Tech Library - Cam timing
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 cyclinder which allows atmospheric pressure to feed
the air/fuel mixture past the inlet valve and into the engine via the inlet
port.
COMPRESSION At BDC the exhaust valve must remain closed and the inlet valve
must be closed. Whilst the piston is on the up stroke the air/fuel mixture
is compressed dramatically ready for ignition by the spark plug. The cycles
repeats with the power phase next.
If an engine was set up like this with valves opening and closing at TDC
and BDC it would run at low revolutions but such basic valve timing is not
good enough for an engine to develop any usable power. If you have grasped
the basics and want to know more it’s time to move on to what happens
in a real motor.
ADVANCED CAM TIMING THEORY
I found a diagram helpful in understanding what is happening. With reference to figure 1, I have divided the two crankshaft rotations into two circles joined at TDC this should be followed as a ‘Figure of Eight’ through the various cycles.
Starting at TDC compression follow the left hand circle clockwise until
you return to TDC exhaust. Then follow the right hand circle anti-clockwise
back to TDC compression. I have divided each circle into four 90° segments
giving eight phases for the purpose of our advanced discussion. The actual
degree settings for cam timing will be discussed later on.
We start at TDC compression. As discussed previously both valves are closed
and the air/fuel mixture has been compressed into a small area. Because the
mixture takes a finite time to become completely ignited we must start the
ignition before the piston gets to TDC. I won’t get into ignition timing
in depth here but suffice to say the faster the crankshaft rotates the further
before TDC the mixture needs to be ignited. The net result of this is to
ensure that the maximum push of the fully burning mixture peaks just as the
piston starts its downward stroke regardless of piston speed.
POWER For the first 90°-100° ATDC both valves remain closed whilst
the mixture burns causing pressure to rise pushing the piston down on its
power stroke.
EXHAUST 1 At around 80° BBDC the exhaust valve starts to open. The main
reason for opening the exhaust valve earlier than BDC is so that it will
be fully opened by the time the piston reaches the start of the upward exhaust
stroke. Most of the power from the burning mixture has been used at this
point so there is virtually no loss of power by opening the valve early. Also
we can use the small amount of combustion power left to begin the exhaust
phase early even though the piston is still moving down.
EXHAUST 2 & 3 From BDC to TDC the exhaust valve is wide open and the
rising
piston is forcing the burnt gasses out of the cylinder
INLET 1 At around 60° BTDC the inlet valve starts to open. As for the
early opening exhaust valve we do this to give the inlet valve a chance to
be fully open by the time the piston is reaching its maximum downward velocity
and it gives a head start to filling the cylinder with fresh air/fuel mixture.
By the time the inlet valve is fully open the exhaust gasses are moving fast
out through the exhaust port and the inertia of this column of gas causes
a slight depression in the cylinder which allows atmospheric pressure to
feed in the fresh mixture. Exhaust systems are designed with this in mind
and the term ‘Extractors’ is fairly self explanatory when you
understand what is happening in the engine.
INLET 2 & 3 From TDC Exhaust/Overlap to BDC the inlet valve is wide open
and the piston rapidly moving down creates a depression allowing atmospheric
pressure to feed fresh mixture into the cylinder.
EXHAUST 4 The exhaust valve remains open till around 60° after TDC. This
is done to purge the exhaust gasses. The rapidly moving incoming mixture
not only fills the cylinder but forces the last of the exhaust gasses out
through the closing exhaust valve until the returning exhaust pulse stalls
the flow. The swirling air/fuel mixture at this point also provides some
cooling for the hot exhaust valve.
INLET 4 From BDC to around 80° ABDC the inlet valve remains open. Although
the piston is now moving upwards the inertia of the incoming air/fuel mixture
is stronger and results in a mini super charging effect where the mixture
is initially compressed into the cylinder.
COMPRESSION After the inlet valve closes at around 100° BTDC the rising
piston continues to compress the fresh air/fuel mixture in readiness for
ignition and the start of a new cycle.
When you realise that this full cycle happens around 70 times per second
at high revolutions you can appreciate that cam timing is a very critical
factor when designing an engine. The timing figures are dependant on the
camshaft(s) which are ground specifically to suit the type of engine.
As you can imagine many factors effect the factory setting of cam timing
such as, the intake and exhaust systems, fuel type used, compression ratio,
maximum RPM of the engine, piston and combustion chamber shapes, torque and
maximum power conciderations, fuel economy, etc.
