This came off a forum I used to belond to, it will give you a good idea how the cam works, what the numbers mean, and how to use this information for selecting a cam for you build. I see so many questions on 'what cam to use' so hopefully this will help. This thread really progressed and was a lot of fun so if your interested in the link to the complete thread let me know. It will be a lot of reading so get comfortable, grab a beer or soda, a bag of chips and enjoy the reading.(The link is at the end of this post)
Doc

buckhtr77]Cam 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 expamples 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 rpmpower, 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 th

 

Thank you, Doc, for the explanation. Here is a picture from the SE '08 Parts Catalog that shows cam timing--hope it helps. Although it is for the SE 257 cam, the opening and closing of the intake and exhaust valvesare nicely presented:



Also, I found the camshaft comparator that allows the cam specs of three different cams to be compared simultaneously a nice feature:

http://www.bigboyzcycles.com/tccams.htm

 

Howdy Folks,

Thanks to both of you guys, This should be an auto link to any "what cam" question IMHO. Most people that ask cam questions get advice that is based on the answering persons personal prefferences and not on what needs to be understood. I just wish that the link didn't have astrics in it. Doc, you may want to repost the link and replace the . with the word dot. Who knows, you may get away with it, LOL

Best to All

Pixs

 

ok now that we have the simple form.Lets delve into the ralaionship of cam to piston speed due differant rod lenght,which in turn changes piston dwell time at the top of the storke,plus there the piston speed.We need to know where the valve is in relation to the piston.Does it folw better with more speed and would less or more aggressive lobe be better.How about if we change the rocker arm ratios,how about all of this again at 4 degress advance and then 4 retard.
Ok sorry forgot this was simple veiw,but damn its fun to play with all these variables and see what if any differance we can make.HAVE FUN you got love that all these clumps of metal made so many ways can make som much differance in the amount of power our toy has.
Yea I have a problem IM addicted to horsepower.

 

thanks much DOC
Good stuff

 

Like I said there is a lot more to this but what I posted will get youa good idea what to look for in a cam selection fora street machine.

Your right though, 1.73 rocker arm ratio will open the valve further but what most people don't know it also accelerates the valve opening too and this effects the piston to valve relationship plus the valve to rocker arm geometry has to be rest by raising the rocker box and most cases a lash cap added to the valve stem or even a longer stem is needed.
Stroke to rod ratio is another thing all in itself....again you are correct in saying the dwell time at TDC and BDC is extended, I used a 8' rod in a 116.5 Pro Modified Sporty to give me more time to burn the fuel at higher rpms before the piston out traveled the flame front and to achieve max cylinder pressure at 17* ATDCwith 26* total ignition advance. yes I've been down that road also, the smell of the fuel will make you an addict....lol

 

I was just bs ing around some Im one of those who freaking loves the most in depth details of engine building and love to expolre every part for more power.Like I said i have a problem need help.
As for addiction running blower cars(not street toys real blower cars) alcohol funny,dragsters and little bit of nitro well just warp your brain .Have fun and lets reverse the rotation of the earth with some real torque.

 

All I had was a 116.5 Pro Mod Sporty....this would get me in the gate just so I could go around smelling the eyeWATERING nitro and the prostockVP fuel burning everywhere.....oh well back to racing.....it's been 6 years since I stepped foot on the track but our little 116.5 small tire, naturally asperated sporstser, that turned 182 hp and 176 tq the best run was a 8.6 with consistent 8,8 but today the same class is in the 8.3...so the bike is under dust in the wharehouse along with the memories and fun times. [&o]WAAAAH AHAHHA !!![&o]

 

Hey this should be kept as a stickie....what do you think?