How to choose a camshaft

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From Comp Cams

Contents

[edit] Overview

The camshaft can be thought of as the brain of the engine, and it has a very large effect on the amount of power an engine makes as well as where in the rpm range that power occurs. The main focus of this article will be on a conventional OHV (overhead valve), cam-in-block engine configuration using two valves per cylinder, as shown above.

[edit] The valve train

Mechanically speaking, the camshaft is linked to the crankshaft and turns 1/2 the speed of the crank. As the cam turns, the eccentric-shaped cam lobe lifts and lowers a cam follower, or lifter. The lifter is linked to a fulcrum, or rocker arm by a pushrod. The rocker arm directs the motion of the cam lobe to the valve, lifting the valve open and closing it shut with the aid of the valve spring(s).

The shape of the cam lobe dictate when the valve opens and closes in relation to crankshaft position (aka the cam timing), and how far the valve is opened (aka the valve lift), as well as how long the valve is open and closed (aka the duration).

This article assumes that the basics of how and what a camshaft does is understood. If further explanation or a refresher is needed, see Camshaft 101: How do cams work?. The article and video will make many of the ideas and terms used in this article much clearer.

For some more advanced information on camshaft operation and definition the specs of a camshaft, see Secrets of Camshaft Power by Marlan Davis (Car Craft, December, 1998).

[edit] Camshaft specifications explained

Typical cam card
Cam dimensions1.jpg

When you look at the cam specifications card (typically referred to simply as a cam card), it will list several numbers that will dictate how this particular cam will operate in your engine.

[edit] Lift

The valve lift is found by multiplying the lobe lift by the rocker arm ratio. Lobe lift is how far the lobe of the cam will move the lifter in a linear fashion. Lobe lift is measured by subtracting the base circle diameter from the maximum height of the eccentric (including the base circle). The lobe lift is ground into the cam, however the actual lift seen at the valve can be changed by using a different rocker arm ratio. Read more: The Basics Of Lift, Duration, And A Whole Lot More by Jeff Smith (February, 2009 Chevy High Performance).

[edit] Port flow

The amount of lift that can be used by a particular head depends on how much flow the cylinder head ports can deliver and at what valve lift the port flow stops increasing. More lift is generally better, provided the valves, retainers, pushrods and springs are properly matched to the cam profile and rpm the engine will turn, and if the port flow will support the valve lift without the port "stalling" or going into turbulence that keeps the flow from increasing. If head port flow stalls or starts decreasing above a certain lift, there is no reason to try to use more than that amount lift. But more lift is better, up to the point where the heads start losing flow.

Head flow for common domestic head castings can be found here. Heads are flow tested at different valve lifts, and many times the ports are tested at different amounts of "depression" (usually measured in inches of water or "in/H2O"). The results will be expressed as cubic foot per minute (CFM) of air flow. When comparing heads and their ports, be sure the depression is similar (28 in/H2O is a commonly used depression), or be prepared to convert the results from one depression to another depression, using a calculator.

Another difference that is often found when comparing flow is the size of the cylinder the head is sitting over (a larger cylinder usually means better flow numbers). Some heads are wet flowed (the air is mixed with a fluid to simulate a working engine) while other heads are dry flowed (just air is used). And yet another difference that may be found is whether an intake manifold is in place during testing (rarely done), or if an exhaust tube is in place (more common), or if clay is used to radius the openings (fairly common). Unfortunately, these differences can make comparing different heads much more difficult.

[edit] Duration

This is the amount of time (stated in crankshaft degrees) that the cam will hold the valve open. Advertised duration is greater than duration at 0.050" lift, but the duration at 0.050" lift is more useful when comparing cams or estimating how the cam will perform.

Some cams have the same duration and lift for the intake and exhaust valves. They are typically called single pattern cams. Those with different lift/duration numbers for the intake and exhaust are typically called split pattern or dual pattern. The seat-to-seat duration is ground into the cam and can't be altered without physically changing the camshaft lobe profile, although changing the rocker arm ratio changes the open duration a small amount.

Increasing duration will tend to shift the power and torque curves upward. Longer durations lend themselves to higher RPM operation, because at higher RPM the time the valve spends open is less than at lower RPM. Keeping the valves open longer (more duration) allows the cylinders to fill with more air/fuel mixture. Since the valve may be open considerably longer than the intake stroke, a lot of duration tends to reduce power and torque at lower RPM. At lower RPM the intake valve is open too long for maximum efficiency, because some of the air/fuel mixture gets pushed out with the exhaust, along with some air/fuel mixture getting pushed back into the intake manifold (called "reversion").

Another thing to remember is that larger engines tend to lessen the effect of having a a cam with more duration. The same duration cam in a small displacement engine will have a higher peak RPM than if it was installed in a larger displacement engine. For example, if a cam provides a 6500 RPM peak hp in a 305 SBC, the same cam might peak at 5500 in a 400 SBC.

