Does anybody have silicon moulds (poured into the port, then removed once set) or cross section pics of a gx head (I have seen cross sections for other heads, but not any a12 gx heads - likely because nobody in their right mind would cut one into sections for such a purpose. Anyhoo - I'm wondering how the gx heads are shaped in the bowl area, down to the valve seat. I'm curious as to whether or not that point would still represent the biggest restriction to flow, and therefore the power potential of either the gx or a14/15 (all else being as equal as practical) in std form not being markedly different.

On how far to machine the deck of any head to bump up the compression ratio, there's enough variance in the heads in question (possibly chamber shape, but more importantly the possibility they have been previously machined) - look realistically there is no safe way to guess, and it really warrants checking hte capacity of each cylinder (might have been rebuilt and bored to the next oversize, which means slightly more capacity, so higher compression) and measuring any piston dish, and then of course measuring the volume of the chambers. Then doing the math to calculate the actual static compression ratio.

As an example (that I have used before) on minis, there's a certain model hepolite piston for the 1275 a-series which lists it's nominal compression ratio as 9.75:! - for the 40 thou oversize pistons, and a std head capacity, they actually measure out to be over 11:1.m There's just no way to know without measuring, thankfully the measuring and the mathematics isn't too hard.

Now once you have the current compression ratio, you can start with the harder maths (that can be done with some free software btw) - you see you have to consider other things.

In general all else being equal an alloy headed engine will tolerate more compression than one with a cast iron head.

A head with decent quench areas will safely run more comp than a fully open chambered head.
excessive carbon deposits can form hot spots, or worn valve guides can let oil into the cylinder which contaminates the intake charge either of which will invite the onset of detonation much sooner.

small tangent - most of hte distributors are over 30 years old now, so the springs on the mechanical advance mechanism may have sagged a little, which means the mechanical advance comes in too soon - which possibly shows up with the symptom of the motor starting and idling well, but pinging (significantly) from just off idle (with throttle open a fair bit) to about 3500-4000rpm, and it running smooth as silk above 3500-4000

I've added those last two in because they are scenarios that can occur and deceive the owner that they have to run less compression than they currently have on the old motor, because it's pinging badly already.

To a lesser extent, bore size has an effect - generally the smaller the bore, the tighter the ball of air/fuel is squeezed just before the spark/burn cycle, so it's less likely to have trail ends detonating. Or in other words smaller bore, safer from detonation. This won't matter much across the range of a series blocks, but it does mean that (if it had identical chambers etc) an a12 engine would safely run more comp than something like a big v8 with a 4+ inch bore.

And one last major factor - namely 'dynamic compression ratio'. At lower rpm, the gases don't start to get compressed (since they'll be shunted back up the intake to some extent) until the point where the intake valve closes, and the piston is already well on its way up the bore. Meaning that if we closed the valve at exactly bottom dead centre, it'd achieve a running/effective/whatever compression ratio more or less equal to the static comp ratio. but if the intake doesn't close till say, 30 degrees after BDC, it will push some intake back up and out, and now have a smaller amount of air/fuel to compress, and it'll act like a much smaller capacity squeezing the gases into teh same chamber.

So dynamic compression is a comp ratio worked out with cam timing figures plotted in there, and gives a clearer picture of what is really happening at lower rpms and what comp ratrio you see. So from that it is probably clear that the bigger/longer duration cam you run, the more compression you can safely run . What in fact happens is with wilder cams, you actually 'need' more compression to get the engine to work efficiently at higher rpms.

Now that doesn't mean you run some outrageous drag race only cam profile, of course not, but it does mean, after taking into account the other factors you have to then really match up the cam timing and compression ratio to one another.

