The motor found in the 2G (second generation) non-turbo Mitsubishi Eclipse was the Chrylser-made 420A, unlike the factory turbo charged Eclipse which featured a Mitsubishi 4G63 engine. Designed sometime around 1995, the motor has several strong points such as:

  • A very strong bed plate four-bolt main lower end design. This means that the block itself is separated into two sections along the crankshaft centerline. The conventional design on many blocks uses separate main bearing caps, which lowers the overall strength of the bottom end.
  • The stock crankshaft is incredibly strong, and capable of withstanding high power output. Knife-edged from the factory, the crankshaft has an overall lower rotating mass which allows higher RPM operation.
  • The head is designed by Lotus. Utilizing a four valve design leads to a very well flowing head. Because of the design, the head leads to great detonation control resulting from its great port / chamber design and great cooling capability.
  • A direct crank-trigger ignition provides accurate spark timing.

Although the block was well designed, it has several shortcomings seen mostly in the strength of the pistons and rods. Mahle pistons were provided stock from the factory, but are still not strong enough to handle moderate boost levels. The factory rods, which were created using a casting process, are not designed to withstand long-term boosted applications.

Opting to fix these problems, I chose to replace the pistons and rods with forged parts. Mainly, the stock rods and pistons are being replaced with Eagle forged rods and JE forged pistons. However, I opted for a long-rod design. For more information on long-rod engines, please click here. The bottom end parts which will be replaced on my motor include:

Other important parts such as the oil pump and gaskets will also be replaced.

The head, as stated above, flows quite well from the factory. For now, it will remain stock. Once the bottom end is tried and tested, the top end will get reworked.