bit of technical advice please.

[OldMav]

I wonder who that would be lol .

Anyway I will add a bit more tech without prejudice about coolers. The STD cooler is fine for the STD TD42ti. Fitting a bigger cooler give very marginal results due to the HT18 turbo is flat out pumping 12 psi to start with at a reasonable output temp for the cooler to work. All it will do is lower boost pressure slightly due to cooling effect but stuff all volume is increased due to the higher output temp from the turbo. At 15 psi no cooler has a chance to do its job due to the tiny increase volume with excessive output temps. Above 12 psi is just to far outside of the HT18's efficiency map to gain any real effective results it just doesn't have the pumped volume for this near all the volume supposed pressure here is generated by heat.

MMmm now to tackle the other posts without prejudice. We are talking diesel so rich lean is not the proper terms here they are the domain of petrol or sparked ignition systems. So for auto ignition systems lean is good above 30 AFR is good and surprise surprise lower EGT's less heat. Now low AFR or rich is bad news this means fuel burning in the exhaust manifold and onward to the turbo which melts Sh#t. High EGT's usually with a turbo not working past its limit you add more boost this adds more oxygen and better still a cooler to lower this a bit more so more oxygen per set volume of air hence fuel burnt more completely in the combustion process hence more power and lower combustion temp due to a faster flame front. But this sort of thing is not possible with a HT18 working past 12psi as it pumps no more volume just pressure induced by temp. Now the other thing that happens when we have a turbo working within its map. We have a thing called valve overlap which is the same as petrol its called the scavenge process where by both valve are open so what happens with a adequate turbo is we have enough air volume to flush out and spent gas and a bit extra to slightly cool the spent exhaust gas lowering EGT's so even more fuel can be added because we don't have any spent gas still in the chamber reducing our volume of oxygen this will gain better effective power or in our case here torque.

Arr well nothing to dramatic to start my first post on eh.