Simon,
It all depends on what you want your engine to do.
A dual plane manifold separates the intake charge from
every other cylinder in the firing order. This gives the carb a better
'signal' for drawing fuel from the venturis.
If you want a 'street' set up this would give good low RPM
performance as well as improved top end.
A single plane manifold will flow more fuel and air, but
is better suited for high RPM's.....i.e: a race motor. One problem with
it is what happens on cylinders 5 and 7. They are
next to each other and fire one after the other ( G M products ).
Number 5 can use the lion's share and lean out the charge to number 7.
Some try to compensate for this by jetting the left rear of the carb
richer...maybe 2 sizes or so.
Offenhouser's DUAL PORT 360 tries to be the best of both
worlds. It separates the primaries in the carb from the secondaries.
Giving the carb a good signal...and the engine a strong low RPM response
while making everything available
to all the cylinders at wide open throttle.
I have not personally used a Dual Port manifold, but have heard
from friends who did that it worked well.
Greg
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