Cams and More Cams

More info from Doc1

Question:
Can someone please explain what all the numbers involved with cams means? I look at the charts and have really no idea what the numbers mean. All I really know to look at is the lift. I have no idea what all the terms mean. How does difference in these numbers for different cams affect how the TQ and HP spread out across the RPM range? I would appreciate some help in this matter. Below are 2 examples of what I've been looking at in the charts for different cams. One is a 590 lift cam and one a 650. What do all these other numbers mean? Thanks again for help.



- Open-Close Lift Duration Centerline TDC Overlap
Intake 22/44 .590 246 101 .209 42
Exhaust 48/20 .590 248 104 .188 -


- Open-Close Lift Duration Centerline TDC Overlap
Intake 26/52 .650 258 103 .235 50
Exhaust 56/24 . 650 260 106 .212 -[/quote]


Are you ready for some reading!!!!!
First cam example:
The intake opens at 22 degrees before TDC (top dead center)and it closes at 44 degrees ABDC (after bottom dead center) The exhaust opens at 48 degrees ( before bottom dead center)and closes at 20 degrees (after top dead center). The lift is .590, thats how far the valve is off the seat. The duration is 258 degrees, this is how long the valve is off the seat, or open, in crank shaft degrees. TDC lift is how far both valves are off the seats at TDC during overlap, (this is important because the valve comes the closest to the piston at this point). When you get into the .220 and higher your loosing some torque on bottom end. Overlap is when both the intake valve and the exhaust valve are open at the same time, in this case it is the intake open (26 degrees and the exhaust closing (20 degrees) add those two together and you get a 42 degree overlap. Over lap uses the exhaust system scavaging low pressure to help fill the cylinder with a fresh charge of air even before the intake stroke starts. The more the over lap the more top end it will give at a price of giving it up on the bottom end. Also the larger the over lap the more important the exhaust system becomes. (A stock cam has -6 to +6 degree over lap depending on the year and if its a carb or fuel inj.) The CENTER LINE number is an imaginary line that simulates the max lift on the nose of the cam, we use the center line numbers when we are timing the cam to the relationship with the piston at TDC. (i.e. this is the + 2 degree advance timing we talk about in the threads to advance or retard the cam timing. The intake closing (44 degrees here) is the MOST IMPORTANT valve event because when it closes the cylinder starts to build cylinder pressure. A later closing intake valve improves higher rpm power, but it will cost you on the bottom end because you lose cylinder pressure. An early closing intake valve improves low end power but hurts the top end performance. The SECOND MOST important valve event is the exhaust opening (56 degrees here). The later the exhaust valve opens the better performance you will have on low to mid range because it makes better cylinder pressure. The cam will have a lower to mid range power band and used for a heavier bikes or low end torque. The earlier the exhaust valve opens helps top end power because it has more time to scavage the cylinder. The cam is good for a lighter bikes that runs at a higher rpm power band.
Now that explains what the numbers are and what they represent. Now lets put them in some kind of respective order.
There are three categories of cams, MILD...MODERATE...MAX EFFORT.
The MILD cam is classified as being in the duration area of 210 degrees to 240 degrees, the MODERATE cam is from 241 degrees to 265 degrees, and the MAX EFFORT cam is 266 degrees and up. The MILD cam is normally what we call a bolt in cam...i.e. it needs no head work or high lift springs to operate in its breathing capabilities. The MODERATE cam will need some port work and spring changes to meat its breathing requirements. the MAX EFFORT cam needs MAJOR head work and it runs top end only so its for the guy that is twisting the throttle wide open all day long.....or for the big inch motors and drag racers.
A stock twinkie cam has a duration of 218 degrees....where does that fall? It has a overlap or 6 degrees...does it need special pipes to make it work? ( I forgot the opening and closing # sorry)
So when you compare cams now I hope this will give you a guide line. A thing to remember as a rule of thumb for every 10 degrees of duration you add to a cam you will lose 500 rpms of power on the lower end, for every 10 degrees of overlap you add to a cam you will lose 200 rpms of power in the lower rpm range. The more duration and overlap the more we up the compression to gain some of these losses back. Now it does get more involved than this explanation but it will get you a good idea on cam selection.
OK now take the first cam example and compare them to the second example and tell me what you see....which one has more top end power and which one will develop more low end torque