Understanding Spark Plug Gap

The spark plug is the device that causes the fuel/air mixture in the cylinder to ignite for a proper burn so understanding the spark plug gap is a crucial element. All internal combustion engines have a spark plug, or in the case of diesel engines, a glow plug, to initiate combustion in the engine. Spark plugs in a gasoline engine have an additional purpose of removing heat from the combustion cylinder. Both duties are only accomplished when the proper spark plug is installed correctly and with the correct gap.

Spark Plug Selection

There are two schools of thought on spark plug selection. The first is to replace the same spark plugs that the manufacturer initially installed. The manufacturer designed the engine to run with a spark plug with the designed heat range, designed ignition gap, and appropriate torque. On the other hand, a different spark plug manufacturer can be selected. The new spark plug needs to be of the same heat range, with the same ignition gap, and the same appropriate torque. Normally the new spark plug will either be a multiple ground type, or an upgrade of materials that help the engine deliver the spark it needs with the stock ignition system.

Reading the Removed Spark Plug

Normally the spark plugs are replaced appropriately every 30,000 miles. The inspection of a removed spark plug should show a light gray to light brown discoloration. The negative probe should not be more than partially burnt away, and the gap should be only slightly more than the originally installed gap between the center electrode and the ground electrode. These electrodes should also be properly aligned. A missed alignment will cause one or both electrodes to be burnt away.

Proper Spark Plug Gap

A gap that is too close will not deliver the proper amount of spark to satisfactorily cause the air fuel mixture to burn cleanly. While a gap that is too wide may not allow the ignition system to fire the plug at all. Spark plugs that are not gapped properly will cause the engine to either skip firing, or ignite too soon in the compression stroke. Properly gapped spark plugs also remove heat from the cylinder ignition chamber. ” The temperature of the end of the plug’s firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. The spark plug works as a heat exchanger by pulling unwanted thermal energy from the combustion chamber and transferring heat to the engines cooling system. The heat range of a spark plug is defined as its ability dissipate heat from the tip.”

Setting a Proper Gap

While plug manufacturers try to produce every plug the same, doing that is about impossible in a mass-production environment. Plus, there is no accounting how that plug is handled on all the steps along the path from the factory to your hands. That is why checking the gap between the electrode and ground strap on every spark plug you use is important. It will not exactly create new horsepower, but it will keep you from leaving some in the pits.

The critical distance between the spark plug’s electrode and the ground strap (that piece of steel that curves out over the end of the plug) is called the “plug gap.” The act of setting this distance correctly is commonly referred to as “gapping” your plugs. If the gap is too small, there is not enough room for enough of the air/fuel mixture to get between the electrode and ground strap, so when the plug does fire there is a chance nothing will ignite. On the other hand, if the gap is too large the spark that does appear will be weak and may not be strong enough to ignite the charge.

When setting the spark plug’s gap for installation, use the automobile manufacturer’s recommended gap. The engine designer has tested the engine on a dynamo to calculate the entire ignition system as a complete package for the engine’s best results. Any deviation from these specifications will move the engines optimal performance away from the engine’s best tune-up performance. These specifications will keep the engine from pre-ignition and detonation, which is the rapid and uncontrolled burning of the fuel in the combustion chamber while the piston is still rising.

For most racing applications, you usually want the plug gap to be between 0.020 and 0.040 of an inch. Most engine builders seem to settle around 0.035 of an inch. Factors such as the type of ignition you run, cylinder heads, fuel and even timing can affect how much gap will work best for you. Generally, you want as much gap as you can get by with to ensure ignition each time the plug fires. For more specific recommendations, talk to your engine builder and even call the tech help line for your preferred plug manufacturer.

Proper Cleaning

Following the removal of the spark plugs each plug whole should be cleaned without allowing any rust or other materials to fall into the cylinder. The installation of a new spark plug should be performed with a rubber inserted plug socket. Hand tighten the spark plug, then tighten one half to three quarters turn for proper torque. This should be sufficient from allowing the spark plug’s washer from letting any gases to escape prior to and during ignition.

Finally

A spark plug a relatively simple piece of equipment, but it must perform its job in incredibly tough conditions–especially in a high performance car. If the plug can produce a strong enough spark to light the air/fuel combustion mixture, then we are happy.

When you throw in the fact that you are asking a spark plug to operate in a difficult environment, that makes it even harder. More engine compression means a denser air/fuel charge that is harder to ignite. High flow intake ports and–once again–high compression means lots of swirl and movement of the charge inside the combustion chamber which makes ignition harder. And high-rpm engines allow the distributor little dwell time to build up a strong spark, which–you guessed it–makes firing the charge harder. Hopefully, this will give you a bit of knowledge in understanding spark plug gap.