In the early '70s, Mopar changed the concept of a tune-up with the introduction of the breakerless, capacitor-discharged ignition system. Thus began the evolution of electronic engine controls. since then spark plug technology has advanced as well, but spark plugs remain the core of any ignition system. Let's face it, unless you're running a diesel engine, your engine has them. They have a great bearing on your car's performance, and can reveal many symptoms of an engine's condition when they are properly "read."

"We recommend spark plug replacement at OE specified intervals or sooner if spark plug problems are suspected. When you pull the old plugs out, the color and condition of the plug's tip can provide a good indication of what's going on in the combustion chamber," says Matt Hallis, product technical support specialist with the Bosch Corporation.

Being able to read spark plugs has always been considered "black magic" of sorts for many, but properly reading a spark plug is a must for any kind of performance tuning. Reading spark plugs can also give you an early insight into any problem that may be just beginning to rear its ugly head.

Orders Up

In order to properly read the plugs and use the readings to improve performance, it's important to keep them in order with the cylinders they were removed from. In a good running and properly tuned engine, the plug tips should all be very close in coloration. A tan, slightly reddish deposit on the spark plug is normal. Other color or condition on the tip may point to fuel, ignition, vacuum, or larger, more fatal engine problems.

If your spark plugs are covered with black and fluffy soot on the insulator, this indicates an over-rich fuel condition, which may be caused by an out-of-adjustment carburetor. If the plug has small, white deposits on the insulator, nine times out of ten this may indicate oil consumption. If that's the case, now you can look deeper for possible problems with a cylinder's valve seals, worn valve-guides, a badly worn or broken piston ring, or even a badly scored cylinder.

A very light tan or, even worse, white, blistered insulators may point to a mixture that is too lean. If proper carb tuning doesn't seem to help, a vacuum leak may be the cause of this condition. Other spark plug conditions include wet fouling, which indicates no spark is getting to the cylinder, or a physically damaged electrode, which indicates serious internal engine problems.

Going Premium

Not all spark plugs are created equal. We may like to think that the spark plugs hanging in the automotive section of Wal-Mart are as good as any plug on the market, but that's simply not true. Buying a quality spark plug means doing a little research. Say you are using a set of Edelbrock heads. Edelbrock has done extensive testing with their heads, and you can bet that different plugs have been part of that testing. Sure, there may be some variables in your engine that dictate a different heat range, but their recommendation will get you close, if not right on.

Hot and Cold Flash

Depending on the driving situation, some people feel a hotter plug is needed to resist fouling when their primary driving habits consist of around-town driving. For highway jaunts, a colder plug would be chosen to resist the chance of detonation.

So this raises the question, "should I install a hotter plug?" When we were dealing with previous plug technology and antiquated ignition systems, a hotter plug may have seemingly solved a tuning issue. Today's high-voltage ignition systems have the capacity to fire the specified plugs under most conditions. If there is a problem with fouling, there's a good chance the problem is caused by something other than the plug (as explained in how to "read" the plugs).

Cooling a Bigger Squeeze

But what if you are running a lot of compression, should you choose a "colder" spark plug?

In some cases, when the compression ratio of an engine is increased, lowering the heat range of the spark plug may be advisable. For instance, going from a compression ratio of 9.0:1 to 10.5:1 would usually call for a plug that is one heat range lower. Whereas a radical increase in compression to-let's say 12.5:1-would call for trying a spark plug that is two heat ranges lower than factory specified. Another thing to keep in mind is an increase in compression usually requires a change in ignition timing according to the fuel that is used. With so many combinations of engines, there is no cut and dried chart showing exactly what your engine needs. Finding the right plug can be a matter of hit or miss, but we're here to help you get it right.

High-performance applications have been the breeding ground for many of today's advancements in spark plug technology. The demand for spark plugs with a broader heat range was heightened by electronic spark control systems that advance timing to the verge of pre-ignition; increasing the need for a spark plug to resist pre-ignition. The advent of unleaded fuels with generally lower octane ratings helped to bring about the need for spark plugs with broader heat ranges. A broader heat range enables the spark plug to reach a self-cleaning temperature at lower loads and avoid overheating under higher loads. This change in fuel availability, and availability of broader heat range plugs, necessitated a greater resistance to erosion and corrosion to produce longer plug life. A standard spark plug might still burn cleanly after 12,000 miles, but its gap may have eroded by as much as .010-inch. As a result, the market for a premium plug was born. Enter the use of platinum for the plugs electrode. Platinum is extremely corrosion and erosion resistant, and the gap remains virtually unchanged for the life of the plug.

So how do we know what the plug in front of us is saying? Is the engine rich? Are there issues with the oil control? Is the engine shot? Well, let's try to shed a little light on the subject. This article will by no means make you an expert at plug reading, but, hopefully, by the end of the article you will have an idea of what your engine is trying to tell you. All you have to do is read the signs it's giving you.

Robert Bosch Corporation
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