Something else to remember with the Spitfire is that there were two
distinctly different ZS manifolds. The very common small runner, and
the much rarer large runner. I strongly suspect that the manifold is
the strangle, not the ZS carburetor. If you do the math, you will see
that a 1.75 ZS has nearly the same throat area as the dual HS2 SU setup.
yet the HS2 (or 4) far out performs the typical ZS.
John Kipping has stated that his experience running HS6 on the non-US
(small runner) manifold yields nearly identical results as the various
dual SU setups. I have also encountered early squaretail Spitfires
running the single ZS that ran quite well. These early squaretails
quite possibly were using the larger runner manifold.
Fwiw, it doesn't seem that top speed is much changed on my Spitfire in
going from the single ZS and small runner manifold to the HS2 and
manifold from an earlier 1300 Spitfire. What has dramatically changed
is how relaxed the engine feels at speeds about 40 mph. Granted the
RPM's are the same, but the engine is no longer straining the way it
once was. With the ZS highway driving was a rotten experience, with the
engine always sounding and feeling like it was killing itself trying to
keep up with traffic. With the HS2's, the engine is relaxed and not
straining, while still buzzing like a top at the same RPM's.
I do now have a large runner ZS manifold, and may some day install it
for some comparisons.
>>> Glenn Trunnell <trunnell@earthlink.net> 06/01/02 07:44PM >>>
Hi everybody,
The work on my SU HIF6 conversion continues apace, but I wanted to find
out what kind of performance other peopler saw with their Spits. My
main question is this: previously with the single ZS my car would turn
about 5300 in fourth gear. Will anybody's pull the full 6000 with the
stock carb and emissions? How about with Dual 1.5 SUs or one of the
various webers? I previously owned a couple of MGBs. The first a 74
was good for about 105 mph with dual HIF 4s the second an '80 wit a ZS
175 topped out at 85 mph. The only difference between these two cars
was the carburettion. So I am curious about a baseline for the Spit as
I have only ever driven mine.
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