[T3] Spark Plug heat range

Wed Jul 2 10:33:02 PDT 2014

Great write up Jim. I agree it is a very confusing thing to understand. When I mention I use W6 it was for my 1915 I built for racing and the w7 to w6 is what was recommended to me buy those who helped me build it. Now I did drive this as a daily driver Arizona so this heat rage in combination with a 10.5 to1 compression ratio was almost perfect. One thing you don't want is ping in our motors and if your experiencing ping this is one option to reduce that as well as high octane fuel, timing and even dwell. Running hotter plugs in an old motor is sometimes used to help burn off the oil and deposits that can accumulate from leaky valve guides and rings from ware. My opinion is if you're having to do that it's time to really consider at least a top end rebuild. 

Daniel Du Vall
72 Squareback
74 (Standard) bug
04 Golf TDI
99 Chevy Z71
Destination unknown!

-----Original Message-----
From: type3-vwtype3.org [mailto:type3-vwtype3.org-bounces at lists.vwtype3.org] On Behalf Of Jim Adney
Sent: Wednesday, July 02, 2014 12:07 PM
To: type3 at vwtype3.org
Subject: [T3] Spark Plug heat range

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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