One of the most important components in any engine is the camshaft. Whether the camshaft is in a pushrod engine or an overhead cam engine, it controls the opening and closing of the valves. This, in turn, controls the flow of air and fuel into and out of the engine which determines engine performance, fuel economy and emissions.
In pushrod engines, the camshaft is either chain or gear driven off the crankshaft. In OHC engines, the camshaft may be belt or chain driven from the crankshaft or an intermediate shaft. The drive ratio is always 1:2 so the cam turns at half the speed of the crankshaft. This is because the crankshaft in a four stroke engine makes two complete revolutions for every power cycle (intake stroke, compression stroke, power stroke and exhaust stroke).
Cam-related problems can occur for a variety of reasons. As an engine accumulates miles, the timing chain stretches. The added slack in the chain has a retarding effect on cam timing, which reduces compression and torque. It can also retard ignition timing if the distributor is cam-driven. Most OHC engines that use a chain drive have some type of automatic chain tensioning device, but pushrod engines do not. Consequently, the timing chain and gear set often need to be replaced in high-mileage pushrod engines.
In OHC engines with belt-driven cams, the main concern is belt failure. If the belt snaps, the cam stops turning and the engine quits. Some valves will be held in the open position, which may result in bent valves and/or damaged pistons if the engine does not have enough clearance between the pistons and valves to freewheel.
To minimize the risk of such damage, most vehicle manufacturers recommend replacing OHC timing belts at specific mileage intervals for preventive maintenance. On older OHC engines, 60,000 miles is the typical replacement interval. On newer OHC engines, it is 100,000 miles.
Cam failures can occur if there are lubrication problems in the engine. Lifters create a lot of pressure and friction on the cam lobes, so the lobes and cam bearings must receive lots of oil. If oil pressure is low or the oil is dirty, the cam may suffer accelerated lobe wear and ultimately lobe failure resulting in a dead cylinder (no valve action).
This type of cam damage can also be caused by using the wrong viscosity motor oil. In overhead cam engines, it is a long way from the oil pump to the top of the cylinder head. On cold mornings when the oil is thick, it can take quite a few seconds for adequate oil pressure to reach the cam. That is why most vehicle manufacturers recommend using 5W-30 oil rather than 10W-30 or 10W-40 for cold weather driving.
Cam breakage or seizure is another problem that can occur in OHC engines. The cause may be inadequate lubrication but in many instances it is caused by head warpage.
When an OHC engine gets too hot, the cylinder head tends to swell and bulge up in the middle. This changes the alignment of the cam bores in the head which may cause the cam to bend, bind, seize or break. If an overhead cam won't turn freely in the head when the belt and cam followers are removed, either the cam is bent or the head is warped and needs to be straightened and/or align bored.
Camshafts are often replaced to increased engine power and performance. But choosing an aftermarket performance cam is not as simple as it sounds. A lot of things must be taken into account to find the "right" replacement cam. These include not only the engine and vehicle application, but also the engine's compression ratio, type of fuel delivery system, other modifications (intake and exhaust manifolds, exhaust system, etc.), cylinder heads, transmission and differential gearing, and even the size of the tires.
But most important of all, what exactly do you want from an aftermarket performance replacement cam? More power? More torque for towing? Better mileage?
As you look through the various performance camshaft manufacturer catalogs, you will notice two things. The first is that there are many, many different cam grinds from which to choose. The more popular the engine application (small block Chevy, for example), the greater the selection of cams that are offered. The other thing is that there are specific recommendations for each type of cam. So the best advice here is to follow the camshaft supplier's advice.
A typical stock replacement cam (either a new cam or a reground,) is essentially a duplicate of the OEM grind. This type of cam is used to restore the original performance of the engine, and is a safe choice for a stock rebuild.
The next step up are the cams with slightly "enhanced" profiles. These include the mileage/economy cams, towing cams and mild performance cams. Idle quality, driveability and emissions remain about the same as stock but the engine puts out more power and gets somewhat better fuel economy.
Once you get beyond stock replacement cams, the selection process gets more complicated because each grind is designed for a specific type of application. Vehicle weight, drive gear ratios and type of transmission (automatic versus manual) take on greater significance as do the modifications to the engine itself (compression, displacement, carburetion, valve size, valve train, etc.).
The most common mistake to avoid when choosing a cam is to "over-cam" an engine. Installing a hot cam in an otherwise stock engine can create a bad mismatch between components, and that hurts rather than helps performance. A long duration cam with a lumpy idle may sound really hot, but may not provide as much low end punch as a stock cam. Emissions can also be a problem with long duration cams.
