The Camshaft
If motor oil is commonly referred to as the life blood of an engine, the camshaft can only be described as the soul. No other single component has such a large effect on the total characteristics of engine package. A camshaft can be so mild that the engine runs as smooth as silk at a very low RPM, or it can be so radical that the engine will barely even run at idle, and there is any range of choices in between. The camshaft is so important because it directly controls valve timing, one of the biggest areas of research for automotive manufacturers. Many manufacturers spend huge amounts of money to create variable valve timing systems, which essentially allows the timing to change with RPM. These systems allow an engine to be more efficient over a larger power range, unfortunately the small block and the LT1 don't have any of these systems.
In terms of the camshaft the LT1 engine is identical to it's small block predecessor, any hydraulic roller camshaft that will work in a normal small block will work in an LT1, at least it will physically bolt in. The drive mechanism for the distributor on a late model LT1 requires special consideration but it shouldn't be a serious issue. Being so closely related to it's small block cousin, the LT1 engine uses a mere two valves per cylinder and a single camshaft located in the engine block. This single camshaft design means that valve timing is completely fixed, once a camshaft is installed the valve timing is set and cannot be changed without swapping the cam out for a different one. This means that camshaft selection is of critical importance and is a game of compromises. A camshaft that makes very good power at a low RPM will typically not make very good power at a higher RPM and vice versa. Any one camshaft is only technically perfect at one instant in the RPM range, at any other point it is a tradeoff. When selecting a camshaft one must decide what performance characteristics are important and choose a camshaft accordingly. A bad camshaft selection can make all other performance parts worthless.
To understand camshaft selection, some terminology must be discussed. Camshaft specifications are typically given in terms of duration, lift, and lobe separation angle, in that order. The camshaft in my car is the LT4 Hot Cam, it has a specification of 218/228 .525/.525 112. This means that it has an intake duration of 218 degrees, an exhaust duration of 228 degrees, intake and exhaust lift are both .525 inches, and the cam has a lobe separation angle of 112 degrees. Even with the numbers spelled out they don't really mean much until you start to understand how each value affects performance.
The first set of numbers in a camshaft specification are the duration values, which are always listed intake first followed by exhaust. These numbers refer to the number of crankshaft degrees that the valves are open. Since a cam lobe profile can be very complicated it can be very hard to tell the exact open to close duration, thus the standard is to measure duration a predetermined lift value. The standard lift value is .050" or fifty thousandths. One might hear this duration referred to as the "duration at fifty." This simply means that this is the number of crankshaft degrees that it takes for the lifter to travel from .050" lift on the opening ramp, across the peak and back to .050" lift on the closing ramp. Duration plays a major roll in engine performance because it essentially controls how long the valve is open. While an engine is running at a very low RPM a short duration is desireable. At a low RPM the air charge does not have a particularly high velocity and relies only on the vacuum of the piston to pull the air into the engine. As such the valve does not need to open until after the piston has already started to travel down the bore, and it needs to close right at or before the piston begins it's upward travel. At low RPM if the valve were still open when the piston started to travel back up, the air charge could actually be pushed out of the cylinder and back into the intake.
At high RPM's the duration numbers need to be larger because as the piston moves faster there is less time for the cylinder to fill with air. By opening the valve slightly sooner and holding it there longer the air charge can begin to fill the cylinder more quickly. Furthermore at a higher RPM the air charge can have a substantial velocity, so much so that air can actually continue to rush into the cylinder even after the piston has begun to travel back up the bore. This allows maximum airflow into the cylinder and thus more power at a higher RPM. For the reasons listed above though this optimal setup for high RPM doesn't work so well at low RPM. Thus a tradeoff must be made in order to allow acceptable engine operation for the entire RPM range. Exhaust duration works in exactly the same way. On modern camshafts exhaust durations numbers are often larger than intake durations. This is used to compensate for a typically less efficient exhaust system than intake system.
