WHAT CAMS DO
Most of us know that a cam opens and closes the valves to allow an air/fuel mixture into the cylinder ans exaust gases out after
combustion.The concept of a camshaft is an ancient device that converts rotary motion to linear motion.
We also know that the bigger and fatter the lobes of the cam, the higher and longer it will open the valves. Allowing more air/fuel mix into the engine and more exaust out.
The height of the lobe, or the distance from the valve seat that the valve opens, is called LIFT, and is given in thousandths of an inch.
The width of the lobe, determines the amount of time, relative to the crankshaft cycle, that the valve is kept open. This is know as DURATION and is given in degrees of crank rotation.
There's much more to making power in an engine than getting lots of fuel and air in for combustion and getting the burned gasses out. It is correct to say that you want as much air into the cylinder as you can on the intake stroke with the correct percentage of fuel. But, it is the pressure of the heated air in the closed cylindeer that pushes the piston. The amount of air pressure and the period for which it acts on the piston, known as Mean Effective Presure or MEP, determines the power of the engine.
The lift and duration of the intake valve, along with the efficiency of the rest of the intake system, will control the amount of air/fuel ratio that gets into the cylinder. All other things being equal, this would determine the volume of the charge in the cylinder, and thus peak cylinder pressure after combustion.
However, it is the amount of time that the two valves are closed, thus effectivly sealing the cylinder, that determines how long the pressure acts on the piston. The point at which the intake valves closes and the point at which the exaust valve opens are both critical to making powerin the engine. These are just two of the timing events that can be altered on a cam.
When the intake valve opens, it takes a little time to get the column of air in the port to start flowing through it. The exaust has a similar problem, but to a lesser degree because the cylinger is pressurized when the exaust valve opens. While the RPM of the engine increases, the lag time of the intake charge tends to remain the same. Once the charge is moving it's momentum will keep it will keep it flowing into the cylinder even after the end of the intake stroke as the piston begins to rise on the compression stroke, if the intake valve is kept open a little longer. Therefore as an engine runs faster and faster, the cam timing should occure later and later to keep pace with the air/fuel charge, which ends up moving slower and slower relative to the engine speed.
It must be decided which operating range that you want to optimize in your engine, and then select a cam that willoptimize airflow and cylinder pressure in that range, given your specific engine and vehicle combination. The problem being that once the cam is installed, it optimizes only one driving range and not others.
There are three basic ways to alter camshaft timing:
LOBE PROFILE, LOBE SEPARATION and theCAM INSTALLATION POSITION.
The first two are determined when the cam is manufactured and cannot be changed wothout re-grinding it, and then only to a certain degree..(no pun intended).
The latter is determined by the alignment of the timing gears on the crankshaft and cam, and itcan be altered.
If we look at each of these cam timing variables in order, what do we find?
The size and shape of the lobe profile dermines how high and how long the valve opens. Lobe lift is the actual hieght of the lobe above the base circle, or, the distance the cam will lift the lifter. Gross lift is the lobe lift multiplied by the rocker arm ratio, giving the theoretical Valve Lift, called also "lift at the valve".
For example; In a small block Chevy, the rocker ratio is 1.5:1. Therefore a cam with the lobe lift of.300 inch, would produce a gross lift of.450 inch at the valve. In flathead engines and overhead cam engines without rocker arms, the lobe must generate all of the valve lift. Net Lift, a term not often used in discussing street cams, is the true distance the valve actually lifts off it's seat. This is equal to gtross lift minus valve clearance, (on solid lifter cams), pushrod flex, and other losses. Obviously one will want as much valve lift as one can get. However, maximum lift is limited by available valve-to-piston clearance, valvespring capability, and camshaft journal size, (the lobes cant be bigger than the journals), or the cam won't go into the block.
Duration is the period during the cylinder cycle that the valve is open. It is expressed in degrees of crankshaft rotation. Remember that the cam rotates at one half crank speed. It has to, because we are talking four cycle engines here. Each valve opens once during two crank revolutions.
