by Mike Harris
One of life's greatest mysteries is the camshaft. At the heart of the motor, it rests, and with each pulse determines how your engine performs, sounds, and lasts. It is one of the more commonly misconceived devices. Somewhere along the line, 75% of the population started believing that bigger is better. A lot of the time that is just not true.
There is a lot more to a cam than just the advertised lift, duration, and lobe separation. The cam may say it has a range of 3000-6500 RPM, but do you know for certain that the overlap will be right for you? How many times have you heard someone say, "Nitrous and Supercharged engines like a wide lobe separation angle like 112º - 114º". Have they been able to tell you why? I can point to several cars that I have worked on, both nitrous and supercharged, that have had 108º lobe separation with the nitrous cars running very heavy doses of NO2. To know what the car really needs as far as a cam, a lot of things besides just compression ratio, intended rpm, gear ratio, and the like need to be known. To properly design a cam or at least get into the ball-park on a cookie-cutter, you need to know the flow on the intake and exhaust of the heads, intake manifold flow rates, intake runner length, rod to stroke ratio (I'll cover that in another article), header tube size and more. All of this would take a year of writing to explain, but what I want to touch on is a few basics to get you "in-the-know" and visualizing what is going on.
Lift is the single biggest profile of a cam that most people look for in their selection. This simply refers to how far the valve opens (comes off the seat) at its highest point. If you don't open the valves wide enough, they cause a restriction. However, based on head flow, intake flow etc., opening too much will not help at all and in a lot of cases actually hurts. Go turn the water on in your kitchen sink. The first turn or so, the water comes out quickly and increases; however, the last bit of turning to full open doesn't change anything. Get the point? You can only flow as much as the smallest restriction. Another dynamic that can happen, is the head reaches a stall in flow at a particular lift. This is exactly the way it sounds, it decreases velocity and can even result in reversion.
Duration is the next in line since a lot of gearheads like to gloat on, "Hey my cam has got over 300º of duration!". I am not impressed unless they know why they needed that amount or if it was recommended by someone that did. In a nutshell, this simply refers to the amount of time the valve is open. The degrees are a reference to crankshaft degree so a 280º duration means that the valve will be open for 280º of the crank's 360º rotation. More here can be beneficial, but it can also kill a motor too. In a high rpm motor, more duration gives extra time to flow the mixture in or push waste out. This boils down to timing events (which I will touch on later), but due to the incoming or exiting velocity of the mixture/waste, you can actually continue the sequence without regards to the piston's stroke. The problem with a low rpm motor, is that the valves are open at the wrong time to take advantage of the same things mainly due to the incoming air speed.
When we talk about about duration though, there is "advertised duration" and ".050 duration" figures. Most cam companies use .010 to .012 valve lift to get their advertised duration. How? Take a lobe from the cam and draw a line on each side of it where this (we'll say .010) valve lift occurs. This will be the opening and closing sides of the lobe. If you were to graph crank degrees in relation to the lobe, you would see that the degrees in between your marks would equal the advertised duration. It can be very misleading though because not all companies measure at the same point. This is exactly why you see the ".050" duration numbers. This is a way to judge one cam against another in a standardized manner, thus leveling the playing field. There is a caveat to this also. When you read the card, make sure you see if it says "duration at .050 TAPPET lift" or "duration at .050 VALVE lift" as there is a difference. The rocker arm ratio changes the way the valve sees the cam, thus a 1.5 ratio rocker arm multiplies the cam lobe lift by one and a half. Now, if it is by valve lift and you are comparing to a cam measured at tappet lift, multiply the lobe lift by the rocker ratio to get the valve lift. (.010 tappet lift x 1.5 rocker ratio= .015 valve lift) This is the same way you would figure valve lift if you only had the lobe lift. You can also compare rocker ratios this way to see the effects of a larger ratio. For example... cam lift with 1.5 rocker ratio is .526 and you a curious as to what a 1.6 rocker would change the lift to. Take the lift (.526) and divide by current ratio (1.5) which yields a .351 lobe lift. Now multiply by the new ratio of 1.6 and the lift now reads .562, and you have answered your question.