MORE TERMS & CONSIDERATIONS
Nearly all cam timing figures rely on having a known clearance between the
rockers and the valves which is greater than the normal or Running clearance.
This is called a Checking clearance and is normally 1.00mm (40 thousandths
of an inch). Timing at checking clearances means that you will have to temporarily
change the shim or adjuster on each valve prior to measuring the timing but
more on this later.
Some factory timing figures are given at Running clearance even though there
is no appreciable gas flow below 0.5mm lift. This is done to provide enhanced
figures which make Duration and Overlay appear much longer which is an advertising
advantage when buyers think that longer must be better! If no checking clearance
figures are given with the factory timing specification then you have to
assume that timing is at running clearance. ie normal operating clearances
are used.
The number of degrees from when the valve start to open to when it finally
closes is called the Duration of the valve timing. Typical factory figures
for the bevel drive 900SS are around 320° for both exhaust and inlet
valves although these are running clearance figures and so are optimistic!
Another term mentioned above is Valve Overlap. This figure describes the
number of degrees where both the inlet and exhaust valves are opened at the
same time. The reasons behind this were discussed in the ‘Advanced’ section.
The duration and overlap can be worked out from the factory figures although
as you will see later these do not always match the cam that is fitted to
the bike! Lets take the bevel 900SS factory figures and do some calculations.
Remember all degree figures are for the crankshaft and must be halved if
applied to the camshaft.
Note, factory timing at running clearance for this model so figures are enhanced!
The exhaust opens 80° BBDC and closes 58° ATDC. The exhaust valve
duration is 80° (To BDC) + 180° (BDC to TDC) + 58° (ATDC) = 318°.
The inlet opens 63° BTDC and closes 83° ABDC. The inlet valve duration
is 63° (To TDC) + 180° (TDC to BDC) + 83° (ABDC) = 326°.
The overlap is the period of degrees where both valves are open. For this
cam the overlap is 63° (BTDC inlet opens) + 58° (ATDC exhaust closes)
= 121°.
The other figures we are interested in is the point of Maximum Lift (ML)
of each valve. Normally this will be half way between the opening and closing
degree figures if the cam is symmetrical. So work out where the point of
ML sould be we divide the duration of each valve in half and then add that
figure to the opening degree figure.
The exhaust valve duration is 318° divide by 2 = 159°. The exhaust
opens 80° BBDC (= 260° BTDC) plus 159° (Half exhaust duration).
Therefore Exhaust Maximum Lift should be at (260°-159°) = 101° BTDC.
The inlet valve duration is 326° divide by 2 = 163°. The inlet opens
63° BTDC plus 163° (Half inlet duration). Therefore Inlet Maximum
Lift should be at (163°-63°) = 100° ATDC.
It is common to refer to these cams as ‘101°/100° lobe center
cams’ and as you have probably seen there is a symmetry between the
ML figures either side of TDC. This is called Lobe Center Symmetry and most
engines have the two ML’s at equal distance away from TDC. We will
use this symmetry to decide if the cam is advanced or retarded when we calculate
the real ML figures.
Figure 2 shows the relationship between the opening, closing and maximum
lift of each valve for the factory specifications the 900ss Bevel Engine.
We now come to a problem not unique to Ducati which is that the cams fitted to the engines do not necessarily match the ones specified in the manual! For whatever reason most 900SS bevels were actually fitted with cams with a lobe center symmetry of ‘96°/96°’ which are not quite as good a cam as the ‘101°/100°’ items. Don’t worry about which cams are fitted to your machine as when we measure the ML it will soon become obvious what you have got! The point is don’t assume that the factory figures relate directly to what is acually fitted to your machine!
Figure 3 shows the timing diagram for the 900SS with ‘96°/96°’ cams. Note that the opening and closing figures shown on the diagram assume that these cams have the same duration as the ‘101°/100°’ cams although this may not be the case. As I have no factory figures for the ‘96°/96°’ cams the only known points are the two ML’s.
Next we will find out more about the three different ways of measuring the valve timing, how to find an accurate piston TDC, how to accurately find the point of maximum valve lift and finally how to measure it using a step by step guide with real life examples!
The Black Mystic Art of Cam Timing!
