From left to the right: Standard 'Lawnmower', B10, P3, 'F'cam.
OK, since one of the pics that's going up on the gallery is of 4 different camshafts and most specifically the lobes thereof I suppose I'd better give a little bit of a shpiel on what cams actually do. For those with a good mechanical knowledge you can go back to downloading cyber-porn and playing with your rubbery friends.
This is NOT supposed to be a thorough treatise on cams and their functions so any nit picking, point scoring or even pointing out of rampant inaccuracies is verbotten !!!! OK ;-).
So what is a Cam?
A cam is a component so shaped that by it's rotation it causes another part in contact with it to move in a different manner.
There are lot's and lots of different types of cam but those used to actuate the valves in a conventional poppet valve 4 stroke engine are pretty simple.
OK, go and mug a primary school kid. G'warn, you're a rough tough bikie.
You don't need anything important, just nick their geometry set, most particularly their compass, you know that thing you stuck in Julie Grainger in 3rd form and had to go and see the headmaster about, yeah, that. Pencil in one bit, pointy thing on the other, good for drawing circles.
If you can't find a primary school kid then probably the next best option is to get a 20c piece and a 5c piece, oh, and a pencil.
OK, now, taking care not to hurt yourself or anyone close by draw two circles next to each other, one with a radius of about 10cm and next to it one with a radius of about 25cm. They can overlap a tiny bit if you're only allowed a small bit of paper but just side by side is fine.
Next, take a straight edge, if the PS student had a ruler so much the better otherwise any old straight edge will do, side of envelope, cornflakes packet, whatever. Now use this to draw a line that is at a tangent to both circles from the sides of one circle to another. When you have done this you will have something that looks not entirely dissimilar to a cam lobe. Good Eh!!!!!
This is basically the sort of cam used to open the valves in our types of motor, it's pretty simple, it's tough and it's been used for centuries.
Now we have to divide it up into various parts.
Starting with the big circle, the bit furthest away from the pointy end, this is called the 'Base Circle' when the follower, rocker or valve is running around this bit the valve will be closed. At the other end, the pointy end, the part from where the tangential line touches the small circle
is called the 'Nose Circle'. Now the flat sides of the tangential lines between the two are known as, depending on the direction of rotation, the 'Opening Flank' and the 'Closing Flank'.
Usually on the part of the bigger circle just before and after the tangential lines meet it from the opening an closing flanks there are also areas which are unlikely to be circular that will gently introduce the follower to the rapid acceleration/decelleration caused when the cam
starts pressing on it. These areas are called the 'Ramps'.
OK, are you happy so far?
As the cam rotates the follower will run off the 'Base Circle', on to the opening flank of the cam, via the opening ramp. At this point the valve will start to open and will continue to do so until the follower reaches the top of the 'Nose Circle' of the cam. At this point the valve will be at it most open, commonly known as 'Full Lift'
As the cam rotates and the follower passes this point the valvesprings will press the follower down so it will move down the closing flank, over the closing ramp and back onto the base circle at which point the valve will be fully closed once more.
Generally speaking the ramps will operate over about 30 degrees of cam rotation and during this time the valve will begin to open or close fully but it's movement will be so small as to be fairly difficult to asess accurately. It's for this reason that when setting the timing on many camshafts that vary from the manufacturers own specifications and they have to be 'Dialed In', that is installed to other than the original cam timing marks, there will be a different clearance, (the 'Setting Clearance') specified to the one usually given for valve lash adjustment which is commonly known as the 'Running Clearance.' This of course depends on the cam and should be supplied with the camshaft at the time of purchase otherwise you have no idea on how to time it.
In the same way that if you set your ignition points gap too small the spark will be retarded, if you install your cam, even if it's to the specified timing in degrees, if the setting clearance is too big the valve timing will be retarded and if it's too small it will be advanced. While with most common camshafts a couple of degrees either way probably doesn't matter too much if you are installing a real 'Buggies Dickus', 'Mr. Fur-Chest' type cam it can make the difference between building a snorter and having a nasty pile of smoking bits !!!!!!