The next value on the camshaft specification is the maximum lift value, also listed intake first, exhaust second. This number is exactly what it implies, the maximum valve lift encountered during the listed duration. Lift is almost always listed for a given rocker ratio. The Hot Cam for example is rated at .525 valve lift with 1.6:1 rockers. This means that the lobe of the camshaft only has a peak lift of .328 inches. I'll cover rocker arm ratios later, for not it isn't of critical importance. Lift can be essentially equated to free horsepower, provided the rest of the car is set up to accept it. Recall from the cylinder head section and one can see that in most cases extra lift means extra airflow potential. A valve that is open for a long time at a very small lift value may very well flow less air than a valve that is open for a short time at a higher lift value. The only real drawback to lift is that it requires better valve springs and a steeper ramp rate on the camshaft. There is a practical limit on lift, but that limit is different for every duration. If a lift value is too high for a given duration the lifter will simply not be able to follow the cam profile. This isn't a serious problem though unless a custom camshaft is desired. Any existing cam profile offered by a manufacturer has been tested and will work.
The last common value in the cam specifications is the lobe separation angle. The lobe separation angle refers to the number of crankshaft degrees between the peak of the intake valve and the peak of the exhaust valve. In a performance camshaft there will almost always be some degree of overlap. Overlap refers to the amount of time that both the intake and exhaust valves are open. Too much overlap can allow unburned fuel to flow right out the exhaust valve where it does nothing for producing power. One might wonder why overlap would be desired at all, which is a perfectly fair question. The question comes back down to RPM. At a low RPM very little if any overlap is desired, the piston is moving so slowly that the intake air charge has time to exit the exhaust valve and do no good at all, this can also increase the amount of remaining exhaust in the cylinder and the engine will run very rough. The common "loping" sounds of a car with a radical camshaft is caused by the inefficiency of the camshaft at that speed. At a higher RPM though overlap can actually help produce power. At high RPM's the exhaust tends to leave the cylinder at a very high velocity, this can actually create a negative pressure in the cylinder which can help to pull in air for the next combustion cycle. By increasing overlap at a higher RPM more power can be made in that range. Overlap is also dependent on duration, for a given lobe separation angle a camshaft with a larger duration will have a larger overlap.
Now that camshaft specifications are understood, a discussion can be made about camshaft selection. The single most important consideration when selecting a camshaft is the desired application. Is the car going to have stock cylinder heads or ported? Is the car going to have stock exhaust or modified? Does the engine need to have street manners or is it a race only application? After all of these questions are answered some general guidelines need to be followed.
Lift - Regardless of application run as much lift as possible. Maximum lift will really be determined by three factors, cylinder head flow, spring capability, camshaft duration. Cylinder head flow numbers are more or less fixed so they are the best place to start. After examining the cylinder head flow numbers determine a maximum lift value, IE try not to exceed the peak flow value too much, this is rarely a problem on a street engine. Determine the desired duration and choose a cam lobe that gives maximum lift for this duration without exceeding them maximum lift value. Lastly choose valve springs that will allow the desired lift, be sure to maintain an absolute minimum of .050 inches between peak lift and coil bind height. Adjustments to total lift can be made by adjusting the rocker arm ratio, consult the rocker arm ratio section for more details.
Duration - Intake and exhaust duration should typically be scaled together, on most commonly the exhaust duration is kept slightly larger than the intake duration, but it doesn't have to be. Only in very rare cases should the exhaust duration be less than the intake, should a cylinder head flow extremely well on the exhaust side compared to the intake, IE the exhaust is overly efficient. This is a situation that rarely happens on the LT1 cylinder head. For a normal displacement LT1 engine with stock rotating components the practical RPM limit is around 6500 RPM's, so a camshaft should be chosen accordingly. For a street driven engine I would highly recommend keeping the intake duration at or below 230 degrees. Above 230 degrees sacrifices in low end torque become so bad that the engine becomes very temperamental on the street, fuel economy will also suffer greatly because a higher RPM must be maintained for cruising speeds.
Lobe Separation - Lobe separation depends greatly on duration. As a general rule I would advise against any separation smaller than 112 degrees for a street engine. Below 112 degrees overlap is so large that low end torque and idle quality begin to suffer a great deal and it becomes harder to make the engine pass emissions testing. A lobe separation of 112 is acceptable for mild durations, and gives a noticeable idle but it is generally not excessive and should pass emissions fairly easily. For durations above about 225 I would suggest opening the lobe separation angle up to 114. This will offset some of the overlap caused by the larger duration and will smooth out the idle and make tuning for emissions easier. The increase in lobe separation will cost some upper end horsepower but it will increase mid range torque somewhat so the total power curve will be very acceptable.
