Re: AMC 4-cylinder in a Spirit (Revisited)

[adh@xxxxxxxxxxxxxxxxx (Sandwich Maker)]

" From: farna@xxxxxxx
" 
" 
" I'm pretty sure Andrew hit the nail on the head -- or at least in the
" general area. Cam and tuning (computer programming in this case) make a
" big difference. The Jeep engines are set up for the lower gears,
" therefore shouldn't be geared to high for best performance and mileage.
" The car engines were setup differently. I'd be cautious about dropping
" the rear axle ratio to the same as similar engined cars. I'm certain it
" can be dropped below 4.10, but my experience with Jeep tuned engines
" tells me you don't want to drop it that much. 3.55 should work well in
" the much lighter car (as you pointed out), even a 3.31 might be
" acceptable, but dropping to a 3.07 would likely be a mistake. 
" 
" The 4.0L used a 3.07 with a five speed, and I can tell you from
" experience that it's to high a gear even for a 4.0L (had a 91 4.0L
" Comanche, 4x4 five speed, 215/70 tires). It started a bit slow even in
" first gear (except on a level) and there was way to much shifting
" between 4th and 5th when driving in moderately hilly terrain -- not big
" hills. It felt like it needed more gear. The saving grace was the 4WD
" -- it had a low range in the transfer case. I had to use that to take
" off up an incline with a heavy load -- first gear wouldn't do it! That
" was with a 4.0L -- I can imagine what a four cylinder would be like!
" I'm confident I'd have got better mileage with 3.23 or 3.31 gears
" instead of the 3.07s. Some of the gearing is marketing. Another problem
" is the EPA check course isn't very realistic. Under ideal conditions
" I'm sure the 3.07 gears are acceptable (they were), but that is maybe
" 20-25% of driving -- and that's being generous! If you want to give up
" anything resembling preformance, go with the 3.07 gears. It will be
" sluggish and won't deliver optimum mileage for most drivers. 

but if you're willing to change cams...

ecms have a measure of latitude to cope with changes.  most of the
investigation with the 'stroker' [4.0 with 258 crank] crowd has been
with more radical cams to cope with the higher compression since there
isn't a good cheap stroker piston.  but the mopar 258 mpi kit was
based on a stock '93 4.0 ecm, and at least one stroker is running a
258 cam, with great off-idle torque just like a 258.  it doesn't even
give up much power - mopar claims 160hp for the mpi 258; one mag
measured 170.  with the 4.0's better head...

that would pull those 3.07s easily, with good mileage.  eagles with
similar tires had 2.73s.  i would expect, naively perhaps, that a
2.5 would respond similarly to a cam change.

it would be interesting to see what changes, if any, the 2.5's cam
went through in its lifetime.  afaik the 4.0 had only two: a fairly
radical grind used '87-'95 and a less radical split pattern '96-up.  
________________________________________________________________________
Andrew Hay                                  the genius nature
internet rambler                            is to see what all have seen

adh@xxxxxxxxxxxxxxxxx                       and think what none thought