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What is the original boost on a 2.0 hdi 90?
And would it be very damaging to the turbo to adjust it up by 0.3-4 bar?

Pretty sure the boost is controlled by ECU so you cannot simply adjust it yourself - only by remap.

On the 90hp it's mechanical, 110hp is ECU controlled. Standard boost is about 14psi. Without adjusting the fueling you won't see much gain.

Indeed, remaps are more about injecting more diesel than going for high boost (they run high boost anyway and because there's no throttle butterfly you get maximum combustion air at all times).

More reading: http://www.hdi-tuning.co.uk/ecu-remappi ... power.html

To increase the boost on a 90 I assume you'd have to fit a bleed valve to fool the wastegate into thinking the turbo pressure is lower, holding it shut for longer. This is different to the 110 which has an electronically managed wastegate which will open dependent on what the MAP sensor is telling the ECU (max boost is 2000mbar including atmospheric pressure). Fitting a bleed valve would have to be accompanied by a remap, as the ECU will not currently know how much fuel to inject when the boost pressure is above that of the factory limit. Remap's aren't just about changing the amount of fuel injected, if you increase fueling you have to increase boost to ensure the correct stoichiometric ratio is maintained during combusion. The 90 I believe can only be run safely up to ~120bhp before intake temps become too high, upon which an intercooler will need to be fitted.

My 110 is currently running 2400mbar of boost (20 psi above atmospheric) with no problems at all with intake temperatures even running the standard (quite small) intercooler.