Masked plugs

Sir,

I was very interested to read the paragraphs entitled “Full Circle” describing the positioning of sparking plug points in the Riley Nine and Chevrolet racing engines.

It may interest readers to know that this was a standard feature of Lea-Francis engines, I believe, from 1926. Their twin-cam designs do not allow for the plug points to reach the combustion chamber, since the bottom 1/4-in. or so is not threaded. This engine was in quantity production from 1926, and fizzled out in 1929 despite the fact that the second series, designated LFS 2, were excellent. Similarly in the blown Hypers a short reach plug is used, and was specified in the instruction manual. This also fails to reach the combustion chamber by about 3/16-in. In the Ace of Spades engine, which came out in 1929, the spark is delivered to the combustion chamber from a small spherical ignition chamber by a hole of about 1/4-in. diameter. This design features a hemispherical combustion chamber with inclined valves driven by an OHC.

The feature was continued in the Rose-designed LeaF engine of the late thirties which was almost identical to the Riley 12/4 and which was subsequently developed to such effect by Connaught & Co. Perhaps Mr. Rose was impressed by the performance of his earlier experimental design for Riley, and incorporated as a standard feature when he changed to the LeaF camp. On the other hand he may have been guided by previous Lea-Francis experience.

In my own experience of the Meadows engine in a Hyper LeaF I have found that there is no advantage to be gained from using long reach plugs, and that if a standard cylinder head is used in which the plugs are at an angle of about 30 deg. pump petrol causes excessive knocking above a compression ratio of 5 : 1 (supercharging at the standard rate). If the plugs are brought into a horizontal position the c : r can be raised to 7 : 1, but only using short reach plugs.

Robert Elliot-Pyle – Robertsbridge.