To my understanding, and the Doc and others can correct me if I'm wrong, but a 1275 head on a small bore is a good thing, because of the
smaller combustion chamber and
bigger valves. Yes, the block needs pockets to allow for the valves (which weren't designed to fit in the smaller bore), but that disadvantage is minimized compared to a small bore head by the smaller standard combustion chamber, and is easily overcome by the better airflow allowed by the bigger valves. That's why Vizard recommends a 12G940 on a small bore over a 12G202 or 12G295.
Given that a 1275 head is better on a small bore than a small bore head, it would follow that a 1275 head is better on a big bore than a small bore head. Why? Bigger valves and smaller combustion chambers, as shown by Jan. And the valves (generally, depending on the head) aren't as shrouded as a stock 202. Bear in mind, when you port a 12G202, its combustion chamber increases, so you end up around the same combustion chamber size as the 12G295 head. After all, porting a 202 is done to make it more like a 295, if not better, and the final combustion chamber shape is similar. The 12G202 after porting or the 12G295
might flow better, but, at the end of the day, the higher compression given by the wider, flatter combustion chamber of the 1275 head will give more power, and the bigger valves help balance out any flow advantage a worked small bore head might have. The best engine results will generally come from a combination of big valves and a small but open combustion chamber.
In fact, ideally, that's the way all engines should be made. Economy can still be attained by using smaller carbies/throttle bodies/injectors, etc.
This sort of thing - changing to a smaller head - is sometimes done on other engines - 350 heads onto a 400 in V8's for example, as a cheap way of upping the compression. This gives more torque, particularly at lower to middle revs. Compression is our friend - more compression, more torque. That's why turbo's and superchargers are a good thing - they up the pressure in the cylinders even further, which is why people sometimes reduce the compression before they put a blower on. The lower the compression, the more boost you can run (but the worse the car is to drive off boost). The only negative with more compression (or boost) is you have to use higher quality petrol - good thing it's a Mini not a V8!
Putting the 350 heads onto a 400 isn't all roses, though, as it costs them at the top end, because, just like here, the 400 heads will have bigger valves. Top end power is all about gas flow. Compression is still important, but if you can't get enough gas in, you won't use all that compressive capacity. On a Mini engine, your highest/best compression ratio come from a 1275 head, as well as the biggest valves, so there's not really any point going to a small bore head.
Hope that makes sense. Sorry if it's all a level below you...
