The Black Mystic Art of Cam Timing!
Part One
By Peter Shearman
INTRODUCTION
Like Electric’s many people think that cam timing adjustment is a ‘Black Art’. They think that you have to be some sort of a Magician or Semi-God to be able to do it! Like most things in life experience is the best teacher and the only way to get experience is to have a go yourself.
Year ago (many!) I didn’t have a clue how to adjust Dellorto carburettors but by watching someone do it, asking questions and finally trying it for myself I learned how. If you are willing to have a go then you will always learn something. Even if you make mistakes, and we all do, you will learn from them.
I wasn’t game to try cam timing until a couple of years ago when I rebuilt the 900 engine. Before I tried it I did a lot of reading and watched it done on a belt drive at a service day before finally tackling it myself. Like Desmo shimming it isn’t very hard you just have to give yourself plenty of time to take it slowly and double check all your figures. The good thing about cam timing is that even if you completely mess it up you can always go back to the standard factory timing marks and start again but don’t try running the engine until you are sure the timing is where you want it!
This article is devided into three instalments. The first explains the role of valve timing starting with the basics and moving on to more complex explantions. The second part describes how to measure your existing valve timing and the final part gives a guide on how to change the valve timing
BASIC FOUR STROKE PRINCIPLES
For those with little knowledge of what goes on in a four stroke engine this first section will cover the simplified basics of operation including the part that valve timing plays in the four stroke cycle. If you already know all this just move on to the advanced section.
A four stroke engine crankshaft rotates twice (2 x 360° = 720°) for each cycle of operation. During this cycle the piston moves up and down the bore twice which gives us four strokes! When the piston is at the top of its stroke this is called Top Dead Center or TDC for short. When the piston is at the bottom of its stroke this called Bottom Dead Center or BDC for short. TDC and BDC are reached twice during each cycle of operation.
During these two rotations of the crankshaft the camshaft only goes through one rotation. This is achieved by driving the camshaft at an overall 2:1 reduction ratio from the cranshaft. The camshaft controls one valve cycle which covers two rotations of the crankshaft so whilst the crankshaft goes through 720° the camshaft only goes through 360°. You need to remember this relationship when it comes time to move the camshaft to change the timing.
Valve timing is usually given using two figures. The first is the number of crankshaft degrees degrees before/after TDC/BDC that the valve completely closes. This gives rise to further abbreviations of, BTDC (Before Top Dead Center), ATDC (After Top Dead Center), BBDC (Before Bottom Dead Center) and ABDC (After Bottom Dead Center).
Four stages are passed through for each four stroke cycle and these are listed below in simplified terms to explain the basic four phases involved. We will go into these phases in greater detail in the ‘Advanced’ section.
Let’s start at TDC at the end of the compression stroke.
POWER At TDC (Compression) the spark ignites the compressed mixture resulting in a burning of this mixture to create the power to drive the piston down the cyclinder. Both valves must remain closed for this power stroke.
EXHAUST At BDC the inlet valve must remain closed and the exhaust valve must be open whilst the piston in on the up stroke. This movement forces the burnt gases out past the exhaust valve to the exhaust port and the exhaust system.
INTAKE At TDC (Exhaust/Overlap) the exhaust valve must be closed and the inlet valve must be opened. The pistons downward movement causes a pressure below atmospheric in the cyclinde
- - - - - - - automatiskt ihopslaget inlägg - - - - - - -
Part Two
By Peter Shearman
MEASURING THE TIMING
Continuing on from last month we will now delve into the various ways of describing and measuring the cam timing.
Before we proceed further it is important to note that cam timing figures are affected by worn cam profiles, worn or incorrect rocker follower profiles, rocker to valve seat height and wear, amount of lift, etc. Ensure that all the above items are checked and are within factory specifications to get the best accuracy with your cam timing.
The intention of this article is to guide people through measuring the cam timing of their road going motorcycle(s) with a fair degree of accuracy. If you are doing cam timing on a racing machine then you may need to do a lot more research and work with much more accuracy to achieve the best results from your engine. This would also include dyno testing at various stages as the best ‘theory’ does not always prove the best in practise!
There are three main methods for specifying and measuring the cam timing and each will give different figures. It is important when specifying any cam timing to state where the timing was checked and why it was done at that point. Good cam timing figures will not only give information on degrees opening and closing but also what clearances were used and/or at what lift this was measured.
Where do we measure the timing and why?
The start and finish of a normal cam’s profile consists of the opening and closing Clearance Ramps. The opening clearance ramp is very gentle on lift firstly to close up the rocker to valve clearance and past that to accelerate the valve gently off its seat. If you try to accelerate the valve too quickly it leads to dramatically increased wear and tear on all valve train components. This has even more relevance in a valve spring motor where the inertia of the spring is overcome as well as the valve. Similarly when the valve is almost fully closed you want to slow it down gently so that it doesn’t hammer hard into the seat that again would cause increased valve and seat wear.
