Re: [AMC-list] Mileage with AMC/Jeep 4 Cyl 2.5 EFI or 4.0 in an 80's AMC

[Frank Swygert <farna@xxxxxxxxxxx>]

Date: Wed, 20 Mar 2013 15:19:21 -0400 (EDT)
From:adh@xxxxxxxxxxxxxxxxx  (Sandwich Maker)

" From: Frank Swygert<farna@xxxxxxxxxxx>
"
" I have a hopped up 4.0L and get about 22 mpg cruising at 70 mph. I've
" got 3.55 gears and an AW4 auto trans from a 2WD Cherokee. I feel it
" would do great with 3.31 gears, but don't want to pony up the dough to
" change the gears just for a couple more mpg.

your car is a classic, a little heavier than mark's american.  that
must account for some of the difference.

i get 23@65 and 24@55 in my gf's 4.0/auto/2wd xj.  when i posted that
on an xj list, another poster replied he gets 25@65 in his 4.0/5sp/4wd
xj.  they also are heavier than an american or spirit though about
equal with a classic, as well as worse aero.

are you sure that 3.31s would be meaningful over 3.55s?  that's like
cruising at 65 instead of 70 for rpm.  my experience suggests you'd be
lucky to pick 1 mpg up for the swap.

" You should be about in line with a four cylinder Cherokee in a Spirit,
" maybe 2-3 mpg better, so you're talking mid to upper 20s as far as
" mpg, with 20-24 as an all around average. I'd use a five speed and
" something like a 3.55 axle. That should be about right for the four,
" a little taller than the 4.10 used in Cherokees with the four.

i agree though, if you don't recam the engine for more low end.  both
the 4 and 6 are cammed rather higher than the older sixes, in line
with the modern era's creeping performance expectations and auto
tranny prevalence.  the 4.0/5sp xj's geared with 3.07s, but
[anecdotally] loses very little with 4.10s, as they actually put the
engine more in line with it's most economical rpm at highway speeds.
=============================

I'm not positive it would gain more than MAYBE 1 mpg with 3.31 gears, that's one reason I won't spend the money to do it. All I'd really gain is about 5 mph more cruising speed, which isn't worth it. I tried running 3.08 gears with the same setup (but with a 4.6L and not the low speed torque cam I have now) and got 2 mpg LESS than with the 3.55 gears. I have run a 3.31 rear axle with a three speed OD (BW OD) with the 4.6L and that did get about 2 mpg more than what I get now. May have just been that it was a stick instead of auto though.

I'd stick a torque cam in the four too. The stock cam should be okay, but a low speed torque cam would be better for a cruiser/daily driver. Doesn't matter if it's the GM or AMC four. If the Iron Duke is worth rebuilding that would be the cheapest way to go. The EFI head might work, but I know the TBI system used a different block with the electronic pickup made onto the crank and going through a hole in the side of the block. That could be replaced with an aftermarket sensor and trigger wheel, but would be a lot of expense. I don't know how the later MPI systems worked though. That might be an option. Of course it will work well with a non-feedback carb. Might lose 1-2 mpg that way, but the main thing is EFI is just so much better for start and go running.

--
Frank Swygert
Editor - American Motors Cars Magazine
www.amc-mag.com

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