--===============2028525827880285170==
--0000000000007bb2d405902bb2ba
Content-Transfer-Encoding: quoted-printable
Sorry Friends,
I did not mean for the text to read like an ad for Pertronix. That text
was written for a friend having ignition problems and it WAS a
recommendation for him to buy the Pertronix distributor. I have just found
that the module and the full distributer worked so well for me. I should
drop the name from the text and use the generic terms.
Rick
On Wed, Aug 14, 2019, 5:44 PM Richard Lindsay <richardolindsay@gmail.com>
wrote:
> Hello Friends,
> Here is the next piece that I wrote a bit ago. I hope you find them
> informative.
>
> In this work, the function of the ignition condenser is debatable. Som=
e
> have considered these words apocryphal. Some have offered alternative
> discussion. And some have found them based in elementary physics. You
> decide.
>
> Rick
>
> Ignition System
>
> The ignition system on older British sports cars is comprised of a
> number of components, all of which need to be in top operating condition
> for maximum performance. Working backwards from the engine, these parts a=
re
> the spark plugs, plug wires, distributor, coil, and in some cases, a
> ballast resistor.
>
> Spark plugs, or 'sparking plugs' as they are quoted in the British car
> manuals, are the components that ignite the charge. An electric spark fro=
m
> the center electrode to ground heats the charge, ionizing it and starting
> oxidation. The 'flame front' then travels across the combustion chamber. =
As
> it does so, the MEP, or Mean Effective Pressure, increases to a maximum,
> pushing the piston down the bore. More on that later but first, here is t=
he
> anatomy of a spark(ing) plug.
>
> The plug is an electrical device. It has a central electrode extending
> from a connection at the outside, to a tip inside the combustion chamber.
> It is supported by and sealed within a ceramic insulator. The exposed
> length of the ceramic insulator sets the 'heat range' of the plug. Short
> insulators conduct heat away from the tip more quickly. Long ones cool mo=
re
> slowly. Too cold and deposits are left on the tip and insulator. Too hot
> and the tip material may burn and remain too hot, causing pre-ignition of
> the charge. The engine designers understood the heat characteristics of t=
he
> various engines and have specified the ideal heat range for plugs in each
> application.
>
> Plug choices are like oil choices. Every manufacturer has a marketing
> campaign claiming their plug is the best. And like the oil additive
> industry, there are a number of added-on doodads to plug tip design.
>
> The most basic plug electrode design, and the design for which these
> older engines were intended, is a plug with a copper or copper plated
> central electrode and a steel ground or earth electrode extending from th=
e
> edge of the threads, going up and over the central tip. The gap between t=
he
> tip and the ground will be discussed later.
>
> Arguably, the biggest improvement in spark plug design is the
> application of rare metals to the tip design. Platinum and iridium are us=
ed
> because they withstand high heat better than copper. Copper tends to melt
> and burn away with use. These high-end plugs will probably outlast the
> engine, unless contaminated.
>
> Other manufacturers, eager to capture market share, have engineered ne=
w
> plug designs utilizing multiple ground electrodes in various orientations=
,
> each claiming superior performance, greater power and extended life. They
> are a waste of time and money in this author's opinion. Premium platinum =
or
> iridium plugs are the author=E2=80=99s plugs of choice. Old design copper=
electrode
> plugs are perfectly acceptable too, but expect to re-gap or change the
> plugs as part of the routine maintenance.
> The 'reach' of a plug is the length of the threaded portion of the
> body. It is the bit that screws into the head. The reach must match the
> engine maker's design. Too short and the spark doesn't occur within the
> combustion chamber. Too long and the plug extends too far into the
> combustion chamber, inviting physical damage. Plug reach must match the
> engine designer's specification.
