Camshafts are one of the most important components in a modern internal combustion engine. This is a simple guide on what all the numbers mean and translate to when you go looking for a camshaft.
Terminology:
Duration - The duration of a camshaft is measured in degrees of rotation. There are two predominant methods for measuring and expressing the duration of the camshaft, advertised and "at fifty". Advertised is measured starting with the actual moment the valve moves from it's seat, and ends when it is fully closed against the seat. A better, and more realistic representation of the cams profile, is measured "at fifty", or at 0.050" of valve lift. This figure will be less than the advertised duration, and is most commonly used in domestic applications.
Lift - The lift of the valve measured in inches. Often times the cam specs will list the lobe lift which is the mechanical displacement of the pushrod by the cam. This figure needs to be multiplied by the rocker ratio of the engine in order to derive the actual lift of the valve. For example, a 0.250" lobe lift with a 1.3 rocker ratio translates to an actual lift of 0.325".
Centerline - This is the midpoint, measured in degrees, of the duration of a camshaft lobe. The intake and exhaust lobes will have different centerlines.
LSA or Lobe Separation Angle - This is the measure in degrees between the centerlines of the intake and exhaust camshafts.
Overlap - This is a measure, in degrees, during the event where the intake and exhaust valves are both open. Sometimes this is expressed using at 0.050" numbers, others at advertised duration.
How cams work, and what makes them effective.
As the crankshaft turns it rotates the camshaft. The lobes of the camshaft push onto the pushrods and this pivots the rocker in the valvetrain. The rocker then pushes down onto the tappet (mechanical lash) or hydraulic lash adjuster and valve spring which in turn pushes the valve downwards.
First a note on valve lift versus flow. As the valve begins to open it doesn't begin to actually flow until a certain point. This is due to a boundary layer that builds up at the valve seat. As the valve begins to open more it will begin to also flow more. This is where port geometry and the intake side of things begin to really affect how much lift potential is present. As the valve increases in lift, generally you will see greater amounts of flow potential, to a point. In most OEM engines the heads will generally flow well until the 0.400" - 0.600" lift region where they tend to return less and less gains. This is due to the port itself and not the camshaft. This is also the core basis on why the duration @ 0.050" is a much more effective number to work with.
During a normal engine cycle you have a series of pressure events and valve moments designed to make the most of them. The intake valve actually opens before the piston reaches TDC - Top Dead Center. This is to allow the valve to be open to it's full potential once the piston begins it's down stroke. The piston displaces in the cylinder, creating vacuum which then pulls the air down into the cylinder. It is very common for intake valves to close on the compression stroke of the motor due to the elasticity of air; by hanging the valve open a little after the piston has reached the bottom you can pull in a slight amount of additional mixture. Once the upstroke begins both valves are closed and the engine goes through it's firing cycle. Pressure at this point is very high in the cylinder and the exhaust valve opens. Contrary to popular belief the majority of the exhaust gas is blown down the port, due to the differential of pressure between the exhaust port and the chamber, rather than being pushed out by the piston.
The timing of these events is important because any residual pressure in the system will cause reversion on the intake stroke, and cause mechanical pumping losses on the exhaust stroke. The camshaft is what controls this timing.
Overlap is either your best friend, or the bane of your existence. First off, just a few degrees of overlap at advertised duration is not enough to cause any actual scavenging effect in the cylinder because the valves are restricting the flow potential at their low lift moments. You need measurable overlap at 0.050", or greater, to actually see a strong scavenging effect, but this can only occur if the exhaust manifold is designed properly. If there is alot of residual pressure in the exhaust manifold, read: backpressure and restriction, then you will see very little from overlap. With a good header, collector, and crossover (For example, X-Pipe or H-Pipe) you can tune the exhaust pulses to utilize the negative pressure wake following the exhaust pulse to pull a vacuum on an adjacent exhaust tube which aids in drawing exhaust out of the cylinder by increasing the pressure differential. In a turbo car you will always have higher exhaust manifold pressure than intake manifold pressure, so overlap is not something that you want alot of because it will backflow exhaust into the cylinder and possibly into the intake manifold. In a supercharged engine, you can make greater use of overlap because of the much higher intake manifold pressures compared to a naturally aspirated engines. At lower RPMs alot of overlap tends to dilute the mixture and cause a rough idle in all engines.
Optimizing a camshaft really is matching the rest of the system to the camshaft. Larger durations, narrow LSAs, and big lifts work well with properly designed headers and intake setups, and generally are at home in high RPM applications. You have to be honest with your setup and choose a cam that is optimal with what you have. Overcamming the motor makes it lethargic and really takes away from it's overall potential. Big cams sound cool, and might impress others, but nothing is more laughable than a choppy idle V8 running a 15 because the owner wasn't honest with himself.
Other considerations are the profiles themselves, which are generally built to suit a particular valve train setup. Some manufacturers will push the boundaries and sometimes end up with cams that have harmonic issues, but in the V8 world I'd imagine this is less so than others due to the longevity of the design.
