“Know your car” Series #2
Where are they? The spark plugs sit on top of the cylinders on a MX-5. Looking at the engine you can see a set of spark plug leads running down the engine with 4 rubber boots in the centre of the engine’s rocker cover. The spark plugs are set about 80mm under those rubber boots.
How Does It Work?
Part 1 Delivering the spark
A spark plug is designed to receive an electrical current at its top and pass the current down into the tip (or electrode) which is inside the engine cylinder. When the electrical charge reaches the tip of the electrode it is forced to arc across the gap between the electrode and the side terminal. This is like a bolt of lightning that ignites the air/fuel mixture in the cylinder and causes the combustion cycle of the piston.
Source Diagram from NKG
To create the arc a very high voltage is required i.e. from 40,000 to 100,000 volts. As the spark plug has to carry an electrical charge from its top to its tip a ceramic insulator is required to isolate the centre wire from the metal jacket of the spark plug.
The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is determined by the plug's heat rating. A typical spark plug tip operates effectively in the 500 to 850 Celsius range.
If below 500 C the ceramic insulator won’t be hot enough to burn off carbon deposits and it will foul. Over 850 C will cause premature detonation, and in extreme temperatures it can melt the parts of the plug within the engine or fracture causing catastrophic engine damage.
To suit the correct thermal range for an engine the spark plugs come in various shapes and sizes. The shape of the ceramic around the exposed tip of the plug's electrode is important in determining a plug’s thermal performance. Take a look inside the spark plug and notice the shape of the white ceramic section.
A hot plug's exposed ceramic section is shaped like an inverted ice-cream cone (with the electrode poking through the cone's pointed end). As the ceramic's outer layer extends down inside the plug it's long exposed sides present a relatively large surface to the hot gasses in the combustion chamber. As it absorbs heat it influences the temperature of the metal electrode. The ceramic's shape also means that there is a gap between the plug's metal jacket and the ceramic. Therefore the contact area between the ceramic and the plug's metal jacket is reduced. This smaller contact area results in a slower transfer of heat from the plug to the cylinder head. In short, a hot plug absorbs more heat, the tip runs hotter and it loses heat more slowly than the cold plug.
Cold type plugs have an almost plateau shaped top to the ceramic section and may fill the hollow of the plug almost to the brim. There is very little exposed section between the ceramic and the inside of the plug's metal jacket. This arrangement presents a small surface area to the hot gasses within the combustion chamber. This keeps the tip of the electrode cooler. There is a larger contact area between the ceramic and the plug's metal sides and this provides for better heat transfer from the plug to the cylinder head.
The higher the number the colder the plug in the NKG range.
Source Diagram from NKG
Mazda recommends that the MX-5’s use 5 or 6 heat range NKG spark plugs for the NB8B and NA 1.6’s. NKG parts BKR5E-11 or BKR6E-11 are for the NB8B 1.8 litre and a further option for the cooler BKR7E-11 is for the 1.6 litre NA’s. A movement from a 5 to a 6 heat range plug (colder) results in lowering the electrode tip's temperature by approximately 70°C to 100°C. This would be beneficial if the "5"'s electrode was 900°C, but retrograde if it were running at 550 °C
Modifications such as forced induction or higher compression result in higher engine temperatures. Therefore take advice on choosing a colder plug to handle the increased engine temperatures if you have these modifications.
Maintenance. Spark plugs in engines using lead-free fuels and electronic ignition systems, as in the MX-5, last for around 30,000 Kms before spark plug service is required. The MX-5 service schedule for the NA’s require inspection (and replace if required) every 15,000Kms whereas the NB service schedule requires replacing every 30,000Kms, with no intermediate inspection.
When replacing the spark plugs make sure that they are correctly gapped (1.1mm in 1.6 and 1.8 litre engines) and use the recommended plugs (I use NGK brand). The 11 in the NKG part number signifies 1.1mm gap.
A small amount of molybdenum based thread lubricant on the first few threads of the plug is recommended. The gasket between the spark plug and the engine block is crushable. Spark plugs should not be overly tightened. They need to be torqued to the correct tension. See the spark plug box for details. The 14mm thread diameter and alloy head mean the spark plug torque is 18.0~21.6 lb-ft for your MX-5. Make sure the engine is cool before removing of replacing the spark plugs. If you are using thread lubricant then the torque can be reduced by one third.
Rob (Techno) Spargo
Mazda MX-5 Club