Camshaft basic specs and concepts
By Chris 96 WS6 ([email protected])
The camshaft is the single most important part in an overhead valve engine in terms of how much horsepower and torque an engine will make and at what RPM it will make it. There is a lot of confusion out there about camshafts and how to choose one. In this article I will explain the basics of cam function and the definitions of the various specifications, which should help readers when choosing a cam.
To keep this article short and to the point, I'm not going to get in to detail on lifter types and rocker arms, springs, etc. This can be left for a future article or for further reading on your own. Since most of us drive late model vehicles I will make the assumption that we're dealing with hydraulic roller cams that are typical in the LS1, LT1, and 87+ TPI engines.
Lift and duration are the two cam specs that get the most attention. Lift is simply how far the cam lobe will open the valve at its peak. More lift generally equals more flow, but too much lift is very hard on valve springs and lifters and if your heads have a definite flow peak then lifting the valve beyond that point usually does not give increased power. Lift is generally expressed as a decimal value of less than an inch, and a distinction must be made between lobe lift and valve lift. Lobe lift is the physical measurement of the lift of the actual cam lobe. Valve lift will always be higher because the rocker arm multiplies the lobe lift through the use of the fulcrum. Valve lift can be calculated by multiplying the lobe lift by the rocker arm ratio (e.g. .300 lobe lift x 1.6 rocker = .480 valve lift). Valve lift in excess of .600" is usually not worth any extra flow on a street Small Block Chevy head.
Duration can have a huge effect on how well the engine runs and where it likes to make power. Duration can be described as how long the valve is open, and is expressed in crankshaft degrees. (the camshaft turns ? of crank speed, but the valve opening is relevant to piston position so it is best to think of it in terms of crank degrees).
Duration specs are given in two measurements, Advertised (also called seat-to-seat) timing, and duration at .050" lift. Seat-to-seat duration is simply measuring the duration from the time the valve first comes off the valve seat in the head to the time it closes again. There is some variation to the way different cam companies measure this spec, so the industry came up with Duration at .050" lift. By measuring the duration from the time the valve opens .05" to the time it closes back down to .05" open, the specs can be standardized so you can better compare two cams. Most "old school" hot rodders still talk in terms of advertised duration, which is why you will hear them talk about 280/290 duration cams as compared to a .050" spec cam that might be described as 220/230 duration.
Intake and exhaust lobes can have different duration specs (split-duration) or both can have the same duration (single-pattern). Whether you will need a split-duration or single-pattern is usually determined by how well your heads flow, specifically the exhaust flow compared to the intake flow. It is also important to note that two cams with the same Advertised duration can have different .050" specs, and vice versa. This is accomplished by changing the ramp rate of the lobes.
In general, the greater duration a cam has, the better it will breath and therefore the more power it will make at higher RPM. Opening the valves earlier and holding them open longer gives the cylinder more time to fill with air/fuel mix at higher RPM. The faster you spin the engine, the less time the valves are open, so the added duration increases that time. The other positive effect is that a larger duration lobe can also hold the valve near peak lift longer, which enhances cylinder filling even more.
However, there are downsides to increased duration. Going too large in duration will result in poor idle quality and bad street manners. The reason for this is that the valves are now open too long at low RPM and the cylinder is bleeding off pressure it uses to build torque. A typical large duration cam is going to hold the intake valve open longer to ensure adequate filling, but at low RPM this is a detriment to making good power because the piston is on its way back up in the bore and therefore not able to build as much pressure prior to ignition.
The key to determining a good duration choice for your cam is to decide what the vehicle is going to be used for and then choose a cam that is suited to that use. Obviously a weekend warrior is going to need a compromise in terms of streetability and all out power, whereas a track-only car will allow the use of much larger duration figures.
Lobe Separation Angle
Lobe separation angle (sometimes referred to as Lobe Center Angle) is a topic of much confusion. Simply put, Lobe Separation Angle (LSA) is the distance, in crank degrees, of the centers (point of peak lift) of the intake and exhaust lobes. What this tells you is how far apart the two lobes are spread. Typical street LSAs for aftermarket cams are in the 112-114 range. The LSA is important for determining the amount of valve overlap a cam provides, which is the amount of time between the exhaust stroke and the intake stroke that both valves are open at the same time.
