The 4G37 and 4G63 engines are gasoline inline-fours. The 4G63 has an iron engine block with an aluminum cylinder head and is equipped with two balance shafts. The turbocharged version of the 4G63 (sometimes referred to as the 4G63T) has a lower compression ratio (7.8 in 1990–1994, 8.5 in 1995 - 1999, and 9.0 in the naturally aspirated version) and oil squirters under the pistons for better cooling from extra heat introduced in the system by forced induction. The turbocharged 4G63 engine received an internal update during the 1992 model year. The engines built from 1989 through April 1992 have 6-bolt motors. Beginning in May 1992, Mitsubishi revised the engine to a 7-bolt design. The bolts refer to the number of bolts on the flywheel that connects to the crankshaft.