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