The purpose of the camshaft is to synchronize the timing of the intake and exhaust valves, and the amount of opening and closing, to yield optimum combustion pressure in the cylinder. This results in the most pressure being applied to the piston/rod assembly to rotate the crankshaft to produce torque. This timing is a vital function in the engine because the camshaft controls how much to open the valves and at what rate, and when to open them and when to close them. It's a straightforward mechanical timing device essential to engine efficiency. Here is an overview to help you choose the right one for your racing.

Mistake No. 1
One of the most common errors you can make is to "overcam" an engine, which means to install a camshaft with too much lift and duration (explained later) for the type of racing for which the engine is used. The result is like putting too large a carburetor on an engine-the engine is inefficient (lazy) in just the rpm range where you need it to operate the best. This ideal rpm operating range is where the camshaft will produce the best power, and it is approximately 500 rpm wide.

You are trying to pick and install a camshaft that will put this 500 rpm window at the point on the track where it can best be used given your car's gearing and tire diameter. When you choose a camshaft, you can raise or lower this rpm range.

Of course, we can't cover every engine model or racing type. We're going to use the typical in-the-block camshaft setup of the overhead valve (OHV) V-8 engine using pushrods operating one intake and one exhaust valve as our working example to explain camshaft terms.

While we'll cover some of the basics of camshaft operation here, it is a very complex subject and beyond the scope of this article-and frankly, probably beyond what most of us want to know about camshafts. We want to get on track and race, not design camshafts. Consequently, the best advice we can give for choosing a camshaft is to become friendly with your performance aftermarket camshaft manufacturer's tech line. They have invested years of researching, developing, and testing camshafts and valvetrain components for every racing application. By answering their questions, you can get a camshaft very closely matched to your racing and engine type.

Four Stroke Engine Cycle
Before we examine the camshaft's function and its relevant terms, it's prudent to review in general terms the four strokes of a typical racing engine cycle so we can see how the camshaft is essential to their operation and achieving maximum power. Remember that each stroke lasts for one half of a crankshaft revolution, and because the camshaft turns at half the speed of the crankshaft, each stroke is only one-quarter turn of the camshaft. This will be an idealized and simplified discussion. Not all of the following mechanical action operates instantly, nor right on the nth degree of crankshaft rotation (there is a reason OHV and pushrod valve actuation is called "monkey motion" by its detractors), but it will be sufficient to get us started.

Power Stroke
Let's start with the piston at the top of the cylinder (Top Dead Center, or TDC) when both valves are closed and when the spark plug has just fired. This is the start of the power stroke-the air/fuel mixture has exploded and the expanding gases move the piston down. When the piston nears the cylinder's bottom (Bottom Dead Center, or BDC), the exhaust valve starts to open. Ideally, the mixture is fully burned and the remaining cylinder pressure starts the evacuation of the cylinder.

Exhaust Stroke
The piston moves past BDC and starts upward. This begins the exhaust stroke. The shrinking cylinder volume forces the spent gases out the exhaust port, and as the piston continues up to TDC, the exhaust valve fully opens, travels through its maximum lift (extension), and begins to close.

Intake Stroke
The next stroke is the intake stroke. As the piston is getting closer and closer to TDC, the exhaust valve hasn't completely shut and the intake valve begins to open. This point is labeled valve "overlap," and it is a critical governor of engine performance. If the intake valve opens too early, the intake charge can travel back up into the intake manifold (diluting the incoming air/fuel charge) and can limit power at low- and mid-range engine rpm. If the intake is opened too late, it can lean out the cylinder and reduce engine performance.

Concurrently, if the exhaust valve closes too early, it will trap burnt combustion mixture in the cylinder, which is beneficial for emissions control, but not for power. But if the exhaust valve closes late (the piston is farther down the cylinder on the intake stroke), low-end rpm performance is severely hampered. The fresh air/fuel charge just passes through the combustion chamber without an opportunity to be fully compressed and perform work. This helps cool the exhaust valve as it closes, but that is a poor trade at the expense of power.

When the piston passes TDC and begins traveling back to BDC, the intake valve continues to open. A pressure difference occurs-nature abhors a vacuum-and atmospheric pressure forces the fresh air/fuel charge past the fully opened intake valve. The piston is not sucking in the air/fuel mixture; atmospheric pressure is pushing it into the cylinder.

Compression Stroke
The piston once again changes direction and starts up to TDC on the compression stroke. The intake valve begins to close, and the piston continues toward TDC. Both valves eventually fully close, the air/fuel mixture is compressed by the piston, and cylinder volume is decreased as the piston reaches TDC. A spark is applied, ignition occurs, and the engine cycle starts again on the power stroke.

Is it any wonder the camshaft has sometimes been called the "brain" of the engine because it has to mechanically control all these valve events with precision within a few degrees of rotation?

Camshaft Terms
We'll overview some of the basic camshaft terminology here and explain why it is important to your engine's peak performance.

Lift: The distance a valve travels. In our valve lift curve plots, lift is specified in inches along the left vertical axis. The horizontal axis is the number of degrees of crankshaft rotation, and each of the engine strokes are specified below it to reference where the valve events are taking place in the engine cycle.

The gross valve lift in this example is 0.470-inch maximum (the peak of each valve travel plot). Camshaft lobe lift, or lobe lift, is the gross valve lift value divided by the rocker arm ratio: in this example, 0.470/1.5 = 0.313-inch lobe lift. Valve lift amount, and the rate the valves open and close, are critical to how much engine torque is produced.

