We have come a long way with the American pushrod V8 over the past several decades. Improvements in strength, air flow, horsepower, and torque have also been accompanied by improvements in efficiency, emissions, and durability using modern technology. The result has been GM’s LS and LT engine platforms providing ample power and grunt and still meeting strict EPA efficiency and emission standards. All of this while “stubbornly” sticking with the two-valve, pushrod layout. One piece of modern technology that has made this possible is the variable valve timing system.

With the timing cover off, you can take a look at the stock L8T cam phaser. The large bolt in the center also houses a spool valve that directs oil into different chambers to advance or retard the camshaft.

GM introduced variable valve timing (normally abbreviated to simply ‘VVT’) to its pushrod V8 lineup in 2007. When the Gen-V LT platform was introduced in 2014, it became a standard feature on all LTs including the L8T we have been dyno testing with Brian Tooley Racing. The system is often misunderstood by performance enthusiasts and removed or deleted when doing performance upgrades like a camshaft swap. But can this piece of modern engine tech be harnessed instead of thrown away?

What Is VVT And How Does It Work?

Variable valve timing uses the engine’s oil pressure to advance or retard camshaft timing. The camshaft bolt acts as an actuator valve for a camshaft phaser in place of the usual upper camshaft sprocket. A solenoid on the timing cover pushes on the tip of the camshaft bolt to move the internal spool valve in or out, directing pressurized oil into different chambers of the camshaft phaser.

The three-bolt solenoid on the timing cover actuates the valve in the cam bolt by pushing it in or out. To the right is the camshaft position sensor and the connector on the left goes inside to the oil control solenoid for the L8T’s variable displacement oil pump.

When the spool valve is pressed all the way, one side of the chamber is filled with oil to completely retard the camshaft. When the solenoid is not pressing on the spool valve, the other side of the chamber is filled to completely advance the camshaft. It can also position the spool valve in the camshaft bolt to any position in between, to fine-tune camshaft timing as needed.

This is the large cam bolt with the spool valve inside. Positioning the dark blue part of the valve dictates which set of holes the oil will go.

What determines if the camshaft timing needs to be advanced or retarded? The engine’s computer. It is programmed to make these adjustments continuously as the engines operates based on engine load and RPM. It will retard the camshaft for upper-RPM power while at wide-open-throttle, advance the camshaft for low-RPM torque, and move the cam to different positions in between as needed for part-throttle cruising gas mileage and reduced emissions.

This video below shows a quick animation of the VVT system’s function. While it isn’t an LS or LT engine being used to demonstrate variable valve timing operating, it still gives you a general idea.

Since its introduction almost twenty years ago, variable valve timing has been incorrectly and unfortunately lumped together by LS and LT enthusiasts with the highly unpopular displacement on demand (DOD/AFM/DFM) system and its failure-prone lifters. While these two systems both involve your camshaft, VVT is a completely separate system and does not suffer from the same durability and longevity problems as DOD.

This is a more common site for most, an engine without VVT. In this case, an LS. In max-effort applications, it can be necessary to limit the phaser’s adjustment range, lock out VVT completely, or remove the components altogether.

How Can This Be Used For Performance?

If the factory VVT system only changes cam timing and does not change the duration, lift, or lobe separation angle, how can it be used in a performance application? In a stock application, the factory programmed the engine’s computer to utilize this system mostly to increase efficiency and reduce emissions, but with the combination of an aftermarket performance camshaft and computer tuning, we can reconfigure this system to get the best of both worlds.

When LSX Magazine and Chevrolet Performance reached out to us for the L8T testing, we were excited to be involved. We knew the L8T could withstand big horsepower, and we looked forward to finding its limits. We were also interested in seeing how it would respond naturally aspirated to camshaft changes. — Brian Tooley

Remember, in general terms, when we go larger on camshaft selection, we tend to trade some lower-RPM performance for bigger gain in the mid- to upper-RPM range. But, if we choose to keep VVT instead of deleting it altogether, we can tune the VVT system to retard the cam in the upper-RPM range to maximize horsepower and advance the camshaft in the lower-RPM range to regain some torque we would have otherwise lost.

What Is The Purpose Of A VVT Limiter?

In stock form, even though the camshaft has modest duration and lift specs, provisions have been made on the top of the pistons to avoid piston-to-valve clearance issues. With the VVT system’s ability to advance the cam as much as 55 degrees, the exhaust valve can get very close to the piston. When upgrading to a larger aftermarket performance camshaft, with different lift, duration, and timing events, allowing the VVT system to advance the camshaft that far will certainly cause expensive damage to your engine.

To get to the inner chamber of the cam phaser, you will need to remove these five bolts. Be careful! You will have to lock the large clock spring in place and make note of which bolt goes where, as they are different from each other.

On all of the BTR camshafts dyno tested with our L8T, a BTR VVT 4-degree limiter was installed to keep the system in check. That still allowed for a few degrees of advance and retard to maximize performance from idle to redline, but kept the valves from giving the 6.6 pistons an unwanted kiss. The BTR VVT limiter is a simple and inexpensive metal block machined to fit perfectly in one of the chambers in the cam phaser. BTR provides VVT tuning information for their camshafts to make things easy.

Here you can see what the chambers look like inside the cam phaser. With the BTR limiter in place, there is only a few degrees of movement versus the original 62-degree total range.

There does come a time when the VVT system cannot handle camshafts with aggressive lobe profiles and high valve spring pressures. At that point, a lock-out can be installed in the cam phaser to disable the system. This option is reserved for racing applications where maximum peak horsepower is paramount and lower-RPM power losses aren’t a concern.

During our L8T intake and camshaft shootout a few weeks ago, as well as the upcoming L8T CNC-ported cylinder head dyno testing, BTR did not have to disable VVT, and the results speak for themselves. So the next time you pick a camshaft for your VVT-equipped LS or LT street engine, consider keeping variable valve timing and use it to your benefit!

A closer look at the installed BTR VVT limiter (the brighter silver metal block at the top).