I'd be the first to agree that 'splitting' an exhaust manifold is nowhere near as theoretically correct in terms of exhaust gas flow, compared to 'Fenton' or equivalent free-flow manifolding.
That said, if you can't get, or can't afford, a set of best-quality headers, adding another exhaust stack by splitting the manifold will help your engine to some extent just by providing a second exhaust pipe, to cut down on the back-pressure of the original system.
I've done up a number of split manifolds, one time and another, not nearly so fancily as Mike Yoder's work, tho.
Its actually a very simple, easy job. I'm planning to split a couple of manifolds for 218/230 Dodge/Ply engines in the near future, and, if anyone in the S. F. bay area of California might be interested, I'll invite them to visit, and see how its done.
If you'd like to do up a split manifold for yourself, the basic process is ever so easy. First, practice a good bit with the DC arc-welder and 'machinable ni-cast' rod, til you can get a nice smooth bead, with 'just enough' penetration, and no or minimal under-cutting, on the thin iron of a manifold casting.
No one can really explain this in print, you have to 'get the feel' for yourself by practising on scrap manifold castings. Don't let your welding-supply person try to talk you into using the cheaper 'non-machinable' ni-rod...the 'good stuff' is not cheap, to be sure, but you won't need all that many pounds of it.
Using a Mopar flat 6 as an exemplar, cut off the flange from a 'donor' scrap manifold, and file/grind/mill it flat/parallel to its gasket face. Clean both the manifold and the new flange by glass- or sand-blasting, or at least go over the surfaces to be welded with a carboloy rotary file in a die-grinder to get to clean iron below any rust/surface inclusions.
Clamp up the new flange in position.....I usually place the new flange in the centre of the four-bolt pattern, but others prefer to place it ahead of the bolt pattern.....thats purely a personal preference.
Warm the manifold a bit by any convenient means.....little propane torches work well...you needn't get it very hot, just 'take the chill off'.
Run the weld by doing a very short....slightly larger than a dime....bead, with the ni-cast, and, immediately....fast as you can move....peen the bead as it cools with the pointy end of a common 'slag hammer'.
Peen the weld with great vigour, as the idea is that you are stretching the weld bead slightly as it cools, to compensate for the natural shrinkage of the weld.
Go away and do something else for a few minutes, til the manifold is 'cool' again. Then, run a short bead on the opposite side of the flange, and peen as before.
After another few minutes, put a bead half way between the first ones, 'on the quarter' so to speak, peen and let cool.
Continue til you have a good sound weld all the way around the flange, then drill out the manifold casting to open it to the new flange, using a suitable hole-saw, and fair the edges with a carboloy rotary-file.
Form a suitable baffle-plate from 1/8" steel, weld it in place, and the job is done.
The only moderately difficult aspect of this job is that of maintaining sufficient patience to allow the casting to cool between the short beads.
If done correctly, the warpage will be extremely minimal, not enough to affect anything, but its best to assemble the exhaust manifold to the intake, check for flatness/parallelism, and send the manifold assy out for grinding if its out more than .005-ish over full length.
As an expedient alternative, clean/polish the gasket surfaces of the block and manifold, gently snug the manifold assy in place using only the two nuts on the long centre studs, and try the manifold-to-block fit with feeler-gages.
If you haven't a suitable surface grinder, or a nearby shop who can grind the manifold surface for you, its possible, with some care, to bring down the 'high spots' with a disc-sander, used judiciously.
When 'close' by feeler-gage, put some prussian-blue on the block, and fit the manifold assy to the block, noting the highs marked by the blue.....sand down the highs a thou or two, and try again.....with a bit of care, you can get a perfect (80pct blue or better) fit, so you'll never have to worry about a manifold gasket failure in service. This may sound tedious and time-consuming, but its really much easier than it sounds.
cheers
Carla
