OK, we seem to be moving towards raw performance here...
Robini, you seem to see my point a bit. You can't compare different planes. 767/747 is indeed an extreme case. You MUST minimize all variables in order to make a fair comparison. You can't talk about payload in performance. No performance tables I've ever seen have dealt with payload, only gross weight. The plane only knows what it's weight is, and it will perform accordingly, it doesn't care if the weight is payload, fuel, or part of the structure. Weight is weight.
I can't see the point of comparing the two at MTOW; that is of no help when you try to decide which has the better performance for a given mission.
Can you suggest a more generally applicable value to use?
3) A twin, being more over powered with all engines running, will necessarily appear to have better takeoff performance when there is no engine failure.
This is true. Twins have a 50%surplus, and quads only have a 25% surplus.
After writing that, how can you argue that an A340 will outperform an A330?
Comparing them at the same weight is useless. An A330 at 500,000lb is nearly maxed out. An A340 at the same weight still has a FAR way to go before MTOW.
For the record, I've been discussing the A332, since more have been built. When they're both empty, the 330 is lighter. It will climb faster. When they're full, the A340 is MUCH heavier. It climbs MUCH slower.
This isn't by any means an academic issue. The 340 has a reputation for climbing slowly- hence my initial question.