retuning the EFI?

[Swamprat]

i have the 4.0L I6 hi output in my XJ, and i came up with an idea to get better gas mileage. i got the idea from my gas powered rc car. see i run my rc cars lean so i can go longer between tanks, and seeing as jeeps dont get very good gas mileage. i thought that if you could retune computer so it runs leaner. (but not too lean) i know that i would run hotter, so i might have to get new plugs and wires.....so possible? not? any tweaks you wanna throw in there?

[wolfpackjeeper]

there is almost no retuning of the computer on an XJ. The only way you can do it would be with a stand alone fuel system. And the gains would be marginal. You can already lean one out by building a MAP adjuster, you see more of a performance gain then a mileage gain.

[Swamprat]

oh well i didn't know that....but thanks

[wolfpackjeeper]

yea, I was trying to have an eeprom burnt for mine and had two different speed shops say"Mopar? Good luck, let us know if you find anything"

[Talyn]

the OBD II system already runs lean in closed loop. Something like 14.5. Maybe at cruise you could get away with something leaner, but I wouldn't want that for any other time.

[jeepinwi]

At idle and at cruise, the engine tries to maintain a 14.7:1 AFR. At full throttle, the engine can make best power at 13-13.5:1. The stock computer does ok, but gets too rich above 4,000 rpm's. You could try using a piggyback FMU like the Apexi SAFCII.

[Talyn]

Correct, I found that with some mods I had on my 4.0L (K&N, 98 head, 62mm tb, gibson exhaust and thorley header) it was running richer than 12:1 above 4000 RPMs. I think they programed it that way as a safety margin. But if you are trying to save gas you shouldn't be in open loop that much for it to make much of a difference.

[Boostwerks.com]

It runs "rich" for a reason. Mainly to account for high ambient temps during the summer, and the rare bad tank of gas. If you lean it out much at all (if you want to notice any increase) your going to have to run at least 91, and slightly advance timing.

IMHO, don't mess with the AFR. Without proper knowledge on how to tune engine management systems, you will almost always cause more harm than good. What I do reccomend however, is to clean the head, fuel system, and crankcase. Then focus on a good oil change, replace the filters, plugs, cap/rotor/wires, and if you can afford it... a decent mandrel bent exhaust along with a decent header.

To effectively raise the 4.0's average MPG you'll have to substantially change it's VE, and even then you won't end up with much. Put the 4.0 in a honda insight and you'll have 30mpg (aerodynamics being my point).

[Swamprat]

haha honda, any ways thanks guy, i never really knew if this would work, so im not gunna try it, well at least on my jeep...

[Talyn]

I really don't see why people are so enthralled with the small block.Its been done over a million times, everyone and their brother has one, and stock they don't make great power. Gas mileage isn't that great and its just an old design.

[wolfpackjeeper]

it is an old design, but it still remains a great one, you can flog a SBC for years with no problems. Needs to be a built motor before it starts being worth switching to. But after you do they have great aftermarket support.