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 a Checking Clearance between rocker and valve of 1.00 mm (ie
1.00 mm lift). This figure of 1.00 mm is a cam timing standard and usually
if no specific checking clearance figures are mentioned then it is fairly
safe to assume they would have been made at this lift but beware ‘factory’ figures
as discussed previously!
If you use methods 1 or 2 you will end up with an opening and a closing degree
figure for each valve. These are compared directly with the factory figures
(assuming they match your cam) to show if your timing is retarded, advanced
or close enough!
If you’re not convinced that the figures and the cam match then some
simple calculation, as discussed earlier, will give you the duration of the
valve opening and the theoretical point of ML. To check this figure or if
you don’t have or trust the factory timing figures for your cam then
you should use method 3 where you will end up directly with the point of
Maximum Lift.
3. Timing by Lobe Center Angle.
If you have no factory figures for your cam then a more accurate place to
measure cam timing is at the point of Maximum Valve Lift (ML). This point
is easier to determine without resorting to changing clearances although
like finding TDC for the piston there is a trick to doing it accurately as
both the piston and the valve do not move for a few degrees of crankshaft
rotation at these points.
This is also the simplest method as you are only dealing with a single figure
for each cam and the point of ML can be measured easily and accurately. When
you have the figures for maximum lift of both the inlet and exhaust valves
it is a simple calculation to work out the Lobe Center Angle (LCA) which
by definition is the number of degrees between valve ML and TDC (Exhaust).
If the cam lobes are symmetrical then LCA’s for both valves should
be the same ie The half way point between the ML’s of the exhaust valve
and the inlet valve should work out to be TDC (Exhaust) for a normal engine
if the timing is correct. If the figure you get is before TDC then the valve
timing is advanced ie the valves are opening sooner than specified. If the
figure you get is after TDC then the valve timing is retarded ie the valves
are opening later than specified. (Note that in some specialised engines’ cam
lobes may not be symmetrical hence LCA’s may not be equal. Consult
the cam timing figures or manufacturer for more information.).
Before starting any measurement you will need some special equipment to do
the job properly.
A large degree wheel with a length of 8 mm threaded rod or a long 8 mm bolt
else the factory tool for turning the engine with a degree wheel attached.
A rigid pointer that will remain in place once set.
A positive stop tool for finding piston TDC.
A dial gauge with mounting system. (The club has a dial gauge & magnetic
stand)
A steel plate to attach the gauge and stand to engine
A calculator or slide rule for averaging timing readings.
Finding the Exact Top Dead Center.
Before you start you will need to determine the piston TDC accurately so
that you can set the degree wheel to reflect piston timing. Whilst the
crankshaft is moving a few degrees either side of top dead center the piston
is virtually stationary so any measurement at this point will not be accurate
enough for the purposes of cam timing. The accurate way to measure TDC
is by using some type of ‘Positive Stop’ device.
The idea behind this is that if you can stop the piston at the same position
on its stroke both before and after TDC and mark the degree wheel at both
points then TDC must be exactly half way between these two marks on the degree
wheel. You can make your own positive stop device with an old spark plug
body drilled out and a piece of threaded rod or bolt inserted and fixed in
place. See references at the end of part 3 for articles on making and using
this type of tool.
Attach the degree wheel to the crankshaft. You will need some way of turning
the crankshaft without upsetting the degree wheel either use the factory
disc tool, which I found had too much play, remove the side cover so you
can get a socket onto the crank nut or select 5th gear and turn the rear
wheel. Attach the pointer to an engine cover screw so that it lines
up with the degree readings on the edge of the timing disk.
Remove both spark plugs and using a blunt probe locate approximate TDC on
the first cylinder you wish to check. Move the timing disk on its bolt so
that the pointer lines up with 0° then tighten the disk so that it cannot
move on the bolt. Turn the engine forward slightly so that you can screw
the positive stop device in without hitting the piston. Then carefully turn
the motor back until the piston touches the stop device. Mark the degree
wheel next to the pointer then turn the engine forward through almost a full
turn until the piston again just touches the stop device. Mark this point
on the degree wheel then calculate and make a third mark exactly half way
between the first two. This is the real piston TDC point, so remove the stop
device and turn the engine until this mark is next to the pointer and now
your engine is exactly at TDC.
