I've made an adapter and flow tested different manifolds. The Johnson LP is about the best, but there's a 2 pc Johnson that flows worse than the Allis.
The Leroi is also a little behind the Allis manifold, it's round and sharp corners kill it, though the exhaust is nice.
There really isn't much difference in the Allis ones, probably more difference from one to another than one model to another. The WD-45, D-17 and Gleaner E ones, slight variances but the later ones have a higher divider in the exhaust which is a good thing.
The aftermarket Vinson is pretty good, a bit better than the Allis variants.
As far as porting on one, there's a quick 25% or more without haveing to work verry hard. If one really wants to work welding and makeing a custom parts there's lots to be found. But the Vinson has a little advantage as the carb neck is wider and can be flared back on it more so than the others.
Weather you use one of the Allis ones or a Vinson, cutting back the corners and forming a D shaped radius, or a flat bottom on the round port, flareing the carb neck back and also cutting the largest radius you can will show results.
When I cut quite a radius on the bottom of the intake runner, where the manifold turns to flow into the head I drill a hole in the face of the intake manifold where the runners go straight into the head. There's a dimple cast on the outside of the manifold there. Tap it and thread a bolt in for filler to anchor to. I tip the manifold at a 45* angle and pour some epoxy in to fill the top corner after cleaning and blasting. What I'm doing is filling the dead spot, the air will follow the bottom of the port, it takes quite a bit of filler to effect the flow. But the more gentle that short side radius can be the more air the manifild will move. FOr some of them I cut the back side apart and fabricate a complete new one, sometimes I end up welding the back side up so I can cut more radius, make a custom carb flange and neck. It just depends on the application and what's needed.
Weather your engine can use it or show the results depends on where the weak link is. It's alwayse interesting to do a head, get the numbers, do a manifold and then test them togather.
On the valve subject. John Deere engines like the 202 through 276 4 cyl's and the 3 and 6 cyl variants of that family use a valve that isn't much more than the Allis stock ones are. It's an easy upgrade as there's no custom guide work, and stock seats, etc all work out with some trimming. I just had a head apart one day and thought things looked close, did a little measureing and it makes for agood substitute valve that is common. I'll measure for sure but I think 1.77" is the intake valve diameter for the Deere. I've got another set in a box for someone doing a little upgrade without extensive work.
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