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Camshafts ? Choosing the correct camshaft for your application

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            The goal of this article is to explain how to choose the correct camshaft for your engine application.  To do this I will have to start at the beginning with what a camshaft is and how it works.  Although you will be presented with a lot of information in this write up, this article only begins to scratch the surface of total engine dynamics.

 

            The camshaft is quite a simple piece of equipment.  In fact, it itself is it?s only moving part.  On the other hand, the job that the camshaft does is extremely complex.  When you think of a camshaft, you should think of it almost as a computer program for your engine.  Programmed into the camshaft are all the essential things your motor needs to run, i.e., how much air fuel mixture enters the cylinder, how fast the exhaust exits the cylinder and the dynamic compression ratio of the engine.  The main job of the camshaft is to open and close the engine?s valves.  It does this through a series of lobes, or cams.  Lobes are ?V? shaped bumps protruding from a round cylinder.  Each valve in the engine is associated with one of these lobes.  As the camshaft rotates, the lobs press on rocker arms, which open and close the valves.  In the case of older pushrod engines, the lobs push against rods, which in turn push on the rocker arms.  If you had a SOHC (single overhead cam) 16-valve engine, there would be 16 lobes on your camshaft.  Other engines use a DOHC (duel overhead cam) setup.  In DOHC engines two camshafts are used, one on the intake valve side, and one on the exhaust valve side.  DOHC setups can run more efficiently because there is a lot more room to work with the lobes and valve size.  In some cases such as in engines equipped with VVT (variable valve timing) there are more lobes then valves.  These extra lobes take over and control valves at certain engine RPM, which we will cover further in this article.  Lobes have control of when, how long, how far and even how fast the valves open in the engine.  This is expressed in timing, duration, lift and ramp rate (acceleration of the valves).  Each of these variables has a different effect on the engine at different RPM levels.  Since the camshaft and lobes are static (meaning they don?t change), you have to know exactly what you want to do with the engine before choosing the camshaft that?s right for it.

 

            The camshaft spins by way of a belt or chain attached to the crankshaft.  For every 360 degrees of crankshaft rotation the camshaft turns 180 degrees.  This is because the pistons cycle is made up of four parts.  For every full rotation of the camshaft each piston in the engine goes up and down four times, or two times per valve opening.  Down on the power stroke, up for the exhaust stroke, down again for the intake stroke and then up for the compression stroke.  If you were to run an engine at an extremely slow speed, lets say 10 RPM, the ideal camshaft would have the intake and exhaust lobes set so that when the piston started on its trip down from TDC (top dead center) on the intake stroke, the intake valve would open, allowing the air fuel mixture to be sucked into the engine.  As the piston neared BDC (bottom dead center), the intake valve would close, allowing the piston to compress the fuel mixture on its way back up to true TDC (when both intake and exhaust valves stay closed).  After the power stroke and the piston has once again returned to BDC, the exhaust valve would open so that the exhaust could be pushed from the cylinder as the piston returned to TDC on the exhaust stroke.  After this, the process would repeat itself.  The reason this camshaft?s profile of timing and duration is ideal for slow RPM is because the piston would be sucking air in and pushing exhaust out at such a slow speed that no velocity would build up in the entering and exiting air.  At higher RPM, however, the airs own velocity becomes a factor.  To explain this, let us pretend the same camshaft profile is used; however, the engine is turning at 2000 RPM.  Starting from the beginning, the intake valve starts to open as the piston leaves TDC on its downward intake stroke.  Since the piston is moving at a very high rate of speed, the air that is being pulled through the intake valve is also moving very rapidly.  As the piston nears BDC, the air being pulled into the cylinder is still moving at quite a high velocity, but our cam profile starts to shut the intake valve.  This slams the door closed on the air rushing in and stops it in its tracks.  The most efficient way to let an engine run at high RPM is to try to keep the air moving as much as possible.  This means shutting the valve late, and opening it early.  Lets try this again with a modified intake lobe.  We will start with the piston at TDC on an engine running at high RPM, lets say 5000 or higher.  As the piston travels down, it pulls air in through the open intake valve until it reaches the bottom (BDC). On our old cam the intake valve would be closed at this point, but let?s leave it open a little longer.  Since the air rushing in is traveling super fast (mach .2 - .45) even though the piston has stopped moving down and is now actually on the up stroke, the pressure inside the cylinder is still less then the pressure coming through the valve.  By leaving the valve open longer you can move a lot more air into the cylinder by use of the air?s own velocity.  The ideal point at which the valve closes is when the pressure inside the cylinder equalizes with the pressure coming through the valve.  Leaving it open any longer would reverse this and push air back out through the intake valve.

