I was on the Samba and stumbled onto an old thread where I had written a bit about what camshafts do, and with the occasional questions about cams on here I thought some may benefit from it. I've changed it up a little bit for here (it was originally in response to a someone looking for a cam for a specific combo) so here goes:
Think of the camshaft as the timing device in the engine. It decides when the valves open and shut, how long they stay open (duration) and how much they open (lift). Ideally a cam/rocker combo is chosen after such details as displacement, intended powerband and use, heads, carbs and exhaust are settled on. If you look at a cam's specs you'll see the advertised duration (or just duration), duration at 0.050" lift, and lift at the cam. I don't recall the duration of a stock cam (250 degrees?), but I have seen the duration at 0.050" listed at 214 degrees and lift at the cam at about .290"(about .330" at the valve with 1.1/1 rockers). It also has very soft ramp angles (great for reliability, not so great for performance), lifting and seating the valves gently back on their seats.
This cam provides a dependable powerband from idle to a little over 4000 rpm. VW used this cam in their later (1300-1600 cc) type 1 motors in part because it provided enough power to push the car satisfactorily and it made the motor VERY HARD TO BREAK. It will run all day at 3500-4000 rpm (which is what it does at highway speeds). Try doing that in a V8!! The small intake, small valve, small port heads and low flow exhaust are all used to make the motor the torquey, indestructable low rpm unit we all know and love.
Adding 1.4 rockers to a stock cammed motor equipped with dual carbs and a header is a good way to up the power; the higher ratio rockers increase duration slightly by 8 or 10' (one day I will plot it out) as well as adding about .060" lift at the valve. This is a very good match for stock heads, which don't flow any more past about .400" valve lift. The top end (or redline, if you will) is extended slightly and there is more power through the midrange to the peak rpm without destroying the torque curve. Have a look at http://kaddieshack.com/1600dynoresults.html to see what adding dual carbs and a header, and then 1.4 rockers does for the performance of a stock 1600.
When you add duration (the number of degrees or amount of time the valves are open) you move the powerband of the motor. Put an Engle W110 (284 degrees, 247' @ 0.050" and .425" lift at the valve) in a 1775 with dual carbs and a 1 3/8" header and the thing makes great power to 5500-5700 rpm (exactly where depends on which carbs and if the stock valve heads have any clean up work in the ports) and the small loss in power in the very bottom end isn't noticed in normal driving.
Put in an Engle W130 (308', 267' @ 0.050", .455 lift at the valve) and now you have a motor that has a potential powerband of 3500-6500 or 7000 rpm. Now of course the motor will need dual Webers or Dellortos, well ported 40x35 heads and a 1 1/2" exhaust to use all that duration and lift. In traffic with a stock stroke motor you might notice a slightly "soggy" bottom end; it all depends on how enthusiastic you are. More duration will just move the powerband up higher.
Stroking a motor can hide a few extra degrees of duration. A stock stroke combo that uses (for example) an Engle W110 can quite often be happy with an Engle W120 (294', 253' @0.050", .431" lift at the valve) when a longer stroke crankshaft is added. The added stroke adds torque (low end power) as does the change in rod ratio when you use a longer stroke crank with stock length rods. With some properly ported 40x35 heads, more lift also helps thoughout the rpm range. 1.25 rocker arms are a good idea to get the most out of that expensive porting work you paid for; with the W120 they'd provide about .490" lift, and this change again adds more overall power while extending the rpm peak from about 6-6200 to 63-6500rpm.
I've used Engle cams as examples because the W series is easy to understand; there are other cams that will fit your needs. In all the examples I mentioned the lift @ 0.050" because that number is the best indicator of what a cam will do. Some cam manufacturers have cams with long ramps and more advertised duration (I've noticed some CB/Eagle cams are like this) but the @0.050 numbers will tell you the effective duration (and approximate powerband) of a cam. Again using Engle's W series, think of the W100 (236'@0.050") as mild and the W140 (274'@0.050") as wild. Look at Berg's catalogue http://www.geneberg.com/cat.php?cPath=165_2773 for a good description of the Engle W series and their applications. Looking at the duration@0.050" numbers for each cam and you'll see the progression:
stock-214' @0.050"- 4000rpm peak
W100-236'- -5000rpm
W110-247- -5500rpm
W120-253'- -6000rpm
W125-262'- -6500rpm
W130-267'- -7000rpm
W140-274'- -7500?
And there is a W160 (it's a drag race cam, power to 9 or 10,000rpm depending on heads, carbs....
Even with a big motor, there are compromises. If you value being able to jump in your tub at a moment's notice and travelling long highway trips (and never worrying about "unscheduled maintenance"), something like a W125 or W130 is probably not for you. Higher revving motors take more maintenance (and sometimes part$$) to keep running well, and although you may love the rocket ship like performance the higher rpms provides, even high strung big motors don't really love toodling around at rpms much below their lower rpms ranges for long periods.
PS-My favorite cam for Webers and big valve heads is the W125 (262'@0.050" and .455" lift). I ran it in a 1750 (69x90) with 44IDF's and some wickedly ported 40x35 heads and a 1 1/2" merged exhaust and it was my main transportation for about 4years. Amazingly well behaved in traffic (for a stock stroke motor) and ripped over 6500rpm with great power. Yes, the bottom end was a little soggy, but I always thought a stroker would have cured that
.........
I hope someone gets something out of this. Al
PS- There are other factors in camshaft manufacturing (lobe centers, ramp speed, base circle diameter are the ones I can think of right now) that affect how a cam performs and lives, but I was trying to keep it reasonably simple...
