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Did the A-15 GX come out of a B-210GX? If so, I may have bad news for you. It is my understanding that the B-210GX was nothing more than a sticker package on a five-speed economy model B-210. If so, the engine isn't anything special. In fact, correct me if I'm wrong guys, wasn't the "real" GX engine an A-12? Also, didn't the "real" GX have dual SU carbs?
Having said that, I'd still say you made the right choice. Or at least didn't make a wrong choice. Both engines are good and will really wake up a 1200. No, your smile should not fade too much.
Sorry to be a downer, hope I'm wrong.

i didnt know about that, i pressumed someone had taken a GX head off a 1200 and used it on an a-15

jb1200 - If its a good gx it will have twin SU carbs and double valve springs, you may be able to see through oil filler whether there is a second sprind inside the first on the valve stems just inside the rocker cover

I am proposing to build an A15 aswell. I'm on the fence with building an A14 though, because they can rev more.
Assuming I were to use a genuine GX head with 44mm carbs, which one would give more power? With one the torque would come at low revs, but the other would rev more and give more top end.
Any comments?

I think you're right dimlight, I don't think the GX engine was originally available in the US. I have heard that there were alot of them available from those Japanese engine exchanges.
I'm sure the A15 will work just fine. There's no replacement for displacement

i reckon it strongly depends on the intended use of the car as a non reving a-15 would be nowhere near as good in a race application but for a road car and engine life an a-15 is probably better because you dont have to rev the crap out of it to get somewhere quickly

I know we all say that the A-15 doesn't rev like the A-14, but really, does a 5mm longer stroke make THAT much difference? Also, is 100cc really that much more displacement in the "no replacement" argument? Sorry to be so pessimistic about the "conventional wisdom." I'm just not sure it really is that big a deal. I could be wrong though.

youd have to admit theres a big diffence in engine charachteristic between the a-12 and a-15, Yes?

well i see the a-14 as the inbetweener cause the a-12 will rev like all hell and go if you keep the revs up there, but down low its a slug

The a-15 exactly the opposite it launches from low revs and never really reaches high revs

the a-14 gives a happy medium where you can use torque down low but also let it scream

it obviously depends on how the engine is built (cam choosen ect. ect.) but in general thats the way i look at it.

Without researching all the physics behind it, I reckon 5mm extra stroke will make a significant difference. I know it doesn't sound like much, but picture the thing turning 50 times every second (that's only 3000rpm) and you'll start to get an idea of the increased inertia caused by moving the crank journal and rod big end 5mm frurther away from the crank centreline. Piston speeds are increased dramatically travelling 10mm further for each stroke (ie 20mm further for each crankshaft cycle), and rod angularity is increased placing greater loads on it and the piston skirt. Although this may be offset by using a longer rod, I suspect the A15 rod is more than likely shorter to allow for the increased stroke and to improve torque. If my tenuous grasp of physics is correct, shorter rods will increase the max piston speed as well.

I totally agree. 5mm is significant. The rod length is same for A15 as A14. The rod to stroke ratio is similar to Ford's 302 engine in the A15. Nobody would consider that to be a non-revver, but the bore to stroke ratio is much smaller in the A15. I would expect that to have a compound effect. Who has got a A15 with dual carbs and big cam and can tell us what it's like to drive?

Ah you guys aren't thinking this out straight.

First of all, the A15 stroke is very short compared to other hi-revving engines.

Secondly, the bore-to-stroke ratio is more interesting in its effects.

Lastly, as mentioned, there is no replacement for displacement.

From the half-logic I keep hearing one might begin to think the A10 is the best "peformance" a-motor -- since it has the shortest stroke!

One of the reasons the A14 is favored by racers is because of class rules. Which doesn't apply to street cars.

The A15 head has the same size ports as the A12 GX head. The biggest effect on feel is the torque comes on sooner due to the bigger CCs, then levels off sooner as it's flowing more air than an A14. it's partly psychological and partly due to the factory tune -- unless anyone's taken objective measurements (for example, timed both 0-100 km) this is all just guessing as to whether a stock A14 is "faster" than a stock A15.

Getting adjustable cam gears and advancing [edit: retarding] it a little should perk an A15 up in the high-rpm section (has anyone tried this?) along with advancing the dizzy timing and richening the jets a little. The late 70s motors generally were a little weaker in those areas to gain lower emissions. Disconecting the EGR valve and air pump should also help performance.