  • More duration is best for: lighter cars, lower rear axle gearing (higher numerically), higher stall converters, bigger head ports and flow, higher compression (to compensate for the low cylinder pressures at lower RPMs), and lower transmission gearing.
  • Less duration is best for: heavier cars, tow vehicles, higher rear axle gearing (lower numerically), lower stall converters, smaller head ports and flow, lower compression (to prevent too much cylinder pressures during cranking) and higher transmission gearing.

[edit] Calculating duration

If the duration is not stated on the cam card, it can be easily calculated by adding the intake opening point in degrees to the exhaust closing point.

If those points are not known, you can estimate the duration by using the advertised duration and the lobe separation angle:

  1. Add the intake and exhaust advertised durations, then
  2. divide the results by 4, then
  3. subtract the lobe separation angle, then
  4. multiply the results by 2

[edit] Lobe separation angle (LSA)

The lobe separation angle, sometimes called lobe displacement angle, is a measurement in camshaft degrees that states how far apart the maximum lift points of the exhaust and intake lobes are. This number is ground into the cam and can't be altered without physically changing the camshaft lobe profiles.

Las drawing.jpg

A CHP magazine article comparing identical camshafts except for the LSA (along with comparing open vs. split plenum intakes): Camshaft Lobe Separation Angle Performance Test

[edit] Narrower LSA:

A narrower LSA will increase overlap. This has a tendency to reduce engine output at lower RPM and increase engine output at higher RPM. A narrower LSA tend to make more peak power but a little less average power.

  • Moves torque to lower RPM
  • Increases maximum torque
  • Narrow power band
  • Builds higher cylinder pressure
  • Increase chance of engine knock
  • Increase cranking compression
  • Increase effective compression
  • Idle vacuum is reduced
  • Idle quality suffers
  • Valve overlap Increases
  • Natural EGR effect increases
  • Decreases piston-to-valve clearance

[edit] Wider LSA:

A wider LSA tend to make less peak power, but a broader powerband. Changing the LSA also changes the valve timing events; opening the exhaust valve sooner and closing the intake valve later, both of which affect how the engine ingests air.

  • Raise torque to higher RPM
  • Reduces maximum torque
  • Broadens power band
  • Reduce maximum cylinder pressure
  • Decrease chance of engine knock
  • Decrease cranking compression
  • Decrease effective compression
  • Idle vacuum is increased
  • Idle quality improves
  • Valve overlap decreases
  • Natural EGR effect is reduced
  • Increases piston-to-valve clearance

[edit] Overlap

Area 1 is for street towing, 2 is regular street, 3 is street performance, 4 is street/strip, 5 is race, and 6 is Pro race.

"Overlap" represents the amount of duration in camshaft degrees when both the exhaust and intake valves are open at the same time. For a single cam engine this factor is ground into the cam and can't be changed without physically altering the camshaft lobe profiles. On a dual overhead camshaft (DOHC) engine overlap can be altered with adjustable cam gears. Adjusting one or more cams closer to TDC increases overlap. Increasing duration at the same LSA will increase overlap. Decreasing LSA at the same duration will also increase overlap.

Overlap is usually not found printed out on the cam card, but it's easy to calculate.

  • Add the intake opening point BTDC to the exhaust closing point ATDC.

If the intake opening and exhaust closing points aren't known, you can estimate the overlap by using the advertised duration (or duration @ 0.050" lift, etc.) and the lobe separation angle.

  • Add the intake and exhaust durations,
  • Then divide the results by 4,
  • Then subtract the lobe separation angle,
  • Then multiply the results by 2
  • The result is the overlap

Overlap and LSA are closely tied together. Increasing overlap contributes to a race cam's choppy idle, along with the intake valve closing point and the exhaust valve opening points. The extra time the valves are open at the same time causes what is called reversion, which is a situation in which the exiting exhaust gasses are partially pushed back up into the intake runner at low speeds. This causes big fluctuations in vacuum and uneven fuel metering if a carb is used (EFI metering isn't affected but reversion can still be a problem). Once the engine reaches higher RPM, the overlap is helpful since it adds to the time the cylinder can be filled with air/fuel mixture. Also, a tuning effect can come into play where the fast-moving exhaust gasses create a slight vacuum which helps to pull in more air/fuel mixture and remove more spent exhaust gasses from the cylinder, which is called scavenging. Overlap also has a large impact on the amount of intake manifold vacuum an engine makes. Less overlap allows more idle vacuum, and vice versa.

