Dual Plugging
dualplugging.htm-31
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>>>>The following is my personal opinion on this subject:!!!<<<<
***A technical, somewhat scholarly, and worthwhile information on the merits of dual plugging, with a somewhat different viewpoint than mine, can be had from: Oak Okleshen, at askoak@aol.com, please ask for the papers on Dual Spark Plug Ignition. Oak did a LOT of dual-plugging work and had a professional background of engineering work in combustion chamber investigations....and was involved heavily with the original dual-plugging projects
Here is a URL for Tom Cutter's rather old article on dual
plugging. While it has some errors and other things that
need updating, this is valuable if you are interested in much of
the theory and background.
http://pub217.ezboard.com/fairheadtechforumfrm12.showMessage?topicID=17.topic
tpcutter@aol.com
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***Please refer to
other articles on this website, dealing with the pulse-air system
and ignition system, ETC.***
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I have responded many times to questions on and off the Airheads
Internet Mailing LIST about dual-plugging. I have also have
had many E-mail's regarding automatic advance units, whether for
dual plugging or not. I've made comments on setting timing and
details regarding the automatic advance units, and modifications
to those units as made by others. I also 'admitted' that although
I had dual plugging on some of my two airheads, and likely will
do it to the otherwise stock 1984 R100RT at valve job time,
"I did not think it necessarily warranted for all
riders." One would have thought that I had
posted instructions on how to desecrate the Sistine Chapel.
I got interesting replies, to say the least, almost all being off the LIST. I won't go into why I think that was so. What I do want to do is to make some definitive PERSONAL OPINION comments on dual-plugging and automatic advance units.
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Introduction:
In my estimation, dual plugging is, or can be, a low to
moderately expensive solution to a possible problem, yet
sometimes it is the best solution. What
dual plugging does, or is supposed to do, is to start TWO flame
fronts at the same time. Because of this, flame
propagation in the combustion chamber may well be more
even, and the burn time will be faster,
OVERALL. If
the engine had the original spark timing, the engine would now
have its 'over-all' combustion event start too early, that is,
too soon before the piston reaches the top of the compression
stroke, this would cause problems, and thus the spark must be
retarded, particularly at the slowest rpm, to compensate.
The purpose of an automatic advance unit is to allow and give a
retarded spark at low rpm and a more advanced spark for higher
rpm. The stock automatic advance unit (see extensive
notes elsewhere's on the various versions used in the airheads
over the years) matches the needs of the stock engine relatively
well. In
the stock engine, with one spark plug per cylinder, the important
ignition timing is the maximum mark (dot or line, F or Z).
The S marking is of secondary importance. Depending on the
year, the S may be just that character, or have a line, or be
surrounded by two lines. The S markings are NOT used
for ONE type of ignition timing method with dual-plugs, and ARE
for the other type. In the
dual-plugged conversion, the spark MUST be retarded for the
initial static condition; and also with the engine running;
AND, the maximum markings (F or Z) are less important, may
not be visible in one version of the timing modification.
Before I keep having to add lots more words, a description of the two timings for dual-plugging is needed. For one, the one I particularly like (both work FINE!), the idle rpm timing is the OT marking; sometimes a degree or two more advanced than that (UP the window is advanced). It just so happens that the OT mark is very close to what is needed. for ONE type of this timing. For that type of timing, the Z or F markings do not get into the window, but are at the bottom of the window. For SOME bikes, a few extra degrees of full advance are advantageous, so the mechanical stops for the ATU weights are modified, so the weights can move outward a bit more. Thus, the F dot or Z marking can be brought into the window at the bottom, as one example. NOTE! that due to how dual-plugging actually works dynamically, less than the normal maximum advance is OK, thus modifying for extra range of advance is not any actual necessity. A dual plug installation has its biggest effect at lower rpm, and as engine rpm increases, the dual plug idea of retarding of the spark has less effect.
One can, as an exercise in knowledge, after setting the OT mark in the window at idle rpm, increase rpm and try to see where the maximum timing mark ends up at. One must remember that engine rpm has a DIRECT and huge effect on just how long (TIME) the burning fuel is in the cylinder. For these various reasons, the spark must be retarded a fair amount at low rpm. That is true for ANY engine.
