okayyeeee.....
dunno about the whole A12 top end on an A15 bottom end.....! that's just a recipe for a slug of an engine.
i would think more gain would be had in the area of inlet runner lengths, exhaust header design and camshaft selection.

inlet runner design - google for David Vizard Calcualting Inlet Runner Sizes. massive read. i think it went something like this.

1) you need to have the same amount of runner volume as the combustion chamber you're filling.

2) start with a 17.8cm runner length for peak torque to occur at 10,000rpm. for every 1000rpm down the rev range you want peak torque to occur, you add 4.3cm of length to your runner.

3) so, say your A15 has a cylinder volume of 375cc, then for peak torque to occur at 10,000prm we get a runner length of

2(3.1416 x r) x 17.8 = 375cc
reverse the formula and we get a 33.53mm

Quote:Here's an idea:

- Late A12 camshaft (short duration)
- A12 round port manifolds
- A12 high compression head (heart-shaped chambers)
- Stock Hitachi carb, jetted a bit rich

I haven't tried it, but the small ports should increase low-speed torque. On the top end, it'll go like an A12, just down low that A15 should have considerable more grunt from the extra cubic inches.

I have decide to put my a15 into a Suzuki SJ410, cause I'm a bit of a 4wd nut.
I would like to know what can be done to this motor to get more low down grunt.