When comparing cams, you will find a number of specs listed. These include lift, duration, overlap, lobe separation and timing.
Valve lift is how far the cam opens the valves. Increasing valve lift increases the distance the valve opens, which makes it easier for more air and fuel to enter the cylinders. Gains in airflow will be achieved by increasing lift up to the point where either the area of the valve opening or port becomes the limiting factor on airflow, or mechanical interference is encountered between the valve and piston, valve springs or spring retainer and valve guide.
One way to specify valve lift is to measure "lobe lift." This is how far the cam lobe actually moves the lifter, which is the maximum height of the cam lobe above its base circle. But this is not how far the valve actually opens. To get that number, you have to take into account the ratio of the rocker arms minus any valve lash. "gross lift," which is the figure most often cited in catalogs, is lobe lift times the rocker arm ratio. Gross lift gives you the theoretical valve lift of the cam. "Net lift" is how far the valve actually opens when you subtract valve lash in the valve train.
Duration is how long the cam holds the valves open, and is specified in degrees of crankshaft rotation. From here on the definition of duration gets fuzzy because of the different ways it can be measured and advertised.
Duration depends upon how and where it is measured. As the cam turns around and a lobe begins to push its lifter up, the valve starts to open, but not instantaneously. A couple of things happen first. The valve does not start to open until all the lash in the valve train has been taken up. The cam lobe also rises gradually from the base circle which makes it difficult to measure the exact point where the lifter begins to move.
One way to measure duration is to start when the lifter has risen 0.004 inches above the base circle of the cam lobe. The number of degrees of crankshaft rotation are then counted until the lifter comes back down to within 0.004 inches of the base circle. This method is often referred to as the "advertised duration." They call it this because the duration numbers it generates are much larger (and sometimes misleading) than those generated by the following techniques.
The Society of Automotive Engineers (SAE) states that duration is to be measured at 0.006 in. above the base circle for hydraulic cams, and 0.006 in. plus the specified valve lash for mechanical solid lifter cams.
The other common method of specifying duration is to measure it at 0.050 in. above the base circle. The 0.050 in. specs are the ones most commonly cited in aftermarket catalogs, and are the ones we will use when talking about specific duration recommendations.
What does a duration spec tell you about a camshaft? It tells you the camshaft's potential for generating power within a certain rpm range. Generally speaking, the longer the duration the higher the operating range of the cam. Short duration cams are good for low speed torque and throttle response, while long duration cams hold the valves open longer for better high speed breathing.
Camshafts with up to 220 degrees of duration (at 0.050 in cam lift) are considered best for stock unmodified engines and those with computerized engine controls. Once you go beyond 220 degrees of duration, intake vacuum starts to drop appreciably which upsets idle quality and affects the operation of computerized engine control systems.
Some duration specs do not tell you anything about its lobe profile. Two different camshafts may have identical lift and duration specs, but the lobes on one cam may be ground differently from those on the other. One camshaft may have more of a peak shaped lobe while the other has a "fatter" lobe. A "V"-shaped lobe will breath differently from a "U"-shaped lobe because it does not hold the valve at its maximum opening as long. Valve float can also be a problem with lobes that change shape abruptly unless valve spring pressure is increased. The profile of the lobes on one camshaft may also be the same on both the up and down sides of the lobe (which is the norm for most stock and street performance cams) compared to an "asymmetrical" grind (different profiles on each side of the lobe) on the other cam.
Another spec you need to look at when selecting a cam is the relative timing of the intake and exhaust valves. This can be expressed either as "valve overlap" (the time during which both the intake and exhaust valves are both open) or "lobe separation" (the number of degrees or angle between the centerlines of the intake and exhaust lobes). Decreasing the lobe separation increases overlap, while increasing the separation decreases overlap.
Most stock replacement camshafts with durations of less than 200 degrees will have lobe separations of 112 to 114 degrees. Higher duration cams for mid-range performance typically have 110 to 112 degrees of lobe separation. With racing cams, you will find lobe separations that range from 106 to 108 degrees.
Overlap occurs when the intake valve starts to open before the exhaust valve has finished closing. Increasing overlap can be a desirable thing in a higher rpm performance application because the outgoing exhaust actually helps scavenge the cylinder to draw more air and fuel into the combustion chamber. But too much overlap at low rpm kills low-end torque and throttle response by reducing intake vacuum excessively. It can also create idle emission problems by allowing unburned fuel to be drawn through into the exhaust.
