SPARK PLUG HEAT SELECTION
By KillerRONS.COM's Scott Offermann and James Monroe
At KillerRONS we are questioned daily about proper spark plug selection and usage in conjunction with alcohol systems in general. This is a difficult question to answer with absolute certainty but in this tech bulletin we will offer general answers and supply the reasons behind these answers. In doing that we will explain the differences in types of spark plugs, the meaning of heat ranges and describe how the proper “gap” is determined. Again, these guidelines will be a starting point only. Ultimately, the correct spark plug should be determined by the tuning process on your particular engine. Look for a “Basic Tuning” article coming soon for your Ron’s Fuel Injection system that will touch on many of the things discussed here in a more depth manner.
SPARK PLUG TYPES
There are many types and styles of spark plugs. For simplicity’s sake we will focus on conventional style spark plugs that are used in most race engines. Race engine spark plugs must provide sure spark, offer great ignitability and support rapid acceleration in the most extreme conditions. At KillerRONS we recommend a spark plug with a cadmium or zinc coating as this makes it easier to read the burn pattern on the spark plug. NGK Spark Plugs are a great choice and are recommended by KillerRONS and will be used in the examples for this technical bulletin.
Most manufacturers offer a choice between an “extended” tip and “recessed” tip spark plug in the same heat range. This refers to how far into the combustion chamber the tip of the spark plug hangs. Combustion chamber design, piston dome size and piston dome shape play a role in what style plug is utilized in an engine. In some instances, a recessed tip is necessary due to clearance issues. Check with your engine builder if you are unsure whether you should be using an “extended” or “recessed” tip spark plug.
DEFINITION OF “HEAT RANGE”
Before we can get into what heat range plug we recommend you should have a good understanding of what “heat range” actually means. A common misconception is that a spark plug produces heat and that a “hotter” spark plug means that more heat is produced and vice versa. This is not true as a spark plug doesn’t produce any significant heat at all.
That being said, it is true that a spark plug does get hot. This heat comes from the process of compressing the gasses and burning the fuel. The heat range of a plug refers to its ability to remove this heat from itself and transfer it through the cylinder head and into the cooling system. The spark plug’s efficiency in doing this is determined by its physical construction (tip insulation) and material construction. With that knowledge in hand we will make the two following statements.
A “cold” spark plug has better heat dissipation capabilities and is able to move more heat from itself into the cylinder head and ultimately to the cooling system.
A “hot” spark plug has less heat dissipation capabilities and has less ability to remove heat from itself than a “cold” plug.
The video link below gives an excellent description of “hotter” and “colder” spark plugs..
The spark plug industry has determined that the ideal temperature of a spark plug under operating conditions is between 950*F and 1450*F*. Less than 950* and the chances of a plug “fouling” becomes greater. At the same time a spark plug temperature exceeding 1450* increases the likeliness of pre-ignition and detonation. This can be devastating to an engine so we believe it is best to error on the “colder” side when choosing a spark plug heat range. The great news for alcohol users is that due to its clean burning characteristics there is a significantly less chance of carbon fouling at temperatures below 950*.
HEAT RANGE FACTORS
There are many factors that affect what heat range spark plug is best for a given application. It depends upon many things but ultimately it comes down to cylinder pressure and an engine’s cooling system efficiency in relation to the spark plug.
Things that can increase cylinder pressure are ignition timing advance, static compression, dynamic compression, horsepower, camshaft, cylinder head efficiency and fuel type. Generally speaking, more cylinder pressure requires a “colder” plug. This is due to a law of physics called “Heat of Compression”. Simply put; when you compress a gas its temperature raises along with the pressure. Cylinder pressure is affected by the following variables.
Static compression is simply the compression ratio designed into an engine. More static compression increases cylinder pressures (heat) and contributes to an engine tipping towards the use of a “colder” plug.
Dynamic Compression is determined by the camshaft design along with the efficiency of the induction system. Camshafts with an early intake valve closure either due to short duration and/or installed centerline will produce more dynamic compression (heat) in certain areas of the RPM band. If this is coupled with high static compression a “colder” plug may be required.
Horsepower is largely determined by the efficiency of the induction system. It is a reference to how good the cylinder heads, camshaft and manifold are at filling the combustion chamber. Highly efficient induction systems trap more air/fuel mixture in the combustion chamber, which results in more dynamic compression. The better the cylinder heads are and the better everything works together the more cylinder pressures (and more horsepower) is produced. All of which mean more pressure/heat and therefore the need for a “colder” plug than a lower horsepower engine.
