I have concluded that the primary reason that modern American V8’s develop sooooo much more power than our aging V12’s is… valve lift. A Chevy LS has more than .600" valve lift, partly accomplished with rockers with a 1.6:1 ratio or better. A Ford Coyote has more than .500" lift on four valves/cyl, accomplished without rockers, it’s all in the overhead cams. By comparison, the .375" of lift our 2-valve V12 has looks pretty weak, and is almost certainly the power limitation.
Here’s the idea I propose: Regrind the OEM camshafts for a 1/4" smaller base circle. This will probably require a tad more duration. Replace the valves with valves with stems 1/8" longer. Replace the valve springs with springs that are 1/8" longer.
Presto, change-o, we have .500" lift. On a pre-H.E., instant tire burner. On an H.E., somebody will probably need to figure out how to get the pocketed exhaust valve to work well with the additional lift. I presume some massaging of that pocket would be in order, which in turn will probably reduce the compression ratio a bit. Probably can compensate with a hair more timing advance.
Also need to ensure that the valves don’t get involved with the pistons, without reducing the lift!
Note that the tappets would end up sticking out of the tappet block by 1/8". I kinda doubt if that’s a problem.
All this would seem simple enough to accomplish if suitable valves and springs can be found for reasonable prices. I’d be willing to bet such parts exist somewhere on some commonly-available junkyard engine or another. I have no idea how to find them, though.
BTW, we’ve had discussions on whether the exhaust systems on our Jaguars are limiting, with Roger Bywater assuring us they are not. Perhaps worth noting, though, that the exhaust systems on modern American V8 Mustangs and Camaros are typically 3" dia., and that’s per bank, dual exhaust with an X-pipe. I don’t think Ford or Chevy would pay the premium for such large pipes if they didn’t accomplish anything.