> Regardless of the type of plug chosen, assuming the reach and heat
> range are correct, the plug threads should be lubricated before
> installation. Copper infused thread lubricant, applied sparingly to the n=
ew
> plug threads, is ideal. This is very important for engines with alloy
> heads! The plug's steel threads can easily damage the head=E2=80=99s soft=
er alloy
> plug-hole threads. This can happen either by physical damage, such as
> cross-threading or over-tightening, or by the action of dissimilar metals
> placed in contact. Thread lubricant minimizes head damage. The author use=
s
> lubricant even for iron heads since the copper particles marginally impro=
ve
> heat flow, thereby cooling the plug. Perhaps that's over-kill but it
> certainly makes plug changes easier.
> Before installing the spark plugs, the electrode gap should be set. A
> typical gap is 0.025", or 0.64mm, and this is often the gap found, straig=
ht
> out of the plug's box. However, the engine maker=E2=80=99s specification =
should be
> followed and the gap set properly. In all cases, the gap is set by bendin=
g
> the ground electrode, never by touching the central electrode. That eleme=
nt
> is surrounded by brittle ceramic and may be damaged.
> The physicist in the author requires that he write a little bit about
> the anatomy of the spark. The spark is comprised of two phenomena; the
> capacitive component that lasts only a few microseconds, and the much
> longer electrical component that produces the big, fat, hot spark. It is
> argued that a properly tuned and warmed up engine can run on the capaciti=
ve
> component only, but they never do. Rather, the high voltage electrical
> spark is needed to heat the charge, thereby vaporizing any wetted charge =
on
> the electrodes, and further promoting ionization of the charge between th=
e
> electrodes. And this is where proper electrode gap becomes important. A
> wider gap provides a hotter spark as is required to adequately heat the
> charge, especially for cold starts. However, a wider gap requires more
> secondary voltage to spark. This topic leads us to the ignition coil.
> High voltage from the coil's secondary winding travels through the
> distributor and the plug wires, out to the spark plugs. The distributor a=
nd
> plug wires will be discussed later but first, here is a discussion of the
> coil. The ignition coil is an autoformer. That is, it is a transformer wi=
th
> one side of the primary winding and one side of the secondary winding
> connected together. This side of the circuit is connected to the coil's (=
-
> ) or 'CB' post. The later descriptor means Contact Block and is a better
> designation since this is also the connection for older positive earth ca=
rs
> like the MG T-series of cars. In that case, the ( - ) marking is a misnom=
er.
>
> A transformer is a voltage multiplier. Any deeper discussion of
> electromagnetic theory is not helpful for our purposes. Stated more simpl=
y,
> the coil's secondary contains thousands more turns of wire than does the
> primary. Both are wound around a soft iron core. The ratio of turns,
> primary to secondary, defines how much the primary voltage is 'stepped up=
'.
> An automobile coil typically produces about 20,000 volts at the secondary=
,
> from its 12 volts primary. This is the voltage that is fed to the plugs.
>
> Some ignition designs incorporate a ballast resistor in series with th=
e
> coil's primary circuit. The reason for this design will become apparent
> shortly. A conventional coil is designed for 12 volts, applied directly t=
o
> the primary. The primary coil winding is typically about 3 ohms resistanc=
e.
> So at 12 volts, a current of 4 amps, flows. ( I =3D E /R ) All is well an=
d
> good for normal operation but what about start up? During cold weather th=
e
> starter has to work harder to spin the engine. And when doing so, it
> demands more current from the battery. In fact, the battery voltage is
> pulled down to 9 volts or less during these cold starts. We've all seen t=
he
> lights dim during starting. Unfortunately, reduced primary voltage also
> results in reduced secondary voltage, thereby weakening the spark right
> when it needs to be the strongest! The ballasted coil design addresses th=
is
> problem.
>
> Under normal operation, a ballasted ignition works just like a regular
> ignition, but the components are different. Designs vary but basically, a
> ballasted coil's primary resistance is on the order of 1.6 ohms, rather
> than the 3 ohms of a conventional coil. Inserted in the primary circuit
> path is a ballast resistor of about 1.4 ohms. The sum of the primary coil
> winding and the ballast resistor in series, is still 3 ohms. The ignition
> current is the same 4 amps as with a conventional coil. The same hot spar=
k
> is provided when the engine is running. The tricky and useful bit of a
> ballasted ignition happens at cold start.
