Some classes of vintage sports car racing require that the competitor stay with the original SU carbs. In fact, over the years we have found that there is really no advantage to using anything else. However, there does seem to be a lot of discussion about what type of SU and what manifold to use. We’ll talk a bit about that here.

We concede that the TR4A long manifold is superior to the shorter rake-style that was used in the earlier four-cylinder engines. However, the current practice of using the long intake manifold with the long H6 carbs is counter-productive.

I put a flowed TR4A cylinder head on the flow bench and attached a TR4A long inlet manifold. I then attached first a long H6 carb with the piston locked at maximum lift and took air flow readings at every .100" of valve lift, up to maximum. I then duplicated this procedure using a short HS6 carb and found that the HS had a 2-3% flow advantage at every step. While there might be some possible ram effect advantage with the H6, it would seem that the tuned length would be the worst of all worlds.

On the track, the HS6 has demonstrated its advantage on every car it’s been tried on. There could be more to this than just the carb, though. Some of that advantage might be that the shorter length of the HS6 allows room to use the very large and aerodynamically clean Hurts Air Ram Bells.

Both tests were conducted on carbs whose pistons were not modified in any way. It is possible to carefully shape the leading edges of the pistons to streamline and thus increase the flow of air to the manifold. In conclusion however, it may be said that regardless of the combination of parts that we tried, the HS6 gave a better flow under any and all conditions and is the obvious choice for maximum performance.