I am glad you are interested in my project. I hope you will participate in the design process with me me.
I have built GMC intake manifolds in the past. My best effort is my triple Weber 32/36 manifold for my GMC. This manifold provides a very smooth intake system. Please see the Harpor SP? intake manifold subject below.
I also built a manifold adapting a Holley 395 cfm 4 barrel to a GMC. It never really worked well. The biggest mistake I made was to use a smooth transition between the plenum and the runners.In the process I ignored the need to keep air/fuel mixture moving rapidly toward each intake port. The result was low speed bogging because the manifold runners were too big, especially at the point where plenum and long runners meet. I also made a very large plenum which did not help.
This time around, I plan to design a Q-Jet to GMC manifold where the the cross sectional area of the runners is constant and equal to the area of the head intake port. This is = 0.785 x(1.75)^2 = 2.404 sq. in. The runner will be made from 1 7/8 OD tubing which has an I.D. of 1 3/4 inches. The critical point of this design is the transition from the the long round 1.75 I.D. runner to the rectangular plenum.
The plenum volume will be made as small as possible, like the production Q-Jet manifolds shown above. The outlet opening on each side of the plenum will be wide enough the allow the smooth flow of the air/gas mixture from the Q-Jet's small and large bores simultaneously. This outlet opening bottom will match the bottom of the flat plenum.
I am thinking about a a rectangular opening into the plenum 1/2 inch high by 4.8 inches wide. This opening maintains the 2.4 sq. in. area like the long round intake runners and hopefully keeps the air/fuel flow rate up for good performance.
I have purchased the steel tubing and steel plate to mount the carb to the manifold and to bolt the manifold to the head. I have also roughly formed a rough a sheet metal transition piece for use between the 1 7/8 O.D. runners and the 1/2 x 4.8 in. openings on each side of the plenum. I have not done anything about the plenum wall material but think this will be a strip of 1/16 steel x 1 inch high and bent around the perimeter of the Q-jet carb bores.
Any comments or suggestions on my theory about keeping the air/fuel mixture flow rate up or on this design would be appreciated. I'm in the preliminary design stage so changes can still be made.
I know you might think I have too much time on my hands but I really enjoy building his kind of stuff. Thanks. Frank Hainey, (Radar).