>
> When starting, voltage from the ignition switch is applied directly to
> the coil, bypassing the ballast resistor. Assuming the starter is pulling
> the available battery voltage down to about 9 volts, the 1.6 ohm coil now
> sees 9 volts at it's primary, for a current of 5.6 amps! So even with the
> starter pulling down the available voltage, the ignition still provides a
> hotter spark for easy starting. Of course, the numbers quoted here are
> generic and they vary with manufacturer, but the concept is the same.
> Ballasted ignitions are implemented to facilitate cold engine starts.
> The ignition's secondary voltage is not continuous. Rather, it is a
> pulsed system operating under the control of the distributor. This device
> is actually two devices integrated into one case. The high voltage or 'hi=
gh
> tension' stated in old-speak, is distributed to the spark plugs, via the
> plug wires, by this part of the distributor, thus the name. The other par=
t
> of the distributor pulses and times the spark by controlling the coil's
> primary circuit. Both are now discussed.
> High voltage distribution is achieved by a rotating contactor, aptly
> termed the 'rotor'. This device is fed secondary voltage at it's center v=
ia
> a contact in the distributor cap. Located around the inside of the cap ar=
e
> electrical contacts leading to the connectors on the outside and to which
> plug wires, and therefore, the plugs are connected. The distributor rotat=
es
> at one half of the engine crankshaft speed because of the engine's Otto
> Cycle or 4-stroke design. The rotor spins around the inside of the cap
> allowing a precisely timed spark to occur between the rotor and the
> appropriate contact in the cap. That voltage then fires the spark plug on
> the correct cylinder at the optimal time to ignite the charge.
> The other function of the distributor is to pulse the coil's primary a=
t
> the right times, and to provide a circuit path for the coil's secondary.
> The first part of this process is rather well understood by most mechanic=
s,
> but the later bit remains shrouded in mystery. Here is an explanation.
> Inside of the distributor is a cam with the same number of 'lobes' as
> the engine has cylinders. Riding on the cam and mounted on the 'breaker
> plate' are a set of contacts, also called 'points'. The lobes of the cam
> cause the points to open and close. When closed, the coil primary is
> grounded allowing a magnetic field to grow to saturation in the coil. Whe=
n
> the points open the magnetic field collapses and induces a high voltage i=
n
> the secondary winding. It is this voltage that provides the sparks.
> Therefore, the spark timing is set at the time the points open, not close=
.
>
> Before discussing timing, the purpose of the condenser should be
> understood. One side of this device is grounded to the breaker plate. The
> other side of the condenser is connected to the coil-side of the points.
> When the points open, the coil is electrically isolated from earth ground=
,
> except through the condenser, and thus its purpose. The coil's field
> collapses induces a high voltage in the secondary. The purpose of the
> condenser is to provide a current path for that circuit. Many workers cla=
im
> that the condenser is there to prevent secondary arcing across the points=
,
> but that is a benefit, not the reason for the condenser.
> Of primary importance is when the spark plugs fire, and this timing is
> also controlled by the distributor. The breaker plate, described above, i=
s
> arranged so that it may rotate a few degrees. The rotor is attached to a
> shaft that also may rotate a few degrees, if by a different mechanism. Th=
e
> former is modulated by a vacuum capsule, or perhaps two of them. The late=
r
> is rotated by a set of rotating weights, acting against two springs,
> sometimes of different spring constants. In the vernacular, the capsules
> are called the 'vacuum advance'. The flying weights are called the
> 'centrifugal advance'. In both cases, the word 'advance' is the key. And
> before discussing ignition timing, the characteristics of the burning
> charge will be discussed.
> The goal of any ignition is to arrange for the MEP (Mean Effective
> Pressure) caused by combustion, to occur where it can do the most work.
> Therefore, timing the ignition is actually timing the MEP to the optimal
> piston position. Fortunately the engine designers have worked through the
> maths and have done all the experiments to simplify our task to reading t=
he
> manual and making a few adjustments! That said, it is still of value to
> understand why the timing must be varied.