Fuel Type also has an effect due to the density of the air/fuel mixture. Alcohol carries its own oxygen which, we believe, is one of the reasons it has the ability to produce lower elapsed times and higher MPH’s on the racetrack than gasoline. This denser mixture brings about higher cylinder pressures and ultimately more instantaneous heat in the combustion chamber. This is why alcohol typically requires a “colder” plug than gasoline.
Spark Plug Cooling
Spark plug cooling efficiency is largely determined by the material the cylinder head is constructed of and the design of the water passages around the spark plugs in the cylinder head. Some cylinder heads are better at cooling the spark plugs than others. Some even have the capability of directly plumbing coolant to the center of the head. Basically, a cylinder head that does a good job of taking heat away from the spark plugs may tolerate a slightly “hotter” spark plug. Conversely an engine that is run with a dry cooling system will require a “colder” plug.
Another variable is the quality of the connection between the spark plug and the cylinder head. Heli-coiled spark plug threads may need a “colder” spark plug. Improperly tightened spark plugs will have a poor thermal connection and will run hot but this is not to be confused with needing a “colder” spark plug.
The bulk of the engines that utilize a normally aspirated alcohol injection system require an NGK–8 (Autolite 3934), NGK –9 (Autolite 3933) or NGK-10 (Autolite 3932) spark plug. Below are some general guidelines we have formulated based upon on-track experience and racer feedback. Armed with the information within this bulletin along with testing you will be able to finalize a spark plug that will help optimize the performance of your engine and help extend its service life.
NGK –5 or –6 = 300-500hp engines with less than 11:1 compression
NGK –6 or –7 = 500-700hp engines with 11 to 13 compression
NGK –7 or –8 = 500-700hp engines with more 13-15 compression
NGK –8 or –9 = 700-1000hp engines with 13-15 compression
NGK –9 or –10 = 1000 and up hp engines with 13-15 compression
NGK –10 = 1200 and up hp engines with more than 15 compression
Supercharged alcohol and nitrous-assisted applications make more power because the density of the intake charge is increased. Superchargers do it mechanically and nitrous does it chemically. These applications (especially nitrous assisted) may require a step or two “colder” spark plug.
SPARK PLUG GAP
Spark plugs come from the factory with a pre-set manufacturer plug gap that may or may not be correct for your engine and ignition components. The main factors in determining the correct spark plug gap are the power of the ignition system and the cylinder pressure.
Less powerful ignitions will require a smaller gap whereas more powerful ignitions will allow you to utilize a larger gap. Higher cylinder pressures (either due to the density of the mixture, static compression ratio or dynamic compression) will typically require a smaller gap.
In our experience a gap of between .030-.035 works well for most combinations that utilize a capacitive discharge type of ignition such as MSD. Care must be taken as to listen to what your engine is telling you. For example, too much spark plug gap can result in misfires and hard starting. Too little gap can result in a loss of power.
SPARK PLUG READING
As mentioned earlier a spark plug has a window of temperature in which it needs to operate. We know that cylinder pressure and horsepower have a large role in dictating the proper heat range but there is another very important variable. This extra variable is the air/fuel mixture. Lean mixtures will cause the spark plug to run hotter and rich mixtures will cause it to run cooler. Due to heat spark plugs will develop a “witness mark” on the ground electrode that will give a clue as to what is happening in the combustion chamber. Too lean, too much ignition advance and/or too hot of a plug will cause this mark to be longer and climb too much of the ground electrode. Too rich, too little ignition advance or too cold of a plug can cause this mark to be very low on the electrode or even non-existent. Your goal is to tune the engine and balance these factors, which will inevitably result in an engine that produces the most power while it operates efficiently and problem free. The focus of this bulletin is not to go into the depths of tuning. We will cover spark plug reading in relation to tuning in our “Basic Tuning” tech bulletin coming soon.
Spark plugs themselves will not make any great power increases and there is a margin for error in most engine combinations. That being said, the wrong spark plug selection and improper gapping can cause a wide variety of problems in the performance and ultimate health of an engine. We hope that you found this tech bulletin informative and we offered you more than just “because we said so” answers. If you have any questions please contact us via email or phone and we will be glad to assist you.