6L “LQ9” Engine Overview
LQ9 is the RPO (regular production option) code for the 6.0L V8 engine GM uses in several high performance truck applications. This engine is very similar in design to the current generation of medium-displacement V8s used in GM’s cars and light trucks. The engine shares many features with the LS1 engine used in the Camaro/Firebird/Corvette car lineup, as well as the LS2 engine currently used in the C6 corvette, GTO, SSR, etc.
Here’s a brief description of the LQ9 engine from the bottom up: The oil pan is a cast aluminum piece that is part of the engine’s structure and bolts to the transmission bell housing. It is sealed to the block using limited compression gaskets, as are the other aluminum covers such as the timing chain cover. The engine is equipped with 6 bolt main bearing caps that fit tightly between the deep skirts of the iron block. The crankshaft is rumored to be forged steel, and the hypereutectic pistons with graphite coated skirts are attached to the crank by means of a powdered metal connecting rod. The rod is manufactured in one piece, and in order to split the big end of the rod to allow installation on the crank, a small cut is made on each side of the big end of the rod. The rod is then “cracked,” and the big end splits in two where the small cuts have created stress risers. This rough split ensures that the rod caps are always perfectly aligned on installation. The small end of the rod employs a floating wrist pin. This combination brings overall compression to 10.1:1. Since this engine has no distributor, the gerotor oil pump is mounted to the front of the engine, behind the timing chain cover, and is driven directly from the crankshaft. The LQ9’s cylinder heads are aluminum pieces that are attached to the block with torque-to-yield bolts and sealed with multi-layer-steel gaskets. Above the heads is a composite intake manifold that contains 8 fuel injectors and a large single-blade throttle body. An air gap exists between the manifold and the engine block, allowing for a cooler intake air charge. Exhaust gases are routed away from the engine using cast manifolds with large heat shields. These manifolds flow very well and I decided to use them in my truck instead of a set of shorty headers.
The valvetrain is the only part of the engine that I have converted to aftermarket parts, with the exception of ARP connecting rod bolts. The camshaft is a Comp 212/218 .558/.563 115 LSA piece that gives a good idle and road manners, along with a significant increase in performance. The stock pushrods have been replaced with Comp hardened rods, and the stock “beehive” valve springs were changed out in favor of Comp 918 springs that will stand up to the high lift of the performance camshaft. The factory roller rocker arms and hydraulic roller lifters were left in place. The ignition system of the LQ9 is comprised of eight individual coils mounted atop the valve covers. Short plug wires provide spark to the iridium tipped spark plugs.
Finally, the LQ9 would not be complete without its sophisticated computer control system. The PCM (powertrain control module) for the LQ9 is located in the engine compartment and controls almost all aspects of the engine’s operation. As with most modern automotive control systems, this one can run a plethora of diagnostic tests, from checking for fuel tank leaks to misfire counting. The computer incorporates several other innovative features. One of which is the ability to change the spark advance curves of the engine based on the octane of fuel being used and the maintenance condition of the engine. The most important feature of the PCM, however, is that its calibrations can be changed with a laptop computer by attaching a special cable to the under-dash diagnostic port. More on this in the computer tuning section.
The end result of this advanced engine is a mill that produces over 400 HP while still maintaining excellent manners and decent gas mileage. Reliability is great so far in my application, and I have no reason to anticipate any problems with this engine.