Seems like I may have inadvertently started a discussion here. In my
original reply about Mikuni vs SU flow I wasn't saying that the
Weber/Mikuni/SK/Dellorto carbs didn't usually produce more power, they
usually do. But it is not because they flow so much more than the SUs
in and of themselves. If you flow a single cylinder of either setup with
similarly sized manifolds and identical heads, the flow difference will
be minimal. That doesn't mean the flow pattern, fuel distribution,
tract length, and response times, etc. are all going to be equal. They
aren't and this is where the real differences come into play, not in the
carb's raw flow numbers. Unlike V8s most sports car carbs are flowed per
throat, because that is how the cylinders see the carbs. And when you
flow them that way the SU often flows more air than the Mikuni/etc carbs
do. An HS4 (1.5" Brit SU which flows about the same as the 38mm Hitachi
SU) flows 210cfm. The 40DCOE (with big 36mm venturis) flows 175cfm per
throat.
I have seen Z cars that worked best with Mikunis, some that worked best
with Webers, and some that worked best with SUs. I worked on one L28
that made about 10 hp more on SUs than on the Mikunis. So much has to do
with the tuner's abilities to wring the most out of the particular carbs
that their familiarity with the carbs will often have more to do with
ultimate power output than carbs or flow or anything else. Also how the
engine is built and how well it integrates with the intake will have a
big effect. The big drawback I see to the SUs is that tuning is much
harder than with the Weber/Mikuni/etc. carbs. Finding a needle with just
the right profile to get maximum power for a particular engine under all
conditions is very, very difficult and time consuming. Setting up
Weber/Mikuni/etc. carbs is easy by comparison. So many tuners get better
results from a Weber/Mikuni/etc. setup than from an SU setup. But if you
flow the two setups side by side, you'll likely find there is little
difference in actual measured steady state flow.
--
Marc Sayer
Journalist, Photographer, Dog Trainer (APDT member #062956)
Passions -
Great Danes, auto racing and fast cars, my wife
Dogs -
Gracie, Tank, Delilah, PJ, & the rescue dogs
Cars -
82 280ZX Turbo, 71 510 Trans Am vintage racer, 93 Ford E150 Tow/dog van
My Homepage - http://gracieland.org
Deaf Dane Rescue Homepge - http://gracieland.org/DaneRescue/
Any Dane at the races is a Great Dane!
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