The 6G72 is a very durable engine which responds well to moderate boost pressures (15 psi or so) and free flowing intake and exhaust enhancements. Internals will generally become overstressed at extremely high cylinder pressures for extended periods as they well exceed design constraints of the engine. The use of nitrous oxide and boost pressures of 20 psi or above may require an eventual rebuild using stronger forged pistons, high quality rings and possibly aftermarket rods, depending upon the application.
The stock heads are very well designed and quite durable. Head gasket failure is virtually unheard of. Head studs may stretch under extreme cylinder pressures. In this case specialty fasteners from a company such as ARP may be required.
The only real area for improvement is in flow. The valves are adequate for moderate horsepower increases but undersized for high output applications. The intake and exhaust runners will also benefit from port matching to mating components (plenum, exhaust manifold) and flowbench porting. Both upper and lower intake plenums can be extrude honed to for additional airflow gains.
Changing camshafts and/or cam timing (with adjustable cam sprockets) can move the volumetric efficiency of the engine in the RPM range. In stock form, this is in the lower RPM range. For high performance applications it is desirable to move the VE higher into the RPM range to coincide with the efficiency of larger turbos. Few options are readily available at this time but regrinds can be done to supplied specifications by a number of prominent machine shops.
The 1991 and 1992 model 6G72 DOHC engine used two bolt main caps. Models from 1993 onwards had much stronger 4 bolt mains. High boost applications pursuing serious horsepower output are well advised to use the newer 4 bolt main engine.
The stock rods are strong forgings making them suitable for most applications. Rod failure in a well maintained 6G72 is not common. Aftermarket rods fabricated from 4340 billet steel are available from Crower which should stand up to even the most demanding horsepower requirements.
The factory steel crankshaft is extremely durable. A common issue when rebuilding the 6G72 revolves around fitment of crankshaft bearings. Some cranks are nitrile heat treated and cannot be ground without destroying this coating. The general consensus is that if a crank is damaged on the bearing surfaces and requires machining for reuse that it is best to purchase a new crank. Aftermarket bearings such as Clevite 77 are available for the Mitsubishi factory crankshaft.
The stock pistons are cast units. Boost pressures much above 15 psi will eventually cause piston failure. Forged psitons are available from JE, ProPower and Arias which will offer much better durability for high boost applications. Combined with Total Seal rings this combination should be durable enough for the most radical of engines including the use of high boost and nitrous oxide. The stock pistons use press fit pins. When properly installed this should be adequate. Floating pins are usually preferrable but will require aftermarket rods since the stock rods do not have enough metal to accomodate machining for the required pin bushing.
Pistons are available which provide an additional 100cc of displacement using a bore size of 93mm. Some head machining will be required to properly fit these pistons. The additional 100cc may be provide and additional 10-20 HP.
Experimental 3.5L engines with stroked crankshafts have yet to be proven reliable. If or when they are, these engines, albeit very expensive, will add a substantial power and torque improvement across the entire RPM range.