H6 vs. HS6

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.