With the B18B you already have the higher compression. The best single thing you can
do to gain a few HP is to change to the dual downpipe exhaust manifold.
Note that there is only 3HP difference between the B18B and the B20B, which is the best
direct comparison engine between B18/B20.
The B20E and F are fuel injection engines that have the dual downpipe exhaust and somewhat larger valves, especially the intake valves. They also have the D or K cam as opposed to the C cam in the B18B and the B20B.
The B20F is lower compression, so it has around 120hp where the B20E has about 130hp.
Compression can be raised in the B20F by milling the head, making it fairly similar to
the B20E.
All the above assumes OEM components throughout.
--
George Downs Bartlesville, Oklahoma