You will probably find that the mark does not coincide exactly with 0° on
the degree wheel due to our first setting being approximate so to make life
easier during timing measurements bend the pointer slightly so that it now
points to 0° (Do Not move the engine!). Rub out all your marks on the
wheel and go through the whole process again using the stop device and now
you should find that the third mark lines up exactly with 0° on the wheel.
Make sure that the engine is at TDC with the degree wheel reading 0° before
continuing! Note that if during any part of the measuring procedure you suspect
that the degree wheel may have moved in relation to the engine then recheck
TDC using the above method before continuing.
If you are using ‘running’ or ‘checking’ clearances
then you do not need to set up a dial gauge at this time. These degree readings
are taken when the ‘clearance’ just closes up during engine rotation.
Measuring at Running Clearance.
If you are using running clearances then obviously these must be set exactly
to the factory specifications before checking the timing. Turn the engine
over until the rocker starts to move then continue to turn the engine slowly
until the running clearance is reduced to zero. Note down the degree reading
at this point that will be your ‘valve opening’ figure. Continue
turning the engine over until the valve has just closed and note down the
degree reading at this point that will be your ‘valve closing’ figure.
Compare your figures with those in the manual to find how close your timing
is to factory specifications. Remember that these figures are not very accurate
or realistic and that measuring the lobe center angle will give you better
figures to work with.
Measuring at Checking Clearance.
If you are using checking clearances there are two ways to achieve this.
If you have screw adjusters or a selection of shims you can either close
the clearance up to zero and measure the timing at 1.00 mm valve lift or
open up the clearance to 1.00 mm and measure when the valve starts or stops
moving.
If you don’t have a selection of shims there is an easy way to achieve
the checking clearance. Start by fitting a loose shim in place that leaves
a clearance greater than 1.00 mm, then fill the gap with a combination of
feeler gauge blades till you get a snug fit. Next subtract the specified
checking clearance from the feeler gauge stack (1.00 mm) and this will leave
you with the correct checking clearance! With the blades in place turn the
engine over until the gauges are a snug fit and take your ‘valve opening’ reading
off the degree wheel. Remove the gauges then continue turning the engine
over until the valve closes and keep turning till the feeler stack just slides
into the gap again then take your ‘valve closing’ reading off
the degree wheel. Compare your figures with those in the manual to find out
if your timing is close to factory specifications.
Measuring at Maximum Lift.
If you are using the ‘maximum lift’ method then you need to set
up the dial gauge. The pointer must rest on the rocker directly over the
valve and the instrument be solidly fixed so that it cannot move at all during
a full rotation of the engine. A flat piece of steel plate drilled with two
holes to match the rocker cover bolt holes will provide a sturdy support
for the magnetic stand base. Adjust the gauge so that full movement of the
valve does not exceed the limits of the gauge.
Finding the point of maximum valve lift needs an approach similar to finding
piston TDC as at this point moving the engine through several degrees will
not produce any movement at the valve. Turn the engine over slowly and watch
the dial gauge carefully as the valve moves down and note the reading on
the gauge when the valve reaches its maximum depression (lift). Pick an arbitrary
valve lift figure say 0.05 mm less than the maximum lift and rotate the engine
forward through almost a full cycle (two rotations) until the dial gauge
reads the arbitrary lift figure just before the valve has reached maximum
lift. Mark the degree wheel at this point then continue rotating the engine
forward past maximum lift until the dial gauge once again reads the arbitrary
lift figure. Once again mark the degree wheel at this point and as with finding
the piston TDC calculate a third mark exactly half way between the first
two that gives you the exact point of maximum valve lift.
Maximum Lift Example.
The following example shows measurements and calculations from my 900SS bevel.
Note that I used an arbitrary lift figure of 0.02 mm but this should have
no effect on the final timing figures. Take all measurements with the engine
moving in its normal forward direction as ‘play’ in the cam drive
system will give different readings in reverse that will not be relevant.
Vertical Inlet
Rough readings; Opens approximately 45° BTDC, ML approximately 107° ATDC
Forward 0.02 mm before ML = 85.5° ATDC Forward 0.02 mm after ML = 123.5°
ATDC
123.5° - 85.5° = 38° divided by 2 = 19°
Therefore the Vertical Inlet ML = 85.5° + 19° = 104.5° ATDC.