The OTHER type of timing is simply an application of a different viewpoint. For this method, the timing is usually set to approximately the original S mark at idle. The timing unit is usually modified with restrictors for maximum advance. This type of timing has an advantage that most never hear about....the closed throttle position on the CV carburetors is not quite so critical.
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Because the rise in effective pressure in the
cylinder happens more quickly due to two flames being started,
the rider will see, if nothing else
changed after conversion to dual plugging, a much
higher idle rpm, no matter the ignition timing method. To
bring the idle down to proper rpm, the tuner will adjust the
carburetor butterflies more CLOSED. This,
in itself, makes the critical teeny area where the butterfly
lower edge interacts with the carburetor idle ports, more
critical. Carburetors with poorly fitted or
backwards (wrongly) fitted butterfly valves will have
problems. Also, there is the possibility that an idle port
(that is a liquid fuel passage NOT air, in the Bing CV, opposite
in the NON-CV) will become more critical due to velocity effects
across the port, and the idle mixture jet may have to be
increased in size. This is easily done, and usually
eliminates any trace of stumbling at very small throttle openings
FROM idle. The adjustment of cables and idle
stops...the so-called synchronizing of the carburetors, becomes a
bit more critical with dual plugs. I have not found
that the idle mixture adjustment changes much in
sensitivity.
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Dual plugging DOES:
1. Reduce spot heat in the combustion chamber, likely due to
more even distribution of the heat, and
thus is probably easier on valves and valve seats, especially in
the R100 models. I have speculated that the OVERALL
amount of heat may also be decreased, assuming the ignition
timing is set correctly. Thus, a case might be made
that dual-plugging decreases engine heat. However, item #2,
just below, is more important, and likely much clearer.
2. Under heavy throttle use the valve seats
likely will not tend to warp towards the spark area.
3. Likely better performance, particularly
in the later, LOW compression ratio models.
4. Probably better gasoline mileage,
particularly in the OLDER HIGHER compression ratio models.
OFTEN allows the use of regular, versus previous use of premium
gasoline's, saving fuel $$....see #7.
5. Allows higher compression ratios and
thereby more performance for the later low compression models,
and will also allow QUITE high CR if one wants to use premium
fuels.
6. Looks neat.
7. May pay for itself over the long long term in mileage, wear
and tear on starter motors, batteries, etc. ...see #9.
8. In case of coil failure, you have an
extra coil, to fire both cylinders.
9. In almost every instance the engine
will start far easier, particularly in cold weather.
10. See the mentioned article by Oak.
11.
Dual plugging with the recommended ACCEL coils does NOT!! make
the ignition work harder!!!!
12. With today's lead-free fuels, the plugs are
not changed very often, as there is no need to...this is not a
specific advantage for dual-plugging, it is for all Airheads, but
with dual-plugging, it SEEMS that plugs last even longer.
There
are disadvantages, and some special concerns:
(1) Cost for the conversion and parts:
If you have the heads off for a valve
job, or to replace pushrod tube rubbers, IMHO that is a good time
to think about it.
(2) The automatic advance system will be more of a compromise
than the stock system, but hardly bad (much more on this later in
this posting).
(3) The BARELY off-idle performance may suffer a tad, although
that IS easily and cheaply curable, typically by changing the
idle pilot jet size.
(4) You have to be careful not to
overtorque the lower plugs (if 1/2 inch reach types are used) and
to use the proper length of plug, and now you have twice as many
plugs to change. Note that one can use the same 3/4 inch reach
spark plug as used on the top plug, if one heliarcs aluminum
material to the head and then machines it, at the new lower plug
area. You can also use 3/4 inch reach plugs with a sleeve spacer.
The lower plug TENDS to operate slightly richer (or, cooler, take
your pick of words) ...especially if the engine is older and
burning a small amount or more of oil......compared to the top
plug. So, as far as heat range is concerned, this is easily seen
from LOOKING and comparing the two plugs of one cylinder, and one
step hotter heat range plug may be required at the lower
plug. Inspection and comparing of the plugs
after a few minutes at heavy throttle and rpm, will tell the
story.