VEry basically with a stock cam, the gains in output from compression ratio increases aren't massive (at least within the scope of what is available going from low octane unleaded to 95ron) - so I'd suggest not going far past 9.5:1. You'd probably get away with 10:1 as long as the valve guides are good, timing is spot on, and it's a closed chamber head. As you advance into bigger cams, something still streetable, but far more 'oomph' up top - something perhaps around 260-270 adv duration - you'd be looking at 10-10.5:1 I'd well suspect you could go to 11:1 on 98Ron and that sort of cam, but that's speculation (and experience on other similar engines) but the gains wouldn't likely warrant it over the 10-10.5:1 . If you were massively motivated, you could hypothetically run as much as 12:1 or more on a stock cam and pump fuel, but supplement with water injection to keep it safe during the times you feed in more than about 60% throttle. Of course that is more work, and some up front cost, and you'd need to be certain to refill the water reservoir, because if it ran dry, and you ran wide open throttle you'd toast the engine pretty quickly.

There are too many variables, also bigger ports dont mean more power.
Depends what rpm you are looking to make power and a zillion other variables.
A highly worked A series is often a pig on the street if not designed properly.
There are just so many factors. (rpm range wanted, torque vs hp at what rpm,
reliability, road or track or a bit of both)

Best examples fall under 2 areas for NA

Use a long rod setup with short runners, big valves and smallish ports and small quad carb/throttle sizes for High rpm performance (like motorbikes)

or

Use short a rod setup with monster ports, huge valves and longer manifold runners and Large carb/throttle body

For NA it needs big money for:
little gain in hp, reliability and drive-ability
For boost less money for:
Bigger gain in hp, equal reliability and better driveability (if using small turbo).
Topgear has made a fun 96kws at the wheels with a budget setup on an a12 so an a15 should make 25-30% more with right turbo.

160hp NA A-series will eat about 5-8000 AUD$
160hp T A-series will eat about 3-4000 AUD$

Valve Sizes: GX heads (A12) have the same valve sizes as non-GX (A12). A14/A15 heads have larger valves.

A14 and A15 heads are the same. There are about four or five different versions. All A14/A15 heads have larger valves than GX, although some 1982 A14/A15 have the smaller intake.

The details are documented in Valve Sizes

I visualy cpmpared a12GX and a15 head and the chambers are the same. the a14 ovalport head (or atleast one of the variants) on the other hand has mutch larger round chambers. the advantage of the GX (to my knowledge) over the a15 head is the port size/desighn and the smaler valves arent the disadvantage people seem to think e'specialy for a streeter as they create higher gas velocity, with (to my knowledge) givs better throtel responce accross the rev range, smoother acceloration, and better acconemy.

how much can i take off the head if i want to make compression more but still run on normal petrol (we only have 93 octane here) isn't the gx head's valves bigger than the a14 & 15?

It doesn't. But he's talking about putting it onto an A15. With the better chamber design of the GX head, the combustion should be more stable. So the engine should be able to stand more timing. (all things being equal) I CC'd my GX head, and I think from memory that it was 29 or 30cc's. I've never done an H89, so I couldn't compare.

What makes you think the GX combustion chamber is smaller(less cc) than standard A12 "heart shaped chamber" head?

Even just fitting that gx head will give you more power due to a increas in your compresion ratio. Better fuel economy due to the gx head combustion chamber has the perfect quench area. Its like the best of both worlds!

The GX head has the "best" shaped combustion chambers right from the get go. So On a N/A motor with all the basic mods, you're going to achieve better results than you might with the H89. (A15 head)
Do bear in mind however, that the A12GX head only has the smaller A12 valves. However the seats can be opened up to take the 37 and 32mm A14/A15 valves, without the (Considerable!) expense of having larger valve inserts installed into your head. As Dave already said, the GX head has the biggest inlet ports of all the A series heads. And whilst it's not uncommon to increase the port size on a lesser head to the same extent, by starting with the better unit any improvemements made to it will leave you better off than you would have otherwise been. (Dollar for dollar)

Although expensive Im almost convinced that in the name of fuel economy in a powerful A series, a knife edged crank and lightened reciprocating parts is
worth the investment if you are keeping the engine for life.
A friend of mine has a toyota 7K with many modifications and swears
by the knife edged crank, lighter pistons with lightweight aftermarket rods
and billet flywheel. Its quite an expense to recover but he says the response
and better fuel economy are worth it.