More overlap (less vacuum) can cause tuning headaches with modern OEM engine management electronics and EFI. It can also make tuning a carburetor more difficult. More overlap makes a choppy idle and tends to make peakier power for the same reason as a narrow LSA does. More overlap and the subsequent lower intake manifold vacuum might mean giving up vacuum-driven accessories like power brakes. Some cars even use vacuum to operate the HVAC, headlight covers, door locks, and windshield wipers, so consideration for those devices has to be given if choosing a cam for a vehicle so equipped. If the vacuum produced is insufficient, a vacuum pump can be installed.

[edit] Lobe intensity

One more point about the cam profile is lobe intensity. For a given duration, more lift means the lobe ramps (the opening and closing faces on the sides of the lobe) are steeper (more intensity). That is to say, the cam lobe has has to accelerate the lifter faster to get to the peak lift within the available amount of duration duration. Faster ramp speed can give more "area under the curve", which usually equates to a broader, less peaky powerband. The downside for flat tappet cams is that the steeper ramps mean they contact the lifter at a greater angle, so the potential for wiping out a cam lobe or lifter is greater. Manufacturers are well aware of this, so they try to design the lobe profiles to optimize power, yet maintain good durability. Cam profiles like the Comp Cams XE-series and Lunati's Voodoo line are both at the edge of how fast the valve can be safely opened and closed. That's why they caution against using a higher ratio rocker arm when using these grinds. More on lobe intensity can be seen at this page by Harvey Crane of Crane Cams.

[edit] Intake centerline (ICL)

This number represents where the intake lobe's peak lift occurs in relation to crankshaft rotation. It is the point of maximum lift of the intake lobe and is measured in crankshaft degrees. A cam ground "straight up" means that the exhaust lobe's peak lift will happen at the same amount of degrees before top dead center, as the intake valve will peak after top dead center if the intake and exhaust durations are the same. ICL is machined into the cam. When cam manufacturers machine the snout of the cam for the cam sprocket, they will drill the holes with the cam slightly advanced, retarded, or straight up. When installed with stock components, the ICL can't be altered.

Aftermarket timing set gears often have provisions for altering the cam timing by advancing or retarding the cam position in relationship to the crankshaft position. If the cam card shows the LSA is the same as ICL, the cam is said to be ground "straight up." If the ICL is less than the LSA, it is said to be ground "advanced". If ICL is more than the LSA, the cam is said to be ground "retarded". It is much more common to see a cam ground advanced or straight up than retarded.

Example: If the cam has a 110 degree LSA with a 106 ICL, the cam is advanced by 4 degrees. More on this under "Phasing the camshaft", below.

[edit] Exhaust centerline (ECL)

This number represents where the exhaust lobe's peak lift occurs in relation to crankshaft rotation. It is the point of maximum lift of the exhaust lobe and is measure in crankshaft degrees.

[edit] Phasing the camshaft

While is is true that you cannot change the lobes of a camshaft after it is ground (unless you weld and re-grind the lobes), you can alter the characteristics of the camshaft in your motor by installing it in either a retarded or advanced position relative to the crankshaft. For instance, if you have determined that you are making too much horsepower down low and can't hook the tires up, you might want to trade off a little of the lower end power for some higher end power. In this case, you would install the camshaft slightly retarded (although harnessing the power would be the preferred thing to do- a 0.10 second better 60 foot time equates to about a 0.15 second reduction in 1/4 mile ET).

Although all four events (intake valve opening, intake valve closing, exhaust valve opening, exhaust valve closing) will be affected by changing the camshaft phasing, the most important one will be the intake closing point. If you retard the camshaft, you will be closing the intake later, thus bleeding off some of the cylinder pressure and resulting in less low end power. Vice versa if you advance the camshaft. More bottom end, less top end. A rough estimate is a 4 degree change in cam phasing will change the cranking pressure by 5 psi (advancing increases pressure, retarding decreases pressure). To put that into perspective, a rough estimate says a one point change in static compression ratio (as in going from 9:1 to 10:1, or vice versa) changes the cranking pressure by 20-25 psi.

Intake centerline can be altered either by the crankshaft grind or the use of a camshaft sprocket that can alter if the cam is installed advanced or retarded. A later ICL (retarded cam timing) will tend to move the power curve upwards, due to closing the intake valve later. With the faster engine speeds, the intake valve can stay open later without the risk of pushing intake gasses back into the intake runners. An earlier ICL (advanced cam timing) will tend to increase low end torque because at low speeds, closing the intake valve sooner will trap more intake air at lower RPM.

Altering the cam timing by advancing or retarding the ICL can fine tune where the power comes on in the RPM band. Altering ICL should be left to those in the know, and most off-the-shelf cams have been designed by cam companies who know what they're doing. Generally speaking a change of more than 4 degrees either way is a good indication that a better cam grind could be chosen.