> The charge does not immediately flash to fully combusted, as our
> experiences with flammable liquids might imply. Rather, the charge is
> ignited at the spark plug and the 'flame front' then progresses across th=
e
> combustion chamber. The MEP is not achieved instantaneously either so the
> ignition of the charge has to be in advance of the MEP. This would all
> seems easy enough to understand if the engine operated at one speed
> only...but it doesn't.
> At slow engine speeds, such as slow idle, the spark timing has to occu=
r
> so that the MEP does the most work. In most engine designs this is the
> 'static timing' and is typically a few degrees BTDC or 'Before Top Dead
> Center'. Why before? The spark ignites the charge early enough to allow f=
or
> the travel time of the flame front and for the pressure to build to the
> MEP. As the engine speed increases, the travel time of the flame front do=
es
> not, but the piston is moving faster! Therefore, to place the MEP at the
> optimal piston position, the charge must be ignited earlier. And this is
> the sole purpose for the timing advance mechanisms in the distributor. Th=
e
> author could write more about the centrifugal advance curve or how the
> vacuum advance mechanism compensates for engine load, but those are topic=
s
> better left for later.
> One of the typical failure modes in old British car ignitions is the
> points. With use, the contacts tend to burn and the insulating wiper ridi=
ng
> on the cam wears. In both cases, the timing suffers eventually ending in
> ignition failure. For about $120 the whole ignition primary can be
> upgraded by using a Pertronix Ignitor instead of the points and condenser=
.
> This device includes a circular adapter that fits over the cam. Imbedded =
in
> the adapter are tiny magnets that spin adjacent to the ignition module.
> Impulses from the magnets cause the internal circuit to pulse the ignitio=
n
> electronically, with no moving parts! This system provides a maintenance
> free ignition primary.
> Unfortunately, the rest of the old distributor components are typicall=
y
> worn or damaged. Most old British cars enjoyed by collectors are
> 40-years-old or older! A lot of wear can happen in 40+ years. Bushings we=
ar
> from lack of lubrication, pivots wear in the advance mechanisms, and
> previous owners may have done quite nefarious things! The author has even
> found ball point pen (biro) springs substituted for the springs on the
> centrifugal advance! There are three choices available to remedy these
> problems: totally rebuild the distributor - a task beyond the skill set o=
f
> most amateur mechanics, buy a period-correct replacement distributor,
> either new or professionally rebuilt, or buy a new Pertronix distributor,
> already equipped with an electronic primary trigger system.
> Symptoms of distributor wear include the inability to set a smooth,
> slow idle, and timing instability at low engine speeds as indicated by a
> timing light. The author recommends the Pertronix distributor. His MG TD
> and TR3b both have these upgrades and both have beautifully smooth slow
> idles, and trouble free ignitions.
>
--0000000000007bb2d405902bb2ba
Content-Transfer-Encoding: quoted-printable
<div dir=3D"auto"><div>Sorry Friends,</div><div dir=3D"auto">=C2=A0 =C2=A0I=
did not mean for the text to read like an ad for Pertronix. That text was =
written for a friend having ignition problems and it WAS a recommendation f=
or him to buy the Pertronix distributor. I have just found that the module =
and the full distributer worked so well for me. I should drop the name from=
the text and use the generic terms.</div><div dir=3D"auto"><br></div><div =
dir=3D"auto">Rick<br><br><div class=3D"gmail_quote" dir=3D"auto"><div dir=
=3D"ltr" class=3D"gmail_attr">On Wed, Aug 14, 2019, 5:44 PM Richard Lindsay=
<<a href=3D"mailto:richardolindsay@gmail.com";>richardolindsay@gmail.com=
</a>> wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:=
0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir=3D"auto"><=
div dir=3D"auto" style=3D"font-family:sans-serif">Hello Friends,</div><div =
dir=3D"auto" style=3D"font-family:sans-serif">=C2=A0 =C2=A0Here is the next=
piece that I wrote a bit ago. I hope you find them informative.=C2=A0</div=
