The Nose's Job
The aftermarket is filled...
The aftermarket is filled with good-quality plug magnifiers. Here's acheap alternative: It's a photographer's loupe. Basically, it's a simpleplastic magnifier that's available in a number of differentmagnification powers. The example shown is an eight-times version.They're downright cheap and available at any camera store.
Projected-nose spark plugs are preferred for several reasons, butperhaps the most important is the plug's ability to act like a hot sparkplug at low rpm levels, then cool off and act like a cold plug at highengine rpm. The projected-nose spark plug is cooled by the incomingair/fuel charge that effectively blows by the extended spark-plug tip.At low engine speeds, the increased insulator length improves thespark-plug temperature characteristics, allowing the plug to burncleanly without fouling. Further, the projected nose helps place thespark plug in a well-suited position to initiate the ignition process.
The High-Power IgnitionSystem Difference
The use of modern day high-powered ignition systems such as computerignitions, multiple-spark ignitions, and many of today's digital systemsalso creates problems in spark-plug selection, but not in a way youmight first imagine. High-quality, high-power ignition systems producerelatively large amounts of coil current. Because of this, thespark-plug condition is not as critical as it once was (in the days ofsingle- and dual-point distributors). While the spark-plug condition isless critical, it doesn't mean the spark plugs are "set and forget"components. Reading of the plugs with these ignition systems iscertainly more difficult, but it isn't impossible.
Indexing is quite common in...
Indexing is quite common in racing. What is it? Here are the basics: Inorder to index spark plugs, the insulator body must be marked with afelt marker (don't use pencil because it can create a carbon track forsparks to follow). The first step is to place a long mark on the side ofthe plug where the ground electrode attaches to the spark-plug body.
Many Corvette enthusiasts make the switch from a conventional ignitionsystem to a high-powered system of some sort, then claim the spark plugsare reading lean. Think about this: The air/fuel mixture hasn't beenchanged; how can the ignition system affect the air/fuel mixture? Itcan't, but because of the better burning properties of a modern ignitionsetup, changes may be required in the air/fuel mixture.
So why are the spark plugs so clean? Because the ignition system isefficient. In order to read spark plugs used in conjunction with acontemporary ignition system, MSD offers the following suggestions.
1. Pick up a copy of a spark-plug tuning guide (all of the majorspark-plug manufacturers offer some form of a high-performance pockettuning guide). Examine the correct color and appearance of your sparkplugs and compare them to the guide. This should give you some insightinto things like detonation, overheating, and coloration.
Instead of rummaging through...
Instead of rummaging through boxes and boxes of spark plugs in an effortto locate the perfect (and sometimes unobtainable) combination of plugthreads that match the head threads, use aftermarket indexing washerssuch as these models from Moroso and B&B Performance. The washer kitsare typically supplied in 0.060-, 0.080-, and 0.100-inch thicknesses.
2. There are several methods to prepare for reading spark plugs, but inthe case of a high-performance Corvette, there is only one right way toaccomplish it. The engine must be shut off "clean." This means youshould run through a quarter-mile and click off the engine, allowing thecar to coast to a stop at a safe location. Don't drive the car anyfurther. The idea is to stop the engine clean, then remove the plugs andinspect them. Obviously, this type of clean shutoff will be easier withmanual-transmission Corvettes than it is with automatics. Nonetheless,it's necessary because the trip back down the return road can wipe outany valuable reading information provided by the spark plugs.
3. The point of effective heat transfer generates a ring on thespark-plug insulator nose. The ring almost looks like a shadow and ismore easily observed with a spark-plug inspection light or loupe. Thering forms quickly and is an indication of burning in the combustionchamber. The closer to the tip the ring forms, the richer the fuelmixture. The closer to the spark plug shell, the leaner the mixture. Ifthere is no ring and the spark plug insulator tip is bone white, thefuel mixture is too lean (this information applies only togasoline-fueled race cars). The total range from rich to lean (ring nearthe tip to ring near the shell) may only take a change of 7-10 percentin the air/fuel mixture.
When making the switch to a high-powered ignition system, you may findthat the actual fuel curve will have to be "fattened" by 5-10 percent,because the ignition system is now burning more efficiently. Inaddition, the total spark timing might have to be reduced byapproximately 3 degrees.
As you might have guessed, reading the spark plugs isn't a black art,but it isn't an exact science either. You have a number of variablesopen to you: Air/fuel mixture, spark-plug heat range, ignition timing,fuel type, transmission type, and a host of other things that aren't inyour control (ambient temperature, weather conditions, altitude, and soon). The idea is to make the most of your situation and tune from there.Let your elapsed times be the final judge.