Hi,
IMHO, when either one is in good condition and properly tuned there's little driving/performance difference between ZS 175CD and SU HS6. They operate the same, give about the same performance and are virtually interchangeable. I wonder if people tend to be biased for or against one or the other just based upon a bad or good experience, because the carbs really aren't all that different.
SU's are a little easier to work on and get spare parts for. There is a wider range of needle choices for SU (but SU needles can be adapted to use in ZS). And SUs have been far more widely used so more people are familiar with setting them up and keeping them in tune. These are about the only clear advantages I can see.
For a fast street TR6, the *triple* carb setup with either the SUs or ZSs should make a very good arrangement and give quite noticible improvement over the stock dual carbs and manifold. Do some port work on the head, install a somewhat hotter cam, maybe increase CR a bit, add a true "extractor" header with a smooth flowing exhaust system, and make sure the ignition system is up to the job. I bet you'll be smiling each time you press your foot down on that RH pedal!
Yes, I'd wager either SUs or ZSs would be better than the Weber downdraft carb arrangement. Perhaps the difference is the funky manifold used for this setup, adding length and turns that probably mess with air flow.
On TR6, a triple Weber sidedraft (DCOE) would be the next step up over a triple SU or ZS setup. Dedicating a carb throat to each cylinder just makes for maximum potential tuning. Also, Weber DCOE have an accelerator pump, which both ZS and SU lack. But to do this right really means putting the car on a dyno and working through all the details.
On the other hand, compared to Weber sidedraft, both ZS and SU are more "self-correcting" for changing conditions such as air density variations due to altitude, temperature and humidity.
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