 

            The same principle is at work on the exhaust valve side as well, only in reverse.  When the piston nears the bottom on the combustion stroke, the exhaust valve opens so that the exhaust gas can be pushed out when the piston returns to the top.  As the piston reaches the top and begins to go back down, leaving the exhaust valve open longer will actually suck the remaining exhaust from the cylinder, and even start to pull a fresh air fuel mixture in from the opening intake valve.  The exhaust system actually acts like a siphon to pull the exhaust from the cylinder.  At this brief moment both intake and exhaust valves are actually open at the same time.  This is called the overlap.  Increasing the degrees of overlap tends to produce more high-end power.  In some extremely high performance engines the overlap can be quite great, taking up almost 90 degrees of camshaft rotation (50 before and 40 after TDC).  On these cams made for high revving engines, the intake valve starts to open while the piston is on the up swing of the exhaust stroke.  Since the engine is running at such a high rate of speed, the velocity of air continuously moving through the intake helps push the exhaust from the cylinder, and in turn after the piston reaches TDC the open exhaust valve helps pull the fresh mix into the cylinder.  In this case, the engine is built to run at full speed to achieve maximum efficiency and does not run well at idle.  This is where you hear the term loping, which refers to a car?s very rough idle due to cam timing and duration.

 

            So then, where does lift come into play?  Lift refers to how far the valve lifts off its seat.  You may think well, the wider the valve opens up, the better right?  That is not the case however.  As you saw before, the air?s velocity plays a very large part in the efficiency of the engine.  If the valve were to open too far this velocity would not build up.  Think of it as blowing through a straw.  You can force the air to move extremely fast and build up a lot of velocity.  Now blow through a paper towel roll, you can move a lot of air, but not very fast.  The key to lift is to find just the right balance of air movement and velocity at your most efficient rpm range.  This velocity range is somewhere between mach .2 and .5.  At anything over mach .6, volumetric efficiency will start to fall off sharply, meaning more lift is required to slow the air moving through the valve.  Valve lift is not the only thing that affects mach numbers. The valve size, rod and stroke of the piston, intake, intake runners and ports in the head all play a role on the total mach numbers.  Also, these mach index numbers are for intake valves only.  Exhaust valves are a whole other beast because of the extremely high air temperature and gaseous content they are dealing with, which changes mach numbers and velocity.  It is not uncommon for exhaust valves to work with exhaust flow velocities reaching or exceeding mach 1, the local speed of sound.

 

            At this point the problem with the static design of the cam becomes very apparent.  You can either build a motor for high revving power, or low-end torque.  Even the altitude and air temperature play a role in how efficient the engine operates.  Now, what if there was a way to change lift and duration of the camshaft on the fly.  You could set up a motor for good low-end torque while still not sacrificing high-end performance.  Ah, but there is a way!  VVT (variable valve timing)

 

            For the past 15 years or so car manufacturers have been playing around with various forms of VVT, and in the present day most, if not all, have it in some form on at least one of their production engines. Variable valve timing refers to changing how the valves act on the fly with the engine running.  In its most simple form, single stage operation, the camshaft switches between two profiles.  A low RPM profile for low-end efficiency, and a high RPM profile for maximum power.  This is not unlike Honda?s Vtec, which uses extra lobes on the camshaft at a preset RPM to add more lift to the valves.  In the case of Vtec, hydraulic pressure is used to force synchronizers between sets of rocker arms, which ride on different cam lobes.  In the past few years, manufacturers have been experimenting with IVVT (infinitely variable valve timing).  In this case the valves are infinitely adjustable instead of one or two preset values.  A computer controls lift, and in some cases even duration depending on several conditions, including RPM, temperature, mixture, and altitude.  Ferrari uses cam lobes cut using 2 different planes. This meaning, if you were to look at the cam from the side, you would notice the lobe tips were not flat.  Not only does the cam rotate opening and closing the valves?, the entire camshaft itself slides from side to side changing the lift and duration by changing the size of the lob at the point in which the rocker arm makes contact.  BMW takes the process of VVT one step further and not only changes the valves duration, but also timing, retarding and advancing the camshafts rotation vs. the crankshaft on the fly.

 

            In the future camshafts may well be taken out of the picture totally.  The holy grail of valve operation is to have each valve individually controlled by an actuator.  If this were the case, not only could valves be changed depending on outside conditions.  They could also be changed independently of one another, depending on individual cylinder conditions, making an extremely efficient running engine.

 

            You can now see why changing the camshaft in your engine is not something to take lightly.  I hope that with the use of this article you can better understand what is going on inside your engine, and perhaps come to a more educated decision when choosing the camshaft that?s right for you.

 

 

                                                                                Written by: Matt Woomer
                                                                                
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