Tech Doctor
02-06-2003, 04:47 AM
Well, I thought that I might start to post some technical articles for you guys and gals to read. Here is the first one on camshafts. If you have any questions, just post it and I will get back to you.
Basics of Camshaft Operation
The idea of proper valve timing in an internal combustion engine is one of the most discussed topics in the industry. On one side of the coin there is the idea that you need to open the intake valve as early as possible to get the most air moving into the cylinder as soon as possible. On the other side there is the residual pressure from the last cycle trying to push the air back out the intake tract, polluting the fresh charge as soon as you open it. Timing all the events in the internal combustion engine is one of the, if not THE most important things you can do to increase power at a given RPM.
Many engine builders refer to camshaft design as a "black art", or something only a few people on earth can do properly. This is not true, although there are truly not too many people that have studied the principals of camshaft theory enough to be able to pick the perfect cam and timing the first time. Most of the reasons for the inability of most people to choose correctly when selecting a cam is they want the "rumble" of a gigantic racing engine in their daily ride. That rumble represents an engine with enough open duration to cause a random misfire at low RPM. This is due to the fact that "Overlap" created by such a "large" grind causes the Intake charge to be contaminated. There are a couple ways to address this problem, one is to increase the lobe separation angle and thereby remove some of the "overlap". The second way to address this problem is to obviously make the cam a little "smaller"; this is where you need to pay attention... "A camshaft with less seat-to-seat duration is not always a smaller or more restrictive grind." In camshaft language there are three different ratings that are given on the cam spec cards of today. These different ratings are the way you can tell between a "good" cam and a "not-so-good" cam. The ratings are referred to as: the "seat-to-seat" timing, the .050" figure, and the most recent rating the 0.2” figure. These three ratings are the keys to proper selection of a camshaft. The ratings I am speaking of are all used to find the "AREA UNDER THE CURVE". These figures help to tell you how aggressive the lobe is or how quickly it opens the valve. The basic rule of thumb is to have the seat-to-seat timing as small as possible and the .050" figures at the proper size to make power at the RPM you plan on spinning the engine to while still maintain a ramp profile that will allow some kind of longevity of the lobe.
There are a few other things to know when trying to select a camshaft. One of the things is that when selecting a cam you obviously need to find the one with the fastest available ramp profile (which will open the valve quicker) you can get away with; and in order to do this you need an equation to compare different cams. The equation works like this: Duration @ .2” minus duration @ .050". Then compare the two cams and the cam with the smallest figure will be the cam with the quickest ramp profile, if the .050" figures are the same. There are multiple ideas of what works for a particular engine. The best solution when searching for the EXACT cam for your engine when funds allow is try your best to come up with the proper cam when figuring the duration and lift but when it comes down to it, be prepared to spend a lot of time on a dyno to help bring out those few extra horsepower. Twin cam engines, like our bikes, have a distinct advantage over single cam engines with the shear fact that we can adjust inlet and exhaust timing separately, and also change the valve overlap with the use of adjustable cam.
Cams are the secret ingredient in the internal combustion engine. More dangerous sounding names have been assigned to cams than any other part you'll find in your motor, further compounding their mystery by giving them personalities. Who would dare to ask the "Dominatrix" that lifts your valves what her stats are? I’ve tried to give a brief explanation to most cam attributes that you will find.
When does that valve open anyway? Well, to determine this we have to put dial indicator on the valve to see when it lifts off of its closed position...for both the inlet and exhaust valves. We need a reference point for this and we'll choose the engine's Top Dead Center (TDC) on the particular cylinder we are measuring. TDC is the point were the piston is as close to the cylinder head as possible and the place in the engine's rotation where both inlet and exhaust valves are closed. You'll know they are closed if both pushrods, in the case of a 2 valve motor Harley Davidson, are "loose" and not under tension (there again, what isn’t loose on a Harley engine).
How do we know exactly where this TDC point is anyway? Well, to do this we need to put a circular degree wheel on the end of the crankshaft that has 360 marks, one for each degree of crankshaft movement as well as marks for TDC (zero degrees) and BDC (180 degrees). BDC is bottom dead center, where the piston is as far away from the cylinder head as possible. A piece of wire can be manipulated into place to align up with the TDC mark (zero degrees).
The best way to find the exact TDC is to use an adjustable mechanical stop that screws into the spark plug hole. Put the motor at a point near to where you think TDC is i.e. both valves are closed and screw in the mechanical TDC stop. Gently screw in the adjustable portion of the stop till it contacts the top of the piston and lock it down. From this point you can gently turn the motor clockwise and counter clockwise until it gently hits this stop. Note these two degree figures, split the difference, and put the wire on the new TDC point (zero degrees). Rotate the engine clockwise and counter-clockwise again to verify that the mechanical stop hits the piston the same number of degrees before and after TDC (BTDC, ATDC). Adjust the wire you've arranged as a pointer, if necessary, to point to the TDC mark. You've now found TDC! Ground zero in your search for truth.
