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My next question then: How does lift and duration affect performance, and which cams have the lift and duration that enhance the low and mid-range the most?
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That's not exactly a simple question...
The duration is the period expressed in crankshaft degrees that the cam holds a valve open, and 'open' is defined as the valve being lifted off it's seat 1.0mm on metric bikes or at 0.040" or 0.050" on American bikes.
In general, the longer the duration (valve opening time) the more effective the camshaft is at higher rpm.
This is because the overlap (the period in which intake and exhaust valves are both open) increases with duration and allows improved scavenging of the cylinder head.
Scavenging occurs when the exhaust mass traveling down the exhaust pipe creates a rarifaction behind the initial exhaust pulse that draws fuel air mixture into the cylinder prior to the demand of the cylinder on the intake stroke.
Scavenging is made possible by the camshaft overlap but is limited by intake and exhaust tuning since it relies on the inertial mass of the intake and exhaust gasses. If the intake and exhaust restrictions are high the flow of gasses is reduced and scavenging effects are reduced.
A longer duration on the intake valve also increases the ram air effect of the intake pulse which tends to 'push' some extra mixture into the cylinder as the intake valve is closing.
The ram and scavenging effects are also tied to throttle opening and are most prominent at full throttle settings. Ram and scavenging are sharply reduced at part throttle settings.
The problem with long duration cams at low rpm is that both the intake and exhaust valves are open at the same time but the scavenging and ram effects aren't working. This causes an effective decrease in cylinder compression and corresponding loss of efficiency at low rpm.
The loss of efficiency can result in a rough idle at normal idle rpms and generally requires an increase in idle rpm to compensate. The loss of compression will also cause a reduction of the engine braking effect.
The combination of ram and scavenging tend to produce a non-linear torque curve that peaks at a high rpm with the peak determined by a combination of the of the intake and exhaust tuning lengths. This high rpm peak is combined with a sharp torque dropoff at low rpm and part throttle.
The lift of a camshaft is the relationship of the lobe height to the base circle of the cam and determines how far the valve opens. If you've got a cam with a 30mm base circle and a 40mm base to lobe height, then it's a 10mm lift cam.
In general, more lift means less restriction to airflow, better cylinder filling and if timed correctly, better scavenging and more efficient use of the ram effect.
Lift is directly tied to duration by acceleration -- the amount of time in crankshaft degrees it takes for the cam to reach full lift. The rate of acceleration in turn determines the force required to overcome the inertia of the valve train. The inertia of the valve train and the acceleration forces of the camshaft also determine the requirement for the strength of the valve springs required to prevent valve float and clatter.
The duration then partially determines the maximum lift. If the duration is too short for the lift the acceleration ramps built into the cam grind becomes too steep and the valve 'floats' -- the cam train loses contact with the camshaft and continues opening from inertia after the camshaft has passed it's peak lift point. With the valve train flying loose this way it's no longer under the control of the deceleration ramps built into the camshaft.
Since valve float is the result of valve train acceleration it's also tied to rpm. The higher the rpm, the greater the tendency toward valve float for a given valve train. Extreme lifts and/or extreme rpms generally require stronger valve springs.
In a worst-case scenerio valve float can cause the valve springs to bind and the valve to slam into full travel, the valves to remain open during the combustion cylcle resulting in burnt valves, and the valve train slam into the camshaft on closing; all of which causes huge stresses and can cause broken springs or rip the heads off the valves resulting in massive engine damage.
Valve lift also has an effect on fuel mixture in that the valve opening acts somewhat like a spray nozzle to break up and vaporize the fuel droplets in the airstream and this is dependent on the amount of valve opening. If the air velocity is too low across the open intake valve the fuel droplets in the airstream won't vaporize as efficiently and some cylinder wetting may occur during low rpm operation.
The increase in airflow from a camshaft with higher lift than stock generally improves cylinder filling and raises the torque curve across the the entire rpm range of the engine but exponentially increases the stress on the valve train.
The stock 'blue' camshaft is was designed for the intake and exhaust restrictions of the Classics motorcycles and no other Triumph cam will provide the same torque/horsepower curves.
The 'green' cams as used on the bikes with the bigger filter box perform well in the midrange with no noticeable deterioration of low rpm operation but require modification to the stock airbox or it's replacement and rejetting to benefit from the installation.
The 'red' cams are subject to the same restrictions as the 'green' cams but reduce low rpm torque and require an improved exhaust system to achieve their high-rpm benefits.
Jim
Linear Mode
