I'm back ...
The design of the intake manifold has soaked up more time and thought than all the other pieces of this engine combined. Although I managed to complete its topside, last summer's modeling left me nearly burned out and still with no plenum or coolant design. The topside was modeled after the original Hi-Po, but its high rise carb mount has become a separate add-on in order to reduce machining time. As with this engine's other 'castings', the manifold's top surface was designed to be finished with a 1/4" ball mill, and the interwoven surfaces created by the stacked fillets became off limits for the rest of the design.
My quandary with the plenum and its associated runners was whether to scale them down from the full-size engine or to simplify and reduce their included volumes and perhaps achieve higher vacuum and runner velocities. Either would require machining a complex interior, but with the scaled approach I kept bumping into problems with my off-limits topside.
I believe George used scaling and divided his brass manifold into two pieces across the center plane of the runners. Runners and a plenum were machined into both halves that were later soldered together. After a lot of indecision and research into other model designs, I drilled my runners directly through what eventually became the manifold's 45 degree sides where they intersected a 1/2" wide central slot running the length of the manifold. The carb opening and runners were blended into the 'plenum' which was permanently closed and sealed with a JB-Welded cover.
The other issue involved the need for a path through the manifold to return coolant to the radiator from the rears of both heads. This was accomplished with drilled passages in both the front and rear ends of the manifold that were connected by a longitudinally milled trough. This trough was also permanently closed with a second JB-Welded bottom cover. Both covers were installed with .030" glue lines, and epoxy thinned with a drop or so of acetone. The acetone increased the cure time and allowed the lowered viscosity adhesive to fill the small spaces with minimum trapped air.
A SolidWorks rendering in one of the photos shows the fuel and coolant paths inside the manifold. The mysterious trough around the carb's topside opening will eventually become the reservoir for a recirculating fuel loop.
The first step in construction was to square up a 7075 workpiece. The eight 5/16" runners were then drilled perpendicularly through the sides of the workpiece. The #1 runner was unique and had to be angled to clear an internal coolant passage. The longitudinal slots for the plenum and coolant return were machined next. A ball mill blended the runners and carb opening into the plenum. After the JB-Weld cured, the bottom surface of the manifold was skimmed, and the 45 degree sides were machined. My Enco mill had just enough y-axis travel for a simple vise setup. These cuts took into account .020" head and manifold gaskets, and care was taken to make them as accurate as possible. For good measure, an o-ring groove was added around the coolant transfer passages at the rear of the manifold. The carb opening was finish machined through the topside of the workpiece.
After finish machining the manifold's bottom side, it was trial fitted to the block/head assembly using shims to simulate the head and manifold gaskets. Half thickness shims were used under the heads to account for the .010" excess deck material still on the block. Once the fit was verified, the actual head and manifold gaskets were cut from .020" Teflon sheet.
The cooling scheme was finalized with drilled passages in the rear ends of the heads to transfer coolant to the manifold. These two holes completed the head machining. Up until this point the two heads were identical, but these last two holes made them different.
The next step will be the lengthy machining of the manifold's topside. A work-holding scheme has to be worked out since there's currently no reliable way to support the workpiece for full topside access. - Terry