Kas Kastner, on modifying SU carburetors for more power:

Over 20 yeas of trying to get more out of the stock S.U.'s I found that the time spent in most cases was a total loss. Thinning the butterflies does help a bit but anything else other than velocity stacks has just been pure anguish. My tests were of the back to back variety on a properly registered and calibrated engine dyno.

Kas

In other words, your time and effort is best spent elsewhere if you are trying to gain additional power. SU's are very good carburetors that are almost impossible to improve upon once you get the carb size and needle jetting right.

Also see Dyno tuning SUs with stacks

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