There is a lot involved to get the AY head to make hp
so dont forget the camshaft profile, timing, pistons,
valve and chambers,exhaust and induction have a big part as well.

The speedway guys where making 25% more than the A series
head by using the e15 8v heads. OHV of corse but easier to
make hp are the miata heads in terms of budget and information
available freely to anyone who wants to.
There are many 1.6 making 160hp atw using OBX cams, milling head 2mm!,
racing beat headers, sports cat, 45mm webers with Rowland inlet manifold
(only 168aud plus 40aud landed!) and ecu to run ignition only or cheap
megajolt crankfire ignition.
Same mods on a 1.8 Bp yields just 172bhp atw only 12hp more for 200cc extra.