With the valve (relatively) slowly lifting off the seat there is almost no gas flow until the valve has reached around 0.5mm of lift and it is not until past this point that we get onto the ‘true’ cam profile that results in rapid lift and appreciable gas flow past the valve.
1. Timing at Running Clearance.
Timing at running clearance is when timing figures are given with the rocker to valve clearance set to the standard factory setting for normal engine running. The opening timing measurement is made when the factory clearance closes up and the valve just starts to move and conversely the closing timing measurement is made at the point where the valve has just hit the seat but before any clearance opens up.
Trying to measure the exact moment when the valve starts to move or finishes moving is very difficult with running clearance’s, due to the slow rate of lift, and so can give rise to inaccurate figures. Measurements with this method are taken on the clearance ramps and not the ‘true’ cam profile and the valves are not effectively open till well past this point so the figures arrived at are not a true indication of the engines ‘breathability’. This method gives unrealistically long duration and overlap figures and is used by manufacturers to give enhanced figures for advertising advantage where prospective buyers think longer must be better!
2. Timing at Checking Clearance.
A much more realistic place to measure the valve timing is at a lift of 1.00 mm (40 Thousands of an inch). At this point we are well past the clearance ramps and onto the ‘working’ part of the cam profile. Lift at this point on the cam is rapid making it easier to measure the exact angle when the valve starts to move and it also gives more realistic ‘breathing’ figures as the valve has opened sufficiently for some ‘real’ gas flow to occur.
The standard way to measure this timing is at the point where the valve starts to move using
- - - - - - - automatiskt ihopslaget inlägg - - - - - - -
Part Three
By Peter Shearman
CHANGING THE VALVE TIMING
Last month we had finally found out if our cam timing was out and by how much. Now you need to make a decision on whether or not to change the valve timing. This will be based on how far out the timing is both in relation to between the cylinders and also in relation to where it should be! You may feel that your timing is ‘close enough’ and you don’t want to go any further, but remember any changes that you make can easily be reversed by going back to the original timing marks.
Another point to think about before changing the timing is how easily can it be changed on your machine. If you own a bevel drive then the timing can be changed by ‘playing’ with the cam drive gears. Square case models are the easiest with an infinite range of adjustment available by moving the double straight cut/bevel gear in the timing chest. Round case models are more fiddly but once again changes can be made without spending any money. Belt drive owners will have to get additional keyways cut in their cam drive sprockets or else get change over sprockets with these keyways already cut. Although this is not too expensive the range of adjustment steps is not as great as the bevel drives but if you know exactly how far you want the timing moved then this information may help the machinist to put the keyway close to the right spot. Offset keys are another option available to move the timing by a small amount on the belt drive models.
For ideal performance tuning you need to be able to change the exhaust timing independent of the inlet timing. This can only be done when you have separate camshafts for inlet and exhaust. If you have this capability then you can change the timing to achieve a desired change in performance. Consult the references for more information if your bike has separate camshafts. Most Dukes have exhaust and inlet cam lobes on the same camshaft so individual changing of one in relation to the other is not possible without removing the camshaft and replacing it with a different specification cam or having it professionally built up and reground to new specifications.
If the engine has not been heavily modified then the main reason for ‘dialling in’ the cams is to match both cylinders together and maybe give them a little advance if this is known to improve performance. In the case of bevel drive square case machines around 3-4° advance gives the best all round performance based on peoples experiences. Advancing more than 4° can improve mid range grunt but at the expense of top end power. This may still be an advantage if you race your bike on tight circuits like Winton or Broadford but I would not recommend it for normal road use.
CAUTION Before you proceed with changing the timing please note that you can do permanent engine damage if you turn the engine over with the cam timing way off mark. Always double check your figures and turn the engine over slowly by hand to ensure that the valves are not hitting the piston or each other during two full rotations. If all seems OK then start the bike but if you hear any strange noises shut down immediately and ring your bank manager before investigating further!
Square Case Bevel Engines
I will run through the way I changed the cam timing on my 900 SS Bevel drive. Round cases and belts are covered in lesser detail in separate sections.
Last month we found that in my 900 SS bevel the vertical cylinder was 9° retarded and the horizontal cylinder was 3° advanced. As discussed previously the ideal setting for this type of motor is between 3 to 4 degrees advanced based on lobe center measurements. As the horizontal cylinder was already at 3° advanced I decided not to move this cam as it was close enough to the ideal setting. The next step therefore was to bring the vertical cam into line with the horizontal by advancing it 12° so that it was also at 3° advanced.
