Recently I was talking to a couple of our airworthiness engineers over the lunch table. They are heavily involved in older vehicle restoration including types such as MG TFs and Sunbeam Alpines. I ventured to ask them how you could decipher or make sense of the nomenculture used by SU to delineate the size of a carburettor. As we know a 13/4-inch SU is a 6 e.g. HD6 and a 2-inch is an 8. They admitted they had no idea and I was in no better informed either.

Wandering into the room arrived one of our administrative persons who is also well known in historic racing circles for campaigning a supercharged Austin 7 and being a bit of a devil on hill climbing. The question was put to him and he replied immediately “it is all very easy” and he went on to say: -

“The original SU carby is a 1 inch carby. As they made them bigger they simply quoted the number of 1/8 inches over one inch. Hence the HD6 you mentioned is 1 inch plus 6/8 inch [3/4] inch giving an overall diameter of 1 3/4 inches. A HD8 is 1 inch plus 8/8 inches giving an overall diameter of 2 inches. In addition the H stands for horizontal throat and the D means the main jet is mounted on a flexible diaphragm. By the way always measure the diameter of an SU carby at the outlet end where it joins onto the inlet manifold.”

With that he finished filling his coffee cup and wandered off out of the room leaving three highly technical trained engineering people reeling in his knowledgeable wake