Variable Valve Timing (VVT)

Variable Valve Timing (VVT) is a technology used on GM Gen V LT engines that allows the engine control module (ECM) to change camshaft timing while the engine is running. This is accomplished through a cam phaser mounted on the front of the camshaft, which can advance or retard cam timing based on engine operating conditions. Like Gen IV LS, Gen V LT VVT is hydraulically controlled using engine oil pressure and is electronically commanded by the ECM. The system continuously adjusts camshaft position based on factors such as engine speed, load, throttle position, and temperature. The amount of available camshaft movement varies by application and cam phaser design, but most Gen V LT engines use a phaser capable of approximately 60 degrees of crankshaft timing authority. This flexibility allows a single camshaft profile to perform well across a wide range of operating conditions.

Benefits of VVT on GM Gen V LT engines include:

• Increased low-end torque
• Improved peak horsepower
• Better fuel economy
• Reduced emissions
• Improved drivability and throttle response

There are early and late variants of VVT cam phasers for Gen V LT generally following 2014-2019 RPOs L83 L86 LT1 LT4 for "early" and 2019+ RPO L82 L84 L87 L8T and LT2 for "late". The later version uses a larger diameter Phaser Solenoid (i.e. cam bolt) and a revised phaser assembly.

For a detailed discussion of Gen V LT VVT Camshafts, Phasers, and Phaser Solenoids, see 15:31 to 17:11 of the full LT Swap Masterclass video on YouTube:

Displacement On Demand (DOD)

Displacement On Demand (DOD) is GM's cylinder deactivation system used on Gen V LT engines. Under light load such as highway cruising, the engine control module temporarily shuts down cylinders for improved fuel economy. The system works through special collapsible hydraulic lifters using solenoids mounted under the valley cover.

There are early and late variants of DOD for Gen V LT known as AFM and DFM, respectively. AFM stands for Active Fuel Management and deactivates cylinders 1, 4, 6, and 7 together to switch from V8 to V4 mode. AFM is found on 2014-2019 RPOs L83 L86 LT1 LT4 and is controlled with solenoids mounted on the underside of the valley cover that divert oil by way of towers cast into lifter valley of the engine block. DFM stands for Dynamic Fuel Management and can deactivate any/all 8 cylinders utilizing 17 different firing orders. DFM is found on 2019+ RPO L82 L84 L87 and is controlled with solenoids attached directly to the engine block in the lifter valley. RPO L8T 6.6L engine blocks are cast with AFM style towers in the engine block but are not machined for use since the L8T is a non-DOD engine. The 6.2L LT2 has DFM style solenoids in the lifter valley but control only four (4) cylinders so it is considered AFM by GM. The LT2 AFM strategy deactivates the opposite cylinders as early Gen V engine, thus 2, 3, 5, and 8 are switched off and on. As a result of the different physical configurations of DOD control solenoids, each of the sub-generations of Gen V LT engines have unique valley covers.

For a full walk around and detailed discussion for all four LT engine blocks including their AFM/DFM features and valley covers, see 0:24 to 10:22 of the full LT Swap Masterclass video on YouTube:

Deleting VVT and DOD

It is possible to "delete" VVT and DOD in LT engines, both of which will require ECM tuning for expected performance and results. Deleting DOD entails removing cylinder heads and replacing the collapsible DOD style lifters with conventional lifters, removing DOD solenoids (usually replacing AFM valley cover or plugging DFM solenoid provisions directly), and plugging DOD oil passages in the engine block. Deleting VVT is accomplished through "locking out" the OEM cam phaser or by swapping timing sets to use a fixed cam gear. It is STRONGLY RECOMMENDED to use a package of parts from your preferred camshaft manufacturer when modifying VVT since the cam sensor location in the front timing cover must coincide with the modified or replaced cam gear inside the engine. Reference this write-up posted by a representative from Scoggin Dickey regarding cam sensor location and Gen V cam gear geometry: FB Link

What This Means

Most importantly, swappers must understand which early or late sub-generation of LT engine they have so that proper replacement parts can be purchased. DOD lifters are considered unreliable by most enthusiasts, so deleting DOD is usually among the first mods done. Similarly, tuning with variable valve timing adds significant complexity, so most builders limit or lock-out VVT during their builds.

CLICK HERE to contact us if you have any questions!