PART FOUR: FAILSAFE MONITORING SYSTEM
1. With the ignition and nitrous...
1. With the ignition and nitrous systems set up, we strapped the engine to the dyno and then added another extremely valuable tuning tool, Innovate Motorsports’ ST-12 wideband air/fuel ratio data logger.
2. The ST-12 gave us the ability...
2. The ST-12 gave us the ability to install a wideband oxygen sensor into each primary tube of the headers. The O2 sensors will tell us the exact air/fuel ratio of each cylinder in real time while the engine is running. Reading the spark plugs will tell us the same info, but only after we’ve ran the engine, which could be too late if we don’t get the nitrous tune-up correct the first time we hit the button.
3. (See below.)
3. We’ve used the ST-12 before so we knew that we had a fuel distribution problem that resulted in the corner cylinders (numbers 1, 2, 7, and 8) running very lean, while the middle cylinders (numbers 3, 4, 5 and 6) were running very fat. After talking with the designer of the manifold and the guys who ported it, we settled on the idea that the manifold, exhaust, and camshaft combination was to blame. Rather than replace a bunch of expensive parts, we chose to use the direct-port nitrous system to correct the air/fuel ratio when the motor is at wide-open throttle and racing, which is when the air/fuel is most critical to us. As you can see from Innovate’s software, the corner cylinders are as much as three air/fuel ratios leaner than the middle cylinders. Our solution to our distribution problem was to richen the mixture in the corner cylinders via carburetor jetting changes and to lean out the middle cylinders using the nitrous system. This was accomplished by going bigger with the nitrous jets in the number 3, 4, 5, and 6 nozzles. What we learned through our testing was going up .002 in nitrous jet orifice size leaned the air/fuel ratio of the middle cylinders just under a half a ratio. We began the day with the middle cylinders nearly fouling the spark plugs with a low 10:1 air/fuel ratio. By the end of the test, we had staggered the fuel/nitrous jetting from .024/.026 to .024/.030, which brought the middle cylinders to a more reasonable 12:1 air/fuel ratio. At the same time, the power output of the engine increased dramatically, showing about a 20hp gain with each jet size increase. Going bigger with the nitrous jets while keeping the fuel jets the same resulted in leaner cylinders that made more power. Each time we went up .002 in nitrous jet size in those overly rich middle cylinders, the air/fuel ratio went down. We stuck with the two jet spread in the corner cylinders and ended our tuning session with a happy engine that made awesome, safe power.
Part Five: Spark Plug Detective Work
As cool as the ST-12 is to use, it's a pricy tool that not every dyno facility has at its disposal. That is why confirming what we saw on the computer with spark plug readings was so important. Reading spark plugs is an art with different ways of interpreting the colors and nuances of each plug. Our plugs indeed confirmed what the computer told us. Here are a few examples of spark plugs from cylinders running in various states of tune.
1. While you can clearly see...
1. While you can clearly see the effects of ignition timing on the ground strap of this plug, you can’t see the fuel ring down at the lower portion of the porcelain insulator. The fuel ring will tell the story of how rich or lean the cylinder is running. Without an Otto scope or other viewing tool, the easiest way to see the fuel ring is to cut off the threaded portion of the plug using a hacksaw or other tool.
2. The fuel ring is clearly...
2. The fuel ring is clearly visible on this plug. It’s a dark, sooty, color and stretches almost halfway up the length of the insulator, indicating that this cylinder is running very rich. A clean running cylinder will have a barely visible fuel ring all the way down at the bottom of the insulator.
3. Now this spark plug looks...
3. Now this spark plug looks like it’s barely ever been run. Note that there’s a very faint gray fuel ring at the base of the porcelain, indicating that the cylinder is running with either an ideal air/fuel ratio or almost on the verge of being lean. Ask ten different tuners if this plug came from a lean cylinder or not and you’ll probably get five different answers.
4. Nitrous tuners used to...
4. Nitrous tuners used to think that running the engine with a rich air/fuel ratio was safer than running it lean. Over the years though, most have come to accept that a rich mixture actually damages more parts than a lean one. The same can be said for running too much ignition timing. The timing advance can be seen on the ground strap. Too much timing and the colored line will appear on the strap near the threaded portion of the plug and that’s a recipe for detonation. If you have way too much timing advance the ground strap can be burned completely off the spark plug. Too little timing and the mark will not be visible or will appear at the very end of the ground strap. On this plug, the rainbow-colored line appears right before the bend, otherwise known as the sweet spot for making horsepower without hurting parts.
The Final Word
Since 2006, we've run this engine on pump gas and on race fuel as a guinea pig for dyno testing, and now we've added nitrous oxide to the mix for some real fun. Thanks to the expert advice from the guys at Nitrous Supply, Induction Solutions, Westech Performance, and a few savvy members of various Internet message boards, we were able to add almost 300 horsepower to the engine without making eight new ashtrays out of the pistons.