When it comes to camshafts, and the timing thereof, a lot of guys feel that understanding it is a black magic. But it isn't really that hard to understand. As the camshaft spins, the lobes open and close the intake and exhaust valves in correlation with the motion of the piston. To keep things running smoothly, there is a direct relationship between the shape of the cam lobes and the way the engine performs at different speeds. Basically, when the intake valve opens and the piston starts downward on its intake stroke, the air/fuel mixture developed in the intake is pulled into each cylinder. When the piston reaches the bottom of its intake stroke, the air/fuel is being pulled into the cylinder at a high velocity (speed). If we close the valve immediately after the piston finishes its downward motion, that incoming airflow abruptly stops. But, if we leave the intake valve open just a fraction of a second longer, the momentum of that fast moving air/fuel mixture continues to force more of the mixture into the cylinder (scavenging effect) as the piston starts its compression stroke (return to top of cylinder). What this means is that we want to keep the intake valve open as long as possible, but as you can figure out, keeping it open too long has a detrimental effect on power.
You could spend hours on the computer reading and learning about how a camshaft works, and what the terms like lift, duration, and valve events are, but we wanted do a simple test in regards to camshaft timing. We wanted to know--and verify--what really happens when we install a camshaft at "zero" (straight up), and then simply advance it four degrees. We know that advancing or retarding the camshaft's timing moves the engine's torque band on the rpm scale by moving the valve events farther ahead or behind the movement of the piston, but how much, and in what direction? As car guys, we all like to think that we are expert engine builders and tuners, and a lot of enthusiasts will experiment with advancing or retarding a camshaft from "straight up" (zero) and see what timing position works best for their application. Just to give you an example of cam timing theory, a cam with 107 degrees of intake-lobe centerline will actually be centered at 103 degrees ATDC when the camshaft is installed four degrees advanced.
01 With a couple of quick spins on the Dynojet Dyno, we felt comfortable with our baselin
02 The hardest part of the job was disassembling and reassembling the front of the engine
03 The Comp Cams timing set (PN 7103) in Alex's 340 has provisions (keyways for nine diff
Effects of Changing the Cam Timing
Advancing or retarding a camshaft's timing from its original "zero" position causes the valve events to happen either earlier or later in the engine's cycle. A camshaft that is advanced four degrees will cause each opening and closing event to occur four degrees of rotation sooner than before. This changes the cylinder's ability to build pressure. On the other hand, if the camshaft timing is retarded, the intake valve will close later (usually sometime during the compression stroke). It should be no surprise that this drops cranking compression and hurts low-rpm power. But as the rpm increase, and since cylinder filling is aided by the extreme velocity of the air/fuel charge in the ports, a retarded camshaft will theoretically help power at higher rpm by holding the intake valve open longer--or will it?
We wanted to actually put cam advance theory to the test, so we enlisted Alex Dunlap and his Dart to help us out. Alex's Dart has a 408-inch small-block with 10.5:1 compression, a new Comp Cams hydraulic roller cam with their retrofit lifters, CNC-ported aluminum heads, and an older air-gap style intake with a 750 Holley. It's a typical small-block build that is found under a lot of hoods. Since the cam was initially installed at "zero" (the dots were aligned straight up), we figured this would be a great way to test the effects of a simple cam-degreeing. To make it simple, the timing chain was also from Comp and featured a lower gear with advance and retard keyways machined in. If this worked, we would be gaining power without spending any money (except for a couple of gaskets). So we felt it was worth a try. The adjustment only took us a day, and to find out if we were happy with the results when we finished, you'll need to follow along.
04 If your timing set is not adjustable, you'll need to get the correct offset keys for t
05 You'll notice that the bottom gear has numbers on both the keyway and the teeth. When
06 Once you decide what setting is correct for your application, reinstall the timing set
The word lift refers to actual distance that the cam lobe actually "lifts" the lifter and pushrod, pushing the valve open. This is how much the valve is "lifted" off its seat at the cam lobe's highest point. If the valves don't open wide enough, they will cause a restriction to the air trying to enter or exit the cylinder. However, opening the valve past a certain point will not increase the flow in or out of the cylinder. A good way to demonstrate this is with the garden hose in your backyard. When you first start to turn the water on, the flow increases, but after a turn or so, opening the valve more has no effect on how fast the water comes out of the hose.
This measurement is taken in degrees of crankshaft rotation. Duration correlates to the amount of time (duration) that the valve stays off of the seat during the lifting cycle of the cam lobe. This measurement is taken when the cam lobe actually starts raising the lifter, until it finishes putting the lifter back at its start position. If the lobe separation is a constant, a longer duration will produce more peak power, a rougher idle, less torque at low rpm, and peak power at a higher rpm. As strange as this may sound, more duration can be helpful in engines that run at a high rpm. The extra degrees of time that the valve is open at high rpm gives the air more time to get into (or out of) the cylinder in spite of the piston's stroke.
Lobe Separation Angle (LSA) is the measured angle, as measured in camshaft degrees, between the maximum lift points of the intake and exhaust lobes. If you look at the end of a camshaft, the complete distance around the lifter lobe is 360 degrees. Keep in mind that lobe separation angle is said to be measured in degrees of the camshaft. Lobe separation is what affects valve overlap. This overlap affects the nature of the engine's power curve, idle quality, idle vacuum, etc. If we keep duration at a constant, a wider LSA will give an engine more peak power, a rougher idle, more torque at lower rpm, and peak power also occurring at a lower rpm.
Not only will a wider LSA give you the aforementioned qualities, but it also results in less cylinder pressure, which is great for higher compression street engines.
07 We reassembled the front of the engine and got ready see if the change made a differen
08 Just like our baseline, we made a few back-to-back pulls on the dyno, just to make sur
09 Right out of the gate, our test Dart picked up 3/10 second in the quarter-mile and 4 m
This is also known as Lobe Center Angle, and is the crankshaft degrees that are measured when the intake and exhaust valves are both open. This "overlap" occurs at the end of the exhaust stroke and at the beginning of the intake stroke. Increasing lift duration and/or decreasing lobe separation increases overlap. When an engine is running, the exhaust valve needs to stay open slightly after the piston passes top dead center. This helps keep the momentum of the exiting exhaust gases to maximize the amount of exhaust gas pulled out (scavenged) from the cylinder. The intake valve on the other hand, opens before top dead center, and uses the momentum of the exiting exhaust gas to start pulling the intake charge into the cylinder. More is not always better, as too large of a lobe center angle can result in too little overlap to make good power. Too little overlap causes the lack of complete expulsion of the exhaust gases, and less intake charge filling the cylinder. Smaller amounts of overlap produce a smoother idle, and a slight benefit in top end horsepower. This effect on performance is directly linked to engine rpm. Higher engine speed causes greater exhaust-gas velocity, which relates to greater momentum of the exiting exhaust gases. This is why a longer-duration camshaft produces power higher in the rpm range. It also causes the loss of low rpm power and economy that larger cams experience. But at higher engine speeds these conditions are minimized due to the slight lag time it takes to get the intake charge moving into the cylinder.
The cam's centerline is used to correlate valve timing to the crankshaft's rotation. This is again measured in crankshaft degrees. Centerline is explained as the number of degrees that the crankshaft must rotate from top dead center until the cam has rotated to the peak (or centerline) of a given intake or exhaust lobe. For the engine to run at peak performance, the valves must open and close at the correct time in relation to the piston's position and the crankshaft's speed. The intake centerline is the point of highest lift on the intake lobe. The exhaust centerline is the point of highest lift on the exhaust lobe. The cam centerline is the point halfway between the intake and exhaust centerlines.