The 4D55 was set from the factory to make 13 PSI of boost, that's what made it soooo much better than all it's competitors turbo diesels. Also the 4D55 was designed to be a turbo charged engine. Other turbo diesel engines of the 80's were naturally aspirated engines that were converted and fitted with a turbocharger.
Perfect example is the Isuzu C223 diesel engine found in the early Isuzu P'up's. In it's naturally aspirated form, the C223 was a very reliable motor but it struggled to make sufficient power by producing only 58hp and 93ft lbs of torque. In 1985 Isuzu change the manifolds and added the Garrett TB020 turbocharger to there legendary C223 engine. The new turbocharged engine was designated the C223T and made 74hp and 125ft lbs of torque. While the new C223T looked good on paper, it was a ticking time bomb. That additional hp and torque resulted in catastrophic engine failures even when the motor was in stock form. The most common part failure was the connecting rods on the Isuzu C223T. While Isuzu eventually corrected the problem with their 3 generation connecting rods, the rods were never available to the public and they were only used very small number of the C223T engines as part of a very small recall. I would say less the 1% of the C223T ever received the upgraded rods.
The 4D55 is capable of handling additional boost, but I would not make any modifications without adding a pyrometer first. That's just diesel modification 101. The 4D55 can safely handle 18-20 psi of boost but measures need to be taken to keep the EGT's in the safe zone like adding an intercooler. I really wouldn't worry about adding ARP head studs but I guess it never hurts. While the 4D55 was discontinued here in the US in 1987, it is still being produced in it same basic mechanical injection form and make 104 hp and 177 ft lbs of torque. It's make all this additional hp and torque while retaining the same basic block, head, pistons, connecting rods, cam etc. The only real changes were adding a watercooled turbo and intercooler.
Welcome Visitor! Please take a few seconds and 
