Actually, on Desmo engines the cam DOES close the valves.
Hey, I learneded sumting!
Desmodromic valve operating mechanism is where the camshaft controls both the opening and closing of the valve. One benefit of this is that when the motor is over-revved, the valves are still controlled, whereas when they are returned by springs the valves can "float" and hit the piston. Another is that the manufacturer can use wilder cam grinds for better performance.
Ducati and Mercedes are two major companies who have used the desmodromic system in racing engines. Ducati has the most experience of any manufacturer in the world at successfully applying desmodromic valve control to production machines.
Note that in the diagram, the springs are only used to close the valve the last fraction, to aid starting.
Valve clearances are adjusted by shims. The rocker arms have to be removed to insert the shims.
http://www.usq.edu.au/users/grantd/m...ages/DESMO.GIF
A cam is nothing more than a stick with lumps on it. It's made by every cam manufacturer, just for your application, so they say, and will make your engine run perfect and have gobs and gobs of power!!!! You don't believe me? Just ask them. Budlight:idea:
Cam- It's what you blame your rough idle on, in your '76 Camaro with a hole in the muffler and vacuum leak.
Cam- It's what's for dinner.
Cam- one of many parts I'm going to need if I ever hope to KICK JACKPUNX'S AS*!
Fills the oil filter with metal particles when #6 exhaust flattens out.
Brian
But..when the roller lifter breaks don't forget about the benefits! Shot peened rods. :notam:
..it creates hours of research in the hunt for the right one, caused by distraction and confusion by conflicting manufacturers opposing suggestions, creating endless questions, ultimately generating a hopeful performance in a four stroke engine.......that about sums it up. :squiggle:
One hell of a painfull sextoy.
..it creates hours of research in the hunt for the right one, caused by distraction and confusion by conflicting manufacturers opposing suggestions, creating endless questions, ultimately generating a hopeful performance in a four stroke engine.......that about sums it up. :squiggle:
What you THINK is the right one. That's the clincher.
This is a good start. I'm just tired of so much "bad advise" flowing around the internet and I think it would be good for discussion on some good basic why's and how's.
Chris
We'll start tomorrow, I need to finish a quote on some stuff for a company.
Yeah, let's talk about "real" correct rocker geometry, and the importance of it in order to communicate that precice cam motion to the valves, which is where it all comes together. Making that valve do exactly what it's supposed to do, at the exact time it's supposed to. That's my pet peeve. The cam is the "brain" of the engine. The critical part is the opening and closing of the valves, in relation to the piston, and the cam is what tells them exactly when to open and/or close, how far to open, how long to stay open, and how fast to do it. All of these four factors...opening/closing, lift, duration, and "ramp speed/lobe will determain the operating characteristics of the given motor, weather it's a stock production motor, or an 18,000 rpm F1 motor.
HOWEVER...if you can't properly transmit that information to the valve, the best lobe in the world is worthless. This takes a lifter, a pushrod, and a rocker arm, in most of the engines we would normally deal with, which is usually referred to as an overhead valve, cam in block pushrod type engine. Crankshaft rotation is transmitted to the camshaft via a pair of timing gears and chain. Camshaft rotation, which is exactly 1/2 of crankshaft rotation, is transmitted to linear motion through a lifter or "folower". In order to accurately transmit the information from the cam lobe to the valve, the lifter must follow the cam lobe perfectly. The lifter has mass (weight) and wants to get thrown off the cam lobe when the cam is spinning, so we have a spring on the valve to, first, keep the valve closed, but also to help keep pressure on the lifter, so it follows the cam correctly.The linear motion of the lifter is transmitted to a rocker arm through the pushrod. Pushrods flex, and also have mass, and resist changing direction like the lifter. More work for the valve spring to do. Then there's the rocker which flexes and bends as it transmits the linear motion of the lifter, through the pushrod, into another linear (actually radial) motion contacting the valve tip and forcing it open against the pressure of the valve spring. Whew!!
Now there's a lot of stuff going on there, and that freekin valve is doing it's best to behave, but with rockers a rockin, and lifters a liftin, and pushrods..., well, you get it...a lot of information from that cam lobe is not getting to the valve when it's supposed to, and some not at all. Every little loss in motion, through friction, flex, or misalignment in what we call the, "valve train", equals crankshaft rotation (piston motion) that's not getting taken advantage of, which equals less power. Don't forget, for every degree that's wasted at the cam or not transmitted properly, it's two degrees at the crank. TWICE the error. And every cylinder that something "different" is happening through miss adjustment, mismatched parts, etc, is a cylinder out of sync.
Proper valve train geometry which includes properly designed parts for the right application, are just as, if not more important than the cam itself.
Uh oh. I'm rambling.
[QUOTE=steelcomp]opening/closing, lift, duration, and "ramp speed/lobe will determain the operating characteristics of the given motor, weather it's a stock production motor, or an 18,000 rpm F1 motor.
Thanks for the "Bumpstick 101" education Professor. :hammerhea However, as I recall, an 18,000 rpm (or higher) F1 motor does not have a cam... They have computer controlled pneumatic actuated valves...
S&S (058's buddy
a camshaft is the 'brain' of the engine.......with that being said someone please explain what a 3/4 race cam isthis one has always baffled me......I mean I would be more of a 7/8 race cam kind of guy or maybe even a 54/64 race cam kind of guy.......oh hell might as well go full race cam
:squiggle: guess us country phucs that build engines on garage floors only understand simple terms like lift and duration and lobe seperation. Just when I thought I got something right......damnit......
Omega
Posting Permissions
Reply With Quote