Terminology:
Duration - The duration of a camshaft is measured in degrees of rotation. There are two predominant methods for measuring and expressing the duration of the camshaft, advertised and "at fifty". Advertised is measured starting with the actual moment the valve moves from it's seat, and ends when it is fully closed against the seat. A better, and more realistic representation of the cams profile, is measured "at fifty", or at 0.050" of valve lift. This figure will be less than the advertised duration, and is most commonly used in domestic applications.
Lift - The lift of the valve measured in inches. Often times the cam specs will list the lobe lift which is the mechanical displacement of the pushrod by the cam. This figure needs to be multiplied by the rocker ratio of the engine in order to derive the actual lift of the valve. For example, a 0.250" lobe lift with a 1.3 rocker ratio translates to an actual lift of 0.325".
Centerline - This is the midpoint, measured in degrees, of the duration of a camshaft lobe. The intake and exhaust lobes will have different centerlines.
LSA or Lobe Separation Angle - This is the measure in degrees between the centerlines of the intake and exhaust camshafts.
Overlap - This is a measure, in degrees, during the event where the intake and exhaust valves are both open. Sometimes this is expressed using at 0.050" numbers, others at advertised duration.
How cams work, and what makes them effective.
As the crankshaft turns it rotates the camshaft. The lobes of the camshaft push onto the pushrods and this pivots the rocker in the valvetrain. The rocker then pushes down onto the tappet (mechanical lash) or hydraulic lash adjuster and valve spring which in turn pushes the valve downwards.
First a note on valve lift versus flow. As the valve begins to open it doesn't begin to actually flow until a certain point. This is due to a boundary layer that builds up at the valve seat. As the valve begins to open more it will begin to also flow more. This is where port geometry and the intake side of things begin to really affect how much lift potential is present. As the valve increases in lift, generally you will see greater amounts of flow potential, to a point. In most OEM engines the heads will generally flow well until the 0.400" - 0.600" lift region where they tend to return less and less gains. This is due to the port itself and not the camshaft. This is also the core basis on why the duration @ 0.050" is a much more effective number to work with.
During a normal engine cycle you have a series of pressure events and valve moments designed to make the most of them. The intake valve actually opens before the piston reaches TDC - Top Dead Center. This is to allow the valve to be open to it's full potential once the piston begins it's down stroke. The piston displaces in the cylinder, creating vacuum which then pulls the air down into the cylinder. It is very common for intake valves to close on the compression stroke of the motor due to the elasticity of air; by hanging the valve open a little after the piston has reached the bottom you can pull in a slight amount of additional mixture. Once the upstroke begins both valves are closed and the engine goes through it's firing cycle. Pressure at this point is very high in the cylinder and the exhaust valve opens. Contrary to popular belief the majority of the exhaust gas is blown down the port, due to the differential of pressure between the exhaust port and the chamber, rather than being pushed out by the piston.
The timing of these events is important because any residual pressure in the system will cause reversion on the intake stroke, and cause mechanical pumping losses on the exhaust stroke. The camshaft is what controls this timing.
Overlap is either your best friend, or the bane of your existence. First off, just a few degrees of overlap at advertised duration is not enough to cause any actual scavenging effect in the cylinder because the valves are restricting the flow potential at their low lift moments. You need measurable overlap at 0.050", or greater, to actually see a strong scavenging effect, but this can only occur if the exhaust manifold is designed properly. If there is alot of residual pressure in the exhaust manifold, read: backpressure and restriction, then you will see very little from overlap. With a good header, collector, and crossover (For example, X-Pipe or H-Pipe) you can tune the exhaust pulses to utilize the negative pressure wake following the exhaust pulse to pull a vacuum on an adjacent exhaust tube which aids in drawing exhaust out of the cylinder by increasing the pressure differential. In a turbo car you will always have higher exhaust manifold pressure than intake manifold pressure, so overlap is not something that you want alot of because it will backflow exhaust into the cylinder and possibly into the intake manifold. In a supercharged engine, you can make greater use of overlap because of the much higher intake manifold pressures compared to a naturally aspirated engines. At lower RPMs alot of overlap tends to dilute the mixture and cause a rough idle in all engines.
Optimizing a camshaft really is matching the rest of the system to the camshaft. Larger durations, narrow LSAs, and big lifts work well with properly designed headers and intake setups, and generally are at home in high RPM applications. You have to be honest with your setup and choose a cam that is optimal with what you have. Overcamming the motor makes it lethargic and really takes away from it's overall potential. Big cams sound cool, and might impress others, but nothing is more laughable than a choppy idle V8 running a 15 because the owner wasn't honest with himself.
Other considerations are the profiles themselves, which are generally built to suit a particular valve train setup. Some manufacturers will push the boundaries and sometimes end up with cams that have harmonic issues, but in the V8 world I'd imagine this is less so than others due to the longevity of the design.
