Making your own intake manifold is relatively easy, requiring little more than a Sawzall, an angle grinder, tin snips, a die grinder, a welder/brazing tools, some sheet-metal and 1/4-inch-thick steel plate. This article is specific to dual-carbureted V8 engines, but the basics can be applied to practically any type.
The short-ram manifold described here would be best used for a forced induction setup, but would also be fine with a high-revving race engine. There are, however, are a few things the fabricator can do to make it a little more livable on the street.
The first step is to make new intake manifold flanges. Lay a replacement intake manifold gasket onto the steel plate and lightly dust over it with white fast-dry spray paint. Remove the gasket, and you've got a perfect form for your new flange.
Have at the plate with whatever cutting tools are at your disposal, keeping in mind that the end is to remove anything with white paint on it. If your engine came stock with an exhaust crossover, do not cut out that portion of the flange. Avoid using a plasma cutter or torch, as this may warp the plate. Several trial fittings may be required.
Once you have the flanges cut, smoothed and cleaned, bolt them to the cylinder heads, including the intake gaskets and torquing the bolts to the stock requirement.
Using an angle grinder and die grinder, cut the coolant crossover portion of the stock manifold away. Do not cut through the coolant crossover. The crossover should resemble a tube with a flange on either end.
Once it is free, cut away the flange with a reciprocating saw, staying as close and as parallel to the mounting surface as possible. Trial fit the crossover on your new flange where the water passage is, removing it and incrementally removing material from the tube ends until it sits flush on the flanges. Tack weld the crossover onto place.
If you are fortunate enough to have round intake ports, use 1/8-inch-thick steel pipe from the hardware store for the runners. Some engines may be able to use rectangular stock.
If you must fabricate the runners, run a piece of string along the outer diameter of the intake port with string, and measure the string. Cut lengths of 1/16-inch sheetmetal about 8 inches long and use this measurement for the width. Bend the sheetmetal in the shape of the port until it meets itself. For curves, you can work the metal around a steel bar and clamp it into a large vise for corners. Tack weld the runners along the seam, and then to the intake port.
For high rpm or blower applications, the runners can be as short as 2 inches from the flange. You can make the runners long enough for their bottom edges to meet in the middle, if you need the torque for a naturally aspirated engine--though hood clearance will be an issue.
Lay a piece of sheetmetal along the top edges of the runners on one side. Use spray paint on the underside to mark where your runners will be. Cut the sheet-metal in a rectangular shape, 1/2 inch from the edge of the front and rear runners lengthwise and 6 inches from their tops and 8 inches from their bottoms on height. There is reason for the excess material.
Tack weld the plenum sides in place on the top and bottom. Bend bottom extensions upward till they form an upward facing "V," and trim the pieces incrementally until the meet evenly about an inch above the top of the intake runners Tack weld together. This will divide the plenum in two longitudinally, smoothing out flow and increasing velocity in the same way as a dual-plane intake.
Cut a piece of plate steel so that it sits evenly on the center ridge and the top plenum sides, and bevel to lower edges where it meets the plate. Tack weld in place and cut off the excess sheetmetal. Cap off the front and rear of the plenum with sheet-metal.
Place two carb gaskets on the plenum, and follow the spray-and-cut procedure to make the mounts. Tap the bolt holes with the appropriate die.
Remove the manifold, and finish all your joins by brazing with copper or steel rod. After some grinding and polishing, you've got a finished manifold.
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