><div dir=3D"auto" style=3D"font-family:sans-serif"><br></div><div dir=3D"a=
uto" style=3D"font-family:sans-serif">=C2=A0 =C2=A0In this work, the functi=
on of the ignition condenser is debatable. Some have considered these words=
apocryphal. Some have offered alternative discussion. And some have found =
them based in elementary physics. You decide.</div><div dir=3D"auto" style=
=3D"font-family:sans-serif"><br></div><div dir=3D"auto" style=3D"font-famil=
y:sans-serif">Rick</div><div dir=3D"auto" style=3D"font-family:sans-serif">=
<br></div><div dir=3D"auto" style=3D"font-family:sans-serif">Ignition Syste=
m<br>
<br>
=C2=A0=C2=A0=C2=A0The ignition system on older British sports cars is compr=
ised of a number of components, all of which need to be in top operating co=
ndition for maximum performance. Working backwards from the engine, these p=
arts are the spark plugs, plug wires, distributor, coil, and in some cases,=
a ballast resistor.<br>
<br>
=C2=A0=C2=A0=C2=A0Spark plugs, or 'sparking plugs' as they are quot=
ed in the British car manuals, are the components that ignite the charge. A=
n electric spark from the center electrode to ground heats the charge, ioni=
zing it and starting oxidation. The 'flame front' then travels acro=
ss the combustion chamber. As it does so, the MEP, or Mean Effective Pressu=
re, increases to a maximum, pushing the piston down the bore. More on that =
later but first, here is the anatomy of a spark(ing) plug.<br>
<br>
=C2=A0=C2=A0=C2=A0The plug is an electrical device. It has a central electr=
ode extending from a connection at the outside, to a tip inside the combust=
ion chamber. It is supported by and sealed within a ceramic insulator. The =
exposed length of the ceramic insulator sets the 'heat range' of th=
e plug. Short insulators conduct heat away from the tip more quickly. Long =
ones cool more slowly. Too cold and deposits are left on the tip and insula=
tor. Too hot and the tip material may burn and remain too hot, causing pre-=
ignition of the charge. The engine designers understood the heat characteri=
stics of the various engines and have specified the ideal heat range for pl=
ugs in each application.<br>
<br>
=C2=A0=C2=A0=C2=A0Plug choices are like oil choices. Every manufacturer has=
a marketing campaign claiming their plug is the best. And like the oil add=
itive industry, there are a number of added-on doodads to plug tip design.<=
br>
<br>
=C2=A0=C2=A0=C2=A0The most basic plug electrode design, and the design for =
which these older engines were intended, is a plug with a copper or copper =
plated central electrode and a steel ground or earth electrode extending fr=
om the edge of the threads, going up and over the central tip. The gap betw=
een the tip and the ground will be discussed later.<br>
<br>
=C2=A0=C2=A0=C2=A0Arguably, the biggest improvement in spark plug design is=
the application of rare metals to the tip design. Platinum and iridium are=
used because they withstand high heat better than copper. Copper tends to =
melt and burn away with use. These high-end plugs will probably outlast the=
engine, unless contaminated.<br>
<br>
=C2=A0=C2=A0=C2=A0Other manufacturers, eager to capture market share, have =
engineered new plug designs utilizing multiple ground electrodes in various=
orientations, each claiming superior performance, greater power and extend=
ed life. They are a waste of time and money in this author's opinion. P=
remium platinum or iridium plugs are the author=E2=80=99s plugs of choice. =
Old design copper electrode plugs are perfectly acceptable too, but expect =
to re-gap or change the plugs as part of the routine maintenance.<br>
=C2=A0=C2=A0=C2=A0The 'reach' of a plug is the length of the thread=
ed portion of the body. It is the bit that screws into the head. The reach =
must match the engine maker's design. Too short and the spark doesn'=
;t occur within the combustion chamber. Too long and the plug extends too f=
ar into the combustion chamber, inviting physical damage. Plug reach must m=
atch the engine designer's specification.<br>
=C2=A0=C2=A0=C2=A0Regardless of the type of plug chosen, assuming the reach=
and heat range are correct, the plug threads should be lubricated before i=
nstallation. Copper infused thread lubricant, applied sparingly to the new =
