How To Tune For Nitrous With A Coil-On-Plug Ignition System

For some time now Holley has made an LS based (coil near plug) distributor less ignition system that can be adapted to older conventional engines. In addition to significantly more spark energy, the system can control individual cylinder timing. Each cylinder utilizes its own coil, so we pondered whether it would make more power than using just one coil? Also, if the each cylinder could be controlled independently, then how do we find what each cylinder would want?

So, because curiosity kills the cat, the system was installed on a dyno mule. The dyno mule is a 383 Small Block Chevrolet that has been around the shop for years.

In order to analyze the timing needed for each cylinder, eight oxygen sensors were installed in the headers primary tubes. The data collected from each dyno pull was used to determine the effects of ignition timing and manifold air-fuel distribution.

Once the ignition timing for each cylinder was optimized naturally aspirated, a small shot of nitrous was used to see how it would complement the engine versus the use of a distributor ignition system. A small cart was crafted outside the dyno cell to hold the nitrous bottle and wiring for the relays and controls. Having the bottle outside of the dyno cell helped keep our bottle temperatures more consistent.

For the dyno tests we choose a Holley 750 double pumper P/N 4779-6.

The intake manifold used for this test was an out-of-the-box Holley P/N 300-110. The engine is a factory, 1987 block bored .030” over with forged internals, 11.2:1 compression ratio, Pro-Maxx 205 cc cylinder heads, and a 150 shot of NOS nitrous. The carburetor was a used traditional Holley 750 double pumper 4779-6 that was actually bought secondhand. It was taken apart and cleaned and rebuilt back to “stock” configuration with the correct jets and power valve just like you would have purchased it new.

The Coil-Near-Plug ignition still utilizes a distributor housing which is used for cam and crank signals for the ignition controller. Once the distributor is installed properly according to the instructions, the ignition controller can identify the proper cylinder for the firing order.

Before the engine was installed on the dyno we mocked up all of the new ignition components onto the engine. We got a set of used valve covers and drilled them for the LS style coil brackets. A plate was mounted on the front of the passenger side of the engine to hold the ignition controller and wiring harness. The coils were installed along with the spark plug wires and then the distributor was installed and correctly phased according to the instructions.

After the engine was installed on the dyno, we installed the software on the laptop computer and followed the on-screen prompts for the correct engine configuration and firing order. This will automatically set a starting base timing curve in which the motor should crank and run so you can make additional changes. After the engine warmed up and reached around 170°F of water temperature we made our first pull data logging the normal parameters of the dyno in addition to the eight oxygen sensors.

We started the pull and let the engine reach 5,400 rpm and could not believe what the oxygen sensors were reading. The outer four cylinders numbers 1, 2, 7, and 8 were extremely lean while the inner four cylinders numbers 3, 4, 5, and 6 were extremely rich. By lean I mean air fuel ratios were around 19:1 and rich the air fuel ratios were 11:1. I had heard about distribution problems for years but never imagined the cylinders to be this far apart. To try and overcome the lean condition, we made a jet change to the carburetor. We knew that the inner cylinders were going to get richer but we wanted to try and prevent the outer cylinders from being so lean. It seemed as we were going in the right direction so we made another jet change. After the second jet change the air fuel ratios of the outer cylinders were starting to reach an air fuel ratio of 14:1.

Now that the air fuel ratios were manageable, we found that the engine made the best power at 36 degrees of total timing. Now the timing for the outer cylinders could be changed. After several pulls we found that after 4,500 rpm the outer cylinders ran better with 32 degrees of total timing. The total timing for the inner cylinders was moved after 4,500 rpm to a total of 38 degrees. At 3,500 rpm the inner cylinders inner cylinders were set to 34 degrees and the outer cylinders liked 28 degrees. The engine was run many times and the engine produced more power with the inner and outer cylinders 6 degrees apart.

Once the timing curve was established the Sniper Nitrous system was hooked up to the motor. There are three different horsepower levels to the Sniper Nitrous system: 100, 125, and 150. The nitrous plate was jetted with a 150 horsepower shot and the timing was retarded with the ignition by 6 degrees for every cylinder from our timing map. We started our pulls at 3,500rpm and found that the air fuel ratio was extremely rich for all cylinders. The air fuel ratio for all of the cylinders was around 8:1. We have a stockpile of nitrous jets, so several pulls were made, changing the fuel and nitrous jets until the air fuel ratio leaned out a slightly. When finished we were able to get the nitrous pulls with an air fuel ratio of around 11.5:1.

Looking at the data, using the coil-near-plug ignition is well worth the time and expense. With the help of coil-near-plug ignition the average torque went up by 20 lb ft and the engine gained an average of 20 horsepower. That is a substantial gain versus a distributor.

The coils can be mounted wherever you choose for your application. For dyno purposes we took a good set of used valve covers and installed the coil brackets, coils, and installed the plug wires. The ignition system was mocked up the engine stand before the dyno session so the transition to coil-near-plug would be simplified.