becomes the next concern. The few people I spoke to about building
this motor told me that if you have a non-HO 4.0L, you should spend
the money on upgrading the cam. I have heard that the 4.0L HO cam
can be used, but again, this is just hearsay as far as I am concerned.
I spoke to Crane Cams about this project and they recommended their
part #753901. The specifications compare as follows.
Stock cam (top) and Crane Cam (bottom)
The main concern with installing this aftermarket camshaft is the
increased lift. Will this increase in lift cause the valves to contact
the tops of the pistons? As we can see from the table below, the
maximum increase in lift occurs on the exhaust valves and it goes
up by .079”. However, this concern is not all that different
than when building any other motor with an aftermarket camshaft.
The stroker actually creates a bit of an advantage in this particular
case and you can see how in the previous discussion about connecting
rods. Because the piston actually ends up lower in the cylinder
than with the stock crank and rods, I have actually created more
room for the increase in valve lift.
Lift - Int/Ex
decided that the Crane cam would be ideal for this project and ordered
one. Another thing that needs to be considered with the Crane Cams
is shimming the rocker arm bridges. The bridges need to be shimmed
to help compensate for the increase in lift so the preload on the
lifters is not excessive. If this is not done, the lifters could
be damaged and you’ll end up doing it over again.
and Compression Ratio
One of the concerns
that arise when stroking a motor is compression ratio. Compression
ratio is the ratio of the volume of air in the cylinder when the
piston is at the bottom of its travel to the volume at the top of
its travel. A compression ratio that is too high can cause detonation.
Detonation occurs when the spark ignites the air/fuel mixture normally,
but rather than burning evenly, it explodes. This is due to higher
compression in the combustion chamber than the fuel was designed
to operate with. The gas that we can find at the local gas station
can run reliably in motors with compression ratios as high as 10:1
and some believe as high as 11:1. If the compression ratio does
exceed these figures, the engine can only operate on pricey race
fuels. This is not very desirable for a daily driver. To avoid this,
I needed to confirm that the compression ratio was in an acceptable
range. To calculate the compression ratio, take:
volume + combustion chamber volume
combustion chamber volume
formula yields a compression ratio of 9.4:1 for the 4.5L HO stroker
motor. This keeps it well within the acceptable range for using
normal fuel. The stock 4.0L HO has a compression ratio of 8.8:1
and Jeep recommends a minimum of 87 octane fuel though here at altitude
in the Mile High City we regularly run them on 85 octane without
Notice the 258 connecting rod (top) is shorter
than the 4.0L rod (bottom).
of this motor is no different than any other. The only concern I
had was the valve to piston clearance because of the aftermarket
camshaft. This proved to not be an issue. The 258 crank does not
have any clearance issues fitting is the block or with the piston
skirts at the bottom of their travel. The only problem that I had
was the previously discussed harmonic balancer spacer that I had
to make. I also had to bore out the block .030” oversize due
to excessive clearance with stock pistons. This increased the total
displacement to 4.7L and the compression ratio to approximately
9.5:1. I wasn’t planning on doing this but it shouldn’t
create any adverse affects. Everything else went together smoothly.
the stock 4.0L block is retained, all of the factory accessory brackets
bolt right on. The stock transmission bellhousing is used, as well.
The 4.0L flywheel must be retained because it has the notches on
the outside edge for the crankshaft position sensor. The fuel injectors
that I used were for an HO engine. It is my understanding that the
non-HO injectors will cause the engine to knock but I do not know
that for sure. The stock intake manifold and exhaust header were
retained. No other modifications need to be done because the Jeep
thinks it still has the old 4.0L under the hood.
