I was doing some fact finding on the M30 (round port) head vs the H89 (A15 Oval Port) head and was tossing up between the two.

Apparently the M30 can be ported to flow as much as the H89 however a ported H89 flows even more as the M30, when ported past H89 spec runs into water jackets etc.

So for a worked A12, should I leave the stock H89 ports with a cleanup... or de-shrourd then and open them up more? This would be for a modified steet setup - that's gonna be revved! Or even port the M30 head and just use that?

The stock H89 valves are at 37 In and 29 Ex and will be fine for the setup in mind - better than the M30 35/29 valves.

Both heads use a closed chamber design I believe.

The other thing I'm having trouble with is the combustion chamber cc's - does anyone know what the stock, unmilled head cc's are for the M30 and H89 heads?

Now compression ratios, technically speaking - is it better to mill the head to achieve a desired compression ratio or is it better to change/eliminate the piston dish?

Me using an A12 block with a H89 head would need to probably use A10 flat top pistons (no dish) then even still mill the head so that I have a good enough CR to run 98 fuel.

What are your thoughts?

I've been under the impression that most A series heads are in the ball park of 30cc in the chambers.

I cc'd a H89 at 28.5 and a H75 at 28.5, not sure about how many times they were milled though.

Please dont mill the head for your own sake, use flat top pistons as the
A14/15 pistons are usually dished. If you go flat tops you should get around
10.5 or 10:1 comp providing you calculate pistion cavity and calculate the final comp.
Theres prob 10-18cc in the pistons depending which one you have.
Go for flat top 76 or 77mm from Daihatsu HC or HD pistons and use the a12a rods.
Then youll have extra 100cc and newer better cast pistons proven in 9000+ 3 time
winning Australia Rallye Champion U2ltr Daihatsu Charade.

Hmm, that seems like the logical choice there D, however will the a12a rods handle it?

Also, I'm trying to firstly get a A14/15 whole engine setup going, if I cant do that at least I want to use a A14/15 head on my A12 Block and if I even cant get a head for that Ill just use a Ported M30 - which is easier for me because my Quad bike carby setup is already running on a round port manifold. However, if I can land myself a A14/A15 I'd willing make a manifold for it (with Chris' manifold flanges!)

Also with most head having 30cc chamber volume, I guess the CR is determined via the piston dish then?

Most important would be to get some arp rod bolts and your rods worked if chasing high rpm.
a12a rods will be choice as they have the bigger pin sizes like the a14/15 rod do.
If you are not worried engineering it then go the a14/15 but with the a12 you can get away
with the a14 crank modded counterweights, a12a rods, 29mm pistons etc.

I'll run better bolts on the rods, head, bearing caps and stud the head for the rockers while rebuilding - as much as I want to gain performance I don't want it from losing reliability if you know what I mean.

So the A14 crank has more throw but fits in a A12 block, and using short rods such as the A12a I can get more displacement?! What do you mean by 29mm pistons?

Edit: just realized the A14 crank has the same throw - so whats the benefit of the A14 crank as you mentioned with the modded counter weights D?

the a14 has a longer stroke than an a12, I run a 1200 block with a a14 crank a12a rods and wiseco pistons. the pistons have a raised pin location to offfset longer stroke. the a15 pistons will work if you can find a block that will bore out to 77mm

Ok that makes sense then - I assume your wiseco pistons are custom?

Also, why stick to the A12 block? So you can still call it a 1200?

couldnt find a a14 block also didnt need to redo mounts and a12 block is 10 pounds lighter. yes pistons are custom 14-1 compression ratio with 29 cc gx head with 40mm intakes and 33 exhaust. the rods have arp bolts block is oringed and has arp head studs.