There are virtually no downsides to the
Mitsubishi 4G63 motor. Though various ancillary parts such as
cylinder heads,
water pumps,
oil coolers and so on have been played with with over the years in different vehicles, the 4G63
cast-iron block has remained almost the same since it was designed. With upgraded internal such as
forged pistons and
connecting rods, this modestly sized bottom end may be beat on relentlessly, and
boost levels over 30
psi are not uncommonly applied to it in racing. The fact that there exist
DSMs capable of 9 seconds and faster in the 1/4 mile is testament to the capability of the 4G63.
There are a couple of downsides to the motor, however. The first being the balance shafts or "silent shafts", which Mitsubishi designed into the motor to reduce engine vibration. These exist purely to ensure the comfort of your bottom end while in the driver's seat. The 4G63 was designed with two of these shafts. One of these is in the "front" of the motor, centered with the highest "extent" of the crankshaft, and is belt driven off a secondary crankshaft sprocket. The other is located directly "behind" the crankshaft in the rear of the motor, and is usually driven by a gear inside the oil pump. Each shaft has its own bearing surfaces inside the crankcase on either side of the mains.
The most immediate problem with this design is the belt-driven front balance shaft. This belt is driven by a sprocket located behind the main timing belt sprocket. If this belt breaks, it usually either smacks the timing belt causing it to jump off the sprocket or break it altogether, gifting you with a valve replacement job. The second most immediate problem is that since bearing surfaces are subject to wear, if a shaft journal begins to wear abnormally, the shaft may seize in the block and you may again get some bent valves. The common solution in 4G63 tuning circles is to remove the two shafts altogether. Surprisingly, this results in very little increased vibration in a motor that is balanced by the machine shop before assembly.
Around 1994-1995 Mitsubishi began shipping an updated 4G63 block design called the 7-bolt motor (referring to the number of bolts used to attach the flywheel to the crankshaft). There were a couple of subtle changes involving the size of the oil galleries in the block, and integration of the oil squirters into the bottom end of the cylinder walls. After a few years, many owners began to have trouble with premature crankshaft thrust bearing failure, which caused the crankshaft to develop too much lateral movement inside its caps. Many DSM owners have had their crankshafts and bearings replaced under warranty (many at VERY low mileage numbers). This phenomenon has come to be referred to as crankwalk, and apparently Mitsubishi has tried very hard to deny that this is a real problem. The reasons for this failure are still not well understood, however the most popular theory has to do with insufficient oil pressure and oiling at the thrust bearing surface itself, which may have to do with the aforementioned changes to the oiling system. Magnus Motorsports sliced apart several engine blocks and studied the changes in detail, and came to some interesting conclusions, the details of which can be read at http://www.magnusmotorsports.com/crankwalktheory.htm.
The easiest solution for 7-bolt motor owners has been to simply swap in a 6-bolt motor from an earlier vehicle, making the necessary sensor changes where appropriate.