Decades ago (OK, half a century or more), there was a product advertised in many of the performance magazines called the Varicam. The idea was a set of springs attached to a centrifugal device that would retard the cam with rpm. The idea was innovative and right on target, but the execution left much to be desired, and eventually it disappeared.
Now let’s fast forward to the 21st century. We’re not piloting flying cars, but we do have VVT (variable valve timing). Starting with the Gen IV LS engines, GM engineers figured out a way to direct engine oil pressure to swing the camshaft through an enormous range of movement. The early Gen IV engines could move the cam over a maximum potential swing of 62 crankshaft degrees. This is an idea whose time has come, but like most innovations, it has been slow to be accepted by the performance industry.
We thought it might be a good idea to do a quick drive-by, highlighting the advantages of VVT in the Gen IV and V engines. What we will reveal is how well it works and why it’s best to take advantage of the power opportunities offered by a VVT-equipped engine rather than just perform a VVT-delete before you know what you’re dumping.
Before we get into exactly how VVT works, we thought we should explain why VVT is such a good idea for a number of excellent reasons. We sought out Jason Haines, who until recently was a development engineer at Lingenfelter Performance Engineering (LPE) and has now started his own consulting firm—Product and Service Solutions. Jason has a wealth of experience with GM’s VVT and offered some interesting points.
Variable valve timing offers some wonderful opportunities for the engine designer and builder. With a single camshaft/pushrod engine, moving the camshaft simultaneously alters all four of the valve events: intake opening (IO), intake closing (IC), exhaust opening (EO), and exhaust closing (EC). While all four are essential, the most important valve event of the four is IC. This determines cylinder filling throughout the engine’s rpm band.
If all we had to do was make power within a very narrow 1,200-rpm band—as with a five-speed manual trans drag race combination or a cabin cruiser boat engine that operates 98 percent of the time at a set cruising rpm—then moving the camshaft around is of limited benefit. But street engines are expected to deliver excellent power and throttle manners between idle and 7,000 rpm. This is where VVT really shines.
VVT is all about advancing or retarding the camshaft. To make this simple, we can discuss this using the intake centerline as our reference point. Let’s use a mild street hydraulic roller cam as an example. We will discuss this in relation to the cam’s intake closing point as that’s what affects both power and, to a certain extent, street manners.
Advancing or retarding the camshaft means that we will be moving the intake closing point in relation to the piston. Advancing the centerline will shift all the valve events to open and close earlier. Specifically, advancing the cam four degrees means the intake closing point will now occur four degrees earlier than its original position. If we retarded the cam the same four degrees from “straight up,” this would place intake closing four degrees after its original position.
We’ve created a small chart that lists the effects of advancing and retarding the camshaft that is worth studying. In a general sense, advancing the cam tends to improve low-speed torque while retarding the cam will improve high-speed power. For street engines with a fixed timing position, the engine builder is forced to choose a compromise position mostly based on how the engine would be driven. If the engine was predominantly street driven, this generally meant the cam would be advanced.
To this point, most street cams are actually machined with a minor amount of advance built into the cam. You can quickly tell just by comparing the lobe separation angle (LSA) to the cam’s intake centerline as listed on the cam timing card. If the intake centerline is 108 but the cam has an LSA of 110 degrees, you instantly know the cam has been machined with two degrees of built-in advance.
Not only does moving the camshaft affect where the engine makes power, but it also affects other related issues, such as piston-to-valve clearance. With increased lift and duration, this moves the valves closer to the piston at certain points. This is why several companies build VVT limiters, which reduce the VVT’s range of operation to prevent the valves from smacking the pistons while still allowing a relatively wide range of advance and retard.
We recently tested a Gen V naturally aspirated LT1 on the dyno at Westech Performance in Mira Loma, California. This was our first foray into both gasoline direct injection (DI) and VVT. The advantages of these two developments quickly became apparent with the first dyno pull. We started the test with a set of headers and an electric water pump as the only upgrades.
After some very minor tuning to the wide-open throttle (WOT) air/fuel ratio (from the factory these engines run very rich at WOT) and adding a couple of degrees of timing, this engine made spectacular power. This was a 6.2L LT1 with 11.5:1 static compression and direct injection and the VVT. If you look at the power curve graph, notice that this engine makes 480 lb-ft of torque at 3,000 rpm and then carries that power all the way out to 518 horsepower at 5,800 rpm.
To put this achievement into perspective, a base 454 H.O. Rat motor that is 78 cubic inches larger makes about the same torque at 3,000, but only offers 438 peak hp. Part of the reason for this greater power is certainly due to the LT1’s higher static compression and the direct injection, but an equally large part can be attributed to VVT.