To insure we all speak the same language we have to agree on the same starting point. In this case it's how far the valve lifts before we start writing down those degrees. The industry standard is when the valve lifts .050 inches (fifty thousandths) off it's seated position. The idea in this is that we will start at an agreed on point where there is "measurable" flow, the assumption being that there is no meaningful flow in the first .050" of valve lift. So, get the cylinder to TDC, making sure both valves are closed and set a dial indicator on either your inlet or exhaust valve. Set the indicator dial to zero and slowly rotate the engine and note the degrees at which the valve hits this magic .050" point. This will be .050" of lift after it opens and .050" before it closes.
Since we all speak the same language on this monumental project we know that BTDC is "before top dead center" and ATDC is "after top dead center". BBDC is "before bottom dead center" and logically ABDC is "after bottom dead center".
When you write down the figures you have to add these acronyms to your degree figures. Inlet cams will generally open x degrees BTDC and close x degrees ABDC. Exhaust cams will generally open x degrees BBDC and close x degrees ATDC.
Once you've got your figures at .050" lift for both the inlet and exhaust valves you can plug these figures into the calculator to get information about your cam's duration and installed centrelines.
The amount of time, expressed in crankshaft degrees, describes the window of time between the Inlet Cam's opening point BTDC and the Exhaust Cam's closing point ATDC. This figure can vary between zero degrees on some stock cams to as much as 115 to 125 degrees on some race motors. Increasing the degrees of overlap tends to move the power band up the RPM band. Increasing the overlap can increase peak power, but only if the exhaust system is properly designed to scavenge the cylinder (eg: Arrow Racetech Systems). Decreasing the overlap tends to boost lower rpm performance.
This is the angle between the intake and exhaust camshaft lobe peaks described in camshaft degrees. Generally speaking the majority of cams will fall between 98 and 120 degrees. This angle dictates two important events: the valve overlap around TDC, and intake or exhaust valve closure delay there is in the relevant stroke (inlet/exhaust). Tightening the lobe centre angle produces more overlap around TDC and wider angles mean less overlap.
A little appreciated consideration is the effect of valve lift on engine performance. As the engine speed increases there will be a need to increase valve lift to keep the inlet speeds from exceeding the Mach Index value of .6, beyond which volumetric efficiency falls off. This leads to the intriguing possibility of planning your valve lift in advance relative to your design or performance goals be they street or racing.
Hopefully, this has helped your understanding of cams. There are many variables involved in selecting the proper camshaft, but this should have some good information for the average gixxer.com punter to go from when trying to learn more.
This really only covers the very basics of cams. Got any questions? Go for it.
Dean.
Basics of Camshaft Operation
The idea of proper valve timing in an internal combustion engine is one of the most discussed topics in the industry. On one side of the coin there is the idea that you need to open the intake valve as early as possible to get the most air moving into the cylinder as soon as possible. On the other side there is the residual pressure from the last cycle trying to push the air back out the intake tract, polluting the fresh charge as soon as you open it. Timing all the events in the internal combustion engine is one of the, if not THE most important things you can do to increase power at a given RPM.
Many engine builders refer to camshaft design as a "black art", or something only a few people on earth can do properly. This is not true, although there are truly not too many people that have studied the principals of camshaft theory enough to be able to pick the perfect cam and timing the first time. Most of the reasons for the inability of most people to choose correctly when selecting a cam is they want the "rumble" of a gigantic racing engine in their daily ride. That rumble represents an engine with enough open duration to cause a random misfire at low RPM. This is due to the fact that "Overlap" created by such a "large" grind causes the Intake charge to be contaminated. There are a couple ways to address this problem, one is to increase the lobe separation angle and thereby remove some of the "overlap". The second way to address this problem is to obviously make the cam a little "smaller"; this is where you need to pay attention... "A camshaft with less seat-to-seat duration is not always a smaller or more restrictive grind." In camshaft language there are three different ratings that are given on the cam spec cards of today. These different ratings are the way you can tell between a "good" cam and a "not-so-good" cam. The ratings are referred to as: the "seat-to-seat" timing, the .050" figure, and the most recent rating the 0.2” figure. These three ratings are the keys to proper selection of a camshaft. The ratings I am speaking of are all used to find the "AREA UNDER THE CURVE". These figures help to tell you how aggressive the lobe is or how quickly it opens the valve. The basic rule of thumb is to have the seat-to-seat timing as small as possible and the .050" figures at the proper size to make power at the RPM you plan on spinning the engine to while still maintain a ramp profile that will allow some kind of longevity of the lobe.
There are a few other things to know when trying to select a camshaft. One of the things is that when selecting a cam you obviously need to find the one with the fastest available ramp profile (which will open the valve quicker) you can get away with; and in order to do this you need an equation to compare different cams. The equation works like this: Duration @ .2” minus duration @ .050". Then compare the two cams and the cam with the smallest figure will be the cam with the quickest ramp profile, if the .050" figures are the same. There are multiple ideas of what works for a particular engine. The best solution when searching for the EXACT cam for your engine when funds allow is try your best to come up with the proper cam when figuring the duration and lift but when it comes down to it, be prepared to spend a lot of time on a dyno to help bring out those few extra horsepower. Twin cam engines, like our bikes, have a distinct advantage over single cam engines with the shear fact that we can adjust inlet and exhaust timing separately, and also change the valve overlap with the use of adjustable cam.
