enginehistory.org

[jjuutinen]

Of the major liquid cooled V-12s of WW Two, only the Jumo 213 had centrally mounted (between valves) spark plugs. Since every book on engine design says it is the ideal location, why did others keep mounting the plugs on the sides of the combustion chamber?

[szielinski]

I haven't looked that deeply, but normally having a single OHC in the way would prevent optimal spark plug location. Given that the 213 only had 1 exhaust valve, maybe there was more room outboard to put a spark plug in a good location.

I remember something about early aviation engines using the spark plug through the side to locate the liner so it wouldn't move - perhaps a throw-back to that philosophy ?

Got a cutaway diagram of the head ? (I couldn't find one).

[jjuutinen]

You mean a Jumo 213 cutaway? I don´t have a really good one but some details can be found in Karl-Heinz Regnat´s book on the Do 335. The English edition is available from Schiffer.

[jrussell]

Spark plug location is a function of chamber configeration, as well as other pesky details such as cooling jacket placement, head bolt placement, port location, etc. Juggling all these requirements is one of the most diffucult trade-offs the engine designer has to make as it effects almost every other detail. An example of a engine which a central plug location was not the best location is the late model Hemi (426), which a single plug design (in the center of the chamber) was changed to a twin plug design(one one each side of chamber centerline).In a small block Chevy, the change to a "slant plug" configuration was demonstrably superior( a wedge chamber design), although this location is not even close to being centered in the chamber.In a pent roof four valve chamber the central location is probably the best location, but how many of the WW2 designs were of that configuration?
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