I know we have an open chatter about cams, but i think this is a little more then open chatter, and this way maybe people will read it more.

I just thought since we have an advanced tech and theory forum, I might as well help give as much info. as I can. It may take while for me to write up some more posts, but I will when I have time.

I am going to go over the basics of cams and how they work with your motor.

Three things to look at when buying a cam(s): Duration, lift, and overlap.

Duration- is the number of degrees of the crank rotation that the valve is held open by a cam (don’t forget that the cam spins at half the speed that the crank spins at). A general rule of thumb is that the more duration the cam has, the more top-end power it will create, and this does cause you to lose low-end power. The more duration the cam has the more the valve will stay open during the cranks rotation. The longer the valve stays open the longer the time the cylinder can be filled. This is important at high rpm, since there is less and less time for the cylinder to fill.

Lift- is the height that the valve is lifted of the valve seat. Usually the more lift (with in reason) as the higher the valve is lifted the more flow that can go by. Look at it this way, if you have your window open half way you let in less air then if the window is open ¾ of the way. The draw back to having more lift is that the valve opening and closing speeds become high increasing the chance of valve float. This is why you should upgrade to stiffer valve springs if the cams lift is moderately aggressive.

Overlap- is the point at which both intake valves and exhaust valves are open at the same time (at the end of the exhaust stroke and the beginning of the intake stroke). Overlap is important, because having both intake and exhaust valves open at the same time creates better scavenging of stale exhaust to occur. This is because the flow from the head into the cylinder (from the intake valves) creates a good push to get rid of stale exhaust out of the cylinder. Just like anything though, too much of a good thing is bad. If you get to much overlap the flow from the intake valves will push gas out of the cylinder before it has been used, thus wasting fuel, which equates to power. One more draw back of to much overlap is reversion into the intake ports. Reversion is when fuel/air is pushed back into the intake ports. This reversion causes intake charge dilution at low rpm as the backflow in the intake port gets in the way of cylinder filling. Two things will occur because of this, cylinder pressure becomes poor at low speeds because of the incomplete filling and the fuel/air intake charge becomes diluted because of air rushing back into the intake ports causes low speed misfire. This is why aggressive cams that have big overlapping, create a ruff loopy idle. This misfiring causes the motor to skip a beat in rhythmic fashion at idle and low rpm. Usually firing once every four revolutions when the cylinder gets enough fuel to touch off. This occurrence is called 8-storking, because of the misfiring and the car skipping a beat it fires every other stroke of the 4-stroke cycle.


Explination 2

1. Max Lift or Nose
2. Flank
3. Opening Clearance Ramp
4. Closing Clearance Ramp
5. Base Circle
6. Exhaust Opening Timing Figure
7. Exhaust Closing Timing Figure
8. Intake Opening Timing Figure
9. Intake Closing Timing Figure
10. Lobe Separation

BTDC means Before Top Dead Center

ABDC means After Bottom Dead Center

BBDC means Before Bottom Dead Center

ATDC means After Top Dead Center

Intake Open BTDC is the number of degrees before top dead center that the intake valve opens on the exhaust stroke.

Intake Close ABDC is the number of degrees after bottom dead center that the intake valve closes on the compression stroke.

Exhaust Open BBDC is the number of degrees before bottom dead center the exhaust valve opens on the power stroke.

Exhaust Close ATDC is the number of degrees after top dead center the exhaust valve closes on the intake stroke.

Duration is the number of degrees the intake valve stays open as measured at .050 inches of lifter lift. This is measured by the degrees that the crankshaft rotates. More degrees of duration will make the engine operate in a higher rpm range.

Exhaust Duration is the number of degrees the exhaust valve stays open as measured at .050 inches of lifter lift.

Overlap is the number of degrees that the exhaust and intake valves are open at the same time


Cam Lift is the distance in inches that the valve is pushed


Advancing/Retarding Cam Timing
Advance
Begins Intake Event Sooner
Open Intake Valve Sooner
Builds More Low-End Torque
Decrease Piston-Intake Valve Clerance
Increase Piston-Exhaust Valve Clerance
Retard
Delays Intake Event Closes Intake
Keeps Intake Valve Open Later
Builds More High-End Power
ncrease Piston-Intake Valve Clerance
Decrease Piston-Exhaust Valve Clerance

Specifics
-A camshaft lobe is an eccentric that converts rotating motion into linear (up and down) movement. To do this, a lobe, or bump, is created from a true circle known as the base circle of the cam - also known as the heel. As the lobe rotates, the lifter follows the rise of the lobe, which moves the lifter upward. The maximum amount of rise is known as lobe lift.
-The maximum lift point on the cam is called the nose, while the inclined areas leading up to and away from the nose are called the ramps.
-Stock camshafts offer relatively short duration and low lift numbers since the factory is after a crisp, smooth idle and excellent part-throttle operation.
-If we increase duration, the intake valve is now open for a longer period of time during the induction cycle. This added duration tends to affect engine power by decreasing idle vacuum and shifting the power curve to a higher rpm. This reduces low-speed throttle response and power while increasing power at the higher engine speeds. Too much duration, especially in stock-type engines, will kill power everywhere, and you will end up with an engine that is extremely lazy.

(All the above taken from Web cams and various sites to save me from spending all night typing it all up )


courtesy of "allc5" and "98vtec" for the write ups