Some good Points there Bart. The e15et is a serious dish
and would suit the turbo people well looking for high boost
however its definitely best to start with the h89 head or
if someone can get hold of a fuel injected head h92, as its
just been enlightened to me. Probably the shaved h75 will
still do well with normal camshaft + turbo.

That was was I had been planning to use for an A14 turbo. Those e15et pistons have a MONSTER dish. My concern was though that having the H75 head shaved that much could create valve to piston issues wheh run in conjunction with a monster cam. (I was planning to zero deck the block as well) Baring in mind that the E15et chamber is something like 18cc. (Thats a lot of meat to shave off an A sereis head)

A very poor low Nox design compared to the Z series
pro swirl open chamber dual spark heads :)
Either way the h75 is not a good performance choice unless
its milled for gx1200`s turbo setup.

NOx emissions are related to combustion temperatures and the fuel mix.

Interesting that it will get better Nox ratings
I thought closed chamber heads have much better
Nox ratings due to efficiency.

Is your H89 head factory-stock, or has it been ported?

Yes, open chamber is for lower NOx emission. Not anything to do with fuel quality. For lower octane fuel they make the compression ratio a little smaller, like 9:1 instead of 10:1 which calls for high-grade fuel.

My h89 has 24cc chambers

mainly for poor fuel quality and for lower NOx emission i guess?

gx1200, use a h89 head or shave your h75 to make it
a close chamber kidney shape & use e15et dished pistons.
Open chamber heads where mainly for poor fuel quality.

The open chamber heads are not any lesser compression. Not sure if they'd be better for turbo or not.

With factory A-series engines, the compression is controlled mostly by the piston dish.