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My two cents ...
You run out of practical upper RPM before you hit the wall with even minor improvements over the stock Euro S. IIRC MarkA used to run his Euro based motor up to 7500 in race trim, but I don't think those were motors that lasted a long time. With something like a 7000 rpm redline my guess is "around" 350 rwhp is possible with a Euro based 5.0L
Further guessing that "about" another 50 to 75 rwhp is on the table for a full race design like Louis Ott's ITC or a massive runner sized dual plenum like MarkA runs now on his stroker.
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If you search a bit on rennlist you will find some threads on using different valves, such as nascar cast offs that are cut to fit. Springs and valves have improved since the 80's, unfortunately heads have improved even more and we can't easily change the basic design of our old heads.
The question I ask myself is how far up the 16v tree do I want to climb, when 32v heads easily exceed there performance?
Answer is unknown until I "need" to take off a set of heads, but something like the .500 lift cam in the other thread, bump the intake valve size to whatever the practical limit of the bore is, port Euro heads to match the larger valves, and fabricate a "looks like stock" ID to match the heads intake.
More likely is that I flip flop and work from a 85/86 US 32v base and just bump valve size and leave the cams alone. A 5.4L motor using 968 pistons is also a thought.
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Thinking about the 914, seems like its not that hard to get 550 hp out of the Subaru motor and pick up a load of reliability at the same time.
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