| How-To - Engine and Drivetrain

Adding Holley’s Universal Coil-Near-Plug Ignition System to a Big-Block Mopar

Newer engines like as the Coyote, LS, and G3 Hemi have an ignition coil for each cylinder. This modern approach is called "coil near plug" or CNP, and it replaces the distributor with eight small coils and eight short plug wires. Eliminating the distributor saves space in tight engine bays and reduces mechanical complexity. The eight individual coils have a lower duty cycle than a single coil since they only fire once per cycle, and the short plug wires are less likely to create interference with the radio or other plug wires.

Even though newer engines have been using the coil near plug technology for almost twenty years, it can be a challenge to find the parts to convert on older engine to this configuration. To help simplify the process, Holley recently introduced a universal CNP wiring harness for their EFI kits. The new Holley harness allow a hot-rodder to simply plug a pair of coil packs directly into a Holley fuel injection system. Converting to coil near plug eliminates both the distributor and the ignition box and allows the user to take advantage of individual cylinder timing. The wiring and software are "plug and play," although a bit of old-fashioned hot rodding may be required to build the various mounting brackets.

We had a perfect starting point for a CNP conversion when a 514-inch Mopar race engine came into the shop. The big Mopar had a Holley EFI system on it, but the ignition was a traditional MSD 7 box with a big race coil. By switching to coil near plug, we were able to eliminate that ignition box, race coil and the distributor. We did need to fabricate coil brackets for the valve covers, and we needed a set of custom-made plug wires, but neither issue required much effort.

The conversion process for the Mopar big block was straightforward since it already had a multi-port injection system. We replaced the existing wiring harness with the new Holley 558-117 universal harness and then installed a complete Holley Smart Coil kit. The new harness has connectors for the coils as well as an additional relay and fuse to power them. The Mopar engine had a 4-7 swap camshaft, but the firing order is controlled by the software, so the wiring harness did not need be modified for the different firing order.

Once we had the coils and new harness installed, we were able to get the Mopar big block on the dyno for some pulls. The big Mopar sounded great on the dyno and pounded out several runs at 880 hp. We didn't see a power increase from the CNP conversion, but we eliminated some expensive parts and simplified the wiring in the car.

We mounted the Holley coils on a pair of brackets that were attached to the valve covers. On a BB Mopar, the coils need to be paired together so the wires can feed between the header tubes.
The Holley 556-127 CNP Smart Coil Kit contains eight coils and two sub-harnesses. These coil sub-harnesses plug into the universal main harness.
A Holley sensor reads a 36-1 tone wheel to provide the crank signal to the ECU.
The distributor was replaced with a special Mopar cam sync unit from FAST Man EFI. The cam sync uses a small magnetic sensor to provide the cam signal to Holley's ECU.
A Holley 558-431 Crank/Cam Ignition Harness is used to connect the crank and cam sensors to the universal main harness.
The universal harness that we used in this build will work with either the Holley HP or Dominator ECU. We ran the test engine with the smaller HP unit since we didn't need the expanded capabilities of the Dominator ECU.
The larger 2,000 cfm throttle body was used for the dyno tests since the engine had a 4500 Dominator flange. The smaller 1,000 cfm 4150 throttle body will also plug directly into the Holley harness.
The intake manifold on this test engine was heavily modified by Wilson Manifolds. Wilson welded injector bungs to the runners and then performed a full competition porting job.
The folks at Custom Wire Sets built a set of Firecore wires to fit our Mopar engine. We needed eight short wires and eight long wires to work with the coil configuration.
The custom wires fit perfectly with the 2-inch dyno headers. Peak power was 880 hp at 6,800 rpm with C14 race gas.
This race engine was complicated! Not only did it have multi-port fuel injection and individual timing control for each cylinder, but the engine also had a dry sump oiling system. Between the various harnesses the dry sump lines, the dyno room was full of hoses and cables.
Paul Roberts from Grays Automotive ran the valves between dyno pulls. The 514 inch big block Mopar ran super smooth with the new coil near plug setup.