Beginning at TDC (Top Dead Center), before the power stroke (ignition) in one cylinder, both valves are closed. Then there is the power stroke and the exaust stroke, (one crank revolution), during which the exaust valve opens and nearly closes. At TDC between the exaust stroke and the intake stroke, (called "TDC Overlap"), the exaust valve is still closing and the intake valve is already beginning to open on most cams. That is, both valves are open at the same time breifly. This is called theOVERLAP. The next two cycles are the intake stroke, during which the intake valve is open, and the compression stroke, which completes the second crank revolution. Understanding this sequence is crucial t understanding cam timing.
Advertised Duration is the term traditionally used to describe how "big" a cam is. For example, a "272-degree cam."
This term should describe the actual duration that the valve is off the seat. BUT....cam lobes must be ground with very gradual opening and closing 'ramps' to keep the lifter from 'slamming' on the cam and to reduce shock in the valvetrain. Thus, it is very difficult to measure, with a degree wheel onthe crank and a dial guage on the lifter, the exact point in crank rotation where the lifter begins to move. Consequently, most grinders measure advertised duration at approximately .004-inch lifter rise. But this is not a standard, nor is it an easy point to measure. Advertised duration for cams is a smeaningful as "suggested retail price" for new cars.
A much more accurate measuring point, and the standard for the industry, is duration at .050-inch. It is measured in crank degrees from
the point where the lifter rises .050 above the base.
Even duration at .050 is an inaccurate comparison figure for cams. Two cams can have the same lift and duration at .050 specs, yet
have different profiles. One cam could have steeper flanks and a broader nose, (known as a "fast acting" profile), lifting the valve higher,
sooner and keeping it open higher longer. It is unfortunate that one cannot tell the difference from looking at the cam specs in a
catalog. One indicator is in the difference between advertised duration and duration at .050.
A cam with less difference between these two figures should be a "faster acting"cam. But thie depends largely on how, and where, the advertised duration was measured on the two cams. One way to compare cams is to measure actual lifter rise per every 20 or 40 degrees of crank rotation in the engine, and plot it as a curve on a graph. Plotting the entire lift sequence is the only way to really compare two different profiles.
A Dual Pattern camshaft is one on which the intake and exaust profiles differ, which is often the case. An Assymetric Profile cam has lobes that differ in opening and closing ramp profiles on the same lobes. This is rare, especially in street cams.
Obviously you want the broadest, fastest acting lobe profile you can get. The two primary limitations to lobe profile are lifter size and type, and spring pressure. With a flat tappet (solid or hyraulic), the cam rise is limited by the contact surface of the lifter. If the lobe rises too steeply, the lifter edge would "dig into" the lobe surface and destroy it. The broader the base of the lifter, the steeper the lobe it can remain in contact with. A roler lifter allows much more radical lifter rise because of it's physical design. This is why roller lifter were invented and why they are used in all-out racing engines. Savings in friction are minimal in comparison, (flat tappets rotate in their bores as they ride on the lobe to reduce friction and wear). Given the difference in cost and complexity, there's little reason to reto-fit to a roller cam for the street.
The more radical the lobe profile, the greater the valvespring pressure necessary to keep the lifter on the cam lobe. If the lobe shape changes too quickly, momentum will cause the lifter to fly off the lobe at high RPM. Valve float is a side effect. This condition also leads to rapid cam wear. Increased valvespring pressure however, increases drag, friction, cam wear and stress on valvetrain components. Thus cam lobe profile is physically limited. Don't use more spring pressure than necessary.
The Lobe Separation Angle, also knwn as the Lobe Centerline Displacement Angle -or more commonly but not necessarily correctly- the cams "lobe centers" .....this is the angle between the centerlines of the intake lobe and the centerline of the exaust lobe for one cylinder, as measured on the cam. In other words, it is the timing of the intake lobe relative to the exaust lobe as determined by the placement of the lobes on the camshaft. The standard lobe separation angle is 110 degrees, (cam degrees that is). When a cam is made, it can be ground with "narrower" (perhaps 108- 0r 106-degree) or "wider" (112- or 1124-degree) lobe separation. Once ground, it's set.
