I have both an A12 and an A15 the A12 has higher comp pistons, (Than the A15) and dual SU intake and from what I believe a A12gx head, the A15 is just a short block. My question is since I need to rebuild both, wich for my 72 coupe, I want it to have as much power as possible and still be drivable, and on a very tight budget, I know that the head will work either way and I plan on having larger valves and port work and a different cam (not sure on Size), It will have a 5 speed trans, and the standard diff for now untill i breaks or come across a cheap upgrade.

If the head you have is oval port go with the A15. I built 2 engines back in the 80's that were stock A15 bottom ends with 1200GX head/carb sets. Get a mild mid range torque cam and you will like the package. Good low end and would pull nicely to 7000. In this case the old "no replacement for displacement" rule applies.

My current project is an A16 with dual DCOE 40 sidies.

The A15 would be a better compromise of power and driveability.

A15 produces more torque and across a greater rev range and torque = acceleration.

So for plenty of acceleration A15 better.

Power = top speed, which depending on how it's tuned, I still then the A15 would be the better choice.

Thanks Gary And Mildman. One of theses days Gary we need to meet since we live just across the river from eachother. Also Gary I am asuming that your A16 is a A15 bored to 79mm?? I was thinking of going this route myself but I am have a hard time figuring out what pistons to use not to keen on bying new pistons and then spending money on haveing them modified. But only pistons I have avialable from my supplyer are .060 thou over and are dished, I want to keep the compression up.

From what I can make of it, the US A15 pistons have a lot less dish than the ones used over here. The 1200 or A12 GX head has the smaller combustion chamber size so here's my take on the matter.

Overhaul the A15 shortblock with the small dish pistons.
Overhaul the GX head but use the A15 size valves. The GX heads had nice big ports but the smaller 35 & 29mm valves while stock A15's had 37 & 30mm valves, so naturally these would be the recomended minimum.

In a light car the A15 will pull like a train & the GX head will let it rev like a Ferrari, so it's a win win situation.

Do it.

EDIT
Have a look for my post with the pictures of A15 pistons to see what I'm talking about.

Thanks Dodgeman I did see your post when I was doing a search on this subject. It just seems that my A12 would give more compression, I reliaze that the A15 has more displacement, I guess what I want is the best of both worlds. So on that note is there any pistons that are a direct swap into an A15 if i bore it out, That are Flat tops? I read about several that require milling of the top but I am trying to go with the easiest route.

I was digging through my bock of spare parts And found that I have a set of 40 or 44 mm Solex's that i may use on the coupe if I can find a corect Intake manifold

Your best bet to find out what is going on is to CC the head and the dish in the piston then do a compression ratio calculation. It seems that when I did the numbers using a stock A15 bottom end and the small chamber head the compression came out in the high 9's.

One of the engines ended up in the race car seen in my avi. With the car stripped for autocross it went like stink. The other engine was in my street 2dr sedan with the Jatco 3N71B automatic. Not fast off the line but once above 20mph it went real well. Could run away from the 8 valve Volkswagen GTI's and run even with the 16V GTI's... Had enough torque to be able to power brake and spin the right wheel without moving...

Both engines were quick ass builds that consisted of nothing more than changing the camshaft and bolting on the 1200 GX head and dual SU carbs.

How are you going about your A16 build if you dont mind me asking?

i maxed-out an A-12 years ago with a custom ground cam, head milled .050 for a comp ratio of 11.25/1...this is very possible with the early head with the only problem being OVERHEATING!...runs like a bat out of hell, but you have to up the radiator capacity.

well at this point I am leaning towards a A15 but Need to come up with all the specs on the build so if any one wants to chime in with some suggestions I would apreciate it.