I'm sure everything is measurable in this most measurable of all possible worlds! But as long as the basic colour of the pipes up near the head is about right - ie not too blue nor glowing cherry-red in the dark - I'm happy. Efforts to reduce the amount of pipe up the head will almost certainly result in air leaks and backfiring; I like the most I can get up the ports - and the same both sides if possible - a solid mount around the level of the lower engine mounting, a good clip round the silencer/pipe joint at the gearbox area and the usual solid mount towards the rear for silencers. That way it all stays on. Largely dictated by the frame, as you say Dave. Any variations in temperature or colour are more likely to be due to induction bias with single carb, differences between twin carbs or imperfections in the ignition department. Especially the last. So, if I was trying to figure out a hard thing like varying exhaust pipe temperatures around the head, I'd start with the mag - bearings, cam ring, points gap etc; I'd move to the carb; and I'd only very reluctantly start thinking about the pipes themselves unless I knew I had an air leak at the head joint (which would mess things up a lot and probably start costing exhaust valves in short order).
