Mitsubishi 6G72 DOHC Timing Events
by Jeff Lucius
Cylinder numbering and arrangement
The 6G72 is a DOHC 24-valve 60º V6 gasoline engine that displaces 2.972 liters. The cylinders are arranged and numbered as shown below.
Ignition Timing
The firing order is 1-2-3-4-5-6, with each numerically-successive cylinder reaching top dead center (TDC) 120 crankshaft degrees after the previous one. Paired cylinders have pistons at the same relative position, such as both at TDC, but they are 360º out of phase, such as one is in the exhaust stroke and the other is in the compression stroke. Paired cylinders are 1-4, 2-5, and 3-6. Spark plugs are fired simultaneously in paired cylinders, during the compression stroke for one cylinder and during the exhaust stroke for the other cylinder. The spark during the exhaust stroke is wasted because the basically inert exhaust gas cannot combust.
Sequential Fuel Injection Timing
It is not clear from the available references (listed below) exactly when the fuel injection activation signal begins. The pattern Mitsubishi uses in other engines is to use the either the begining or the end of the crank angle sensor (CAS) signal of one cylinder to activate the injector of another cylinder. The CAS signal is 50º "long" and starts at 5º before top dead center (BTDC) and ends at 45º after top dead center (ATDC) at the start of the combustion stroke. For the 6G72 this means that the start of the CAS signal for cylinder No. 1 probably activates the injector for cylinder No. 5 during the exhaust stroke (see the figure below); and cylinder No. 2 activates cylinder No. 6, and so on. On the other hand, it is possible that the end of the CAS signal for cylinder No. 1 activates the injector for cylinder No. 5 later in the exhaust stroke or even for cylinder No. 6 very late in the combustion stroke. It is unlikely that the cylinder No. 1 CAS would activate cylinders Nos. 1, 2, 3, or 4 because activation would start too late during the intake or compression stroke. If the three possible options are considered, then injector activation starts sometime between 18º before bottom dead center (BBDC) in the combustion stroke to 78º BTDC in the exhaust stroke.
Valve Timing
The intake valves on the non-turbo and turbo engines open at 16º BTDC and close 55º ABDC (after bottom dead center), which is a duration of 251º. The exhaust valves for the turbo engine start to open at 50º BBDC and are closed at 17º ATDC, a duration of 247º. For the non-turbo engine, the duration is 243º with the exhaust valves opening 48º BBDC and closing at 15ºATDC. Overlap for turbo engines is 33º and it is 31º for non-turbo engines.
Summary
The figure below summarizes all timing events (valves, ignition, and injection) for one cylinder.
References
Chrysler Corporation, 1988, 1990 Laser Technical Information Manual: Part number 81-699-9002, various pagination.
Chrysler Corporation, 1990, 1991 Stealth Technical Information Manual: Part number 81-699-0114, various pagination. Available on-line at 2-stim.htm.
Mitsubishi Motors Corporation, 1999, 1992 - 1996 3000GT Service Manual, Vol. 1, Body and Chassis: Pub No. MSSP-001B-96 (1/2), various pagination.
Mitsubishi Motors Corporation, 1998, 1999 3000GT Service Manual, Vol. 1 & 2: Pub No. MSSP-001B-99 (1/2), various pagination.
Page last updated September 27, 2005.