Now we need to do the same measurements and calculations for the Vertical
Exhaust valve
Vertical Exhaust
Rough readings; Opens approximately 67° BBDC, ML approximately 83° BTDC
Forward 0.02 mm before ML = 107.5° BTDC Forward 0.02 mm after ML = 65.5°BTDC
107.5° - 65.5° = 42° divided by 2 = 21° Therefore the Vertical
Exhaust ML = 65.5° + 21° = 86.5° BTDC. From these two figures
we can now work out the type of cam that is fitted to the bike.
The Lobe Center Angle for the Vertical cylinder is = Inlet ML + Exhaust ML
divided by two.
104.5° ATDC + 86.5° BTDC = 191° divide by two = 95.5°
The lobe center angle for the vertical cylinder camshaft is 96°. As discussed
earlier the points of maximum lift should be symmetrical about TDC and so
we can now calculate whether this cam is advanced or retarded and by how
many degrees.
Figure 4 shows the difference between where the cam timing has been measured
and where it should be for the standard timing with equal lobe center angles.
Vertical Cylinder Valve Timing is...
86.5° (Measured LCA Exhaust) - 95.5° (Calculated Lobe Center Angle)
= 9.0° Retarded or 95.5° (Calculated Lobe Center Angle) - 104.5 (Measured
LCA Inlet) = 9.0° Retarded Retarded valve timing means that the valves
are opening later than expected. Now the whole procedure detailed previously
is repeated again for the Horizontal Cylinder. I will skip some of the calculations
and just work with the figures obtained.
The Horizontal Inlet ML = 93.5° ATDC.
The Horizontal Exhaust ML = 99.5° BTDC.
The Lobe Center Angle for the Horizontal cylinder is = Inlet ML + Exhaust
ML divided by two.
93.5° + 99.5° = 193° divide by two = 96.5°
Therefore we have a 96° lobe center camshaft on the horizontal cylinder
so both are the same type of cam as expected! Never assume this without measuring
both cams as stranger combinations of parts have appeared on Ducati’s
straight from the factory! If the bike’s full history is unknown there
is always the possibility that a different replacement cam was fitted at
some stage.
Horizontal Valve Timing is...
96.5° (Measured Lobe Center Angle) - 93.5° (Calculated Lobe Center
Angle) = 3.0° Advanced Advanced valve timing means that the valves are
opening earlier than expected.
From these two sets of figures you can see that the difference in cam timing
between the vertical and horizontal cylinders on my 900SS bevel was 9.0° Retarded
+ 3.0° Advanced = 12.0 crankshaft degrees!
This 12 degree difference is not exceptional as some early bevel twins have
been measured up to 28 crankshaft degrees retarded and with similar differences
in cam timing between front and rear cylinders!
This is often the hidden reason why two externally identical bikes can have
huge differences in top end speed, fuel consumption, and mid range torque. Cam
timing variations will also effect compression readings significantly and
this can explain why a bike with two fully reconditioned cylinders and heads
will still give different compression readings on each cylinder!
Now that you have done all your measurements and calculations you need to
make a decision on whether or not to change the valve timing. This decision
should take into account the following factors.
How far out is the timing between the two cylinders? Factory specifications
give +/- 5° on cam timing. For road use I would leave the timing if there
was less than 5° difference between the two cylinders. Obviously for
perfect engine balance the closer the two cylinder’s valve timings
are the smoother the engine will run.
How far out is the timing in relation to where it should be? Once again using
factory specifications if the timing was within +/- 5° of TDC exhaust
then I would leave it for road use.
Obviously these two figures have to be looked at individually for each cylinder
as you could have a worst case where one cylinder was say 5° retarded
timing wise and the other cylinder was 5° different from it also retarded
which would put the second cylinder 10° out!
You will also have to look at the cost’s involved verses the expected
improvement. If you have a square case bevel then the cost could be as little
as some of your time, but if you have a belt drive and get an expert to do
it all for you then the costs could be considerable. If you’re still
not sure take your figures to someone who knows Ducati engines and get an ‘expert’ opinion.
Next month we will get into the thick of it by actually changing the cam
timing and finding that not only does the engine still run but that it runs
much smoother, has better compression and performs better (hopefully without
any expensive noises).
The Black Mystic Art of Cam Timing!
