When parts break
on a normal person’s vehicle, their thoughts probably go straight
to the cost and inconvenience involved. When the same thing happens
to a 4-wheeler, their thoughts turn to “How can I upgrade
this?” With this theory in mind, when the 4.0L in my 1991
YJ lost all oil pressure and died, my thoughts immediately turned
to “How can I make this a good thing?”
The fuel-injected stroker motor allows our
YJ to run at extreme angles and produce torque at lower
I talked to immediately brought up a V8 swap but this was not feasible
cost-wise for a starving college student like me. I also believe
that Jeep quit putting V8’s in the CJ series for a reason.
With these thoughts in mind, I decided that just a basic rebuild
with a slight twist would do the trick.
The slight twist that I am referring to is stroking the stock 4.0L
to a 4.5L. The term stroking the motor refers to altering the distance
that the piston travels in the cylinder bore and therefore increasing
or decreasing the total displacement of the motor. Obviously, in
this case I am increasing the displacement by .5L. If you subscribe
to the theory “there is no replacement for displacement,”
this is exactly what you are looking for.
The 258 (258 cubic inches - 4.23L) was the predecessor to the 4.0L
(242 cubic inches) that we see in many of today’s Jeeps. It
is known for its excellent low-end torque, which is ideal for wheeling.
However, its less than desirable carburetor, vacuum system, and
highway drivability make it less than ideal for a daily driver.
This led Jeep to design the 4.0L. The 4.0L is basically a hot rod
258, despite its smaller displacement.
took the 258 platform and improved it with fuel injection, better
head design, a header, a different cam, larger cylinder bore, and
shortened stroke. The fuel injection was a godsend for this motor.
It improved the fuel economy, allows the Jeep to run at any angle,
and allows the driver to walk out to his rig when it is 5 degrees
outside and the Jeep starts right up.
rest of the modifications improved the motor’s on-road drivability,
as well. These improvements were not all good for the 4-wheeler,
though. If the torque produced by these two motors is compared,
you can see that while the YJ's 4.0L produces more (220ft-lbs vs.
210ft-lbs), it happens at a much higher RPM (2,500 vs. 1,800). This
is not as desirable for off road where the revs typically don’t
get up to 2,500. If one could combine the desirable traits of both
of these motors, it could be an ideal power plant for the daily-driver/weekend
warrior rig. This is the inspiration for stroking the 4.0L. The
goal here is to retain all of the on-road drivability benefits of
the 4.0L while gaining the low-end torque of the 258.
Editor's Note: The current TJ's 4.0L is rated at 235 lb.-ft.
@ 3200 rpm
I imagine that some of you are still wondering why not go with a
V8. After all, those stroker kits are pretty expensive. A lot of
us have heard the urban legend that this stroker can be built with
the right combination of factory parts. The problem is, very few
people know what that combination is and even fewer people know
where to find those people. It was my goal to figure out this combination
and therefore be able to build my own stroker kit.
The basis for
any stroker motor is the crankshaft. Most who have heard the legend
of this motor hear that the 258 crankshaft is what is needed to
gain the extra displacement in the 4.0L block. The question then
becomes, what needs to be done to fit the 258 crankshaft in the
newer block? The answer is essentially nothing. A comparison of
the mechanical specifications of the two different crankshafts reveals
many similarities. In fact, all critical specifications for the
crankshafts are identical except for the stroke, which increases
by .455” with the 258 crankshaft. So the numbers indicate
that the crank will fit, but does it really? The answer here is
The only difference between the two crankshafts
besides the stroke was the longer snout on the 258 crank
crankshaft that I used was out of a 1983 CJ-7. The only problem
that I found with it was the front end where the harmonic balancer
fit was approximately ¼” longer. This problem was easily
remedied with a spacer between the balancer and the fixing bolt.
It also could have easily been machined down to the proper length.
So obstacle #1, crankshaft compatibility, has been conquered.
The next obstacle
that I faced was what connecting rods to use. The stock 4.0L rods
in conjunction with the increased stroke would cause the piston
to contact the valves so that was not feasible. A comparison of
the 4.0L connecting rods to those found in the 258 again reveals
many similarities. In fact, all of the specifications are identical
with the exception of weight and length. The weight is not of great
concern so we will ignore this detail. The length of the 258 rod
is exactly ¼” shorter. The stroke increase of .455”
is divided between the top and the bottom of the pistons travel.
This means that it would actually only move up .2275” more
than stock (.455” divided by 2). The shorter rod will offset
this extra piston travel up so, in this case, the piston will actually
end up .0225” lower in the cylinder (.2275”-.250”).
All this adds up to one conclusion - on paper, the 258 connecting
rods will work. And again, they do work when put into action in
the real world.