When setting your valve clearances it is vital that the follower be sitting on the 'Base Circle' of the cam. One absolutely foolproof way to do this is to turn the motor until the valve you want to check the clearance on is fully open. When you have ascertained this point then turn the crank through 360*. This way you *Know* that valve has to be in the middle, approximately, of the base circle. I say approximately because ther are such things as asymetric cams where he opening and closing flanks are different and the peak of the nose circle may not be absolutely opposite the middle of the base circle. (On Guzzis I think that the B10 'Performance' cam is slightly asymetric but not enough for it to cause a problem with valve lash adjustment.) With most machines though if you set the piston on the side you want to check the clearances on at TDC compression. Both inlet and exhaust clearances can be checked at this point as both cam followers are sitting on the base circles of their respective cams.
OK, that's the basics. Sometime over the weekend I'll try and give a bit more of an insight into the various properties various shapes and timings of cams can produce in a motor and why.
If people aren't interested, tell me and I'll shut up.
OK. Now we've had a bit of time to let that first bit sink in lets go again.
You should by now have a rough grasp of what a cam looks like and how it opens a valve. Without being pedantic we can accept that the valve spring will close the valve, (I'm *NOT* going to start delving into stuff like Desmodromics here OK.) but the rate at which it is closed, at least if the springs are doing their job, is controlled by the closing flank of the cam too.
So! We have one cam for each valve, (OK, or pairs/multiples of valves in some multi valve engines.) but what relationship do the cams that open andclose the inlet valve have with the ones that open and close the exhaustvalves?
Well, on your Guzzi, and most other commonplace motors an awful lot. With most common cams not only are the opening and closing flanks symetrical but the actual cam timing. The points at which the valves open and close are as well, for both inlet and exhaust cams, but at the opposite ends of the four stroke cycle.
Let's just back-pedal a bit an go over how the cycle works.
AS the piston approaches TDC at the end of the exhaust stroke the inlet valve begins to open. At this point the exhaust valve is also still open but soon after TDC the exhaust valve will close. Likewise, at the other end of the cycle the inlet valve doesn't close until after the piston has passed BDC and is beginning to rise back up the cylinder on the compression stroke but also, on the power stroke the exhaust valve will open before the piston has reached BDC. Why would this be so?
Well, the answer lies in two things, Mass and Inertia.
While Mass and Weight aren't the same thing lets say that they are. As you know, if something weighs a lot you have to push it hard to get it to move. In doing so you impart engergy to it and it will want to keep moving. Basically it's like a 'Battery' for the energy you put into it. This 'Stored' energy gives the whatever it is you have pushed 'Inertia'. Once it's moving it don't wanna stop !!!!!
This holds true all the way from the asteroids we see in silly American films that 'Can't be knocked off course by any power known to man and will lead to the death of all life on Earth when it collides with us !!!!'
(Usualy said by a git in a silly hat and too many brass buttons.) to really tiny little things like atoms. It's the 'Laws of Physics' which as we all know are written by boffins in white coats with slide rules sticking out of their pockets.
Back to our motor !
The reason why the valves aren't timed to open at TDC and BDC is because what they do is allow the movement of gasses. Gasses have Mass which means that they need energy to get them moving and they also, once they are moving, have inertia, which means they will damn well keep moving until something stops them !!!!
Once an engine is running it's a pump. A heat pump ! Always remember you are burning very old sunlight so you can tell the ecofacists to bite their bums :-).
Gasses are rushing into the cylinder, being crushed and then heated by the very old sunlight, and then when they have expanded due to the heat they are allowed to rush out of an enclosed space.
Why do they rush into the cylinder? Because there is an area where the gas pressure is lower in there than it is outside at that stage in the cycle of the motor.
Why do they then rush out again? Because at that stage of the cycle the pressure outside the cylinder is less than that inside it.
The thing is that with all this rushing in and out the gasses, be they hot or cold, are all travelling at a fair old clip. Because the gasses have Mass, once they are moving they don't want to stop. So when a motor is working, although the flow will be jerky, both in and out of the cylinder the gas will try to keep moving due to it's own Inertia.
