Hi Martyn,
I do not agree that twins are better than singles. What matters is the amount
of airflow the engine receives (and requires) and with carbs the amount of air
the carb can flow be it a large single carb or two smaller carbs. Although a
twin 1.5 SU set up will flow a total of more air, on many A Series engines you
will get a better result with a single 1.75 (or maybe a single 2.0?). The
only usual advantage of a twin SU setup over a single is better fuel economy
(gas mileage).
Also, when talking about SU carbs it is perhaps worth clarifying which SU. I
mention this because the later HIF carbs flow more air than earlier HS types.
I am very acquainted with David Vizard's books, as well as Haynes SU manual
and another book on SUs. I also have a library of other tuning books.
Perhaps I can refer you to page 75 of the yellow edition of Vizard's A series
book where a table idicates airflow in cfm for a 1.5 SU H4 carb is 133, it
being a reasonable assumption that two of these carbs make 266 cfm. The same
table indicates a SU HIF6 carb making 240cfm - only 26 cfm less than the twin
setup and I would suggest that the small gain of the twins might well easily
be lost on the manifolds.
For myself I haven't run SU carbs in a decade but as I said before, from what
I have heard (some comments originating very close to Mr Vizard) the single SU
1.75 takes some beating.
Further comment welcomed, especially when supported with dyno figures.
Daniel 1312
45 DCOE, 40mm main venturi (smaller sizes to prove it needed the 40mm
venturis) 185 main jets, 55 pumps, 65 F9 idles - and the mixture bang on, and
engine taking full throttle in 4th at 2,500rpm if not lower with a MD286
scatter cam (K&N air filter used to make sure small children are not sucked
into the intake tracts)
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