Deceleration Fuel Cut-Off option. Use it or not?

[Larry Callahan]

I'm not sure if I should be using this option or not. On my way to work there is a hill I would guess about a mile and a half long and I my Electromotive unit shuts down the fuel in decel mode. Is this common?

Can I hurt anything? Of course my A/F gauge pegs out but I think it's o.k.

The grapevine is pretty long and I could see running for it 10-15 min without fuel going down that hill.

I can disable it and have it dump some fuel via the fuel map if needed. Being my first injected motor I have a lot to learn so watch for a lot of questions while I near the end of tuning. lol.

[TD509EFI]

Larry,

The OEM's used it, and I'll be using it on my BBC with the Holley MPFI. Using it shouldn't hurt anything but can save fuel; the extra engine braking can also cut potential wear on the brake pads. Try it and see what you think.

John

[parsonsj]

It's pretty hard to damage an engine in a no-load situation like that. I run more timing and 15.2 AFR in those situations.

jp

[Larry Callahan]

When you say more timing you mean a larger number like 40's 50's or more kind of thing?

I am still thinking backwards I think.

[parsonsj]

Uh, 36. I'm not going crazy, lol.

jp

[Larry Callahan]

lol

[Larry Callahan]

Jody has me thinking I should not use this option. As it is I have problems with my IR setup going lean just as I crack the throttle on some cylinders.

I think it would be best to just add some timing and lean it up a bit like JP suggests. I will tweak it a bit and drive it to work on Tuesday. I may drop it off at Bones Fab to weld me in another bung fitting for another O2 sensor.

[67 caprice]

It's pretty hard to damage an engine in a no-load situation like that. I run more timing and 15.2 AFR in those situations.

jp

I use the F.A.S.T Fuel. When your driving your car on a flat road note how much vacuum your pulling. Then note on deceleration what the vacuum is. I have 40 deg total timing on deceleration. 15:1 AF It helps clean the cylinders.

[Blown353]

I did the same thing as Russ on my old FAST system by really leaning the coast down cells from max RPM all the way down to 1500 rpm but with even more timing (50 degrees.) I ran 42-45 degrees under light load cruise. I would have preferred a "real" DFCO scheme but FAST doesn't offer it so leaning the coast down VE cells and upping the timing there is the next best way.

A ECU that supports a *real* DFCO scheme is even better (by real I mean it has a MAP threshold to enable it, a small delay timer until it cuts all fuel, and a low RPM parameter to turn the fuel back on so it doesn't die when you come back to an idle.)

It won't hurt the engine at all-- it will save fuel and keep things cleaner... but that's only if you don't have any other tuning issues that show up with it enabled. What Jody mentioned is a possibility-- some engine/cam/intake combinations may have problems with tip-in "coughs" or lean spots if you have the engine tuned to run very lean on coast down. Try the DFCO and if you notice any lean stumbles or coughs on throttle tip-in from the lean/DFCO coast down then you might have to go back to a slightly richer coast down.

IR intakes for sure are much more sensitive to acceleration enrichment parameters than a standard intake with far greater plenum volume so you might need some tweaking in that department.

[camcojb]

Jody has me thinking I should not use this option. As it is I have problems with my IR setup going lean just as I crack the throttle on some cylinders.


I don't think I ever mentioned to not use dfco, I use it on my Big Stuff 3, works great. I was referring to my IR setup needing to be richer at idle than any of my non-IR setups, but could be leaned normally during cruising and light throttle areas off idle.

Jody

[Larry Callahan]

Sorry, I didn't mean to say you said not to use it. It was the talk about IR's needing to be richer at idle. I worry about a pop just as I crack the throttle as I do just off idle. I guess it can't hurt to try it once.

I should set up my closed loop O2 and start playing with it. I would like to make it work if I can.

[camcojb]

Sorry, I didn't mean to say you said not to use it. It was the talk about IR's needing to be richer at idle. I worry about a pop just as I crack the throttle as I do just off idle. I guess it can't hurt to try it once.

I should set up my closed loop O2 and start playing with it. I would like to make it work if I can.
No need to apologize, I just didn't want you to think it can't work on your combo, as it can. The way my DFCO works is I can set the map for start and for stopping the DFCO, so as long as it is correctly set and turns the fuel back on before it reaches idle you should be fine. If you're cruising down with the throttle closed and injectors off (DFCO active) then you're not creating a lean condition as the fuel is completely off, cylinders are not firing.

Also look at your acceleration enrichment tables, because the IR's I've tuned like more fuel and quicker at part throttle than a standard single throttle body does, so changing those maps will likely get rid of your lean pop at acceleration off idle.

Jody