Overlap can be an aid to producing good top end power and torque, because the exhaust gases, hot as they are, are making their way out the exhaust port and through the headers at a high rate of speed, and their exit helps pull in the air/fuel mixture from the intake valve, aiding in cylinder filling (known as scavenging). At low speeds this causes the lope that everybody loves, but it is accompanied with lower intake manifold vacuum, poor gas mileage, and overall reduced streetability.
It is important to note that a 112 LSA on a 210/220 cam does not equal the same amount of overlap on a 220/230 cam with a 112 LSA. Even though the angle between the lobes is the same, the larger cam has fatter lobes, meaning that it will have more overlap even at the same LSA. To achieve equal overlap between the two, the bigger cam would need a wider LSA.
LSA is calculated by adding the centerlines of the intake and exhaust lobes, adding them together, and then dividing by two. For example, a cam with a 108 intake centerline and a 116 exhaust centerline would have an LSA of 112. The LSA is ground into the cam and cannot be changed without grinding a whole new cam, as it necessitates spreading the lobes or tightening the lobes.
The intake centerline (ICL) of a cam is the most overlooked spec. The ICL can have nearly as much of an effect on peak power and the RPM at which peak power occurs as duration. The intake centerline is the position, in crank degrees, that the intake lobe hits peak lift (the center of the lobe). Most small block Chevrolet cams seem to come ground on a 108 ICL, meaning the intake valve reaches peak lift at 108 degrees. ICL is inter-related with LSA, in that the LSA of a given cam is typically considered the "straight up" point for that cam. For example, a cam with a 114 LSA, if ground with an ICL of 114, would be considered "straight up". However, the same cam ground on a 110 ICL is considered to be 4 degrees advanced.
This is a good time to discuss retarding/advancing and the effects of both. Retarding or advancing a cam can be achieved in two ways. The most common is to install the cam with offset bushings in the cam gear to adjust the cam timing, but it can also done by grinding the advance/retard into the cam core itself. You might have a cam ground with a 112 LSA and a 108 ICL...that means that if installed straight up it would be 4 degrees advanced. You could chose to retard or advance the cam from that point using an adjustable timing set as well, but the changes would be + or - from the 108 degree starting point. A cam that has a 112 LSA and 112 ICL would be ground straight up, and to advance it four degrees (resulting in a 108 ICL "as installed") would require an adjustable timing set. For an LT1, any advance or retard must be ground into the cam, as using offset cam bushings would alter ignition timing since the Optispark is driven off the cam gear.
Advancing a cam means you are moving the valve events earlier in the cycle. If done when installing the cam you will be moving both the intake and exhaust events earlier in the cycle. This generally boosts low speed torque at the expense of peak HP. Retarding a cam does the opposite, it will improve peak power by delaying valve events, which will increase peak HP and move the peak HP rpm up, but at the expense of low-end power.
Grinding the advance/retard into the cam offers a couple of advantages. You can advance the intake valve while holding the exhaust valve where it is, which would increase overlap, or you could retard the intake, which would increase the LSA and reduce overlap, but it would also delay the intake valve closing which would improve top end power. Or you can move both lobes and split the difference. This is where the benefits of custom cams come in: you can tailor the cam to your specific needs.
The discussion of intake centerline and exhaust and intake events necessitates discussing the most basic of cam specs: the intake opening and closing events and the exhaust opening and closing events. With some math (or a program like Desktop Dyno) and only these four numbers you can calculate all the above specs.