plug threads, is ideal. This is very important for engines with alloy heads=
! The plug's steel threads can easily damage the head=E2=80=99s softer =
alloy plug-hole threads. This can happen either by physical damage, such as=
cross-threading or over-tightening, or by the action of dissimilar metals =
placed in contact. Thread lubricant minimizes head damage. The author uses =
lubricant even for iron heads since the copper particles marginally improve=
heat flow, thereby cooling the plug. Perhaps that's over-kill but it c=
ertainly makes plug changes easier.<br>
=C2=A0=C2=A0=C2=A0Before installing the spark plugs, the electrode gap shou=
ld be set. A typical gap is 0.025", or 0.64mm, and this is often the g=
ap found, straight out of the plug's box. However, the engine maker=E2=
=80=99s specification should be followed and the gap set properly. In all c=
ases, the gap is set by bending the ground electrode, never by touching the=
central electrode. That element is surrounded by brittle ceramic and may b=
e damaged.<br>
=C2=A0=C2=A0=C2=A0The physicist in the author requires that he write a litt=
le bit about the anatomy of the spark. The spark is comprised of two phenom=
ena; the capacitive component that lasts only a few microseconds, and the m=
uch longer electrical component that produces the big, fat, hot spark. It i=
s argued that a properly tuned and warmed up engine can run on the capaciti=
ve component only, but they never do. Rather, the high voltage electrical s=
park is needed to heat the charge, thereby vaporizing any wetted charge on =
the electrodes, and further promoting ionization of the charge between the =
electrodes. And this is where proper electrode gap becomes important. A wid=
er gap provides a hotter spark as is required to adequately heat the charge=
, especially for cold starts. However, a wider gap requires more secondary =
voltage to spark. This topic leads us to the ignition coil.<br>
=C2=A0=C2=A0=C2=A0High voltage from the coil's secondary winding travel=
s through the distributor and the plug wires, out to the spark plugs. The d=
istributor and plug wires will be discussed later but first, here is a disc=
ussion of the coil.=C2=A0The ignition coil is an autoformer. That is, it is=
a transformer with one side of the primary winding and one side of the sec=
ondary winding connected together. This side of the circuit is connected to=
the coil's ( - ) or 'CB' post. The later descriptor means Cont=
act Block and is a better designation since this is also the connection for=
older positive earth cars like the MG T-series of cars. In that case, the =
( - ) marking is a misnomer.<br>
<br>
=C2=A0=C2=A0=C2=A0A transformer is a voltage multiplier. Any deeper discuss=
ion of electromagnetic theory is not helpful for our purposes. Stated more =
simply, the coil's secondary contains thousands more turns of wire than=
does the primary. Both are wound around a soft iron core. The ratio of tur=
ns, primary to secondary, defines how much the primary voltage is 'step=
ped up'. An automobile coil typically produces about 20,000 volts at th=
e secondary, from its 12 volts primary. This is the voltage that is fed to =
the plugs.<br>
<br>
=C2=A0=C2=A0=C2=A0Some ignition designs incorporate a ballast resistor in s=
eries with the coil's primary circuit. The reason for this design will =
become apparent shortly. A conventional coil is designed for 12 volts, appl=
ied directly to the primary. The primary coil winding is typically about 3 =
ohms resistance. So at 12 volts, a current of 4 amps, flows. ( I =3D E /R )=
All is well and good for normal operation but what about start up? During =
cold weather the starter has to work harder to spin the engine. And when do=
ing so, it demands more current from the battery. In fact, the battery volt=
age is pulled down to 9 volts or less during these cold starts. We've a=
ll seen the lights dim during starting. Unfortunately, reduced primary volt=
age also results in reduced secondary voltage, thereby weakening the spark =
right when it needs to be the strongest! The ballasted coil design addresse=
s this problem.<br>
<br>
=C2=A0=C2=A0=C2=A0Under normal operation, a ballasted ignition works just l=
ike a regular ignition, but the components are different. Designs vary but =
basically, a ballasted coil's primary resistance is on the order of 1.6=