Okay, I got off track at the last part; now on to centerline. This measurement is the amount of degrees that the crankshaft would have to turn from top-dead-center (TDC) before the lobe is at peak lift. This in conjunction with duration can be used to determine the opening and closing events of the cam. When degreeing in a cam, this is what you are looking for. You want to make sure that the cam is actually ground this way and install it so that it meets your needs.
Lobe separation angle is sometimes confused with centerline, but they are two totally different animals. If you drew a straight line through the center of both the exhaust and intake valves, this number would represent the number of camshaft degrees which separate them. It can never be changed as it is ground in the cam. Defining what this number needs to be is done by the lobe profiles chosen, and when you change this angle, you bring a whole new set of variables in as far as the opening and closing events are concerned. This can change the way the motor "sees" the ramps. Changing this also changes your timing events which I will touch on in a minute.
Base circle, before I get too far ahead, is a term for the lowest portion of the cam. It is the area in which the lifter rests, also this is where you would make your lash adjustments. Basically, a lobe starts out as a circle, and is ground on to make the profile you see (egg-shape). But, if the bottom of this is left alone, you would not create lift at all. In fact, you would actually create a trap for the lifter and it would stick between the lobe and the bore. What the manufacturer does is turns down this area, also known as the "heel" so that the lifter actually sits a bit further down in the bore, and when the cam rotates, it is now able to actually lift the tappet. This is also one area for which there is not a standard in the industry. Several manufacturers use regrinds, which will end up making the base circle smaller. On a motor with stud mount adjustable rockers this is not a problem; however, in a motor with non-adjustable pedestal mount rockers it can wreak havoc in getting proper lash.
Ramps? What are ramps? I know I keep talking about them, there are several ramps and important sections of them. This term refers to the portion of the cam lobe that actually raises above the base circle of the cam. It will denote how quickly or slowly the valves open and close.The first is the lash ramp. This is the where the lifter is in a transition area where the lifter goes from the base circle to an area with a more aggressive rate known as the opening ramp. The lash ramp softens the effects of an "at rest" condition to a "work" mode. The opening ramp is next and it is exactly what it sounds like. This is where the cam begins to open the valve. A couple of segments of this ramp are the major and minor intensity areas. Major intensity is the first segment and it occurs within .020 and .050 of lobe lift and in most cases will not start before the first 28 degrees of duration. This allows for a suitable lash ramp for the lifter to stabilize. Exceptions to this would be VERY aggressive roller cams which see regular maintenance as far as pushrods, springs, etc. Ramp rates are made in such a way that they allow a slowing area (lash ramp) for the valve to seat without great harshness and minimum bounce. Yes, I said bounce. Even with high spring pressures, a valve will bounce 3 times before resting on the seat.
Getting back on track, (I wander easily, don't I?), major intensity is the area where the valve sees its quickest movement, or, as I say, aggressive rate. Minor intensity is another segment of this ramp, and it occurs between .050 and .200 and is a bit less aggressive as far as rate of rise. The idea behind the cam is to utilize flow rates of the heads, intake etc. to make the power. By knowing flow at different lifts and where max flow occurs, the grinder uses these segments to get the valve open quickly so that it may spend most of its duration time in the area where it can get those flow numbers. The closing ramp is the final ramp we talked about. It has major and minor intensity segments as well, and should be designed in such a way that it helps to cushion the valve as it closes.
What about the area between the opening and closing ramps? Well, that is called the nose. It is the area where the valve is kept at peak lift as long as possible before the closing cycle begins. With too aggressive a design in ramps and the nose, the lifter can actually run off the ramp and then come crashing down on the nose of the cam. To combat this, rev-kits are made which use springs mounted to the lifter that apply added pressure to the lifter to keep it seated in the bore and on the cam. Attention to ramp velocity, nose design, and inertia of the lifter must be given close consideration. I have seen several "pancaked" lifters from this phenomenon (also known as "loft"), even with rev-kits, and believe me, it is not pretty.
Let's talk about those opening and closing events. Most people think that you open the intake when the piston is going down the bore, close it at the bottom, have a compression stroke, power stroke, and then open the exhaust when the piston is coming back up. Well, sort of, and a lot depends on application, but we are talking performance here; so in that case it is dead wrong. Cam events will make or break a motor, as you have heard, the cam is the heart of the engine.