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’ the
timing gears in the timing chest on the right hand side of the engine. With
reference to diagram 1 below the cam drive is as follows. Straight cut
double gear 14 sits on the end of the crankshaft. The inner gear drives the
oil pump and the outer gear with 24 teeth drives both straight cut gear 9’s,
one for each camshaft, which have 36 teeth.
Straight cut gear 9 is joined to bevel gear 10 via key 11. ie these two gears
turn as one. Bevel gear 10 has 23 teeth and mates to an identical 23 tooth
bevel gear (Not shown) which drives the shaft in the cam drive tube. At
the cylinder head a 21 tooth bevel gear mates with a 28 tooth bevel gear
connected to the camshaft. The reason these seemingly strange combination
of gears is used is to spread the load evenly over the gear teeth rather
than having the same few teeth always taking the load of opening the valves.
This is called the ‘hunting tooth’ principle and has been used
on gear driven camshafts for many years.
DIAGRAM
1, Square Case Bevel Cam Drive
So our overall cam drive ratio is :-
24/36 = 2/3 . . . 23/23 = 1/1 . . . 21/28 = 3/4 . . . 2/3 * 1/1 *3/4 = 6/12
= 1/2.
The camshaft runs at half crankshaft speed as discussed in the first article
in this series.
If we move the 24/36 straight cut combination by one tooth we get a change
of . . . 360°/24 = 15° in relation to the crankshaft.
If we move the 23/23 bevel combination by one tooth we get a change of .
. . 360°/23 = 15.652° but as this combination is spinning at 2/3
crankshaft speed then . . . 15.652° * 3/2 = 23.478° in relation to
the crankshaft.
Now on their own these figures are too large to use unless your timing is
more than 15° out but because gears 9 and 10 are joined together we can
use a combination of moves to give us anything from half a degree up to a
23 degree change.
As an example say we move the 23/23 bevel gear combination by one tooth and
the 24/36 straight cut combination by one tooth also. If we move one set
back a tooth and the other set forward a tooth the difference is 23.5° minus
15° = 8.5°. Depending on which way we move each gear this can achieve
an 8.5° advance or retarding of the cam timing.
Because of the difference in degrees of moving each set by one tooth there
are many possible combinations each of which results in a different overall
degree change. If we put these figures into a computer program and tell it
print out all possible combinations of tooth movement we will get a Printout
many pages long. If in this program we limit the number of teeth moved to
a workable figure say 15 and limit the maximum number of degrees changed
to plus or minus 15° then this gives us one page of possibilities from
0.65° inaround 1° steps up to 15° In my case I needed a 12° change.
The program Printout tells me that if I move the 24/36 straight cut gears
7 teeth in one direction and the 23/23 bevel pair 5 teeth the other way I
will get...(5 * 23.487°) - (7 * 15°) = 117.435° - 105° =
12.435°
By undoing the nut on the end of the crankshaft the alternator rotor and
distance piece can be removed. Turn the engine over until all the timing
dots line up. This will be TDC compression on the vertical cylinder. You
then need to remove the timing gear support plate being careful that all
shims on the shafts are kept in their place and are not mixed up.
Now comes the tricky part which way do we move the gears? Always keep in
mind that we want to advance the camshaft that is we want the valves to open
earlier. The camshaft turns clockwise viewed from the timing side of the
engine. Remember the crankshaft runs anticlockwise (backwards) when viewed
from the same side. The biggest change in degrees is made by moving the 23/23
bevel pair 5 teeth so we must advance the camshaft by 117.435° with this
move and then retard it 105° with the 7 tooth move of the straight cut
gears.
Start by removing the double straight cut gear 14 from its key and without
moving any other components pull the straight/bevel double gear 9&10
out of mesh with the cam drive shaft bevel gear. Rotate gear 9&10 anticlockwise
by 5 teeth and then re-engage it. This means that the timing dot on bevel
gear 10 is now 5 teeth to the left of it’s matching dot on the cam
drive shaft bevel.
Now slide the double straight cut gear 14 onto the keyway and almost into
mesh and you will find that the timing dot on gear 9 is a number of teeth
(not necessarily an exact number) anticlockwise from the dot on gear 14.
At this point we have not moved the camshaft and now we will move it the
12.435° required. This time we have to move gear 9 without taking the
gear out of mesh. ie we move gear 9 and all components in the drive train
to the particular cam shaft. As each tooth on gear 9 is 15° you should
find that you only have to move gear 9 a little less than one tooth clockwise
so that the timing dot on gear 9 is now 7 teeth anticlockwise away from the
timing dot on gear 14.