There is little point in changing the cam phasing arbitrarily. Unless the camshaft is first degreed, so the exact specs are known, changing the cam position relative to the crankshaft is a total shot in the dark, and could just as easily do nothing or even cause the engine to perform worse than if nothing was done. So, before changing the cam phasing, always degree the cam. Degreeing the cam will also show if there were any errors made during the manufacturing of the cam or other valve train components.

Note: Also see Adjusting the cam timing, or "phasing"

[edit] How the cam specs affect engine output

The following is a rough guide to how the cam specs relates to how the cam is used:

  • Stock/near stock cam: 0.260" to 0.273” lobe lift (0.390” to 0.410” lift w/1.5 rockers); duration @ 0.050” lift around 180° to 200°
  • RV/mild performance cam: 0.300 to 0.310" lobe lift (0.450” to 0.465” lift w/1.5 rockers); duration @ 0.050” lift around 212° to 222°
  • Hot street performance cam: 0.320" or so lobe lift (0.460" to 0.480” lift w/1.5 rockers); duration @ 0.050" lift around 230°
  • Racy street/strip cam: 0.333" and higher lobe lift (0.500” and higher lift w/1.5 rockers); duration @ 0.050” lift around 232° and higher

[edit] Solid vs. hydraulic camshaft

[edit] Flat tappet vs. roller

Roller cam, left; flat tappet cam, right

Telling one from the other visually is relatively easy. The roller cam will have much more rounded lobes, like the big end of an egg. A flat tappet cam will be much more pointed, similar to the small end of an egg. Above is a hydraulic roller cam, the rounded lobes are readily appearant compared to the flat tappet cam to the right of it. More at Identifying camshafts.

Often roller cams will be made of steel and will be shiny instead of a flat black color of a flat tappet cam caused by the wear treatment it is given.

Warning Warning: Roller cams cannot use flat tappet lifters, and vice versa. Besides the possible mechanical interference between a flat tappet and a roller cam lobe, the timing events will be skewed much to badly for this to work. A roller lifter on a flat tappet lobe would have very little duration, a flat tappet on a roller lobe would have way too much duration, even if it could work without mechanical interference.

Ft vs roller.jpg

Crower HIPPO (Hi Pressure Pin Oiler) lifter
Endure-X roller lifter

Comp Cams Endure-X roller lifter is designed specifically for street and marine use. The groove directs pressurized oil to the lobes and roller bearings to keep it alive at low rpm, because it is at low RPM that the roller lifter suffers from a lack of lubrication.

[edit] Things that can "frag" a camshaft and lifters

See: Camshaft install tips and tricks

[edit] Custom cams

Choosing a cam is often something that seems shrouded in mystery. The manufacturers have a hundred years of technology to draw from and millions of dollars and man-hours expended on the research, development and testing of camshafts. They have used that experience to come up with thousands of lobe profiles and grinds that attempt to cover the whole broad spectrum of engines and applications. It's possible that an off-the-shelf grind might be perfectly fine, but it can't hurt for you to look into a custom designed/ground camshaft if a particular combination falls between what's readily available. Most all the cam companies will set you up with a custom ground cam for a fee. And most companies have tech lines and web sites to help you pick the right grind. Take the manufacturer's expertise and recommendations into account when deciding on a cam.

[edit] Simulation software

Another helpful item for choosing a cam is the free software offered by Comp Cam, called CamQuest. It lets you compare how their different cams affect engine output. For more in-depth research, purchase some dyno simulation software like Desktop Dyno 2000 or DynoSim. They allow you to alter the cam specs and the results are displayed graphically on a simulated dyno chart.

[edit] To summarize

The whole system has to match: carb, intake, head flow, exhaust, cam, torque converter stall speed, rear axle ratio, tire size, transmission ratios, and vehicle weight. Some of those things are already decided for you within a small range, like vehicle weight and transmission ratios, while others are easily altered like rear axle ratios and tire size. Choosing a cam with this knowledge might make it a bit easier to understand the reasons why a professional engine builder might recommend a certain cam and it might help you make wiser decisions about your cams in the end. Either way, the right cam choice can make the difference between a well-sorted combination and a clumsy, finicky engine that won't put a smile on your face.

[edit] Resources

[edit] Compression ratio calculators

Warning Note: Some dynamic compression rtatio calculators (like KBs) ask for an additional 15 degrees of duration be added to the IVC @ 0.050" lift point figure. This works OK on older, slower ramped cam lobes, but the faster lobe profiles may need to have 25 degrees or more added to be accurate.

Warning Note: If the intake valve closing (IVC) point isn't known, it can be calculated:

  1. Divide the intake duration by 2
  2. Add the results to the lobe separation angle (LSA)
  3. Subtract any ground-in advance
  4. Subtract 180

This result does not need to have any amount added to the IVC point, like the KB calculator calls for.

[edit] Dyno simulator

[edit] Crankshaft Coalition wiki articles

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