[T3] Spark Plug heat range

Jim Adney jadney at vwtype3.org
Wed Jul 2 10:07:13 PDT 2014


When I first got interested in cars and engines, I was completely 
confused about the meaning of Spark Plug heat range. Over time I 
eventually read enough to get over that confusion, but I suspect that 
there are lots of people out there who are just as confused as I once 
was. Here's an explanation I'd been meaning to write up for a long 
time. I've saved this as one of my FAQs.

People often get confused about the meaning of the spark plug heat 
range. Since spark plugs are sold in "hotter" and "colder" versions, 
it's easy to assume that "hotter" is what you might want if you were 
working toward a more powerful, or "hotter" engine, or that a 
"hotter" plug would give you a "hotter" spark. Neither of these 
viewpoints is correct.

The heat range refers to how quickly heat is conducted away from the 
tip of the insulator. The tip we're concerned with is the end of the 
insulator nearest the gap where the spark must jump. The insulator's 
job is to keep the spark from jumping anywhere else, other than 
across that gap.

That insulator is thick enough that the spark will never jump thru 
it, but if the surface of the insulator accumulates enough combustion 
byproducts, the spark can run down that surface to ground. If this 
happens, the mixture will not be ignited. Such a plug is "fouled."

Fortunately, there is a mechanism for keeping that surface clean. The 
ceramic surface will catylize a reaction between any surface 
contaminants and oxygen in the mixture as long as the ceramic is hot 
enough. That sets a minimum temperature for the ceramic to be "self 
cleaning." If the insulator gets too how, however, it will start to 
cause pre-ignition and eventually the ceramic may crack or blister, 
so this sets a maximum temperature.

The result of this is that there is a "self cleaning" temperature 
range. This temperature is the same for all plugs of the same make 
and model, because it is just a function of the ceramic material, but 
it may vary slightly between different makes and models due to 
different ceramic compositions. According to my Bosch literature, 
self cleaning can begin when the insulator tip reaches 400 C (750 F) 
and is fully functional by 530 C (985 F.) Self ignition sets the 
upper limit at about 850 C (1560 F.) Note that these temperatures are 
high enough that the outside air temperature is unimportant. This is, 
for the most part due to the fact that the engine temperature, 
especially the head temperature, does not vary significantly with air 
temperature because the engine temperature is regulated by its 
thermostat.

So how can different plugs have different heat ranges? They are made 
different by making the heat path between the insulator tip and the 
spark plug threads different. This is done by changing the shape and 
inner configuration of the insulator, the shape of the spark plug 
steel body, and the composition of the spark plug center conductor. 
So the heat range of the plug is determined by how easily heat can 
flow from the insulator tip to the spark plug threads.

The important point to keep in mind is that the cylinder head is the 
heat sink for the spark plug, so the thermal conductivity of the plug 
must be matched to the operating temperature of the head. A head that 
runs hotter will need a colder plug and vice versa, in order to keep 
the insulator temperature in the self cleaning range. The correct 
plug will have an insulator that runs in the lower end of the self 
cleaning range when the engine is lightly loaded and in the upper end 
when it is running at full output. It's also important that the 
insulator get to the 400 C self cleaning threshold quickly, even when 
the engine is started in cold weather.

When you buy spark plugs, one of the numbers in the part number 
indicates the heat range, but every manufacturer does this 
differently. VW Type 1 & 3 engines use Bosch W8A (formerly W145, 
W145T1, W145T1.1) W8AC, WR8AC, W8AP, or WR8AP. The next cooler Bosch 
plug is the W7A (formerly W175, W175T1) W7AC, WR7AC, W7AP, or WR7AP. 
Note that the numbers run in different directions in the different 
generations. Unless you're car is an actual race car, you're best off 
with one of today's W8A versions.

Key points:

Spark plug insulators are self cleaning within a certain temperature 
range, which is about the same for all plugs.

All spark plug insulators want to run within that temperature range.

The cylinder head is the heat sink for the spark plug.

Different heat range plugs are chosen to compensate for different 
amounts of cooling from the heads in different engines.

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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