Spark plug recommendatiions Options

[retrofit]

(IMG:style_emoticons/default/WTF.gif) What spark plugs are recommended for a stock '74 2.0?
There are more choices than the typical Bosche.
Allen

[ericread]

I posted this response the other day on another thread, but I thought this might be pertinent to this thread. Yes, you'll notice I cut some corners in this response in an effort to not post a lengthy article.

A spark plugs’ heat range is an index of its capacity to dissipate thermal energy. The different characteristics of automotive engines regarding operating load, compression, engine speed, cooling, and fuel make it impossible to run all engines with a standard spark plug.

The so called “heat range”, which is assigned to each spark plug, is used to characterize these heat dissipation capacities. A plug with a low heat range number (e.g., 2-4) indicates a cold plug that quickly dissipates heat to the engine block and cooling system, while a high code (e.g., 7-10) indicates a hot plug that retains heat. By properly selecting the heat range of the plug, it ensures that the plug will operate between the plug’s designed operating range of 500-900 degrees Celsius. In this range, the spark plug will be self-cleaning, yet will not be hot enough to pre-ignite the air/fuel mixture.

In some situations, a slightly hotter or colder plug may be installed than the one normally recommended. But a hotter plug should not be used unless an engine is experiencing a fouling problem because of the increased risk of preignition and detonation. Switching to a slightly colder plug can reduce the risk of preignition and detonation in performance applications (especially turbocharged and supercharged engines.

Copper and Platinum spark plugs come in many heat ranges. So what's the difference between these plugs? First, let's take a look at the thermal characteristics of commonly used electrode materials:

Zinc == 1.16
Aluminum == 2.37
Copper == 4.01
Steel == 0.70 - 0.82
Platinum == 0.716
Iridium == 1.47

As you can see, copper has the best thermal characteristics, and makes the best material for a spark plug conductor. So why use Platinum. Because while copper is the best conductor, it also burns off faster under higher temperatures. Coating the copper with a fine layer of Platinum allows for a longer life at higher temperatures, but requires a higher voltage spark. Now you can reduce the voltage necessary to get a suitable spark by narrowing the diameter of the electrode. But this lowers the ability of the electrode to dissapate heat. And we are back to the problems of a heat, preignition and detonation.

Whew, it kind of goes in a circle.

Sooooo, although you could use a Platinum plug in the correct temperaure, 1) You might find that the stock ignition system of our 35 year old cars isn't really made to carry the higher voltage these plugs may require; 2) You might find the plugs may contribute to problems of preignition and detonation, and; 3) You will probably find the 914 combination of lower compression and lower heat temperature make it really ideal for using a copper plug

As for actual plug sizes: Yes, there are many lengths of spark plugs. Place a very long plug in a 914 and the plug will contact with the piston head - causing disasterous results.

Eric Read

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