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Ok, i'm fairly new to the world of engine building. I have a 350 i'm gonna be building with a buddy of mine who knows what he's doing....the cam he gave me today has a Duration of 292/292 and gross lift of .480"/.480". Now looking at the lunati cam website (it's a lunati cam) the rpm range is 2000-7000. Just wondering what duration and gross lift mean and how that works and what the 292/292 and .480"/.480" means. I just wanna know what it means when he talks about it or I see it on paper.
Cams design is perhaps the most complex aspect of engine design. There is no exact science that says a cam with these numbers is going to perform exactly this way. I will try to explain what the numbers mean and how they should effect the performance of the enigne. The gross lift is the amount the cam opens the valve. More lift typically means the valve is open more which means more fuel/air can enter the cylinder which means more power. That is fairly simple. Why does every cam not have maximum lift? It's hard on the valve gear. I'll have to explain duration before I can continue. Duration is the number of degrees of crankshaft rotation that the valve is open. There are 720 degrees of crankshaft rotation in a complete cycle. You have probably noticed that cams with short durations also have smaller valve lifts. This is because there are fewer degrees of crankshaft rotation for the valve open and close and the acceleration required to gain such a lift in such a short time becomes too great. You have also probably noticed that cams with large durations have very high working rpm ranges. An engine has an ideal air/fuel velocity in the intake port of about 2500 feet per minute. A long duration cam keeps the valve open for a long time so the fuel/air mixture has plenty of time to meander into the cylinder. So much time, that it doesn't need to move at the idea 2500 feet per minute to fill the cylinder by the time the valve closes. On the other hand the air has to rush in with a short duration cam. As rpm increases, the amount of time available to fill the cylinder decreases and this is when the long duration cam becomes an advantage. You may also see lobe seperation angle or LSA on a cam spec. LSA is the number of degrees between maximum lift of the exhaust valve and maximum lift of the intake valve, typically 110 degrees. At low rpms, as the piston travels downwards on the intake stroke, the exhaust valve will still be open and the cylinder will draw air in from both the exhaust and intake side of the engine. Not very efficient. At higher speeds, the fast moving exhaust pulse moving down the header actually creates a small vacuum that can be used to draw fresh air into the cylinder as long as the exhaust valve remains open. The sciency term for this is exhaust pulse scavenging. Wider LSA's, greater than 110 degrees, mean more torque but at a cost of high rpm efficiency. Tight LSA's can also be used to bleed off cylinder pressures in high compression engines that have problems with detonation at low rpms. I hope this helps, it's a lot to learn at once and it's very difficult to explain.
wow, i must say that is complex, but very helpful. I'm wanting to learn as much as I can. That helps alot....i'll probably read it more than once. Thanks for the info!
76GMC1500, you had me nodding my head until you got to here...
"If you have a 292 duration cam with 110 degrees LSA, both valves will be open for 182 degrees of crankshaft rotation."
Duration and overlap are measured in crankshaft degrees. Lobe Separation Angle is measured in camshaft degrees. You're subtracting apples from oranges.
Both valves can't possibly be open for 182 degrees of crank rotation, not even at zero tappet lift. The motor would never fire and run.
Here's a typical 292 degree cam ground on 107 degree centers and showing 78 degrees of overlap @ 0.004" tappet lift and 18 degrees overlap @ 0.050" tappet lift.
http://www.cranecams.com/?show=brows...tType=camshaft
Here's a 320 degree duration cam ground on 108 degree centers and showing 104 degrees of overlap @ 0.0186" tappet lift and 64 degrees of overlap @ 0.050" tappet lift.
http://www.cranecams.com/?show=brows...tType=camshaft
This is the wildest solid lifter grind that Crane makes for the small block Chevy.
"Wider LSA's, greater than 110 degrees, mean more torque but at a cost of high rpm efficiency."
Sorry to disagree, but just the opposite is true. Narrower LSA's will give more low end torque, but peak sooner and lay down on you on the top.
" Tight LSA's can also be used to bleed off cylinder pressures in high compression engines that have problems with detonation at low rpms."
Sorry again, but it's actually the intake closing point that must be manipulated to alter cylinder pressure.
I was kinda scratchin' my head over that , too. Other than those numbers, the explanation was pretty clear and concise.
This is the best intro article I've found - recommended by techinspector.
Elgin on Cams
Here's another -
Cam Timing
I need more practical experience to really get a grasp on cams.
The guys here have years of experience.
I'm gonna call Crane for a recommendation then pass it by the group for approval, pros, cons and contrasts.
Bert
Ok, I fixed the most blatent error. That's a lot of information to pull out of my head, I had to be wrong about some things.
It's ok, at least you're up to the plate, swinging the bat. Everybody including me pops up a foul ball once in a while :)
thanks so much guys....your knowledge is amazing. I love learning about how a chevy motor works....soaking it all up like a sponge. Another question then, with that size cam (292/292 duration and .480/480 lift) Would you recommond going with double springs or even triple.....sounds like this size cam will be working the springs hard. Right now I have just stock head springs. Oh, the heads are 76cc heads.
Jeepguy,
I think you would be better off if you could find a good (GOOD LUCK) engine builder and have him recommend what you need for your engine. In fact the 292 advertised duration cam is pretty lumpy at idle and will cause low speed driveability problems unless you really know how to set-up your carburation, ignition torque converter. gearing, etc. SB Chevy engines are very forgiving but they can be easily screwed-up also.
Go to a good bookstore and buy David Vizard's book on building SB Chevys and the one on hotrodding the SB Chevy. Read them before you spend a penny.
After 50 years of building, racing, etc. I sometimes wish I had taken up ceramics or basket weaving.
Ok, here's how it works simplified:
lift, how far the lobe rises off the base circle, multiply that by the rocker ratio and you get valve lift, what most cams are advertised at.
Basically the more the better at the expense of parts life. Also every head has a max lift that if they surpass flow will actually be reduced. Most stock performance SBC's it's between 480" and 510" lift optimum.
Duration, how long the valve stays open. the longer the higher RPM the engine can turn. It creates overlap and poor efficiency at lower rpms. Gives that "rough choppy" idle.
It is measured at different amounts of lift, advertised can be .002"-.006" lift, and there's duration@50 or at .050" valve lift{more accurate}
LSA, or Lobe Seperation angle. It lets you know the basic powerband the cam will have. A tight LSA 104will have a rough choppy idle from the extra overlap, reduce the bottom end power, peak really strong in the midrang and fall off on the top end {keep in mind that this "powerband" is not the RPM of the engine, which it determined by the duration}
A wide LSA 114 is more efficient due to less overlap. It will have better bottom end, not much peak power, and good top end. They result in a wide, flat torque curve.
Also they are good for blower motors and Nitrous b/c the exhaust doesn't react with the fresh intake mix as much.
If you have a low compression engine a wide LSA can help it build cylinder pressure and if you have too much compression a tight LSA will help bleed some off {generally speaking}.
:)
Ok, it's amazing how much i've learned already in just the last day from this post. All your describtions have made sense(some more than others :D ) I haven't spent a dime yet. I got this motor from a buddy who is leaving town and had parts to give. Got a 350 block, heads, intake and that cam.....but knowing what i have now cam wise really helps in deciding which route i wanna go in the build. I'm definatly gonna get those books and read up on whatever I can. I know an engine builder....he built my 400 chevy in my Jeep, plan on picking his brain and parts bin. Once again thanks soooooo much.