ohms, rather than the 3 ohms of a conventional coil. Inserted in the prima=
ry circuit path is a ballast resistor of about 1.4 ohms. The sum of the pri=
mary coil winding and the ballast resistor in series, is still 3 ohms. The =
ignition current is the same 4 amps as with a conventional coil. The same h=
ot spark is provided when the engine is running. The tricky and useful bit =
of a ballasted ignition happens at cold start.<br>
<br>
=C2=A0=C2=A0=C2=A0When starting, voltage from the ignition switch is applie=
d directly to the coil, bypassing the ballast resistor. Assuming the starte=
r is pulling the available battery voltage down to about 9 volts, the 1.6 o=
hm coil now sees 9 volts at it's primary, for a current of 5.6 amps! So=
even with the starter pulling down the available voltage, the ignition sti=
ll provides a hotter spark for easy starting. Of course, the numbers quoted=
here are generic and they vary with manufacturer, but the concept is the s=
ame. Ballasted ignitions are implemented to facilitate cold engine starts.<=
br>
=C2=A0=C2=A0=C2=A0The ignition's secondary voltage is not continuous. R=
ather, it is a pulsed system operating under the control of the distributor=
. This device is actually two devices integrated into one case. The high vo=
ltage or 'high tension' stated in old-speak, is distributed to the =
spark plugs, via the plug wires, by this part of the distributor, thus the =
name. The other part of the distributor pulses and times the spark by contr=
olling the coil's primary circuit. Both are now discussed.<br>
=C2=A0=C2=A0=C2=A0High voltage distribution is achieved by a rotating conta=
ctor, aptly termed the 'rotor'. This device is fed secondary voltag=
e at it's center via a contact in the distributor cap. Located around t=
he inside of the cap are electrical contacts leading to the connectors on t=
he outside and to which plug wires, and therefore, the plugs are connected.=
=C2=A0The distributor rotates at one half of the engine crankshaft speed be=
cause of the engine's Otto Cycle or 4-stroke design. The rotor spins ar=
ound the inside of the cap allowing a precisely timed spark to occur betwee=
n the rotor and the appropriate contact in the cap. That voltage then fires=
the spark plug on the correct cylinder at the optimal time to ignite the c=
harge.<br>
=C2=A0=C2=A0=C2=A0The other function of the distributor is to pulse the coi=
l's primary at the right times, and to provide a circuit path for the c=
oil's secondary. The first part of this process is rather well understo=
od by most mechanics, but the later bit remains shrouded in mystery. Here i=
s an explanation.<br>
=C2=A0=C2=A0=C2=A0Inside of the distributor is a cam with the same number o=
f 'lobes' as the engine has cylinders. Riding on the cam and mounte=
d on the 'breaker plate' are a set of contacts, also called 'po=
ints'. The lobes of the cam cause the points to open and close. When cl=
osed, the coil primary is grounded allowing a magnetic field to grow to sat=
uration in the coil. When the points open the magnetic field collapses and =
induces a high voltage in the secondary winding. It is this voltage that pr=
ovides the sparks. Therefore, the spark timing is set at the time the point=
s open, not close.<br>
<br>
=C2=A0=C2=A0=C2=A0Before discussing timing, the purpose of the condenser sh=
ould be understood. One side of this device is grounded to the breaker plat=
e. The other side of the condenser is connected to the coil-side of the poi=
nts. When the points open, the coil is electrically isolated from earth gro=
und, except through the condenser, and thus its purpose. The coil's fie=
ld collapses induces a high voltage in the secondary. The purpose of the co=
ndenser is to provide a current path for that circuit. Many workers claim t=
hat the condenser is there to prevent secondary arcing across the points, b=
ut that is a benefit, not the reason for the condenser.<br>
=C2=A0=C2=A0=C2=A0Of primary importance is when the spark plugs fire, and t=
his timing is also controlled by the distributor. The breaker plate, descri=
bed above, is arranged so that it may rotate a few degrees. The rotor is at=
tached to a shaft that also may rotate a few degrees, if by a different mec=
hanism. The former is modulated by a vacuum capsule, or perhaps two of them=
