Spark plugs and ignition wires:
I have read on other forums people finding after market ignition wires and caps.
This could be a big money saver, but it can also be a source of concern if you don’t
pay attention to the following points:
The K100 is originally fitted with Bosch X5DC (or can be fitted with the NGK
D7EA). Both are non resistor spark plugs and here is the reason:
The original BMW spark plug cap already integrates a resistance of 5KΩ. Similarly,
the connector at the other end of the original ignition cable also includes a resistance
of 1KΩ (The total resistance is 6KΩ).
If you replace the wires with after market non resistor wires and caps and keep the
original spark plugs, you will run into troubles and take a chance to damage the output
transistors of the ignition module (resistance of the wire/spark plug will be close to 0).
It is possible to fit a K100 with after market plugs and wires, but the spark plugs have
to be replaced with a resistive type (Bosch XR5DC or NGK DR7EA). These spark
plugs have a built in 5kΩ resistor.
The X(R)5DC has a thermal index of 5 the D(R)7EA has a thermal index of 7.Even if
the thermal index is different, both are suited to be installed on a K100 engine....
Why is the Bosch having a thermal index of 5 and NGK has a 7?
Each manufacturer uses its own designation for the heat range:
On the Bosch plugs the higher the number (from 4 to 8 ) the hotter is the plug. NGK
works on the opposite and goes from 8 to 4 (a lower number indicates a hotter plug)
The term spark plug heat range refers to the speed with which the plug can transfer
heat from the combustion chamber to the engine head. It has been found the optimum
combustion chamber temperature for gasoline engines is between 500°C–850°C.
When it is within that range it is cool enough to avoid pre-ignition and plug tip
overheating (which can cause engine damage), while still hot enough to burn off
combustion deposits which cause fouling.
The spark plug can help maintain the optimum combustion chamber temperature. The
primary method used to do this is by altering the internal length of the core nose, in
addition, the alloy compositions in the electrodes can be changed. This means you
may not be able to visually tell a difference between heat ranges. When a spark plug is
referred to as a “cold plug”, it is one that transfers heat rapidly from the firing tip into
the engine head, which keeps the firing tip cooler. A “hot plug” has a much slower
rate of heat transfer, which keeps the firing tip hotter.
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