Originally Posted by Alastair

When looking to extract extra power from a Diesel engine, a few things must be remembered or it will all end in tears....

IF you really ARE developing 25 PSI boost which I seriously Doubt, For god's sake, have pity on it before you destroy or seriously damage either the Turbo OR the engine. At the moment, its a 'Runaway' waiting to happen when the turbo self destructs....

Reduce the boost-pressure to an Absolute-Max of 15 PSI untill you do further work.

Starting with a Good Turbo engine (regardless of make) its possible to gain good improvements just by getting all the settings to Correct, factory spec, and ensuring that the engine is in a good state of health.-There is No use or benefit of 'just upping the turbo-pressure' on an engine with say, Mis-set fuel-timing, or a stretched timing-chain or bad compression.....

Things that especially effect the power, would be Injection Timing, Valve-Clearances, good clean air-filter etc...Just the essensial 'service-work' before you even Think of 'Modding/Adjusting'--Doing this work is pretty cheap and should be done anyway and can in some cases drastically improve the drivability/livelyness of your car.

Moving on, assuming that your engine is in good to great health to start with, the boost-pressure can be raised, BUT as others have said, little benefit will be had if the fuelling adjustments Inside the injection pump are not altered. These are difficult to carry out with the pump on an engine, and should be carried out on a proper pump Test-Bench by someone who really knows what they are doing....

If you raise the Turbo boost-pressure More than around 15 PSI, then its really mandatory to fit an Intercooler, to reduce the thermal-loadings on the engine. It also improves the power capabilities as well, as the cooler the intake-charge, the more dense, therefore contains More Oxygen...

More Oxygen, Plus More Fuel, equals More Power--Within Limits....!

(Case in point,-That hideous BMW I own, has an Intercooled 2.5 Turbo-diesel. The Non Intercooled version of the same car develops 125 BHP. The Intercooled engine can run a slightly higher boost pressure safely and develops 147 BHP in standard form....)

Much more than the above considering the Mercedes OM617 engine and its Bosch PES 5M pump and you are looking at extensive IP work, increasing the size of the elements (The Elements are the pumping-plungers and barrels that actually supply the fuel....)--from 5.5mm to 6 or even 7mm sizes. These elements would have to be 'specials' as the way the 'Scroll' is cut in them also affects the timing at different loads and speeds in conjunction with the pump drive gear mounted 'Timing-Device'....The Governor and other parts in the IP would need alteration and appropriate settings to run these new elements correctly. Looking at Lots of dosh for this sort of work, and Huge amounts of power to be gained--How fat is your wallet!!....