It is best to begin with the exhaust valve, because in the four-stroke cycle the exhaust valve does it job before the intake valve. After combustion, the piston moves down on the power stroke (this is where the power comes to spin the crankshaft and thus move your car down the road). At the end of the power stroke, when the cylinder is approaching bottom dead center (BDC), the exhaust valve opens to allow the burned gases to exit the cylinder. This is the exhaust valve opening point (EVO). The exhaust valve stays open while the piston is on its way back up, pushing the exhaust gases out past the valve. To allow complete scavenging and removal of the exhaust gases, the exhaust valve is held open for a short time after the piston reaches top dead center (TDC)--this is true for even stock cams. So, the exhaust valve opening will occur before BDC (BBDC) and closing will occur after top dead center (ATDC).
The intake valve actually opens right before the exhaust valve closes. As discussed earlier this takes advantage, to a varying degree depending on the cam, of overlap and exhaust scavenging. Opening the intake valve before TDC (BTDC) allows the incoming air charge to begin filling the cylinder before the piston begins its travel downward. Even though the piston is not yet dropping, the velocity of the air stack in the intake manifold will ram air/fuel mix into the cylinder (this effect is greater at higher RPM). The intake valve stays open as the piston moves all the way down and will typically stay open for a short time after the piston has reached the bottom, because the piston is dwelling near the bottom and for a moment does not have any significant speed coming back up. Keeping the valve open after BDC (ABDC) gives the air stack a little bit more time to get the cylinder completely full. Once the valve closes and the piston is on its way up, the compression cycle begins and the spark plug fires right before the piston reaches top dead center, then the power stroke begins and we start all over again.
Effects of valve events on power
Exhaust opening -- Opening the exhaust early gives the cylinder a head start on evacuating the spent gasses, but open it too early and it will eat into the power stroke. Generally a higher RPM engine will appreciate the extra open time more. By contrast, waiting to open the exhaust valve later will help extract every last once of power out of the hot exhaust gasses as they push the piston down, but waiting too long will cost top end power because the valve will not be open long enough at high RPM to vent all the gases.
Exhaust Closing -- Closing the exhaust valve earlier will reduce valve overlap and improve low end torque but will have negative effects on high RPM power by not completely venting the cylinder of gasses. However, delaying exhaust closing will increase overlap, improving peak power and torque, but at the price of reduced streetability.
Intake Opening -- Related to exhaust closing point, opening the intake valve earlier increases overlap and also allows the cylinder to begin filling earlier, but open it too early and you can cause reversion, which is when exhaust gases are actually pushed out into the intake because the piston is still on its way up with the exhaust valve open. Reversion has very nasty effects on idle and low speed driveability, but luckily is not typically an issue with the range of durations seen for typical street/strip LT1/LS1/TPI (or 3800 V6 for that matter) engines. Delaying the intake opening will reduce the chances for reversion and limit the effects of excessive overlap, but will obviously eat into the time allowed to get air/fuel mix into the cylinder.
Intake Closing -- This is the most important of the four valve events. Intake closing will--to a large extent--determine at what RPM peak power occurs. By manipulating intake closing you can make small cams make good power. Closing the intake valve early helps trap the air/fuel mix and build good cylinder pressure, which is good for low speed torque. However, at high RPM the cylinder doesn't get as full. Delaying the intake closing makes sure every available bit of air/fuel gets in to the cylinder, which greatly benefits high end power, but at low piston speeds you can lose some of the mixture as intake manifold and cylinder pressure begins to equalize and the velocity of air into the cylinder slows.
Conclusions & Additional Considerations
This should be a good beginning overview of cam function. An important thing to come away with is that lift and duration are not all there is to a cam. Smaller cams can make good top end power by delaying intake closing and/or retarding the cam (Example is the ZZ3 cam. At 208/221 it is considered small, but because it is ground on a 112 intake centerline it will make similar power--with stock heads--to the 108 centerline LT4 Hotcam, but with much less duration and overlap, meaning better streetability). Last but not least, it is important to realize there is no single magic cam that works well for all intended uses, and because each engine is different, the best solution isn't always an off-the-shelf grind.
How to Build & Modify Chevrolet Small-Block V-8 Camshafts & Valvetrains (Motorbooks International Powerpro Series) by David Vizard (Paperback - September 1992)