Therefore, when the inlet valve is shut, the incoming gas will bank up behind the inlet valve like water behind a dam. As soon as the valve begins to open it will try to rush in, ESPECIALLY, as the gasses rapidly leaving the cylinder on the exhaust stroke will, because they want to keep moving, (They too have their own inertia.) will have created a low pressure area in the cylinder even before the piston has reached TDC.
This is the reason why, at the end of the exhaust stroke and the beginning of the induction stroke you have a period where both valves are open. The inlet cam is just beginning to open the inlet valve while the exhaust cam will just be coming off it's closing flank onto the closing ramp. If the cam is matched well to the motor it will deliver a decent low pressure area in the cylinder to draw in gas from the inlet side while, (with the aid of the exhaust pipe), preventing too much of the new, incoming charge, disappearing straight out of the exhaust valve !!!!
After the piston has reached BDC on the induction stroke, once again, the inertia of the incoming gas will keep it entering the cylinder for a bit, even though the piston is in fact on it's way back up, already compressing the incoming charge. The one part of the cycle that works contrary to this is the point at which the exhaust valve opens. This is more dependent on the fact that by the time the piston reaches the point where the exhaust valve begins to open the pressure on the piston is negligable because of the rate at which the piston is retreating before the expanding gas that has been heated by the ancient sunlight.
OK, that's enough for tonight. Tomorrow we'll look a bit at why certain things to do with a cam's profile will effect the way that the motor behaves.
Well, Zebee reckons this is all sounding a bit like ‘White Mans' Magic'.
Sorry to say it but it's just about to get a whole lot weirder.
Last time we were examining the hows and whys of gas movement and how the cams controled the the opening and closing of the valves, when and why.
This time I'm going to try and explain how the profiles of different cams will effect the way that the motor produces power and where in it's speed range that power will be produced.
Lets try looking first at three separate things that are incorporated in any camshaft.
1.Lift. Lift is simply the amount that a cam will open a valve at any point in it's cycle. Using a degree wheel, a bit of graph paper and a dial gauge you can produce your own cam chart quite easily that will allow you to plot the amount of lift per x number of degrees of rotation of the cam. If you are going to be doing porting or head work using a flow bench it's damn near vital to produce such a chart as knowing when the valve is at any particular lift will allow you to work on increasing the flow at the most auspicious part of the valve's opening cycle. It's a common mistake that people think that having a cam with a very high lift will automatically give you much better flow, nothing could be further from the truth and a very high lift cam can produce plenty of mechanical problems of it's own.
Duration. The duration of a cam is simply the amount of time that the cam will hold the valve open. Obviously, the longer the valve is open the longer it will be flowing gas. This would seem to indicate that the longer the duration, the better. Alas, it's not that simple.
Overlap. Overlap is not a property of an individual cam but is a period of time when both cams, the inlet and exhaust, are both holding their respecive valves open. This occurs at the end of the exhaust stroke and the beginning of the induction stroke and it's here that this very important stage of the cycle that it can contribute to the running of the motor. The thing is that unfortunately overlap is one of the most critical items in engine building but at the same time one that can be most effected by other changes like exhaust pipe and inlet tract length.
OK. Nows the time to go and get that picture you drew of a camshaft the other day, you know, the two circles joint together by tangential lines.
What ! You threw it away ?!?! Buddha on a bicycle, do another one then.
And keep your compass and ruler handy you'll need them again in a minute.
One of the best ways to go from here is to get you to draw two of those > cams and cut them out. Do one where the base circle has a radius of say > 20mm and a nose circle of 10mm. Now do another one where the base circle is again 20mm radius but the nose circle is 15. When you do this make the centres of the base and nose circles 5mm closer together on the second > cam because this way the distance from the bottom of the base circle to > the top of the nose circle, (And therefore the ‘Lift'.) will remain the > same.
Now you have to draw a straight line on another bit of paper and get some sort of pin-board and a drawing pin. Take your two cams and pin them, through the centre point of the base circle, onto the straight line. If you have the straight line going up and down it's easiest, just think of that line as the direction that the cam follower will be moving in.
So now you have your two cams superimposed one on top of the other. They should both be the same height, ie, they will have the same lift BUT the one underneath with the bigger nose circle will be appreciably fatter at the top. Now, point both of the cams at 90 degrees to the line you have drawn.