. The later is rotated by a set of rotating weights, acting against two spr=
ings, sometimes of different spring constants. In the vernacular, the capsu=
les are called the 'vacuum advance'. The flying weights are called =
the 'centrifugal advance'. In both cases, the word 'advance'=
; is the key. And before discussing ignition timing, the characteristics of=
the burning charge will be discussed.<br>
=C2=A0=C2=A0=C2=A0The goal of any ignition is to arrange for the MEP (Mean =
Effective Pressure) caused by combustion, to occur where it can do the most=
work. Therefore, timing the ignition is actually timing the MEP to the opt=
imal piston position. Fortunately the engine designers have worked through =
the maths and have done all the experiments to simplify our task to reading=
the manual and making a few adjustments! That said, it is still of value t=
o understand why the timing must be varied.<br>
=C2=A0=C2=A0=C2=A0The charge does not immediately flash to fully combusted,=
as our experiences with flammable liquids might imply. Rather, the charge =
is ignited at the spark plug and the 'flame front' then progresses =
across the combustion chamber. The MEP is not achieved instantaneously eith=
er so the ignition of the charge has to be in advance of the MEP. This woul=
d all seems easy enough to understand if the engine operated at one speed o=
nly...but it doesn't.<br>
=C2=A0=C2=A0=C2=A0At slow engine speeds, such as slow idle, the spark timin=
g has to occur so that the MEP does the most work. In most engine designs t=
his is the 'static timing' and is typically a few degrees BTDC or &=
#39;Before Top Dead Center'. Why before? The spark ignites the charge e=
arly enough to allow for the travel time of the flame front and for the pre=
ssure to build to the MEP.=C2=A0As the engine speed increases, the travel t=
ime of the flame front does not, but the piston is moving faster! Therefore=
, to place the MEP at the optimal piston position, the charge must be ignit=
ed earlier. And this is the sole purpose for the timing advance mechanisms =
in the distributor.=C2=A0The author could write more about the centrifugal =
advance curve or how the vacuum advance mechanism compensates for engine lo=
ad, but those are topics better left for later.<br>
=C2=A0=C2=A0One of the typical failure modes in old British car ignitions i=
s the points. With use, the contacts tend to burn and the insulating wiper =
riding on the cam wears. In both cases, the timing suffers eventually endin=
g in ignition failure. For about <span class=3D"m_9097570549663078898money"=
>$120</span> the whole ignition primary can be upgraded by using a Pertroni=
x Ignitor instead of the points and condenser. This device includes a circu=
lar adapter that fits over the cam. Imbedded in the adapter are tiny magnet=
s that spin adjacent to the ignition module. Impulses from the magnets caus=
e the internal circuit to pulse the ignition electronically, with no moving=
parts! This system provides a maintenance free ignition primary.<br>
=C2=A0=C2=A0=C2=A0Unfortunately, the rest of the old distributor components=
are typically worn or damaged. Most old British cars enjoyed by collectors=
are 40-years-old or older! A lot of wear can happen in 40+ years. Bushings=
wear from lack of lubrication, pivots wear in the advance mechanisms, and =
previous owners may have done quite nefarious things! The author has even f=
ound ball point pen (biro) springs substituted for the springs on the centr=
ifugal advance! There are three choices available to remedy these problems:=
totally rebuild the distributor - a task beyond the skill set of most amat=
eur mechanics, buy a period-correct replacement distributor, either new or =
professionally rebuilt, or buy a new Pertronix distributor, already equippe=
d with an electronic primary trigger system.<br>=C2=A0 =C2=A0Symptoms of di=
stributor wear include the inability to set a smooth, slow idle, and timing=
instability at low engine speeds as indicated by a timing light. The autho=
r recommends the Pertronix distributor. His MG TD and TR3b both have these =
upgrades and both have beautifully smooth slow idles, and trouble free igni=
tions.<br></div></div>
</blockquote></div></div></div>
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