(5) You have more 'stuff'. You have two 'different from stock'
ignition coils that likely are not easy to find if one fails on
the road (granted, the two coils in the Accel 1040404 kit seem to
last forever). But see #8
above. NOTE that for the 1981 and later models, you
need only the two Accel coils, the spark plugs, and the spark
plug wires/caps; as there is NO NEED FOR ANY OTHER ignition PARTS
changes. However, you MIGHT want to modify the ignition
timing unit...but it is NOT really necessary. See prior
comments!
(6) For the earlier points models, the new coils are
probably going to be types that require more primary current than
the original types, and you will likely be adding a points
current amplifier. That amplifier will change the points
current to a much lower than stock value, GREATLY lengthening
points life. The points will still act as the triggering
device, now for the amplifier which then drives the coil(s).
IF you had a failure of the amplifier, you COULD bypass it, to
enable returning to home or wherever. The points will burn
up far faster, that is all. In the 1979-1980 models,
which use the canister POINTS ignition, you COULD change to the
1981 canister electronic ignition, and that WOULD be a NICE
conversion! If you wanted to spend money, you could
do that. You would not have to mount the module where
the early brake master cylinder was....unless, of course, you
converted to a bars master cylinder (not cheap to do, with the
assembly including throttle cable modifications, etc.).
DETAILS:
When you dual plug heads, the new bottom plugs machining is
probably more often done for
14 mm 1/2 inch thread length, perhaps Bosch W6BC (7593), and the
top plug a 14 mm 3/4 inch thread length W5DC (7591).
You may need different heat ranges, depending on your bike's
year, compression ratio, etc. W5 to W8 is the range used on
airheads. Most often, especially with a clean running
engine, both spark plugs heat range remains the same heat number
as it was when stock, even with a modest compression ratio
increase (such as by milling the heads). If moving the CR
from 8.2 to 9.5, that is a larger change, and a colder
plug...perhaps that W5, will be needed. Note AGAIN, that in
most instances of dual plugging I have seen....particularly as
mileage gets large and small amounts of oil are being burned
(typically from valve guides, sometimes from rings), the lower
plugs may need to have a ONE step increase in heat range
value. This is done, if needed, after a spark plug
inspection after a hard hot run on the highway.
BE SURE TO READ MY SPARK PLUG
ARTICLE!!!! DO NOT TAKE THIS RECOMMENDATION
LIGHTLY! SPARKPLUGS.HTM
The top plugs will remain the stock 14
mm 3/4 inch thread length. Some folks have
installed smaller thread diameter lower plugs, claiming
this offers less likelihood of a crack occurring,
etc. Some do the smaller plug because they say the
smaller thread plugs are 'easier' to find at the local autoparts
store (NGK brand, for instance). Properly done 14 mm
is OK, with ME....as...if the lower plug drilling/tapping is done
correctly, I see no problem with 14mm....and that is what I have
always used.
NOTE:
There is also a reverse problem with obtaining the correct type,
size and heat range of spark plugs for the lower plugs, if
you do not use the 14 mm size. You
also do NOT want a resistor plug! The
extra spark plug hole must be drilled squarely at at the correct
place, and someone who has done many of these is the best person
to go to for the drilling and threading. Oak
Okleshen is my favorite person for dual plug conversions.
NOTE: Some folks will weld some aluminum to the partly
machined second plug hole, and then machine it to match the stock
type 3/4 inch top spark plug. This allows the SAME
3/4" reach plug as the existing top plug. I have NO
objections to this, if the welding is done properly and machined
properly. You now don't have the potential error of over-torqueing
the lower plugs.
For the more common machining method, with NO welding, it is true that the lower plug fits in thinner metal area, and therefore that 1/2 inch thread length plug is used. It is always easy for those who are truly mechanically hamfisted, who like to 'force things', to strip out threads.