Changing the cam timing on square case motors is achieved by ‘juggling’
Part One
By Peter Shearman
INTRODUCTION
Like Electric’s many people think that cam timing adjustment is a ‘Black Art’. They think that you have to be some sort of a Magician or Semi-God to be able to do it! Like most things in life experience is the best teacher and the only way to get experience is to have a go yourself.
Year ago (many!) I didn’t have a clue how to adjust Dellorto carburettors but by watching someone do it, asking questions and finally trying it for myself I learned how. If you are willing to have a go then you will always learn something. Even if you make mistakes, and we all do, you will learn from them.
I wasn’t game to try cam timing until a couple of years ago when I rebuilt the 900 engine. Before I tried it I did a lot of reading and watched it done on a belt drive at a service day before finally tackling it myself. Like Desmo shimming it isn’t very hard you just have to give yourself plenty of time to take it slowly and double check all your figures. The good thing about cam timing is that even if you completely mess it up you can always go back to the standard factory timing marks and start again but don’t try running the engine until you are sure the timing is where you want it!
This article is devided into three instalments. The first explains the role of valve timing starting with the basics and moving on to more complex explantions. The second part describes how to measure your existing valve timing and the final part gives a guide on how to change the valve timing
BASIC FOUR STROKE PRINCIPLES
For those with little knowledge of what goes on in a four stroke engine this first section will cover the simplified basics of operation including the part that valve timing plays in the four stroke cycle. If you already know all this just move on to the advanced section.
A four stroke engine crankshaft rotates twice (2 x 360° = 720°) for each cycle of operation. During this cycle the piston moves up and down the bore twice which gives us four strokes! When the piston is at the top of its stroke this is called Top Dead Center or TDC for short. When the piston is at the bottom of its stroke this called Bottom Dead Center or BDC for short. TDC and BDC are reached twice during each cycle of operation.
During these two rotations of the crankshaft the camshaft only goes through one rotation. This is achieved by driving the camshaft at an overall 2:1 reduction ratio from the cranshaft. The camshaft controls one valve cycle which covers two rotations of the crankshaft so whilst the crankshaft goes through 720° the camshaft only goes through 360°. You need to remember this relationship when it comes time to move the camshaft to change the timing.
Valve timing is usually given using two figures. The first is the number of crankshaft degrees degrees before/after TDC/BDC that the valve completely closes. This gives rise to further abbreviations of, BTDC (Before Top Dead Center), ATDC (After Top Dead Center), BBDC (Before Bottom Dead Center) and ABDC (After Bottom Dead Center).
Four stages are passed through for each four stroke cycle and these are listed below in simplified terms to explain the basic four phases involved. We will go into these phases in greater detail in the ‘Advanced’ section.
Let’s start at TDC at the end of the compression stroke.
POWER At TDC (Compression) the spark ignites the compressed mixture resulting in a burning of this mixture to create the power to drive the piston down the cyclinder. Both valves must remain closed for this power stroke.
EXHAUST At BDC the inlet valve must remain closed and the exhaust valve must be open whilst the piston in on the up stroke. This movement forces the burnt gases out past the exhaust valve to the exhaust port and the exhaust system.
INTAKE At TDC (Exhaust/Overlap) the exhaust valve must be closed and the inlet valve must be opened. The pistons downward movement causes a pressure below atmospheric in the cyclinde
- - - - - - - automatiskt ihopslaget inlägg - - - - - - -
Part Two
By Peter Shearman
MEASURING THE TIMING
Continuing on from last month we will now delve into the various ways of describing and measuring the cam timing.
Before we proceed further it is important to note that cam timing figures are affected by worn cam profiles, worn or incorrect rocker follower profiles, rocker to valve seat height and wear, amount of lift, etc. Ensure that all the above items are checked and are within factory specifications to get the best accuracy with your cam timing.
The intention of this article is to guide people through measuring the cam timing of their road going motorcycle(s) with a fair degree of accuracy. If you are doing cam timing on a racing machine then you may need to do a lot more research and work with much more accuracy to achieve the best results from your engine. This would also include dyno testing at various stages as the best ‘theory’ does not always prove the best in practise!
There are three main methods for specifying and measuring the cam timing and each will give different figures. It is important when specifying any cam timing to state where the timing was checked and why it was done at that point. Good cam timing figures will not only give information on degrees opening and closing but also what clearances were used and/or at what lift this was measured.
Where do we measure the timing and why?
The start and finish of a normal cam’s profile consists of the opening and closing Clearance Ramps. The opening clearance ramp is very gentle on lift firstly to close up the rocker to valve clearance and past that to accelerate the valve gently off its seat. If you try to accelerate the valve too quickly it leads to dramatically increased wear and tear on all valve train components. This has even more relevance in a valve spring motor where the inertia of the spring is overcome as well as the valve. Similarly when the valve is almost fully closed you want to slow it down gently so that it doesn’t hammer hard into the seat that again would cause increased valve and seat wear.
