Masked sparking plugs

Sir,

Until I read the paragraph in the September Rumblings, about the Chevrolet V8 in the Can-Am car and its connection with the 1929/30 Rileys, it had never occurred to me that having the sparking plugs recessed in masked holes would have such a significant retarding effect. To check this, I looked up the ignition timing figures for Alvis’s of the 1930s, most of which I believe had their sparking plugs in masked holes, and compared them with a more modern car.

The BTDC figures for the fully advanced position are:

1934 Alvis Speed 20 …..43 degrees (28 fully retarded + 15 manual advance).

1935 Alvis Firebird ……..47 degrees (7 fully retarded + 20 distributor automatic + manual advance).

1937 Alvis 4.3-litre ……..46 degrees (0 fully retarded + 26 distributor automatic + 20 manual advance).

1961/6 Sunbeam Alpine About 26 degrees (‘ fully retarded + 11 distributor automatic (2,200 r.p.m.) + 6 vacuum advance (8.25 Hg.),

From the above, even allowing for more modern fuels and better combustion chamber design, it would appear that the masking of plugs has a significant retarding effect. I have found the large range of manual adjustment in addition to the distributor automatic adjustment on the Firebird and also a Speed 25 (similar in this respect to the 4.3) quite unnecessary and only rarely have to touch the manual lever.

It is generally considered among most Alvis owners that the reason for recessing and masking plugs was to give them some protection from the combustion chamber heat and thus extend their lives. Whether it does in fact do so is open to debate. Certainly I have never before heard of any suggestion that it affects the shape of the torque curve, although from the modern example you quote it almost certainly does.

PETER DANIELS – East Horsley