The STOCK torque for the regular 3/4 inch plugs, listed in SOME books, even some BMW literature, is 16-18 ftlbs, clean and dry threads. Even Oak has published using that figure. I disagree somewhat. Spark plug manufacturers have several arguments why you should NOT use antiseize. I do NOT agree with them, although I understand their concerns. My feeling is that using sparkplugs made of steel, going into aluminum alloy heads, without lubricating them with a lubricating/antiseize compound, IS asking for trouble. Once the plugs are lubricated with antiseize, you MUST NOT torque them to the stock specifications. If you do not have a torque wrench (why not?) then using NEW gaskets (crush washers), torque to JUST crush the gaskets. I HIGHLY recommend an ACCURATE torque wrench!
My
recommended lubricated/antiseized torque values are about 2/3 of
non-lubricated. This means about 13 to 14 ftlbs for the top plugs
and about 10-11 ftlbs for the bottom (1/2 inch plugs at
bottom). Approach torques slowly and
carefully, and be sure the crush washer does crush some, nearly
fully. If you are using an old washer, you should
already KNOW what torque to use. These values
are for 14 mm plugs.
For those with the BMW electronic ignition (1981 on models), be
sure you use the nominal 5000
ohm spark plug caps, such as NGK LB05F spark plug caps. NO
resistor plugs. I do NOT like the idea of the specialty low
resistance spiral-wound wires. Please use quality solid copper
stranded wires core and 5000 ohm caps on the electronic ignition
models! If you do not use the proper wiring and caps you will
likely damage the triggering unit's Hall device from re-radiated
energy, which MAY not show up for some time. You
do, of course, know that the module under the tank has to be
cleaned and fresh heat-transfer compound applied every year or
two!?!
****>>>>One nice advantage
to having the stock electronic ignition (1981 and up) is that the
EXISTING system need only have two aftermarket coils added, and
their spark plug wires/caps. You do NOT need to change or modify
the canister (you MIGHT want to bend the ears of the internals or
use restrictors), nor do you need to change the module. Simple.
Reliable. The Accel 140404 coils are perfect for
this, and mount under the tank easily, in the place of the
original two coils. For those Airhead models that
came with ONE coil (ST, GS, and late models), you need to use a
wee bit of ingenuity, to mount the two Accel coils. The
ACCEL coils ARE my favorites....they never seem to fail.
If you are using points with a points amplifier or a Dyna unit,
etc., you can use 1000 ohm caps, unless
the manufacturer says otherwise. For points models, do not
eliminate the resistance caps entirely, there are more than just
radiated signal reasons for this. Many have no idea
that the cap resistance will lengthen the spark DURATION, whilst
reducing the intensity, the combination needs to be right.
***NOTE: Normally, when one runs out of adjustment
range on a set of points, for timing purposes, one thinks of
badly worn rubbing block on the points, and/or a badly stretched
timing chain. With the stock points plate, with
dual-plugging, you must retard the ignition, and you may not have
enough adjustment area....so you may have to do some hand-filing
of the holes.
>>>>>NOTE:
Please use ONE coil's outputs for the TOP plugs, the other coil's
outputs for the bottom plugs!!!! This IS correct! Failure
to follow this recommendation CAN cause the system to act as if
the ignition was weak, noticeable especially at starting on a
cold morning. <<<<<
If you are contemplating doing a dual-plugging job on your later
airhead with the 8.2 compression ratio, be advised that if you
plan to shave the head, every .005" you shave will up the
C.R. approximately 0.1 point. Around 9.6 is the absolute upper
limit for today's premium gasoline's. You could also use 9.5:1
BMW pistons, and even shave the head a tad to clean it up and
make sure it is flat. Only those who use premium gasoline's
and keep their engines well tuned...and fairly clean of carbon
deposits...should use 9.5 or above. Properly milling, even
a minimal cleanup milling of the head on both surfaces is a good
idea, especially the R100 models. Some have shaved the
cylinder base, probably more of a hassle, and some other things
will be problems, like the O-rings cavity/areas.
Some will find that they can NOT use compression ratios
over 9.4, even on 92 octane premium (98-100 octane, OK)....as
carbon deposits and variations in engines will cause pinging. If
your timing chain is worn, you may require premium
gasoline.