With the valve (relatively) slowly lifting off the seat there is almost no gas flow until the valve has reached around 0.5mm of lift and it is not until past this point that we get onto the ‘true’ cam profile that results in rapid lift and appreciable gas flow past the valve.
1. Timing at Running Clearance.
Timing at running clearance is when timing figures are given with the rocker to valve clearance set to the standard factory setting for normal engine running. The opening timing measurement is made when the factory clearance closes up and the valve just starts to move and conversely the closing timing measurement is made at the point where the valve has just hit the seat but before any clearance opens up.
Trying to measure the exact moment when the valve starts to move or finishes moving is very difficult with running clearance’s, due to the slow rate of lift, and so can give rise to inaccurate figures. Measurements with this method are taken on the clearance ramps and not the ‘true’ cam profile and the valves are not effectively open till well past this point so the figures arrived at are not a true indication of the engines ‘breathability’. This method gives unrealistically long duration and overlap figures and is used by manufacturers to give enhanced figures for advertising advantage where prospective buyers think longer must be better!
2. Timing at Checking Clearance.
A much more realistic place to measure the valve timing is at a lift of 1.00 mm (40 Thousands of an inch). At this point we are well past the clearance ramps and onto the ‘working’ part of the cam profile. Lift at this point on the cam is rapid making it easier to measure the exact angle when the valve starts to move and it also gives more realistic ‘breathing’ figures as the valve has opened sufficiently for some ‘real’ gas flow to occur.
The standard way to measure this timing is at the point where the valve starts to move using
- - - - - - - automatiskt ihopslaget inlägg - - - - - - -
Part Three
By Peter Shearman
CHANGING THE VALVE TIMING
Last month we had finally found out if our cam timing was out and by how much. Now you need to make a decision on whether or not to change the valve timing. This will be based on how far out the timing is both in relation to between the cylinders and also in relation to where it should be! You may feel that your timing is ‘close enough’ and you don’t want to go any further, but remember any changes that you make can easily be reversed by going back to the original timing marks.
Another point to think about before changing the timing is how easily can it be changed on your machine. If you own a bevel drive then the timing can be changed by ‘playing’ with the cam drive gears. Square case models are the easiest with an infinite range of adjustment available by moving the double straight cut/bevel gear in the timing chest. Round case models are more fiddly but once again changes can be made without spending any money. Belt drive owners will have to get additional keyways cut in their cam drive sprockets or else get change over sprockets with these keyways already cut. Although this is not too expensive the range of adjustment steps is not as great as the bevel drives but if you know exactly how far you want the timing moved then this information may help the machinist to put the keyway close to the right spot. Offset keys are another option available to move the timing by a small amount on the belt drive models.
For ideal performance tuning you need to be able to change the exhaust timing independent of the inlet timing. This can only be done when you have separate camshafts for inlet and exhaust. If you have this capability then you can change the timing to achieve a desired change in performance. Consult the references for more information if your bike has separate camshafts. Most Dukes have exhaust and inlet cam lobes on the same camshaft so individual changing of one in relation to the other is not possible without removing the camshaft and replacing it with a different specification cam or having it professionally built up and reground to new specifications.
If the engine has not been heavily modified then the main reason for ‘dialling in’ the cams is to match both cylinders together and maybe give them a little advance if this is known to improve performance. In the case of bevel drive square case machines around 3-4° advance gives the best all round performance based on peoples experiences. Advancing more than 4° can improve mid range grunt but at the expense of top end power. This may still be an advantage if you race your bike on tight circuits like Winton or Broadford but I would not recommend it for normal road use.
CAUTION Before you proceed with changing the timing please note that you can do permanent engine damage if you turn the engine over with the cam timing way off mark. Always double check your figures and turn the engine over slowly by hand to ensure that the valves are not hitting the piston or each other during two full rotations. If all seems OK then start the bike but if you hear any strange noises shut down immediately and ring your bank manager before investigating further!
Square Case Bevel Engines
I will run through the way I changed the cam timing on my 900 SS Bevel drive. Round cases and belts are covered in lesser detail in separate sections.
Last month we found that in my 900 SS bevel the vertical cylinder was 9° retarded and the horizontal cylinder was 3° advanced. As discussed previously the ideal setting for this type of motor is between 3 to 4 degrees advanced based on lobe center measurements. As the horizontal cylinder was already at 3° advanced I decided not to move this cam as it was close enough to the ideal setting. The next step therefore was to bring the vertical cam into line with the horizontal by advancing it 12° so that it was also at 3° advanced.
Changing the cam timing on square case motors is achieved by ‘juggling’
