Single Over Head Cam
Summary of the SOHC 6G72 in the Mitsubishi 3000GT Base model (1997 - 1999)
--Sunmind 07:19, 8 October 2006 (EDT)
The SOHC (Single Overhead Camshaft) engine, although based on the 6G72 engine like its siblings with DOHC (Dual Overhead Camshaft) has some significant differences that affect not only the performance but the ease and effectiveness of modification.
An example is the forged crankshaft which is standard on the DOHC engines, whereas the SOHC has a cast crankshaft. Furthermore, the cast crankshaft on the SOHC may be one of several types and it may be necessary to check the identity stamp in the block and the color code on the crankshaft, in order to replace bearings, for example. Not having a forged crankshaft prevents the SOHC engine from being a "stroker" motor and thus gain additional displacement. Although the SOHC does have a cast crank, this is considered to have a durability higher than would be necessary in a normally aspirated engine. There are SOHC engines that use forced induction (turbocharging, supercharging and nitrous oxide injection) without any apparent failures of the crankshaft. If such failures have occurred, there has been little reporting of such incidents.
The SOHC has two (larger) valves per cylinder, whereas the DOHC engines have cylinder heads with four valves per cylinder. This variation in valve train configuration imparts advantages and disadvantages to both. The multi-valve configuration of the DOHC is believed to flow better than the SOHC, especially at high rpm's and the DOHC valvetrain is made up of lighter components than the SOHC. These factors give an undoubted advantage of the DOHC engine to operate a higher rpm than the SOHC. It would be inaccurate, however, to simply make the assumption that the SOHC is inferior because of the valvetrain configuration. There are contemporary high performance engines that use the SOHC configuration, such as AMG/Mercedes as well as legendary engines such as the Ford 427 SOHC V8 (Cobra).
Not much is known about the modification potential of the SOHC since there is a very limited after market for performance upgrades, meaning most modifications would have to be custom made and therefore much more expensive. For example, there may be only one or two companies in the world that sell billet camshafts for the SOHC and probably the same number that sell reliable exhaust headers. The major piston manufacturers in the U.S. (where the SOHC was primarily sold) did not include the SOHC in their catalogs. The SOHC 6G72 is used for competition motor sports in Australia and New Zealand. Those countries are the principal source of SOHC competition parts; although the cost of shipping, customs duties and currency exchange rate fluctuations can make these parts very costly. A separate section will be written to specifically detail SOHC modifications.
Conventional wisdom, or at least custom, has generally been to not invest time, money and effort in attempting to make the SOHC a high performance engine but rather to opt for an engine swap with the DOHC engine in normally aspirated form or to install a twin turbo motor from the VR4 model. Since both these options are less costly than developing a high performance SOHC engine, it is little wonder that very few projects involving a complete, normally aspirated SOHC performance engine build have been undertaken in the 3000GT platform.