The camshaft lobe lift is fixed by the camshaft grind, but you can easily and relatively inexpensively increase gross valve lift, and improve the performance of an existing camshaft, by changing the rocker arm ratio. For example, if we take the 0.313-inch lobe lift in our example and change the rocker arm ratio to 1.6, the gross valve lift is now 0.5 inch (0.313 x 1.6 = 0.5 inch). The engine responds as if a larger lift cam has been installed, and improved torque can result. Keep in mind, though, that you've changed mechanical relationships with the increased rocker arm ratio. Small increases in rocker arm ratio can usually pass, but major increased rocker ratio can cause coil spring binding or piston/valve and other mechanical interference. Check for this interference before firing up an engine when you've changed the rocker arm ratio, or expensive noises can result.

Duration: The amount of time that a valve is open. It is specified in degrees of crankshaft rotation. Shorter-duration camshafts yield power in a lower rpm range. Longer-duration cams work better at higher rpm, at the expense of bottom-end power as duration time is increased. Camshaft duration is defined at a specified lift point, and confusingly, there are two types of duration usually specified: advertised duration, and duration at 0.050-inch tappet/lifter lift. Duration at 0.050 inch is the one to use when comparing camshafts.

Two values are critical in specifying duration: crankshaft degrees of rotation, and the point of the tappet/lifter rising at which the measurement is taken. Advertised durations are measured all over the map; that is, not at any regular tappet/lifter rise value. In our valve lift curve plots, the advertised duration is measured at 0.006 inch (the line just above the horizontal axis). It can vary from one cam manufacturer to another. Remember, too, that tappet/lifter lift is at 0.006 inch, and actual valve lift is this value multiplied by the rocker arm ratio. In this example, it is 0.006 inch x 1.5 = 0.009-inch lift at the valve.

The opening and closing points of each valve are circled in this plot (just above the horizontal axis), and counting the number of crankshaft degrees between them gives us an advertised duration of 270 degrees for both exhaust and intake lobes. See this value specified by the dimension arrows below the horizontal axis. When the duration is the same for both lobes, the cam is called a single pattern cam.

A dual pattern cam is one that has a different profile on the intake and exhaust lobes. The lift and duration on the intake may be smaller or larger than the lift/duration on the exhaust lobe.

Duration at 0.050-inch tappet/lifter lift in this example is specified farther up the valve lift plot line; and in this example it is 224 degrees duration. Keep in mind that actual valve lift at this tappet/lifter rise value is 0.050 x 1.5 = 0.075 inch. That is where the second set of circles is placed up the valve lift curves.

We can see that in the simplified marketing world of "mine is bigger than yours" that using advertised duration instead of duration at 0.050-inch tappet/lifter lift might be a perceived advantage. Always rely on duration at 0.050 inch to compare camshafts.

Lobe Separation Angle: The number of camshaft degrees separating the peak lift point of the intake and exhaust lobes. Lobe separation angle is fixed in the camshaft after its initial grind and can only be changed by regrinding the camshaft. In our valve plot, the lobe separation angle is 110 camshaft degrees-the number of degrees peak to peak.

Lobe separation determines where peak torque will occur in the engine's power rpm range. A lobe separation of 106 degrees is "tight" and causes maximum torque to start earlier in the engine's rpm range. It will increase quickly and then peak early. A lobe separation of 110-112 degrees is more "open," or broad, and spreads torque across the ideal operating rpm, and improves power at upper rpm.

Intake And Exhaust Centerline:
These centerlines are sometimes confused with lobe separation angle. We noted that lobe separation angle is fixed by the camshaft grind. Intake/exhaust centerline is referenced to piston TDC, thus it is adjustable by where the lobe's centerline is installed. This centerline is the position of the centerline-the peak lift point on the intake/exhaust lobe-in reference to piston TDC. It is the maximum lift point of the lobe related to TDC in crankshaft degrees. It can be changed-either advanced or retarded-and doing so is called degreeing a cam. Camshaft manufacturers supply a recommended intake centerline installation position.

As you can see in the valve lift curve plots, the intake centerline of the colored plot is installed 4 degrees advanced at a 106-degree centerline. The lobe separation angle is 110 degrees. The black curves are the same camshaft installed an additional 4 degrees advanced to achieve a 104-degree centerline. Advancing the intake lobe centerline starts overlap sooner and opens the intake more: It usually increases bottom-end power by moving the ideal operating rpm lower in the powerband. A rule of thumb is that advancing the intake centerline about 4 degrees from its original installed position will move the powerband to begin about 200 rpm sooner. Retarding it the same number of degrees will move the powerband up about 200 rpm. This can be useful for tuning the engine's powerband to a particular track. A camshaft that is installed "straight up" (neither advanced nor retarded) per the cam card that comes from the manufacturer is a good cam degreeing starting point.

Armed with these major terms and parameters in mind, contact your performance camshaft manufacturer for more detailed selection of a camshaft for your racing needs. The time you invest with them will be well spent.

Need To Know
Before you call a racing camshaft manufacturer, you should have these values in hand. The more you can flesh out these parameters, the better the camshaft will be matched to your performance racing requirements.

* Track type/size
* Camshaft restrictions/rules (i.e., flat tappet/lifter required)
* Engine size and number of cylinders
* Bore and stroke
* Compression ratio
* Connecting rod type/length
* Rocker arm ratio
* Carburetor size
* Intake manifold type
* Cylinder head brand/type, or flow data (best)
* Intake/exhaust valve size
* Transmission type
* Torque converter stall speed (if automatic)
* Rear axle ratio
* Rear tire size
* Fuel type and octane rating
* Car weight* RPM powerband operating range of engine

SOURCE
COMP Cams
  • «
  • |
  • 1
  • |
  • 2
  • |
  • 3
  • |
  • 4
  • |
  • View Full Article