Lobe seaparation angle is a relitively new term, replacing the once more common, but misleading, "overlap" spec. Reducing the lobe
separation angle does increase overlap--the time that both valves are open. But it also increases the time that both valves are closed.
People find the lobe separation angle confusing for two reasons: First, many cam companies list the intake centerline (or both intake and exaust centerlines) in similar looking crank degrees, as a method for "degreeing in" a cam. More on this shortly,
Second, the way the lobe separation angle is shown, and measured, on a cam card (and here), is really upside down. If one already understands the lobe centers, don't let me confuse you, But if you're new to the concept, consider the cam turned over, so that the lobes point down., with the exaust on the left and the intake on the right. This is how the cam actually sits, in the block in relation the the lifters for a given cylinder, when the piston is at TDC at the beginning of the combustion cycle. Viewed from the front of thre engine, both the crank and the can rotate clockwise, in a typical V8.
Viewing the cam this way, you should readily see that narrowing the lobe separation angle will cause the exaust valve to open later
and the intake valve to close sooner. These points are critical valve events, because they determine the time the cylinder is sealed
(with both valves close), so that cylinder pressure can act on the piston. Ed Winfield, whom many consider to be the father of
"Hot Roddin", said about cams--"The intake closing point is the most important valve event of all--"
Perhaps so,but if you open the exaust valve too soon, a "blow down" effect occurs in the cylinder, allowing cylinder pressure to escape out the exaust port unused. Narrowing the lobe centers increases low end torque.
The problem is. that narrowing the lobe centers also increses the valve overlap. The longer the duration of the cam, th eworse the problem. If both valves remain open together too long at the transition between the exaust and intake strokes, exaust will tend to blow out the intake valve and vise versa. This make for a teriible idle quality in a street engine. The cylinder is sealed for such a short time, that at low RPM the engine will act as if it has a massive vacuum leak. I don't even want to get started on the pollution output in the form of unburnt hydrocarbons.
For that reason, most streetable camshafts are given lobe sep. angles of 110 to 114 degrees -- to reduce the roughh idle and to increase the vacuum signal for accessories such as power brakes. If one does not care so much about idle quality, narrow lobe centers such as 110 to 108 degrees will give you gobs of torque in the street driving range. It also depends on duration; the longer the duration of a steet cam, the wider the lobe centers must be to maintain any idle quality.
CAM INSTALLATION TIMING
The third basic variable in cam timing is how the cam is installe in the engine relative to the crank/piston position. If you align the marks on the cam and crank timing gears as you install the camshaft, the #1 cylinder should be at TDC, and the cam will be positioned near the middle of the overlap for that cylinder. If the cam is ground " staight up" and if the timing gears are accurate, the cam will be exactly in the middle of overlap when the #1 piston is at TDC. The only way to check if all this is correct, is to dial-in the cam using a degree wheel.
Some manufactures cam cards give an intake centerline figure as the checking point for the cam installation timing. This number looks like, and is often the same as, the lobe separation angle for the cam. But intake centerline, (sometimes listed as "Lobe centerline or the "max lift point", for intake and exaust), is not the same as the lobe center displacement angle. The similarity is a coincedence. If a cam with a lobe displacement angle od 110 degrees is installed in a "straight up" in the engine. At TDC on the #1 cylinder, the cam will be positioned in the middle of the overlap below thw lifters. There are 110 degrees beween the two lobes; the center of the intake lobe is 55 degrees to the left of the lifters, and the center of the exaust lobe is 55 degrees to the right.. If you turn the crankshaft 110 degrees past TDC, the cam, (which turns at half crank speed), will turn 55 degrees, and the centerline of the intake lobe will be directly under the lifter, or at maximum lift. Thus the "intake centerline" checking spec for this cam is 110 degrees ATDC, (After Top Dead Center). The exaust centerline would be 110 degrees BTDC,(Before Top Dead Center). The lobe separation angle and the intake-centerline checking spec happen to be the same number only when the cam is installe "straight up".
