Tech Talk #89 – The Eyes Inside Your Engine


By David Reher, Reher-Morrison Racing Engines

“Spark plugs are the best source of information on what’s really happening in the cylinders.”

If you really want to know what’s happening inside an engine, look at its spark plugs. The spark plugs are eyewitnesses to the combustion process, and their expert testimony provides insights into what is really happening inside the cylinders.

There was a time when the walls of auto parts store and speed shops were plastered with posters that showed examples of used and abused spark plugs. Photos of black, carbon-fouled plugs were labeled “Rich,” while snow-white plugs with their ground straps burned off were described as “Lean.” Reading and interpreting a spark plug’s story in a racing engine requires a more subtle approach – unless, of course, a broken piston or wayward valve has smashed the spark plug to smithereens.

Today racers have sophisticated tools such as exhaust gas temperature probes (EGT) and oxygen sensors (O2) to guide their tuning efforts. While EGT and O2 numbers can be very helpful, they are not infallible. The temperature of the exhaust gases is measured after the fact, and therefore may not accurately portray the actual conditions in the combustion chamber. Oxygen sensors are intended to be used with unleaded fuel, and consequently the leaded gasoline used in racing engines can contaminate them. The use of power adders such as nitrous oxide, the age of the oxygen sensor, its location in the exhaust system, and even the header design (straight collector, merge collector, or zoomie-style individual pipes) can affect oxygen readings.

That’s why I regard the spark plug as the most trustworthy authority. Reading spark plugs may seem rather quaint and old-fashioned in this high-tech age, but short of tearing down an engine to examine its internal components, the spark plugs are the best source of information on what’s really going on in the cylinders.

I use a Welch-Allyn otoscope to read spark plugs. That’s the same illuminated magnifier that my doctor uses to examine my ears during a physical. It’s significantly more expensive than the illuminated flashlights that are commonly used to read spark plugs, but as a professional engine builder, I regard it as an investment in the health of our customers’ engines. And when you compare the cost of a high-quality otoscope to the expense of repairing a detonated engine, it’s a bargain. In fact, I use the otoscope for checking all kinds of machined parts in our shop.

I’ll preface my comments on reading spark plugs by noting that they apply to conventional-type spark plugs used in gasoline-burning engines. The three parts of the spark plug that I examine in detail are the ground strap, the center electrode, and the porcelain insulator.

The ground strap – the metal hook that projects from the spark plug body – is an excellent indicator of combustion chamber temperature. If the heat is high, the strap will show a rainbow of colors, similar to the tip of a welding rod. The condition of the center electrode is another telltale sign of excessive heat. The electrode in a new plug has sharp edges; if the corners of the center electrode are eroded or rounded after a run, that is a sure sign of excessive heat.

Is the tip of the porcelain insulator clean, or does it have specks that look like black pepper or aluminum dots? “Pepper” on the insulator is usually caused by oil, and any aluminum specks are probably coming from a piston that’s on the verge of melting. Specks on the insulator nose are a warning sign of detonation, and that’s not good. Detonation destroys engines.

Now look deep inside the plug to where the porcelain joins the metal body. This is where you can see evidence of the fuel mixture. When the fuel mixture is right, you’ll see a light tan ring on the insulator, about 1/16-inch tall. As the air/fuel ratio gets richer, this ring darkens – until it becomes black if the mixture is excessively rich.

The spark plugs are a cumulative record of the engine’s history. So if the spark plugs are used, they show everything that’s happened since they were first installed. Therefore it’s essential to use fresh plugs if you want to get accurate plug readings. It may not be necessary to check the spark plugs religiously in a mildly modified engine, but if you are running a Top Dragster or Top Sportsman engine that makes 1500 horsepower, you have to pay attention to every detail.

Reher’s rules of thumb on track tuning are simple. If you are going to do a timing loop to determine the optimum spark advance, retard the timing before you advance it! You aren’t going to hurt the engine if the timing is slightly retarded – but if it’s already on the high side, you can cause serious damage with too much spark advance. If the car slows down with the timing retarded, you can safely advance the timing a little and probably pick up some performance. What if you retard the timing and the car runs quicker? That means you were already on the edge.

The same principle applies to tuning the fuel mixture: Fatten it up first – don’t lean it down! It’s safer to run the engine a little too rich than it is to court detonation with an air fuel ratio that’s too lean. Using these techniques, checking the spark plugs, and tracking the tuning results on time slips, you can tune an engine very precisely at the track.

The more powerful an engine, the more attention to detail it demands. A little extra time spent working on the details generally saves a lot of money in the long run. “Well, it’s all right” is never a good thing to tell yourself at a race track.