Hi guys, I have the original Series 1 a12 in my 73 b110 sedan, I also have an a15 with a race head + twin 45 dellortos available. Would the a15 be better or would the a12 do fine? I would like to take the a12 to an engine builder and have him make one ripper of a motor. Driveability is not really an issue as it is not my daily driver. Experienced opinions on both would be nice.

Cheers

if you have options, bigger cc is always better.

I have had both A15 and A12 in my utes

And all i can suggest is the A15 a set of twin dellortos and the decent head and cam are great fun.. high rev's great sound from the twin carbs and that little extra power to play with.

If you go the A12 option sooner or later you will look for something a bit faster..

The A15 can be great fun and i would not be going back to an A12 anytime soon..

Depends what you want from your motor. i have had a few A12's and you cant compare to the A15 for power output.

Doesnt mean you wont get good power from an A12, there are some big worked even turbo A12 set ups that are heaps of fun.

If you can find some local datsun owners maybe a datsun cruise, you can tag along and see for your self.

Obviously 'technology' is a big deal, so ohc, multivalve and all that does skew things a little bit, but when comparing apples to apples (in this case a pushrod a12 vs a15) - then I'll point out something very 'telling' about making engines peform. Pick just about any class of racing you can think of, where forced induction isn't allowed, and you can bank on one of the big (and first) regs being maximum engine capacity.

That ought to tell anyone how important more swept capacity is to the overall result.

the only time where you'd find exceptions to this is where you had some oddball setup where the head is exceptionally restrictive and the larger capacity was delivered all from a longer stroke, and one that is just insanely too long to support decent rpm, a decent rod to stroke ratio (which affects both max safe rpm, due to side thrust issues from excessive rod angularity, as well as 'how' the engine breathes as it alters how quick the piston moves toward and away from top dead centre) and you are also very restricted as to max carb size. Typically only with all of those things being the case, in combination with one another, will you find that there isn't a 'gain' from more capacity.

A case in point might be the a series mini engine (which is what you might call an ancestor of the datsun a series, though the datto motors are way way way better, and I say this as someone quite fond of minis). They come in various sizes up to 1275cc. With some serious overboring, and offset ground crank, you can in fact increase the capacity a fair bit - 1440 and 1480 are about the largest 'usual' capacities (and about 1380cc is the largest with std 1275 stroke) that people go for.

however it is in fact possible, by 'offset' boring (shifting the bore centrelines outwards and sideways just a little with the overbore operation) and a longer than 'usual' stroker crank - to get all the way to 1600cc. The problem is that the mini head, even with the best porting job in the world, doesn't flow fantastically, so will struggle to adequately feed the motor at higher rpms. Additionally, the rod to stroke ratio becomes less than ideal, with far too much side wall thrust - which costs power due to friction, but also leads to more cylinder wall flexing. which is bad enough (the flexing) but when you combine it with the much thinner bore walls - well the flexing is so significant, that not only is it close to structural failure, but even if it survives, all this flexing leads to very compromised ring seal, so you lose power (from combustion gases/pressure being lost past the rings, but this also means it pressurises the crankcase, and will push oil up past the rings of pistons on an intake stroke, and compromise the effective octane of the fuel, and end up limiting the safe compression ratio you can run with any given fuel.

So bottom line (for the mini engine) is that this 1600 odd cc engine doesn't make more power than a properly prepped 1440 or 1480, and won't stand decent rpms, and won't last anywhere near as long.

but that is the bmc a-series. The datsun a series doesn't have practically ANY of thse issues. The a15 has considerably bigger bore in std form than even the 1600cc mini einge) and a MUCH better breathing head, etc etc etc.

So in simple terms, if it isn't restricted in some particular racing category, or especially if it is a streeter, and you aren't going to turbo it, then the a15 will always have a big advantage over an a12

Not only will it make more power (probably around 25% more, which is absolutely huge as far as driving experience goes) but it will just as importantly have similar increases in torque (for similar cam/combos). And that makes it far far far more fun to drive, and far more enthusiastic getting back into full swing after each gearchage. You typically won't have to go to as low a diff gearing (higher numerical ratio) to get as good a performance as you would need to go with diff gearing to make even a very hot a12 combo keep up with it at all.

There are 4.3ish and 4.88 diff gears out there - and for a 'red hot' a12, you'd almost certainly want 4.88s to get the most out of it. They do exist, but are often hard to find and expensive (to get em cheap, keep an eye out for local diy wrecking yards, and if an a12 powered vanette comes in, it should have a h165 iff with said 4.88 gears)

It's been said on here that the datsun gearbox ratios aren't 'perfect' and the 'jump' from 1st to 2nd and 2nd to 3rd is biger than most would like) - so having a 'peaky' engine that has a narrower powerband (vs an a15) will see this ratio progression as even les ideal. If (for instance) you upshift at 4000rpm, with a standard box (and i use 4000rpm merely for sake of example) - well it would (again hypothetically) perhaps pull the rpms down to 2500rpm, a drop of 1500rpm. Not great but reasonable enough. Now with that same box, but now upshifting at 8000rpm, well that would yank the rpms down to 5000rpm - a 3000rpm drop. That's bad enough in and of itself, but it's no secret that massively higher revving engines tend to have a narrower and narrower powerband, so that sort of rpm drop would pull it right down below its powerband, and be a 'dog' as far as performance goes. it is in effect a 'double' hit (bigger rpm drop AND narrower powerband) so it's why racers (where legal and the gears themselves exist) seek out closer ratio gearsets, so they can keep the engine in its 'sweet spot'

now obviously the actual rpm differences between a hipo a12 and similarly modded a15, i.e. you'd (maybe) upshift the a15 approximately 1000rpm sooner than the a12) - so that helps a bit, but also, the a15 with more torque (and typically a wider powerband) will fare far better with 'stock' gearbox ratios. So the difference in peformance with be EVEN bigger than the torque/power figures would otherwise suggest.

Anyway - sorry for drifting off on different tangents here and there - but bottom line - the a15 will outgun _any_ similarly modified a12,

I would go a15 Currently doing a build up on one,Its more torquey wont go back to a12 soon.

Cheers.