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From what I understand, the latest F heads don't flow the best stock, but have a little more metal in the right places and can be worked more for better flow (unless it suffers from 'core shift'). It's easy enough to shave an F head down to get whatever CR you like. But if you think 10.5CR is too high, there is no real good solution to drop the CR on a B20E head, thick head gaskets ruin the squish areas and don't help reduce pinging as much as the lowered CR would lead you to think. however, I'm running about 10.5 in my PV's motor (B21FT dished pistons, and an R-sport head with tiny combustion chambers) and have no pinging problems using 91 octane.
A late F engine will also have an 8-bolt crank, which means it is already using B21 style rods, so if you felt like bumping it up to 92mm bore it would be very easy to do. Perhaps not a great idea if you were going to run a turbo or supercharger with lots of boost, but generally speaking a B20 block has no problems with a 92mm bore normally aspirated. Although on my engine with the 6-bolt crank it worked quite well (I think) to push out the wrist pin bushing (22mm) and ream the steel ever so slightly for an interference fit on the B21-sized 24mm wristpins.
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I'm JohnMc, and I approved this message.
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