Astron II in a first generation Magna
|Manufacturer||Mitsubishi Motors Australia Limited|
|Also called||4G54, 4G54B, G54B|
|Production||1985 - 1996|
|Cylinder block alloy||Cast Iron|
|Cylinder head alloy||Aluminum Alloy|
The Astron II was an improved version of the original Astron and debuted in the mid 1980s. It was available only in 2.6 Litre form.
The Astron II's main difference over its predecessor were in the cylinder head with hemispherical style combustion chambers, larger exhaust and inlet valves and hydraulic lash adjusters. The pistons were flat topped with compression rising to 8.8:1 (up from 8.2:1) to produce 83 kW at 5200 rpm and 200 Nm at 2400 rpm (up from 76kW at 4800rpm and 192 Nm at 2400 rpm) in carburetted form.
The 2.6 Litre engine was praised for being powerful, economical and refined.
Original Astron II
When used in the Magna, these engines typically had the numbers M1 through M5 cast into the head and V5xx engine numbers.
Revised Astron II
A revised Astron II was fitted to fuel-injected TR and TS models. The revised Astron II used roller rockers, a higher compression ratio (9.2:1 up from 8.8:1), larger inlet ports (42mm up from 40mm in the original), a knock sensor and a self-adjusting balance shaft chain adjusting mechanism as fitted to late model original Astron IIs.
These changes increased power to 98 kW at 4750 rpm (103 kW when running on premium unleaded) and torque rose to 212 Nm at 3750 rpm.
Revised Astrons can be identified by their knock sensor situated under the fuel injection plant (original Astron II's do not even have a hole for it), its engine number (all are prefixed by W531 or W532) or the casting number on the head (M6 through M9). The M6 and M7 were used in the TR Magna.
With the release of the TS Magna, the Astron II was modified once again with a switch to the M8 and M9 head which used swirl-type "kidney shaped" combustion chambers with dished top pistons (to match the combustion chambers). These promoted better mixing of air and fuel, less detonation (knocking) and a better flame front.
The M6 through M7 head is said to be the best Astron II head - using the largest valves, ports and having good flow characteristics. The M9 head while similar has less desirable flow through the intake ports due to the shape of the combustion chamber.