Some original equipment cams use a bit more overlap to create an exhaust gas recirculation (EGR) effect to reduce NOX (oxides of nitrogen) emissions. The trade-off is usually some low end throttle response and torque. Replacing this type of cam with one that has less overlap (more lobe separation) can make a significant improvement in performance.
A roller cam uses lifters that have small friction-reducing rollers on the bottom. In stock engines, roller cams are used to reduce friction. In performance engines, roller cams are used to offer more radical lobe profiles. The rollers on the bottom of the lifters can roll up and over a sharper slope than a flat tappet cam.
Flat tappet cams were used in most engines up until the mid-1980s, when vehicle manufacturers began to switch over to roller cams. The valve lifters that ride on a flat tappet cam are actually slightly convex on the bottom. The curvature combined with a slight slope on the cam lobe itself causes the lifter to spin as it rides up and down. This is done to reduce friction and wear.
which is better? It depends what kind of engine you are building. If you are building a high revving performance engine, a flat tappet cam with solid lifters will give the most rpms without valve float. But solid lifters are noisy and require periodic adjustments. A roller cam with hydraulic lifters, by comparison, is a good choice for a street performance engine or a lower rpm engine that is built to produce a lot of torque.
Cam timing is the amount of advance or retard between the cam and crankshaft. Cam timing can be checked by comparing the lobe spread and intake lobe center with a degree wheel and dial indicator.
Most installers assume cam timing is correct as long as the timing marks on the cam drive are lined up properly. That may be fine for a stock cam, but it is not good enough for a performance engine rebuild. The cam may be advanced or retarded depending on the alignment of the cam drive (crankshaft sprocket and cam gear), the amount of wear or play in the cam drive (gear back lash, belt wear or chain stretch), and the geometry of the cam drive. Resurfacing an OHC cylinder head can also alter cam timing, which may require the installation of a thicker head gasket and/or offset the cam drive pulley to compensate.
Why is cam timing so important? Because it affects engine performance. For the engine to run its best, it needs accurate cam timing. As a rule, advancing cam timing 2 to 4 degrees helps low-speed torque and throttle response with little sacrifice in higher rpm power. Advancing the cam also helps compensate for chain stretch as the engine ages. Retarding the cam, on the other hand, may improve performance at high rpm but usually at the expense of low speed torque. A retarded cam is something you do not want in a stock or street performance engine.
Something else to keep in mind is that many aftermarket cams are not ground "straight up" with zero timing offset. Many already have about four degrees of advance built into the cam to compensate for timing chain stretch as the engine ages.
If camshaft timing is not measured with a degree wheel and someone installs an offset dowel or cam drive gear to advance cam timing another four degrees, they could end up with too much advance and possibly valve-to-piston interference problems.
Finally, anyone who is replacing a camshaft with flat bottom lifters should replace both cam and lifters at the same time.
Worn lifters can damage a new cam. New valve springs should also be recommended.
Cam lobes should always be protected with assembly lube when a cam is installed, and the cam should be broken in following the manufacturer's recommended procedure (usually running at 2,000 rpm for up to 30 minutes when the engine is first started).
If you are driving an older classic muscle car or hot rod that has an engine with a flat tappet camshaft, you should be aware of the fact that today's "SM" rated motor oils contain much lower levels of anti-scuff additive called "ZDDP" (Zinc Dialkyl Dithio Phosphate). The level of ZDDP in current motor oils has been reduced to no more than 0.08% phosphorus to extend the life of the catalytic converter. Phosphorus can contaminate the catalyst over time if the engine uses oil, causing an increase in tailpipe emissions.
The lower ZDDP content is not harmful to late model engines with roller lifters or followers because the loads are much lower on the camshaft lobes. But on pushrod engines with flat tappet cams, the level of ZDDP may be inadequate to prevent cam lobe and lifter wear. In some cases, cam failures have occurred in as little as a few thousand miles of driving! This is even more of a risk in engines if stiffer valve springs and/or higher lift rocker arms are used.
To avoid such problems, you should add a ZDDP additive to the crankcase, or use an oil that meets the previous "SL" service rating, or use diesel motor oil or racing oil that contains adequate levels of ZDDP to protect the camshaft and lifters.
If you are installing a new camshaft in the engine, be sure to use the cam manufacturers assembly lube and follow the recommended break-in procedure. But you will still need to add ZDDP to the crankcase or use an oil that contains adequate levels of ZDDP for continued protection.