Installing an MSD Digital 6A box for better fuel economy and power
The Jeep inline six (4.0L) is considered one of the most reliable engines by Jeep fans. Although it’s not a huge horsepower and torque monster, it has been known to last 200K miles or more with proper maintenance. The ignition system on these engines is the same on a variety of Jeep models ranging from 1992 to 1998 and features a cap and rotor style distributor with a single coil.
A typical tune-up would consist of replacing these items, along with new wires, and spark plugs, but we were looking for an upgrade that would take the Jeep ignition system to the next level, increasing spark capacity to smooth out the idle and adding multiple-sparks at low-RPMs to slightly improve fuel economy.
We began with the obvious, replacing the worn cap and rotor on the distributor and upgrading the wires from the factory 7mm diameter wires to a larger 8.5mm. We found an Accel cap PN120330 and rotor PN130330, that are both higher quality than factory replacements and feature extra protection from arc-thru and crossfire. We also replaced the factory coil with a new MSD PN8228 coil, that can deliver a more consistent hotter spark. MSD PN32239 pre-cut 8.5mm wires have a thicker insulation jacket for improved EMI interference and low resistance to improve the reliability of the energy getting to the spark plugs.
We had recently changed our sparkplugs to try and improve the slight ignition stumble we had with the engine, so we kept them for this upgrade as it didn’t make a change to smooth out the idle. After installing everything, the next step was to dramatically increase the spark output of the system with an aftermarket capacitive discharge ignition. The MSD Digital 6A produces multiple sparks up to 3,000 RPMs which effectively cleans out the combustion chamber of any unburnt air-fuel mixture to optimize fuel economy. With a secondary voltage output capable of 45K volts, it produces a hotter spark and provides more peak energy than the factory system alone.
The first problem we ran into, was to find an adequate place to mount the ignition box. On our 1995 Jeep XJ Cherokee, there isn’t a lot of room. It’s the same with other Jeep models equipped with the same 4.0L engine and ignition system. For our purposes, we found a spot on the top firewall that would work to keep the MSD unit safe from water and exposure to heat. Depending on the year and model of your Jeep, there may be better options so pick a spot that’s away from exhaust heat.
This area wasn’t flat and had a slight step, and we couldn’t reach behind the cowl area to add bolts, so we used rivet-nuts to secure the MSD 6A box to the area which made for a clean installation. The included wiring harness has a weatherproof connector that attaches to the MSD box. the other ends connect to the battery positive and negative terminals. Four other connections go to the coil, but on 4.0L engines with a single, two-prong coil, there’s a much easier way to connect to the factory harness without cutting.
MSD makes a wiring adaptor for Dodge Ram 5.2 and 5.9L engines that also works for the 4.0L ignition, as it has the same type of coil. The MSD PN8889 adaptor allows you to easily connect the four wires from the MSD 6A box to the factory harness and plug it into the coil. It made for an incredibly simple installation.
Once completed, our slight stumble at idle disappeared and after two fill-ups, we noticed nearly a .5 mpg improvement in fuel economy. With our 4.0L averaging 15-16 mpg on the highway, any improvement is welcome. Also, the tune-up will give us greater reliability and be capable of providing enough spark energy for any additional engine upgrades we decide to do in the future.
SOURCE: Holley Performance www.holley.com