A CR of about 9.2, is usually usable and OK on 87 octane fuel,
particularly at higher altitudes. Some have no problem with 85
octane at high altitudes. I think that 9.2 from
milling (9.5 pistons, milling cleanup only, if using piston
change for the CR) is a nice MAXIMUM figure for any
consideration of 'regular' grade fuel, and you may well
want to consider about 8.8. Thus, if you have an engine of
the 8.2 low compression type and are not changing to HC pistons,
shaving about .030-.060 will be the amount you likely will
use. There are some advantages to using the
squish-band pistons (9.5) from the 1977-78 era, those heads, and
a SMALL amount of shaving. I am not going to get into that
here. Suffice it to say that as the CR gets above 8.8 or
so, you must pay closer and closer attention to fuel, pinging,
timing, etc. For
most, using the stock heads, and shaving the heads and having the
cylinder match properly for 8.8-9.2 will be just fine.
I used to have an entire
section here on spark plugs. I have moved it to a spark
plug article. BE SURE to read it...it is IMPORTANT!
DO NOT TAKE THIS
RECOMMENDATION LIGHTLY! SPARKPLUGS.HTM
The 1978 and later flywheels have the ignition markings
re-marked, for 3 degrees of extra retard. This causes these later
models to run a tad hotter, and this was done for emissions
reasons. BMW also began to jet the carbs leaner, and modified the
squish band in the cylinders (those horsepower producing
turbulence bands went bye bye after the end of the 1970's).
Shame. A nice squish band does help. If I remember correctly, it
was in 1979 that BMW made the last high compression R100 (9.5)
model for the U.S.....and that was an excellent performer. It is
possible to have the correct pistons and heads, or modify
some. Some have done this. Some foreign shipped
models still came with high compression pistons. I am not
getting into, here, the years, models, piston shapes, etc., of
the short while that BMW had squish combustion chambers.
HERE,
I will begin getting into what has been discussed previously, in
much more depth:: In these discussions, in order to
simplify them SOMEwhat, I will ASSUME that, WHERE IT IS
APPLICABLE, that the stock advance unit is used, either with
slightly increased advance range, or not, and that approximately
the OT mark ignition is being used.
When dual plugging is installed, that alone, in itself, DOES
effectively ADVANCE the timing of the flame in the cylinder. BUT,
it does not do this evenly for all engine speeds.
Having two flame fronts DOES THE MOST ADVANCING AT LOWER ENGINE
SPEEDS. So, retarding the ignition to approximately the OT (top
dead center) mark is a COMPROMISE. The effect of having that
second spark plug diminishes as RPM goes up, and at around 5000
or perhaps even 6000+, that second plug has far less effect. It
is possible, not yet proved to me entirely, that the SINGLE plug
ignition is MORE efficient at high rpm's due to more turbulence
in the combustion chamber. What is apparent from
engineering work, is that one does not NEED the same amount of
actual advance above 3000 rpm, as came from the stock
engine. That is one of the reasons that timing at or near
OT, and NOT restricting the advance weights movement (and even
maybe stretching the movement a bit) works so well.
But, on a practical basis, the ideal or
near ideal situation, is to STRETCH
the advance curve a SMALL AMOUNT, so it is RETARDED at lowest
rpm's from you setting the OT mark at idle rpm; and closer to
nearly stock at highest rpm. That is, one sets the timing
retarded, and stretches the automatic advance pin stops
outward...a wee bit. RESTRICTING the automatic
advance range with sleeves on the pins, etc., is WRONG
for this particular method. NOTE!!!
Stretching the advance range is NOT
a 'must' item.