At this point your Follower, (The up and down line.) will just be approaching the opening flank of the cam. Next, turn first the bottom cam with the bigger nose circle through 30 degrees so the follower starts riding up the opening flank. OK, now turn the one on top with the smaller nose circle also through 30 degrees.
Ooo-errrr ! Wot's ‘appened? The cam with the bigger nose circle has lifted the follower more, innit? Why? Well because of the larger nose circle the angle at which the opening flank is to the centreline of the cam is less acute so as the camshaft turns the follower will move further,(And faster.) than on the cam with the smaller nose circle. With both of these cams you will eventually end up with the same maximum lift and the time it takes to get to maximum lift will be the same in both cases. But with the cam with a larger nose circle the initial lift will be more rapid and greater than on the small one.
So, generally speaking, and that's all I'm trying to do here, a cam with a > fat top to it's lobe will generally give more rapid opening than one that has a pointy lobe.
OK, back to your geometry set and draw yourself another cam with the larger nose circle radius but this time move the two circle centres 5mm further apart so you effectively get more ‘lift'. Pin this cam underneath the other two and do the turny-turny thing again and see what happens?
OK, this stuff is not only probably hard for you to take in but it's the devil to write, it's doing my brain in :-). I think if we take it in little bites it's best so I'll leave it there for now.
Please, if people have any questions, fire ‘em off and I'll do my best to answer them. If we could for the time being stick to stuff I've already covered though I think this would be best.
Right. Yesterday we had a brief look at how changing the profile of a cam will cause the way the valve moves to change.
I was going to go into how the profile and relationship between the ramps, flank profiles and base to nose circle ratios effect things like duration but to be honest it's a very complex area and I've only got a sketchy understanding of it myself. At the end of the day none of us are really likely to need detailed information on this and if we do I'd advise heading off to do a diploma in mechanical engineering as to get a full grasp of it would, I'm sure, require a fair bit of heavy math. And I'm pretty sure you don't want to go there, I know I don't !!!!!
Lets use the KISS principle here, OK?
Now, any camshaft can only be made to work in any particular motor at one engine speed. This is because of a number of factors but the principle one is the tuned length of the inlet and exhaust tracts. At different engine speeds the rate at which gas is pumped through the motor is different.Also the amount of time it takes to complete a cycle varies with the engine speed.
The thing is that the ability for the motor to pump efficiently is governed by the cam design and this can only work in conjunction with the correct pipe length and inlet manifold length as what governs the high and low pressure areas needed in certain parts of the entire motor is the speed of sound and gas inertia. While the second is variable depending on the speed the gas is travelling the first is, for our purposes, a constant.
As the piston approaches TDC at the end of the exhaust stroke the majority of the exhaust gasses will of exited out of the exhaust valve. Once theyhave started moving their inertia keeps them moving so there is a low pressure area in the exhaust port that will keep drawing them out. At this point the inlet valve will start to open and if the inlet manifold length is correct there will be a high pressure area of fresh air, (In fact it's fuel air mix, don't quibble OK.) that has banked up behind this valve since when the valve closed the inertia of the incoming charge kept the gas moving.
As soon as the valve opens this air is going to take the path of least resistance and start moving into the cylinder, aided by the low pressure area in there caused by the exhaust gasses rushing out. Clear as mud ?
OK, you say, what about the speed of sound? Where does that fit in?
Well, as the exhaust gasses and their accompanying heat and noise disappear down the exhaust they are, effectively, in an enclosed space and move in a pressure wave towards the end of the pipe travelling at the speed of sound. When this wave reaches the end of the pipe it bursts out into the open air, > (a low pressure area.) but at the same time a reverse pressure wave starts > moving back up the pipe in the other direction. Now, ideally, this pressure wave will reach the exhaust valve just as it's on the point of closing and if everything is working together it will arrive just in time to prevent any of the new, incoming, charge from leaving the cylinder while also allowing just enough time for the egress of all the old, spent gasses.