I have shown that the parts will actually fit together, the question
now becomes does it work? The answer is an enthusiastic yes! I could
not be happier with this motor. It does everything that I set out
to do. The highway drivability is great, and with 4.10 gears and
35” tires, I cruise on the highway at 70 MPH turning about
2200 RPM in 5th with ample power to spare. This engine will idle
over rocks that stall a 4.0L HO in an otherwise identically-equipped
motor spools up much faster than stock, though I believe this is
due to the cam more so than the stroke increase. I have left many
sports car owners scratching their heads wondering how a Jeep could
leave them behind, let alone one with 35” tires on it. I have
to use 87-octane gas or the engine will knock. These are just driving
impressions that are very subjective, though. Is it actually that
much better? I’ll let the numbers do some talking.
drove it knew that it was better than stock. We wanted to know how
much better, so to find out, I looked through my list of wheeling
buddies and found one with an identical Jeep, or as close to identical
as I thought I could find. My Jeep is a 1991 Wrangler with 18,000
miles on the new motor at the time of testing, 5-speed, 4.10’s,
and 35’s. The nearly identical Jeep belongs to Dave Jepsen
and is a 1994 Wrangler with 89,000 miles, 5-speed, 4.10’s,
and 33’s. Both Jeeps have stock intake and exhaust systems.
The plan was to do some side by side racing down major streets and
through school zones in Denver. The local law enforcement wanted
to race with us, though, so we needed another plan. I borrowed a
set of 33’s off of Cole Ford’s Project Cross Trainer.
Then, using a G-Tech meter, we ran both Jeeps through a series of
horsepower test runs as well as 0-60 tests.
G-Tech meter is basically an accelerometer that you calibrate for
the vehicle’s weight. It compares how fast the vehicle accelerates
to its weight, and spits out a rear wheel horsepower figure. This
number is the horsepower that the vehicle is putting to the ground,
not what the engine is producing. A good portion of the horsepower
at the crankshaft is lost in the drivetrain due to friction. However,
with the rear wheel horsepower (RWHP) for Dave’s Jeep with
a known crankshaft horsepower, I can calculate what percentage of
the crankshaft horsepower is lost. Then with that figure and the
RWHP of my Jeep, figure out an approximation of my Jeeps horsepower
at the crankshaft.
tests went something like this. Both Jeeps were weighed to get an
accurate figure to input into the G-Tech meter. A flat test track
was used to eliminate any errors due to slope. I drove both Jeeps
to eliminate any errors due to differences in driving technique.
Each Jeep did 12 horsepower runs and 3 0-60MPH runs. For the HP
tests, I threw out the high and the low and averaged the remaining
10. The three 0-60 figures were all averaged. The tests were conducted
at 6,000 feet elevation because that is where I live.
stock Jeep put an average of 94.6 horsepower to the ground. Its
0-60MPH time averaged out to be 12.41 seconds.
Superstock Jeep (still trying to think of a cool name) put an average
of 119.9 horsepower to the ground. It averaged 10.1 seconds for
the stock Jeep’s RWHP (94.6HP) and factory horsepower numbers
(181HP) at the crankshaft, I can approximate the crankshaft horsepower
for my Jeep. This figure comes out to be 229.4 horsepower. This
figure is probably slightly inflated due to the mileage difference
on the motors but it does give a good approximation of the improvement.
4.0L engine is a great foundation to build a Jeep around. However,
like everything else on a Jeep, some of us see a need to improve
upon it. The right combination of factory parts allows us to do
just that with minimal costs over a standard rebuild. It produces
enough horsepower to keep up with many V8’s and when I break
an engine mount in Moab, I can just go to a local parts store to
get a new one rather than having to call Advance Adapters to have
one shipped to me. For those of you with older YJ’s or CJ’s
with the 258, read Cole Ford’s article on swapping
in a 4.0L. You already have the parts you need for this upgrade.
I want to stress that I did not just throw this idea together. I
spent a lot of time researching this project so it would work. Jeep
has given us the parts to make the perfect rockcrawler motor. We
just needed to figure out how to put it all together.
Lopez is a staff-writer for Rockcrawler.com and lives near
Denver, Colorado. Tony is a licensed pilot and avid 4-wheeler.
Tony is the one in green.
Tony at firstname.lastname@example.org