Camshaft Basics
 

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The camshaft is often referred to as the “heart” or “mechanical brain” of an engine. Those are two comparisons that would lead you to believe that the cam is an important part of your engine’s performance, and you would be correct. A camshaft has but one simple job; open and close the valves of your engine. But that simple job can seem very complex, and camshafts are perhaps the most understood item under the hood. In this post, we are going to help you understand some of the very basic elements of cams and how those elements alter performance. This post is not meant to be a tutorial on how to choose a cam for your vehicle, but rather it is intended to give you a foundation to better understand the terminology and functionality of camshafts. Always consult a professional when selecting the appropriate cam for your application.


What is a Cam?

First, let’s define the word “cam”. A cam is, simply put, a projection from a circular rotating piece of machinery that is designed to turn that rotational movement into linear movement. Or, conversely it can be used to turn linear movement into rotational movement. You can see in this simple animation that when the valve is riding against the round, rotating portion of the came lobe there is no movement of the valve. But, when the projection from that round surface (the lobe) meets the lifter it is moved up and down. In an engine, cams tell the valves what to do (open or close), when to do it and for how long to do it.


The Numbers

So, you’re staring at a cam card with a bunch on numbers; 277/285 .614/.621 113°. But what does it all mean? In this case, the first set of numbers is the duration, the second is the lift and the third is the lobe separation angle. Now, let us define each of those independently.


Duration

Duration is the amount of time that the valve off the seat, or open. Cams that have the same duration for the intake and exhaust valves are called single pattern cams. Cams with different numbers for intake and exhaust are called dual pattern, or sometimes split pattern. Now, the amount of time is not stated in seconds, but rather in degrees of crankshaft rotation. The crank turns a chain which turns the cam. If the crankshaft were to turn halfway around while the intake valve was open, then the intake valve duration would be 180, because that would be half of the 360° that the crank will turn in one revolution. So, in our example above, the intake valves are open for a total of 277° of crank rotation and the exhaust valves are open for 285° of crank rotation.

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Another thing to note about duration is that there are two sets of numbers that are given for duration. One is “duration at .050″ and the other is “advertised duration”. Advertised duration varies based on where the manufacturer takes their measurements. Therefore, when comparing cams to one another, the only reliable number to use is the duration at .050. Duration at .050″ lift is measured from the point where the cam moves the tappet (lifter) up .050 inch until .050 inch before the lifter is all the way back down. this gives a measure that can be compared from one manufacturer to another.


Lift

There are three different measurements of lift. First, is the cam lift, also called lobe lift. Cam/lobe lift is the number that relates to how far the cam lobe will move or “lift” the lifter from the central diameter of the cam lobe.

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The next number for lift is the Gross Valve Lift, often shortened to just Valve Lift. The number we see when we shop for a cam is usually this number. Gross Valve Lift is the amount that the valve actually opens from the valve seat. It is expressed in a decimal form that is a in inches. Gross Valve Lift is calculated by taking the cam lift and multiplying by the rocker arm ratio. So, be aware that when you read the number it is only true when using the rocker arm length used to calculate that lift. In our example above, we see an intake lift of .614. If we assume this was with a set of 1.7:1 rocker arms we would know the valve lift was .361. That means that, as the cam rotated, the cam lobe moved the lifter .361 inch which in turn moved the short end of the rocker arm .361 inch. That was then transferred across the rocker arm to the longer arm which multiplied the distance and then opened the valve by .6.14 inch.

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Overlap

The overlap is not often found on the cam card. The overlap is the amount of duration in camshaft degrees when both exhaust and intake valves are open. This is important when talking about scavenging. When the exhaust valve is open and the intake valve is open, the pressure is reduced in the cylinder.


Lobe Separation Angle

Lobe Separation Angle (LSA), sometimes referred to as Lobe Center Angle or Lobe Displacement Angle, is the distance in camshaft degrees that separate the intake lobe centerline and the exhaust lobe centerline. The centerlines of the lobes are the maximum lift points for each lobe. Wider lobe Separation Angles, in the range of 112-116 degrees, move the intake and exhaust lobe centerlines further apart. The result is a smoother idle due to decreased overlap. Lower Lobe Separation Angles, say in the range of 106-111 degrees, move the intake and exhaust lobe centerlines closer together and will increase low end and mid range power. The down side to the increased power is a rougher idle.

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How Do These Numbers Relate To Performance?

Remember, this post is not about choosing a camshaft, because that requires the input of several factors not addressed here such as transmission type, stall speed, gear ratios, cylinder head flow numbers, vehicle weight, engine compression, tire size, exhaust system and more. The following are just generalities of what happens when you increase or decrease one of these parameters we have described above.


Duration Changes

Increasing duration will usually shift the power and torque curves up the RPM range. Longer durations lend themselves to higher RPM operation because at high RPMs the amount of time the valve is open is less than the amount of time it is open at lower RPMs. Opening the valve longer allows the cylinder to fill with more air and fuel. At lower RPMs the time it is open is long enough for some of the air and fuel that just came in to be pushed back out because the valve is open longer than optimal for low RPM operation. In general, increased duration means power in the higher RPM range and is best for light weight vehicles, lower rear end gears, higher stall converters, bigger head ports and flow, and cars with higher compression ratios. Less duration makes better power in the low RPM range which is good for heavy cars or tow vehicles, higher rear end gears, stock or low stall converters, heads that flow less, and engines with lower compression ratios.


Lift Changes

Lift does not change the RPM range, only the amount that the valve is open from it’s seat. Lift is best matched to the flow of your cylinder heads. More lift is generally better, but you must be sure that the valves, retainers, and springs are capable of the lift you plan on producing without binding, and the heads flow more as you lift more. For any given duration, more lift means the the opening and closing faces on the sides of the lobe are more aggressive. They accelerate the lifter faster to the peak lift in the given duration. more aggressive lobe ramps often give great gains because they get the valve lifted higher, faster. The faster you can get the heads to their peak air flow, the more air can enter the cylinder.


Lobe Separation Angle and Overlap Changes

LSA changes primarily change the amount of overlap. Overlap and LSA are closely related. Increasing overlap gives engines a rough, choppy idle. The extra time both valves are open causes 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 fluctuations in vacuum and uneven fuel metering. Once at high RPM, the overlap is beneficial because the faster moving gasses create a slight vacuum and help to pull in new intake charge into the cylinder, a phenomenon known as scavenging. Modern EFI engines can be tougher to tune due to increased overlap.

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Nice post APEX!

great post :thumb:

Always a good refresh for noob DIYers like me..

Great post.

Thanks, I hope it is helpful. :thumbsup:

+1

we should link the string of Apexs posts for all the questions

awesome post for the non cam savvy person such as myself.

Well explained

AWESOME post!! Now i just need to read it when Im not drunk off of Makers Mark.... lol

That is a pretty good idea. I will put something together.

That is my goal. To take some of these complex topics and make them understandable.

Thanks you!

:lol: Nice choice. They make that stuff just down the road from my house.

Thanks for posting this.

I do have a question. I've listened to many videos and it always sounds to me as if a cammed car sounds like it wants to stall, why it that? And is the car easier to stall?

I've looked into a cam and loved the sound, but the "stalling sound" always concered me.

Read the Lobe Separation Angle and Overlap Changes part.