Cams are the secret ingredient in the internal combustion engine. More dangerous sounding names have been assigned to cams than any other part you'll find in your motor, further compounding their mystery by giving them personalities. Who would dare to ask the "Dominatrix" that lifts your valves what her stats are? I’ve tried to give a brief explanation to most cam attributes that you will find.
When does that valve open anyway? Well, to determine this we have to put dial indicator on the valve to see when it lifts off of its closed position...for both the inlet and exhaust valves. We need a reference point for this and we'll choose the engine's Top Dead Center (TDC) on the particular cylinder we are measuring. TDC is the point were the piston is as close to the cylinder head as possible and the place in the engine's rotation where both inlet and exhaust valves are closed. You'll know they are closed if both pushrods, in the case of a 2 valve motor Harley Davidson, are "loose" and not under tension (there again, what isn’t loose on a Harley engine).
How do we know exactly where this TDC point is anyway? Well, to do this we need to put a circular degree wheel on the end of the crankshaft that has 360 marks, one for each degree of crankshaft movement as well as marks for TDC (zero degrees) and BDC (180 degrees). BDC is bottom dead center, where the piston is as far away from the cylinder head as possible. A piece of wire can be manipulated into place to align up with the TDC mark (zero degrees).
The best way to find the exact TDC is to use an adjustable mechanical stop that screws into the spark plug hole. Put the motor at a point near to where you think TDC is i.e. both valves are closed and screw in the mechanical TDC stop. Gently screw in the adjustable portion of the stop till it contacts the top of the piston and lock it down. From this point you can gently turn the motor clockwise and counter clockwise until it gently hits this stop. Note these two degree figures, split the difference, and put the wire on the new TDC point (zero degrees). Rotate the engine clockwise and counter-clockwise again to verify that the mechanical stop hits the piston the same number of degrees before and after TDC (BTDC, ATDC). Adjust the wire you've arranged as a pointer, if necessary, to point to the TDC mark. You've now found TDC! Ground zero in your search for truth.
To insure we all speak the same language we have to agree on the same starting point. In this case it's how far the valve lifts before we start writing down those degrees. The industry standard is when the valve lifts .050 inches (fifty thousandths) off it's seated position. The idea in this is that we will start at an agreed on point where there is "measurable" flow, the assumption being that there is no meaningful flow in the first .050" of valve lift. So, get the cylinder to TDC, making sure both valves are closed and set a dial indicator on either your inlet or exhaust valve. Set the indicator dial to zero and slowly rotate the engine and note the degrees at which the valve hits this magic .050" point. This will be .050" of lift after it opens and .050" before it closes.
Since we all speak the same language on this monumental project we know that BTDC is "before top dead center" and ATDC is "after top dead center". BBDC is "before bottom dead center" and logically ABDC is "after bottom dead center".
When you write down the figures you have to add these acronyms to your degree figures. Inlet cams will generally open x degrees BTDC and close x degrees ABDC. Exhaust cams will generally open x degrees BBDC and close x degrees ATDC.
Once you've got your figures at .050" lift for both the inlet and exhaust valves you can plug these figures into the calculator to get information about your cam's duration and installed centrelines.
The amount of time, expressed in crankshaft degrees, describes the window of time between the Inlet Cam's opening point BTDC and the Exhaust Cam's closing point ATDC. This figure can vary between zero degrees on some stock cams to as much as 115 to 125 degrees on some race motors. Increasing the degrees of overlap tends to move the power band up the RPM band. Increasing the overlap can increase peak power, but only if the exhaust system is properly designed to scavenge the cylinder (eg: Arrow Racetech Systems). Decreasing the overlap tends to boost lower rpm performance.
This is the angle between the intake and exhaust camshaft lobe peaks described in camshaft degrees. Generally speaking the majority of cams will fall between 98 and 120 degrees. This angle dictates two important events: the valve overlap around TDC, and intake or exhaust valve closure delay there is in the relevant stroke (inlet/exhaust). Tightening the lobe centre angle produces more overlap around TDC and wider angles mean less overlap.
A little appreciated consideration is the effect of valve lift on engine performance. As the engine speed increases there will be a need to increase valve lift to keep the inlet speeds from exceeding the Mach Index value of .6, beyond which volumetric efficiency falls off. This leads to the intriguing possibility of planning your valve lift in advance relative to your design or performance goals be they street or racing.
Hopefully, this has helped your understanding of cams. There are many variables involved in selecting the proper camshaft, but this should have some good information for the average gixxer.com punter to go from when trying to learn more.
This really only covers the very basics of cams. Got any questions? Go for it.
Dean.