the same cell numbers during deceleration as acceleration in some cases. The tracer (little white bubble) goes right through a few cells, that I have identified, when I lift the gas. I have decreased these numbers, but I can only go so low for the very same cells are used to provide fuel when the gas is reapplied. I am aware not to create a lean hesitation on acceleration thus I am caught. I could be wrong (prob so) about my thinking. I know this is a very tricky thing to work out, so that is why I am asking for some help.

Thoughts so far:
1) Might be the case that when you quickly close the throttle body you are cutting off the air supply before the computer can react(maybe) and so have have a surplus of fuel. But I can be convinced that the computer is very fast and can react with the proper fuel adjustments.
2) What about DFCO is this a real thing in the classic FAST
3) Maybe the IAC is not fast enough to compensate. Is this because it could be clogged and needs to be cleaned, or is there an upgrade IAC that is faster the the stock one.

Thank you


Classic fast, XFI, SPortsman & MS3 programming
Your limited to a fix, as no such thing as DFCO vs rpm on any fast system, and I have complained about this for years. You can keep reducing the numbers on decel to compensate but it probably wont do much if you have a transbrake no engine braking, as that 9/8 af happens real fast. If your moving the throttle the fuel enrichment vs map should come into play, you may want to adjust that table 1st to see you can smooth out the transition getting back on the throttle. You can try the new style IAC as its quick but its going to be costly, and bump the throttle follower so it stays open more on decel. See link for IAC.


I'm a ASSHOLE.....
I agree norbs .....I have hated this issue for yrs..... I have this same trouble....

What i have done is allowed the ve table/KPA to go lower about 2 lines and loose some when you lift the throttle on engine decel it normally pulls more vacuum so it goes lower than the normal engine idle areas so it don't interfere with driveability .. so i simply lower those ve numbers in that decel area till the engine seems to be somewhat happy... but don't lower them so much that you get a lean pop when the throttle is mashed.....