While we were dynoing this 3/4 race engine (280 degree duration cam)
installing the velocity stacks in every case made the fuel "stand-off"
disappear. Without the velocity stacks there was a cloud of fuel in front of
the carb inlets that you could feel the wetness on your hand up to over 1 ft
away. The fuel seemed to be in constant motion into and out of the carb
throat. the fuel did not seem to "blow away" into the dyno room. When the
velocity stacks were installed, the cloud was no longer there, and the engine
made slightly more power from the point where it came on the cam, ie. about
3200 rpm up to around 4500 rpm. Above that rpm the power was always less with
the stacks than without them. d
As I mentioned before, we did not have any "stub stacks" as recommended by
David Vizard as being a benifit on Minis and MG engines. So they are a
questionmark as far as this engine is concerned. It is interesting and
puzzliing when emperical results do not agree with what should the theoretical
results. Everthing I have ever read says that the stacks should make things
better everywhere, not worse. The square corner of the carb inlet should act
to in effect reduce the size of the inlet and reduce the air flow. Maybe the
power level of this engine does not require any more air and so that is why it
did not help.
On Weber carburated engine, every time we have gone up in venturi size,
the power has gone up. But again this was on a full race engine making well
over 170 hp on this same dyno.
By the way even on this engine, with either set of carbs, the air valves
were up all the way they could go by 4500 rpm. So from this point on the
actual taper of the mixture needle is no longer having any effect on the fuel
metering. as only the exact point of the needle where it enters the jet is
controlling the mixture and that is no longer changing as the revs change. Of
course the size of the needle at this point is important and is controling the
amount of fuel flow into the engine.
Regards, Greg Solow
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