Bosch Spark Plug Cross Reference

The OEM spark plug for the V4 was made by Bosch (most years anyway). During the reign of the Sonett, the naming/numbering convention was of the form WxxxT30, where xxx specified the heat range (the rest of the number encoded the electrode material, resistor presence,  and reach). Curiously, higher heat values indicate colder plugs. For example W225T30 is a colder plug than W145T30.

At present, Bosch uses a very different numbering scheme, although it captures the same basic attributes.  In the new scheme, higher heat range values indicate hotter plugs.   The table below shows a cross reference between old style and new numbers. All plugs listed are 3/4 inch reach and have extended copper electrodes (except the W280T20S which has a silver electrode). The factory specification calls for plugs in the range W145T30 to W225T30 (varies by year).  Internally Bosch and their retailers use a Sales Number to refer to each plug.  A sales number does not guarantee that a plug will be stocked by a retailer, but it does indicate that it is stocked in the Bosch warehouse.  Plugs without sales numbers are special order and may not even show up as available to a retailer.  The W3CS is a cold plug designed for severe (high performance) operating conditions.

As a general rule, as compression the compression ratio goes up, the plug temperature goes down.  A cold plug is more easily fouled than a hot plug.