What is JDM?

JDM is an acronym for Japanese Domestic Market. It refers to parts that are normally only available in Japan.

A JDM engine is, therefore, an engine that is normally sold in the Japanese market only. Such 4G63-based engines are now routinely imported by some shops as direct replacements for American-spec engines. This is done because JDM engines often come with larger 16G turbos and other enhanced parts not available on American factory engines.

Buyer beware, however: not all JDM engines are equipped with these superior parts, and some configurations may require extra work to put into an American car. In many cases, even the importing shop may not know what they have until they get it and inspect it very closely. Protect yourself and verify that the JDM engine is, in fact, the best option.

Someone who refers to a "JDM transmission" is often referring to the JDM-spec Galant VR-4 transmission that was made available in Japan with a "FWD-to-AWD switch". This transmission comes with extra bolts installed that allow the owner to change from FWD to AWD with a small amount of wrenching. 

As with the JDM engines, JDM transmission authenticity is always in question unless the unit can be inspected in person by the buyer. The chances of getting the correct part are increased if you are dealing with an expert, DSM-centric shop with experience on the JDM components. While many shops like to claim expertise, the fact remains that unless they have done it before, they probably don't know the differences.