GM manifold is definitely duel plane, generally rated superior to
aftermarket manifolds up to 5000 rpm. It also sits lower than
aftermarket, if hood clearance is a problem. Note that there are several
minor differences in the GM manifolds. Some have a vacuum port in the
rear runner, driver's side. Important if you run power brakes on a '77
up MG.. Saves you from using the carb for a source. All have a fitting
in front for the water temp sensor. Also found on some, is a small
vacuum port on the front runner, driver's side. I have not figured out
what that was for, but suspect it could power a vacuum switch for heater
control/windshield washer.
Jim
Looking for a 4.2 Rover( or parts to build)
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