On Wed, 17 Sep 1997 19:48:53 -0700 (PDT) William Lewis
<wrlewis(at)ucdavis.edu> writes:
>I have been quite interested in the overdrive thread as I will be
>installing OD on my car this winter. I am curious, though, why my
>Jeep
>with the 4.0 inline 6 cylinder and far worse power to weight ratio
>doesn't
>suffer any ill effects running at 2,000 rpm in overdrive? I have 170K
>miles on that lump and don't burn any oil between changes (yearly,
>whether
>it needs it or not). Is it the torque of the engine or some other
>design
>that allows newer cars to tolerate lower rpms?
>
>Bill Lewis
>Searching for enlightenment
I would not say that the power to weight ratio is better on the jeep.
It weighs around 4000 lbs, and makes 185 BHP.
The SV alpine weighs 2100 lbs and makes 95 BHP.
If you consider torque to weight ratio, the jeeps got you beat.
Do the math here.
The SV motor makes 56 BHP per liter.
The newest of 4.0 Jeeps make 46 BHP per liter.
It is all in the opperating range for which the vehicles engines are
designed. Sports cars sacrifice some drivability and durability for
that extra zip and sportyness.
The alpine makes power in the upper RPM range, so that its lower
torque output, can be made use of with higher numerical gearing.
"Read lower gear".
The jeep makes its power at much lower RPMs due to a larger stroke
and its camming. It doesnt rev to very high RPMs, because that would
not be practical on the average yuppiemobile. Torque is about done
at 4000 RPM, and revving it beyond may yield slight HP increase,
but at the point where the HP fails to increase as a function of
increased RPM, the torque output is falling at a greater slope than
the RPM has increased. This is the point of diminishing returns on your
power.
The torque is going fast on the jeep where its just getting started on
the
Alpine. The jeep is quite a bit smoother, and easier to drive though,
all
else being the same.
I dont have specific torque figures for the Jeep engine, but based on the
185
BHP advertized, I would extrapolate the torque to be about 220 lb feet
at around 2500 RPM. Long intake runners, good head design compared
to the old 4.2 liter engine, and factory headers make for pretty fair
engine performance. Older jeep 6s make less than 150 BHP and less
torque too even with a bigger engine.
2000 RPM sounds low for continous opperation even on the jeep, but
you probably get the point. If you want to know for sure, go buy a
manifold vacuum gauge. As Dick T. said, the highest cruising vaccum
will yield the best posible economy, and least engine load.
Cruising with vacuums of less than 12 inches of Hg (41 kpa), is just no
good for any motor.
BTW, I own a 94 cherokee with 4.0 6, and I am not knocking the jeep
at all.
Jarrid Gross
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