can someone please explain cam numbering?

[s13rookie]

im just wondering about the numbers for cams, like 248/11.0, okay i know they are for duration/lift. but what does that actually mean and how do i use those numbers to pick the cams that are best suited for my needs? thanks to anyone for their help, i just dont want to buy the recommended cams, i want to know how and why, im trying to learn as much as i can, thanks, matt

[DSC]

you mean like the measurements or whatever? Like they are open for 248 somethings (mili-seconds?) and open a max of 11 (feet, inches, milimeters?) something else?

I read this somewhere a while back but unfortionately don't remember...doh :(

I'm curious too :cool:

[Jeff240sx]

The first number (if it's a whole number) like 232 or 248 (s14 and s13 cams, respectively) is the degrees that the cam will hold the valve open at least .050". The remainder of the 360* of the circle is the valve closing, closed, then opening.
There is also a number. It's not a whole number, and it sure as hell isn't 11. That is the ammount of valve lift in inches. It usually reads (JWT cams) as .390". That is the max lift of the cam.
JWT cams will read out .390/275.
Also.. when I say valve lift... I really mean valve drop.. but that just sounds funny.
-Jeff

[s13rookie]

i have been looking on jspec to try to find the figures i was using, but to no avail, any way the numbers were, i think 248/11.0mm if that makes more sense

[uiuc240]

yeah, Jeff, he's talking lift in millimeters

so, your 248/11.0 cams are 248 degrees duration, 11.0mm max lift.

make sense? The cam is egg shaped, and the valves stay open for a rather large portion of the revolution...this is where overlap comes in to play. If you have a longer duration, this will inherently give more overlap, seeing as how the valves will be open longer (more time at the same time).

overlap = good for NA, not so important for turbo.

Eric

[s13rookie]

so if picking cams for a nasty turbo vehicle, would i want to lower or raise the amount of (valve, im assuming) overlap, or would i want none at all?

[Jeff240sx]

You'd want no (or very little) valve overlap in a turbo car. The reason for this is so you don't lose all your boost thru the exhaust valves.
-Jeff

[s13rookie]

ok great, thanks for the help, im starting to understand now. probably going to head to borders this weekend and try to find an engine book, to explain some other things im an idiot on