The std switch comes on at 86/87C and goes off at 77C, the problem is not only the span of the temp range(on/off) but the fact that the stat is trying to keep the temp to 82C whilst the fan fights to lower it to 77C.
Having measured temps before and after fitting a alloy rad at all speeds, including on rollers, I have made the follwing observations.
The fans will NOT be on at 70mph, this is rubbish, I have two lights that come on with my fans and they are not on at 70mph.
The std system was crap to start with, maybe the reason it was changed!!
The shield that sits between the intercooler/radiator on std set ups holds all the heat away from the i/cooler but has a drastic effect on water cooling it can easily be on for 10 miles at 50mph on a hot day with this in place taken off it will be off in 2 miles at 50 mph, I took mine off reasoning that flow above normal speeds is very suffecient to keep i/cooler cool, so would only be of benefit when not needed if fitted.
The fan circuit without a doubt needs rewiring, I use a fully waterproof relay/housing on the inner wing on mine.
With the alloy rad/rs500 i/cooler I now have things have improved a lot, but as said problem is the fan switch but also the stat, 77C is way to cool for the engine nearer 90C is preferable.
I do not yet have my vented bonnet fitted, but this can only help to draw hot air out even better.
The temps I qouted are the ones I measured, not the ones that are supposed to be but actual ones.
Adding deflectors will help immensley as air works on the lazy principle of easiest route, so if it can go around something it will instead of through it ie the rad, so thin ply below the front edge of rad to stop air spilling out and also at the side helps a lot as well.
tabetha