Wes Duenkel
November 20, 2017

Gasoline internal combustion engines need three fundamental things: fuel, oxygen, and spark. The sexy aspects of modifying cars are focused on the first two elements: Double Pumper carburetors, sophisticated fuel injection systems, whistling turbochargers, and massive blowers all work to cram more fuel and oxygen into our engines. But without spark to ignite the mixture, nothing will happen.

Spark plugs are often overlooked, but for good reason. Decades of testing have proven: if the ignition system is sufficient and working properly, gains from the latest whiz-bang spark plugs, fat wires, and high-powered ignition systems will be minimal. But the wrong or worn-out spark plugs kill power and drivability. Here are a few tips to make sure your engine is getting the spark it needs.

Here are two spark plugs of different heat ranges. The top is a standard spark plug from a 2014 Focus ST. The bottom plug is a replacement with a colder heat range (necessary when increasing boost with the Ford Performance calibration for the Focus ST). Note how the top plug’s center electrode is longer and more “exposed” to the heat of combustion than the bottom plug. The bottom plug’s center electrode has a shorter, “easier” path to the plug threads, drawing the combustion heat away from the center electrode to keep it from being a “hot spot” for detonation.

Check and Set Spark Plug Gap: The air-fuel mixture is ignited when a spark “jumps” the gap between the spark plug electrodes. A smaller gap produces a shorter spark, and a larger gap creates a longer spark that is exposed more to the air-fuel mixture in the combustion chamber.

However, large spark plug gaps can be more trouble than they’re worth. Why? Recall that air is an insulator (doesn’t conduct electricity well). When more air is between the spark plug electrodes, it requires more voltage to jump the gap. This increased voltage is hard on the rest of the ignition system (wires, coil, amplifier, and distributor cap and rotor). Electricity will also take the path of least resistance, so if too much air is between the spark plug electrodes, the shortest path may be somewhere else—like between the spark plug wires, or the wires and the engine.

Maintaining proper spark plug gap is especially important with boosted or high-compression engines, as the additional pressure inside the combustion chamber squeezes more air molecules between the spark plug electrodes—effectively increasing the spark plug “gap.”

Understanding this phenomenon is important when diagnosing misfires. Some engines will run fine at idle and light throttle, but will misfire badly under load. The culprit may be spark plugs that are badly worn or gapped improperly.

Heat Range: A spark plug’s heat range classifies how insulated the center electrode is from the spark plug threads, and thus, the cylinder head. The heat of combustion warms the spark plug tip, and a plug with a long, insulated path back to the cylinder head has a hotter center electrode than that of a shorter, more direct path. “Hotter heat range” plugs are less likely to “foul” because they stay hotter between combustion cycles and burn off any harmful deposits left behind.

However, if the spark plug stays too hot, it becomes a sort of “glow plug” and can ignite the fuel mixture prematurely. This is called detonation, or “pinging.” Higher combustion temperatures from boost or increased compression sometimes call for a “colder heat range” spark plug, as the risk of fouling is a reduced under these conditions. Spark plugs of a heat range “too cold” can appear black and “sooty,” while spark plugs of a heat range “too hot” will cause detonation and the center electrode can look white, brittle, and/or pitted.

Misfire Diagnosis: Most ignition misfires get worse under load. Check the plugs to make sure they’re not worn and the gap is correct.

If replacing plugs doesn’t solve the problem, check the spark plug wires, as they may be arcing to each other or electrical ground. Here’s a trick: in a dark place, open the hood with the engine running. Mist the engine with water in a spray bottle. If the misfire increases and/or you see little “lighting flashes” around the plug wires, the wires should be replaced or upgraded. Also, check the other components such as the distributor cap and rotor, ignition coil, and ignition amplifier (such as an MSD box).