When you have changed the timing do a quick check to see that you have not
miscalculated and put it too far out. With the vertical piston at TDC compression,
it should still be there if you haven’t moved the crankshaft, check
both pairs of head bevels and you should find that the timing dots match
up and are relatively close together.
Remember as discussed in an earlier article that the camshaft will only be
advanced by 6.217° (12.435° divided by 2) as the camshaft turns at
half crankshaft speed. As we haven’t touched the bevel gears on the
head you will find that the dots still line up although they may not be exactly
touching now as the camshaft has been moved by a little over 6 degrees. This
means that when head work is required you can still match up the top timing
marks and with a little jiggling drop the heads into place the same as before.
If all seems OK at this point repeat the whole measuring procedure to check
that the timing is exactly where you want it to be and that you haven’t
moved the timing in the wrong direction. If you think you have completely
stuffed it up never fear as you only need to align all the timing dots again
to get back the standard timing!
Of course once you have changed the timing the dots on the lower bevel gear
10 and on the straight cut gear 9 will no longer line up with their mating
gears. You should use an engraver or similar to put a new mark on both these
gears (9&10) and also make a drawing of how it looks so that if the engine
has to come apart you can restore your modified timing without going through
the whole measuring procedure again.
Round Case Bevel Engines
With the round case motors there is no straight/bevel double gear and so
changes have to be made to the timing case bevel gears as well as the head
bevel gears to achieve a change. Diagram 2 below shows the cam gear drive
train.
[
DIAGRAM
2, Round Case Bevel Cam Drive.
In round case motors, with reference to diagram 2, the cam drive is as follows.
A bevel gear with 23 teeth sits on the end of the crankshaft and this drives
a matching 23 tooth gear (Set 700) on a short shaft. This shaft drives the
points cam via a two to one reduction gear set (Not shown) under the points
drive base. In the middle of this shaft is a 24 tooth bevel gear which drives
two 36 tooth bevel gears, one for each cam drive shaft. (Set 800). At the
cylinder head a 21 tooth bevel gear mates with a 28 tooth bevel gear connected
to the camshaft. (Set 600).
So our overall cam drive ratio is :-
23/23 = 1/1 . . . 24/36 = 2/3 . . . 21/28 = 3/4 . . . 1/1 * 2/3 *3/4 = 6/12
= 1/2.
If we move the 24/36 bevel gear combination by one tooth we get a change
of . . . 360°/24 = 15° in relation to the crankshaft.
As there is no double gear on this model the only other place we can change
the timing is at the head bevels.
If we move the 21/28 bevel combination by one tooth we get a change of .
. . 360°/21 = 17.143° but as the 21 tooth gear is spinning at 2/3
crankshaft speed then, . . . 17.143° * 3/2 = 25.714° in relation
to the crankshaft.
If we move one set back a tooth and the other set forward a tooth the difference
is 25.714° minus 15° = 10.714°. Depending on which way we move
each gear this can achieve a 10.7° advance or retarding of the cam timing.
Using our computer program again we find there are not quite as many different
combinations for this engine but we can still achieve a fairly close result.
Using our 12° figure and checking the chart we find that we can achieve
10.7° by moving each set one tooth as detailed above but the next closest
is 12.85° by moving the 24/36 gears 6 teeth one way and the 21/28 gears
4 teeth the other way.
The method for achieving a change in cam timing is the same as for the square
case motor but in this case the head bevel dots will not line up and additional
marking will be required. It is also worth noting that a special tool is
required to allow the cam drive shaft bevels to be removed from mesh in the
round case motors. (The Club has one of these tools for loan to members.)
NOTE 1. . . If you are lining up the lower timing marks in the timing chest
of a round case motor you will notice that the central bevel gear, which
drives the two cam drive shaft bevel gears (In set 800), has two timing dots
180° apart which match a single timing dot on each cam drive shaft bevel
gear. When the bike left the factory these two dots on the central gear were
painted either red or green and matching paint was applied to the single
dots on the mating gears. I think green was used for the vertical cylinder
and red for the horizontal. It is probable that this paint will be long gone
when you come to do the timing so be aware that you can have all the timing
marks lined up but still have the point’s cam 180° out! The point’s
cam must be checked and should be just about to open the points for the vertical
cylinder (The points closest to the condensers). If this is not the case
then rotate the crankshaft three times and all the marks should line up again
as well as having the vertical points just about to open. To avoid future
problems clean and repaint the dots before reassembly.