The problem is of course that because the speed of sound at which the pressure wave travels is constant, but the time taken for complete cycles changes with the engine speed unless you can either change the length of the exhaust pipes (Yamaha EXUP valve.) or alter the point at which the exhaust valve closes, (Variable valve timing.) you are always going to end up with a compromise. The same also holds true for the incoming charge and the point at which the inlet valve opens and the inlet tract length/volume.
SO! What do manufacturers without an unlimited budget and a desire to produce a serviceable everyday product do? They go for a compromise.
You'll usually find that most machines, (Like our old Guzzis, are designed to run fairly well in the 4,500 RPM-7,000RPM rev range.) are built to run well in the ‘Midrange'. Why? Because this corresponds to a sensible roadspeed, the cam/intake/exhaust will work to near their optimum somewherein the middle of this range but the cam etc won't be so extreme that the motor either won't run down low or will run out of puff if you make it run much faster than this. It's a compromise but it's a very sensible compromise.
You may well ask then why If the ‘Standard' choice of cam is so good would anyone want to change it?
Well there are a number of reasons. If other parts are changed a ‘Standard' cam may no longer be suitable for the new engine parameters. Other owners may want something different from their engines, perhaps they will be willing to sacrifice a bit of docility at low engine speeds for more power.
More power requires a higher engine speed so you pick a cam that will offer that.
There will always be drawbacks though. Almost without exception you will find that any cam that gives increased duration and lift will also accelerate wear as the forces required to move parts further and faster are going to be greater and higher stresses will be imposed on parts like cam
flanks, followers, valve stems and guides. Also if a valve is held further off it's seat, for longer, it has less time to dump heat, (The only places a
valve can dump heat are through the seat when it's closed and from stem to guide. In your Guzzi motor when you are cruising along at 120KPH your exhaust valve stem will be glowing dull red. If it can't dump that heat it will either burn or the head will fall off, this makes a horrid noise and tends to be expensive :-)=). ) so it will fatigue and wear more quickly.
Using a cam with radical overlap also produces unwanted side effects. What makes high overlap desirable is that it will allow really good scavenging of the old gas and good cylinder fill, but only at high RPM. The stupid ‘F' cam I have which runs 112* of overlap as opposed to the ‘Standard' cam's 40* will, I'm sure be absolutely hideous to try and use on a road bike. It probably won't tick over at all, definitely not under about 2,000RPM and will no doubt spit back and blow the carbs off with the slightest provocation. Fuel ecconomy will be lousy because until the motor is spinning pretty fast and the exhaust length is tuned correctly it will be flinging loads of unburnt fuel out through the exhaust. Oh yes, It'll go like a rhino with a spear up it's arse, but like the rhino it will probably make the bike cantankerous and unpredictable and will need a lot of carefull setting up :-)=).
At the wrong engine speeds, a cam of this sort will, in conjunction with the exhaust pipe length lead to the reverse pressure wave reaching the exhaustvalve at a very inauspicious point and the pressure inside the cylinder will be greater than that in the inlet tract and it will actually cause the incoming charge to be pushed back through the carburettor. The cylinder will be poorly filled and if you're really lucky residual heat will cause it to ignite. This is what causes spitting back and the blowing off of carbs ! The symptom is well known in classic racing circles where big, highly tuned, old singles are reknowned for suffering from whatis called ‘Megaphonitis' at lower RPM.
For now I'll leave it at that, if people have questions I'll try my best to answer them. When Peter Cusack gets the pics I sent him up in the gallery, > (No hurry Peter, I'm not hassling.) there is a reasonable one, I hope, of > the lobes on a ‘Standard' Guzzi lawnmower cam, a B10 belonging to John Y., a P3 and the Stupid ‘F' cam. Providing they come out OK it gives a pretty good idea of what different cams with different profiles and overlaps will look like. What they offer in terms of performance????? Well, that depends on what they are fitted to.
If that is all very vague and unhelpfull I apologise, it's a pretty complex area and I've tried to keep it simple and understandable. Like so many other things, cams aren't a ‘Stand Alone' item. Any motor is a fairly complex juggling act and the way any part, but especially cams, behave is dependent on so many other variables that it's impossible to give a concise and simple description. All I hope is that I haven't let you more confused than you were before !!!!!!!!!!!!!
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