Actually that's true that a single 38mm SU would flow better than a 38mm Weber bore, because of the lack of a venturi. However, I calculated the flow rate based on a 38mm bore using the "slide valve" setting, which means that it assumes a venturi-less design, like your SU's. I also looked up the flow rate online and found this page:
http://www.mgbmga.com/tech/mgb19.htm
Down around the the middle is lists flow rates for the HS4 (38mm) and HS6 (44mm). The HS4 is listed at 142, so double that is 284. The software calculated the flow rate at 310CFM, which is pretty close so I used that.
Having said that, I do think these simulations run a bit on the low side for reported HP. There are a huge number of variables and some of them have to be estimated because there is no easy way to measure them. One big one is head flow. I tried to find some flow numbers for an A20 head but the only thing I could find is a *really* old 3geez post from "Sean" that claimed a flow rate of 190CFM. That might have included some head work too, it was hard to tell exactly. I do have plans to someday build a flow bench so I can test out some theories while I'm building my big valve head (that I've been working on forever). No idea when I'll actually get to it though. Lack of funding has been a serious problem lately, as I know you can relate.
Anyway, for the price it's pretty hard to beat the Delta. I wonder if they could do a custom grind to provided specs? My guess is they would need to make a new master profile, which could be expensive. I should look at the Colt more carefully and try to figure out exactly what makes it do so well in the simulations.
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