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The High Output 4.0 litre motor that came in 91+ xjs and MJ's is a
solid motor, it came with 190 hp (upgraded to
193 in 200) and 225
lb/ft of torque, it is a fairly low end toque monster, but it can
be improved on, I will go into
detail on some modifcations that I
think make the biggest differencein power when trying to get the
most out of your
4.0.
Intake: There are a lot of possibilities when thinking of enhancing
the intake side of the 4.0 motor, as with any
engine, it needs to
breath better, allowing every possible horsepower to be released from
the mighty 4.0 motor.
The first step is a k&n flat filter, other
companies such as accel and holley make this type of filter, it is
the
first level in increasing airflow. You can also remove the
air tube (see cheap performance tricks in the tech inde)
that
restricts airflow and enlarge the front opening of the air box.
These modifications are simple and cheap/free
to do and are a great
first step in improving horsepower in any engine, especially the 4.0.
The next step in the airflow
section is a higher flow cone intake,
lots of companies make these, or you can make your own. Companies
such
as k&n, rustys offroad, turbo city, volant, and accurate power
make intakes for the 4.0 high output.
Exhaust: This is the next good step in improving performance when
attending to horsepower and torque.
The first "stage" of this is to
get a highflow muffler, companies such as: dynomax, flowmaster, magnaflow,
borla,
and cherry bomb all make highflow mufflers that fit your JEEP
XJ. The next "stage" is a catback exhaust system,
several companies make
these, they are: flowmaster, dynomax, rustys offroad, turbocity,
magnaflow, borla, gibson.
The next step in exhaust performance is a
highflow catalytic converter, companies like random technology, carsound,
and
dynomax have highflow cats for sale. The next step in exhaust
performance is a header, several compies offer these
such as; gibson,
borla, heddman, doug thorley, mike leach, and pacesetter.
Ignition: The ignition on the JEEP 4.0 is pretty good from the factory, if
you are just going bolt ons and not to
far into the motor, all you need is
a good set of wires and plugs and a good cap/rotor combination. Companies
that
make ignition wires are: Jacobs, Accel, MSD, Mopar, Splitfire.
Companies that make a good plug for the 4.0 are:
Autolite, bosch, splitfire,
champion. Good companies for quality cap and rotors are: Accel, MSD, Mopar.
If
you really need more spark (ie stroker and supercharger) or get them for
cheap, a good quality aftermarket coil and ignition
box are a good enhancement
(not really for the money though) companies that make them are: Accel, MSD,
Crane Cams,
Petronix, and Jacobs.
Throttle body spacers: These are a good upgrade, they arent worth 90$, but
30-50 is a good price, companies such
as; rustys offroad, poweraid, hesco,
and street and performance electronics make them. They provide a solid 3 hp
increase.
Throttle bodies: There are two different options for a highflow throttle body.
You can buy a 60mm one from
jeepers and creepers and turbocity. The next step
up from that stage is a 62mm throttle body, these get a new throttle
plate as
compared to just taking the 55 mm taper away from a stock throttle body (j&c
and turbocity) accurate
power, rubicon express, and flowmetrics have 62mm
throttle bodies.
Aftermarket chips: There are a few options for an aftermarket computer upgade,
most not worth it, there is the JET
chip, it is not reccomended (when paid in
full price form) it only gives 3 horsepower to a manual equipped xj and 8 to
an
auto. ASE is a better choice here, they advertise a 16 hp gain, but it is
proabably more like 5-7 on a manual
and 10-12 on an auto.
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