The first event we will talk about is the overlap phase which is where the intake valve opens and the exhaust closes. The word "overlap" means just that, where the opening/closing events cross each other. Why would you do that? Well, the spent charge is moving at a very accelerated rate on the exhaust stroke and just as it is about to close, the intake valve cracks open and takes advantage of this. The rush of exhaust acts as a siphon, and although some of the intake charge is lost out the exhaust, it aids in giving direction of the mixture from the intake into the cylinder. By moving the mixture in this way, it is not dependant solely on the suction/vacuum of the piston travelling down the bore. Although some of the charge is lost, this can actually aid in increasing the velocity of the charge and promote better cylinder filling. Still don't understand? Get a glass and two straws and fill the glass about ¾ full. Place one straw in the glass and hold it just above the bottom. Take the second straw and put one end in your mouth and butt the other end against the top opening of the first straw so that part of the opening protrudes higher than the opening of the straw in the glass. Now, blow a hard steady stream of air through (air going over the top of the straw in the glass) and watch the water rise in the straw. If you can blow long enough it will come out of the top. Now do you understand?
There are caveats to everything, I suppose, and this phenomenon is no different. You can close the exhaust too early and not take full advantage of it, close the exhaust too late and reduce volumetric efficiency, and/or open the intake too late and waste your time or even cause some turbulence. Ever hear someone say, "Power adder cars like a wide lobe angle separation like 112º - 114º", but not know why? A wider lobe separation angle will yield less overlap (remember, LSA effects timing events?). Supercharged/Turbo cars need a lot less overlap because they are forced induction and the system is already in a pressurized state. This does ten-fold what overlap can do as far as cylinder filling, intake charge, and volumetric efficiency; however, they do need a little help thus the wider LSA. Too much overlap, and in a pressurized state like this, the amount of mixture lost is much greater than a naturally aspirated car would see, thus working against what the supercharger/turbo is accomplishing. A nitrous car is much the same in that it is artificially doing what the supercharger/turbo do and too much defeats its intended use. A bigger issue is that if the mixture sees too much of the hot exhaust gasses, it can ignite prematurely. Too late of an exhaust closing will cause reversion and bring the hot exhaust back into the cylinder and besides the pre-ignition chances, the turbulence can disturb the atomized mixture and create MAJOR problems. Too late on the intake can create a lean condition, and we know what happens then! As I said, there are caveats to everything, and though this is the norm for power adder cars, there are tons of those using tight LSA's and making big power, BUT, they are application specific, and a lot has gone into making the design work.
Next we talk about when the intake valve closes. No, not at the bottom of the intake stroke, actually, as the piston starts up on the compression stroke! Why? This centers around the fact that the intake charge cannot come to an abrupt halt just because the piston stopped moving down. In this way, we take advantage of the intake charge's inertia to continue filling the cylinder even though the piston has started up on the compression stroke. The trick is not to close it too early and lose out on this effect, but also not to close it too late and begin pushing the mixture back out of the cylinder. This again aids in volumetric efficiency and increases cylinder pressure in doing so. An improperly phased cam will do the exact opposite.
Okay, now the easy one... the exhaust opening. Simple! Open it after the power stroke as the piston is moving up on the exhaust stroke, right? Wrong. The exhaust needs to begin opening shortly before BDC on the power stroke. Why? Well, first off, the combustion process has actually spent its energy about 2/3 of the way down and is moving based on the power stroke of other cylinders at this point. If you wait and open the exhaust as the piston starts up on the exhaust stroke, it will be pushing against the spent gases much the same as it does during the compression stroke. This will rob power from the power stroke of another cylinder to work against this force also. Opening the valve before BDC acts as a "pressure-relief" and then all the piston has to do now is sweep the cylinder clean with its only obstacle being the back-pressure of the exhaust system. One must be careful not to try and take advantage of this too quickly though as it would be possible to lose the effect of the power stroke.
These are the basics around camshafts, and should help you understand what is actually happening and hopefully will allow you to be more choosy in your next cam choice. Much more goes into designing a cam for a motor, but this article is intended to give an understanding of what the cam does and why. A custom grind is always the way to go for real power. It will be tailored to your application versus using a "cookie-cutter" that was developed for another; however, this is not always possible or sometimes just not a need. If this is the case, use the information in this article to help in your decision on buying the next cam and understanding what it will be doing in your motor.