NOTE 2. . . The points drive has a set of 2 to 1 reduction gears under the
point’s body. There is a timing dot on the small drive gear that matches
up with a painted mark on the large gear. As mentioned previously this paint
may be long gone so if you pull the points drive out be sure to mark the
large gear with an engraver, paint or similar to aid correct re-assembly.
As far a round case timing goes the factory figures give a lobe center angle
of 99.5°/98°. When I measured my 750 GT I came up with a lobe center
figure of around 108° so true to form the cam specified in the manual
does not match the cam fitted to my machine. Speaking with Brook Henry he
believes that there were two different cam grinds for both the GT and the
Sport so don’t assume without measuring.
According to my measurements on the GT if the lobe center of 108° is
standard then my horizontal cylinder was around 7.5° advanced and the
vertical cylinder was 17.5° advanced! So there is a 10° difference
and both cams seem over advanced. As stated before more advance gives better
mid range power and this is what GT’s are famous for. At this time
I have not found any information on what would be the best setting for the
GT. As changing the round case timing is more involved I probably won’t
attempt it till the next engine rebuild. If anyone has some detailed information
on the standard GT cam timing or the best place to set it then please pass
this information on to Club members either via myself or through the pages
of Desmoto.
Belt Drive Models
As discussed previously you will need to get your camshaft drive pulleys modified with extra keyways or else get change over pulleys with this work already done. I can’t explain the fine detail of this procedure as I have only seen it done at Cafe Racer one service day. The idea is that the extra keyways are set several degrees either side of standard and by trial and error you select the keyway which puts your timing closest to the wanted figure. Set the engine so that the cam is at the point of maximum lift then lock it in place. Remove the belt and camshaft pulley then turn the engine to the wanted figure for maximum lift. By turning the pulley you should be able to find the keyway that allows the closest fit between the belt teeth and the keyway. Also offset keys are another possible way to change your timing by a smaller amount. However you change the timing don’t forget to double check your figures by repeating the measurement process.
Summary
The intention of this series of articles was to stir an interest in the
black mystic art of cam timing and is really only a starting point for discussion
and research for anyone interested in delving further into this complex and
fascinating subject. Several of the references mentioned go into much greater
detail in specific areas and books such as ‘Tuning for Speed’ by
Phil Irving have whole chapters devoted to this subject.
There are also many Ducati and ‘after market’ cams available
with more radical timing and/or lift to suit racing and other applications.
With your new found knowledge of cam timing you should at least be able to
understand the conversation when dealing with the suppliers of these cams
who should be able to provide you with a cam to suit your specific requirements.
Thanks must go to a couple of cam timing ‘experts’ who were kind
enough to proof read and assist with clarifications and additions to these
articles. The fact that these gentlemen did not wish to be credited
by name lends credence to the fact that nobody will admit to being a cam
timing ‘expert’! Still I thank them for their time and
effort to bring you a more accurate article!
Peter Shearman
References
Secrets of the Ancient Cam Timers by Kevin Cameron. Cycle September 1989.
Cam Timing, How to do it by Kevin Cameron. Cycle September 1989.
Check Your Cam Timing (Belt Drive) by Peter Birtles. Desmoto June 1990.
Check Your Cam Timing (Bevel Drive) by Peter Cross. Desmoto July 1990. Accurate
Method of Finding Top Dead Center by Reubin Hoggett. Desmoto October 1984.
Top Dead Center Tool and Timing Tips by John Withers. Desmoto March 1986.
Ducati Timing Gears by Peter Shearman. Desmoto August 1992.
Computer Generated Timing Gear Change Charts by Peter Shearman.
Thanxs also to Tania Donaldson who scanned in the pictures for this article (and most of the pictures that appear on the DOCV Homepage).
Please note that Ducati-UpNorth.com cannot accept any liability for the accuracy or content of this section. Visitors who rely on this information do so at their own risk. If you are unsure it's worth contacting your local Ducati dealer who will be able to help. Do not attempt a repair or modification if you do not have the correct tools or knowledge to do so.