Hello Markn,
The single biggest difference between the Bristol motor and its BMW 328 ancestor is the use of iron for the BMW head, and aluminium for the Bristol. I once got to look at a 1937 BMW 328's motor and noticed that its pre-war Solex carburettors were different from those used by the Bristol Aeroplane Company later on. The actual height of the pre-war carburettors looked much the same to me, but it didn't cross my mind to do a precise measurement. As you may know, the German designer of the engine, Professor Fritz Fiedler, went to England after the war--after being released from internment--and worked for the Bristol Company, after it had acquired the motor's manufacturing rights from Fraser-Nash--BMW's pre-war British partner. It is under his direction that the head was re-cast in aluminium, as before the war the German Government had not allowed car manufacturers to use the alloy, as it was needed for the aircraft industry, gearing up for war. Fiedler also presided in England over some significant power increases by using camshafts with more lift and duration. With Bristol, he also worked on stricter tolerances for the motor, to incrase its efficiency and thus its power, and on using more exotic metals such as vanadium, so as to increase strength/reliability. Other than that, the basic configuration remained the same, as did the 1971cc capacity...with the exception of the 2.2 litre version of the Bristol engine, as used in some Greyhounds, which traded top-end power for extra torque. I hope this helps a little, and if anyone knows of further differences, feel free to add to these comments.
Laurence in British Columbia
