Special cylinder heads

Power is made in the combustion chamber. With the side valve engine the chamber shape is important as during the induction and exhaust strokes the chamber forms part of the gas route, restrictions etc affect the engines breathing. Also the position of the spark plug can affect the speed of combustion. Finally the volume of the chamber sets the compression ratio of the engine, the higher being the better. However these three factors don’t combine comfortably in a side valve engine and the design is always a compromise between them, usually to the advantage of two at the cost of the other.

For the other two factors, breathing and compression there seems to be two camps. One suggests that breathing is everything and compression should be secondary. Not surprisingly the other camp goes for a reasonable compression keeping the combustion chamber as tight and as small as possible.

With a both pre and post war racing history not surprisingly there has been quite a bit of development of Austin Sevens’ cylinder heads. Other side valve engines had a similar problem and there was much cross fertilisation of ideas. Jack French suggested we look at a modern side valve to see what the latest thinking on design is; good idea, but my lawn mower is reliable (currently) and I don’t want to take it to bits just to look. Still, I can see where the plug is positioned and, being air cooled, can infer some of the shape inside but more easily; it’s surprising how much information is available on that well known auction site, see the Briggs and Stratton head picture below! Many racing engine builders just plumped for a certain special head—perhaps the one they already just happened to have, but there were some discussions at the time on what shapes, compression ratios etc to use. Of course a road special may need slightly different things to a racing A7 as well. Books and articles worth a read are;

· 750 Companion—the ‘green’ book. This in the ‘Getting the best from your 7’ section has a few things from the past notably the Jack French Simplicity series (that were also in The Special Builders Guide) and In Sheeps Clothing. Also the Bill Cowley engine tuning principles and the Holly Birkett inlet valves articles suggest ways on how to get the most from the head.

· Not A7 but the Speedsport book, Tuning side valve Fords by Bill Cooper has good information and photographs on this subject for the Ford 10 engine.

· 1989A A7CA magazine had a collection of pictures some of which are copied here where I haven’t my own.

Pre war special heads

Along with the Austin sports heads a number of companies sold go faster goody heads. I’m not sure of the relative chronology of all of these but some of them are definitely pre 1930 and thus could be claimed to be VSCC compatible.

Post war heads—750 Formula

In the early years of the 750 formula many of the heads above were used on the traditionally styled cars with varying degrees of success. Some new aluminium castings must have been available, the Lotus 3 and Pat Stephens Stoneham special used a special head in the early fifties. Growth in popularity of special building and the racing formula brought specialist companies into the market developing new heads as part of their range of goodies. Cambridge Engineering run by Bill Williams had been in existence pre war and their early head may have started then. They were followed by Sporting Motoring agency—Dante, Speedex, Super Accessories—Supaloy as well as a number of smaller lesser known concerns into the sixties.

Looking at some typical examples and what they were intended for, the breathing heads are predominantly intended for a racing engine that revs to 5 or 6+ thou rpm. Breathing is needed to quickly get some air with petrol into the engine. There’s no point in having a high compression if there’s nothing to compress. Compression heads work well on a slower revving engine that doesn’t need such good breathing. So, in simple terms racing engines work well with a breathing head and a road engine gets the best from a compression head.

In the mid to late sixties the final developments of the side valve head were made on the Seven and its successor the side valve Reliant 750 engines. Final stages were two piece heads to make easy manufacture, see similar examples below. Others machined the bottom half of a cylinder head including the combustion chamber from a slab of 5/8” aluminium, made a lid and then glued the whole lot together with four sides and two tubes of Araldite. Low drag cars needed engines inclined over, the heads had water outlets on the side to match. The ultimate A7 750 was probably the Cowley developed and raced in the late sixties and early seventies. This not only had an eight port block with inclined inlet ports but a twin plug head that probably is the optimum side valve design. Note how this twin plug has them both over the valves—the works head had them across the chamber, see above. Reprinted with permission from the 750 MC Bulletin.

Developments ‘down under’

Similar special building and racing was going on in Australia and New Zealand. Heads were also developed along the same lines. I can’t add any more to the history of these various heads, I’ve included them for interest and completeness. Some do illustrate the split head with a separate top and bottom. This design simplified casting as no cores were needed. All bar the Ludgate, which is definitely fifties and sixties, they seem to be based on the earlier gasket shape and may therefore be pre war.

Another good source is information from the USA. Searching the web on ‘tuning side valves’ brings back very little. Once I’d got the terminology right — ’hopping up flat heads’ - there was quite a bit on 45 Harley’s and the Ford V8 flathead. There’s a few books on Ford V8 tuning (search Amazon with ‘flathead’), the one that seems to me the best to extrapolate into the A7 is ‘Flathead Fever’ by Mike Davison (Australian not American). Being for Hot Rods they do tend to use shiny new proprietary parts — but there is information that helps with A7 ideas. The V8 has domed pistons, valves at 5 degrees to the bore centre line and they are pocketed into the block. The engine is tuned in the same way to maximise breathing and compression.

It was accepted in earlier days that a spark plug located over the exhaust valve will give the best combustion. Pat Stephens in ‘Building and racing my 750’ asked Colin Chapman of Lotus to machine his spark plug position in a raw casting as close to the exhaust valve as the casting would allow.Side valve engines are still being tuned up. In addition to A7 and other cars there’s the flathead Harley and also (believe it or not—my wife wouldn’t—she thinks I’m crackers on something as normal as A7’s) racing lawn mowers with tuned up single cylinder side valve engines. Its easier to try the plug in a different location in an air cooled head. Internet ‘chat’ on the single cylinder engine suggested that 8 different plug locations were tried but the engine was just the same. I believe that a position fairly central in the chamber between the valves is probably about right, anything outboard of this is liable to be disadvantageous. To be honest its not a parameter that can be adjusted other than trying a different head. Although I’ve seen a few un-machined castings about, this position is normally set by the original design and casting of the head.

The Briggs and Stratton head, approximately the same size as an A7 cylinder—but it has more head studs! Note the central plug position.