yes it should raise compression a tad, but the difference in the valves size seems not much but makes a fair difference. I did it and its definitely not as good as the original head.

i think your meant to put a a12 gx head on an a14 because it has higher compression but still has oval ports plus the big valves

yeah but those gx heads are reall rare aye?

iv never seen one

i tested the compression today and its got 110 psi on all cylinders.

thats a bit low. you should have about 160

I agree with frankthedolphintrainer! Geez you coul've picked a shorter name!! hehe.

I'm confused about how the open style round chambered head will raise compression ratio. It will if you shave it down a heap, but otherwise it has a much larger combustion chamber. I sat both styles on my bench for comparo. The open style head appears to be almost exactly the same except someone has come along and milled a heap of metal out of the chamber to reduce the compression.

Someone told me they did this for some engines, destined for countries with tight pullution control. The little A-series engine had been around for many years by this time and wasn't up to the challenge without modification. The reduced temperatures are apparently responsible for reduced nitricoxide gas emmisions, or was that sulpher something or other gas ?? Can't remember now, but it seemed like a reasonable story to me at the time and I've never doubted it.

Oh yeah, if your thinking of having the head shaved to increase the compression, you might want to do a plastersene check to work out how much clearance you have between valves and piston.

The head I'm looking at is a H89 casting, has oval ports and the high compression kidney-bean shaped chambers. It was from an A15.

the a14 with und chambers was made for the el chepo driver, its design was to give less power but much better fuel economy

I recently bought an a14 it came with an a12 round port head witch has the kidney shaped combustion chamber standard A12 inlet valve 35mm - but A14 exhaust valve size 30mm - apparently built by someone who built juinor speedway a series engines - maybe there is something special about the combo


A12 head on A14block thread

Quote:The cylinder heads that were used on the 1200 GX & the A12T [120Y GX] used the largest of the production port sizes & were equal to the smallest of combustion chamber sizes but still used the standard A10 & A12 valve diameters of 35 & 29mm, so fitting an A12T head to an A14/15 engine is not automatically an upgrade as the engine now has smaller valves compared to the A14/15's original size.

Even if the compression ratio was unchanged, the fitting of the 'good' A14 head to an A12 engine is also NOT going to necessarily increase power. It will, however, remove the small port & small valve size restriction of the original head & allow other mods [carb, cam & exhaust] to unlock the potential of the smaller engine.

Anyone who thinks that simply fitting an A14 head is going to provide noticeable power gains just by itself, ... is dreaming.

Have any of you actually measured the combustion chambers? My information says that for A12 engine, closed chambers are not higher compression than open chamber. Instead, the difference is with the pistons: Flat tops give high compression. Dished pistons are lower compression (but not very low, 9:1 is the lowest).

So changing heads won't change your compression much if any. Putting A14 oval-port head on A12 engine simply ruins your low-RPM power. But on the the other hand, high-RPM power is still limited by your stock Hitachi carburetor.