[Fot] SU carb smooth bore or leave rough?

[Enquiries Road & Track]

I have had a few goes at shaving throttle shafts with very mixed
results..typically fracturing where they meet the carby body.  if you
planned to change them routinely, thats not such an issue

Some recent flow bench work leaves me wondering completely about the wisdom
of any of this without real-world testing

We found some head/intake arrangements where the air flow was too high in
certain places and we had to dimple the port to slow it down. Sometimes,
just enlarging the port does not work as intended

The very best SU research ive seen was done by David Vizard . There are
plenty of on-line references to his work, but if anyone can't find what
they want, i can scan relevant parts of his text book (tome) on tuning BMC
A series engines. Ive used his analysis on  various shapes of ram tubes for
many years

of far more importance seems to be the distance from the valve to the
throttle plate, with the ideal varying considerably with the RPM range
being used. As a general comment, it seems like many folk have it far too
long

I have proven that fiddling with the damper in a SU has a dramatic effect
on throttle response, and not always what you think. i'll offer the comment
that its very engine set-up specific. Sadly, this aspect is all-but
impossible to test on a conventional flow bench

Terry
Australia



On Thu, Nov 26, 2020 at 2:35 AM DAVE HOGYE via Fot <fot at autox.team.net>
wrote:

> I know that Huffaker does this.  I believe they also remove the damper oil
> and or the damper completely and shave down the throttle shaft to almost
> nothing.  So, I don't think you'd be trashing them, but I don't know what
> the dyno results are either. As far as I know, they also replace head
> gaskets and engines quite regularly on their BMC builds.
> Dave H.
>
>
> On 11/24/2020 12:41 PM Mike Harmuth via Fot <fot at autox.team.net> wrote:
>
>
>
> I was going over my race carbs (SU H4)and wondered if there's anything to
> be gained  (or lost) by smoothing out the casting (before the jet)? I'm not
> sure if it's designed (or left) that way to promote turbulence and better
> mixing as the air flows over the jet or if polishing it down would help.
>
> I'd hate to trash a pair of good carbs before I get some answers.
>
> thanks
> mike h
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