Here’s how GM does it. All VVT control is actuated using hydraulic oil pressure. This starts with the oil pump, which is different than previous LS pumps. This pump is a sliding vane design. If you’ve ever seen inside an automatic transmission front pump, they are of a similar design. The pump must deliver greater volume as well as higher pressure in order to supply the demand requirements of the VVT system. The Gen IV engines used a higher-volume and pressure pump, but it still remained a gerotor design.
Oil pressure is applied to a valve in the front of the camshaft that also doubles as the cam gear retention bolt. Oil is supplied through a galley in the front portion of the camshaft supplied by oil from a groove in the second cam journal from the front. All cam movement is controlled with an electronic solenoid mounted on the cam timing cover that sends commands to the cam gear bolt/valve. The valve is machined with multiple holes that direct oil pressure to either advance or retard the camshaft.
Inside the cam timing gear is a series of slots that allows movement between the actual chain-driven portion of the gear and where it connects to the camshaft. In the early engines, this allowed as much as 31 degrees of movement. In both versions of VVT, this is far more potential movement than what is actually used. Haines says that most LS VVT engines limit the range of movement to between 20 and 25 crankshaft degrees of movement. Even then, this greater movement is limited to light throttle applications where the cam is retarded to reduce manifold vacuum. By minimizing intake manifold vacuum, less negative work is done, which offers a slight benefit to fuel mileage.
The “parked” or standard position for VVT engines is full advance. So when viewing the VVT table on a HP Tuners or EFILive edit screen, you will see a chart with mostly double–digit numbers. These numbers represent the amount of retard tuned into the system.
This amount of authority also presents a problem. The factory system is carefully designed in conjunction with the stock cam to prevent piston-to-valve (P-V) contact. This isn’t difficult with the short duration and conservative lift numbers of the factory camshafts. But in a performance situation, we’ve all seen that even slight increases in cam duration can have a very positive effect on peak power.
When a bigger cam with more lift is added to a stock VVT engine, many think that this requires that the VVT system be deleted, which isn’t true. There are kits available from companies like Brian Tooley Racing and others that offer this feature. But there are also limiter kits available that will allow the addition of a slightly longer duration camshaft while still retaining VVT function, although with less overall movement.
For example, LPE offers a GT32 cam for naturally aspirated LT1 and L86 engines with specs of 225/241-degrees of duration at 0.050 with 0.643/0.657-inches of valve lift on the intake and exhaust, respectively, and can still be used with the factory VVT. This will, however, require a cam limiter system, which is also available from LPE.
This is where taking advantage of VVT for a performance engine comes into play. Adding duration to increase power tends to soften the amount of torque the engine makes at lower engine speeds. But with VVT, it’s possible to advance the cam slightly at lower engine speed, which improves the low-speed torque, throttle response, and even fuel mileage. Then at higher engine speeds, the cam can be retarded several degrees to offer horsepower advantages. These power advantages can easily exceed 10-15 lb-ft of torque at low speeds and 10-15 horsepower at peak engine speeds. Why leave that power on the table?
This really is the best of both worlds where a savvy cam selection for a street engine can offer power improvements across the entire rpm band from idle to peak horsepower. This will become the smart money move for future performance engines, not just in factory cars but also in engine swaps, especially as the Gen V engines like the LT1 and L86 become available at a lower cost. VVT is an idea whose time has come and the smart builders are already taking advantage of the technology.
|Gen V LT1 w/ VVT|
|The VVT modification in the Gen IV V-8 engine began in 2007 and continues to the present with the Gen V. We’ve only listed the 6.0L and 6.2L engines in this list but there are many more VVT engines in the 4.8L and 5.3L displacements.|
|Gen IV 6.0L|
|* GM crate engine PN 12677741|
|Gen IV 6.2L|
|* GM Crate engine PN 19244102|
|Gen V 6.2L|
|*GM crate engine PN 19328728|
|**GM crate engine PN 19332621|
|Advance vs. Retard|
|Early intake closing creates more cylinder pressure at lower engine speeds|
|Earlier exhaust opening reduces pumping losses to improve fuel mileage|
|Builds more cylinder pressure at lower engine speeds for better torque|
|Decreases intake piston-to-valve (P-V) clearance|
|Increases exhaust P-V clearance|
|Later intake closing point delays maximum cylinder pressure to higher engine rpm|
|Delayed exhaust opening allows longer cylinder pressure push on the piston|
|Builds more high-rpm power|
|Increases intake P-V|
|Decreases exhaust P-V|
|Lobe Separation Angle (LSA) does not change with advance or retard|
Photos: Jeff Smith, General Motors, GM Media Archive