If the cam is moved ahead in relation to the crank/piston timing, it is said to be "advanced"; if it's moved back from "staight up", it is "Retarded." Both lobes are advanced or retarded the same amount, because they are fixed on the shaft. If a cam with a 110 degree lobe sep. angle is specefied to be installed on a 106-degree intake centerline, the cam would be 4 degrees advanced. In other words, the intake lobes would reach max lift 4 crank degrees sooner. If the card called for a 114-degree intake centerline, the cam would be installed 4 degrees retarded.
Cam installation timing, or phasing can be altered in two ways. The cam manufacturer can cut the keyway (or dowel pin) for the cam gear in a retarded or advanced position, so that the cam is advanced or retarded a given amount when you line up the timing marks on the gears. In such cases it is said that the cam is "ground advanced" or "ground retarded"
Given that timing chains are subject to a certain amount of streatch, most cam manufacturers today will grind their steert cams 4 degrees advanced to compensate for this.
The engine builder can also advance or retard cam timing with an offset key for the crank sprocket, an offset bushing for the dowel pin in the cam sprocket, or a multi-position chain-and-sprocket set,usually with both 4 degrees advanced and retarded positions. There are some available that feature as many as 9 positions. In any case you cannot tell if your cam is "advanced" or "retarded" unless you "degree " it. These days, the generally preffered method of checking/adjusted cam timing is to measure at .050 lift the opening and closing points of the intake and exaust crank degrees before or after the closest top or bottom dead center.
If you have a cam measured at .050 lifter rise, and the spec card tells you that the:
Intake opens at 10 BTDC
Intake closes at 46 ABDC
Exaust opens at 50 BBDC
Exaust closes at 6 ATDC
How is this analyized
For cam checking, TDC is TDC overlap (between the intake and exaust strokes) and BDC is at the bottom of the intake stroke, 180 crank
degrees later. If you add the 10 degrees before TDC when the intake opens, the 180 crank degrees from TDC to BDC, and the 46 degrees
after BDC when the intake closes, you get a 236-degree duration (at .050).
You can also see that this cam, set to these specs, would be two degrees advanced. If it were instsalled "staight up", the intake would open at 8 degrees BTDC and the exaust would close at 8 degrees ATDC, the intake would close at 48 degrees ABDC, and the exaust would open at48 degrees BBDC. In the example above, each of these figures is advanced two degrees. See how it works?
This is a single-pattern camshaft that is ground 2 degrees advanced. If you were to install it by aligning the timing marks, it should reach .050 lift at the points specified above. On a dual-pattern cam the numbers don't come out the same when it is staight up, because the durations of the two lobe differ.
Another thing to think about: If you change the lobe sep. angle on a cam, keeping other things equal, where do you install it? If you
were able to narrow the lobe centers from 114 to 110 (impossible without buying a new cam). If you install it with the intake lobe in
the same place, you will be retarding the exaust 4 degrees, which equals 8 crank degrees compared to the first cam. You could also
put the exaust lobe in the original position, which would advance the intake 8 degrees.
As most cam manufacturers specify, you could split the difference; in this case. narrowing the lobe centers 4 cam degrees, would advance the intake crank degrees and retard the exaust 4 degrees. Changing lobe centers and cam installation position, gives you numerous cam timing variables to play with.
Retarding a cam moves the power range to a higher range RPM level; advancing it helps the lower speed ranges. For the street, you don't want to run a cam retarded. Timing-chain streatch is actually a good thing for most engines, since the chain will streach more as RPM increases, allowing the cam to run advanced at low speed, then retard slightly at high speed.
It is possible to further advance a "big" cam about 4 degrees to improve low speed drivability and bottom-end torque in a street engine. Advancing a cam has more of a good effect on low to mid-range drivability than it causes detriment to the top end. If you do, be sure to recheck valve-to-piston clearance, because the intake valve will be opening sooner in relation to piston TDC.
There are many more camshaft basics that have not been touched on here, such as the fact that increasing the rocker arm ratio will increase valve lift and rate of lift without changing the duration.; or that you can decrease duration on a solid lifter cam by increasing the valve lash, and visa versa.