Our BMW automatic advance units are fully mechanical, and quite reliable, in general. What you want, theoretically, is to bend those advance ears outward, or otherwise modify the unit for a WEE BIT more TOTAL advance RANGE. AGAIN, this is NOT a must, as the small change for the high rpm region is of only small effect. Folks who simply must have everything to the Nth degree, can consider modifying the advance weights so that the assembly can move a bit further than just bending the ears will allow. You also might want to consider moving the rpm point of maximum advance, up the scale a bit. This is more likely so with lower grades of gasoline and higher compression...such as with 9.5. Considering the premium price for premium gasoline, it may well pay to increase the rpm for maximum advance a bit. That is done by one or two slightly stiffer advance springs...or, modifying the weights. One can modify the weights and stops for increased RANGE at the same time. This is surely much harder to do on the canister models! For those, the ears can be bent a bit outward via an oval inspection hole,,,trying to do more means disassembly of the canister. You won't get much more advance though through the oval hole, by bending the ears;...a couple of degrees or so. Again, the effect or improvement is small. Note that weights' and stops modification can be done in a way that increases total range. The proper range is a tad unclear, but if the OT mark is centered in the timing window at idle, and the maximum advance mark is at the lower edge, or very slightly below it, at 3600 rpm or so, that is about as good as one can get. In the long run, perhaps trying for these minute improvements may not be worth it, and the stock advance is certainly adequate!
After the earliest /5 (they had advance
stop around 2000 rpm), stock advance units (about 25 or 26
degrees available) go to full advance at about 3000 rpm. Moving
that advance to 3600 will usually cure that last bit of pinging
at sea level if you went fairly high in compression ratio or are
using regular gasoline with maybe 9.2 or somewhat
higher. In fact, this works
pretty well with single plug ignition.
NOTE! Pinging aside, best performance as far as
overall acceleration goes, is obtained with a fast advance
curve...down around 2000 rpm for the maximum. The early
/5 springs did faster advancing. BEWARE...don't play in
this area without being knowledgeable!
In the case of
an engine running 9.5 to even 10:1 compression ratio,
dual-plugging, and the delay of the maximum advance, correct
slightly rich carburetion, a clean combustion chamber, and
carefully tailored advance, will allow spirited performance on
premium pump gasoline's. SOME have been able to run
regular 87 or mid-grade 89 fuel with 9.5:1 pistons, some even
with just enough machining to ensure a flat head.
***To keep your combustion chamber clean, I suggest using Chevron
gasoline's, or adding Chevron Techron to your tank now and then;
and, I recommend a premium multi-grade motorcycle oil. The
better oils do not burn off the first fractional quart fast as do
the poor additives in cheap oils, this means less deposits.
I am fond of Golden Spectro 4, in
20W50. That is an SG additive-type semi-synthetic
oil, that is quite good at low to high temperatures. Full
synthetics are even better in regards to performance AND
deposits, but MAY bring about other problems, such as seal
leakage, although that is rarer these days. Full synthetic
oils like Spectro's, BMW's, and the diesel type oils (they have
ZDDP!!), see the oils article!....are very nice. Mobil 1 in
15W50 works quite well in airheads too.
Coils:
For the stock electronic
ignition airheads (1981+), the STOCK canister and the STOCK
module under the fuel tank, are all perfectly suited to drive two
each 6 volt dual tower coils, wired in series. The coils
for that are made by Accel, and the kit of a pair of the coils is
Accel #140404. The number is for the kit. Each of
these two coils has a 0.7 ohm primary. If your coils
come with ignition wires, discard them if they are carbon core
resistance types, and use QUALITY silicon rubber insulated wires,
that have stranded copper wires cores. Use with proper
resistance type spark plug caps....for the electronic ignition of
the BMW, use 5,000 ohm caps ONLY. Do
NOT...NOT!!!....use resistor spark plugs with resistor
caps. DO NOT!!!...NOT!!!...use resistor spark plugs
and zero ohm caps. READ the spark plug article on this
website....there can be problems with Bosch's new numbering
system and the plugs!
Dyna: Dyna III unit was for the stock 1970-1978
Bosch coils. The Dyna Ignition Booster was for the
1970-1980 points models of airheads....and can be used on dual
plug installations with the proper coils.
Dyna GREEN coils are sometimes seen on
dual-plugging installations. These are 3 ohms. They
were used with the Dyna II-2, the THREE wire sensor Dyna model,
model D35-2.
So...Dyna has had TWO types of installations. One
one type, the sensors have TWO wires, that was model D35-1, and
for the stock bike used the brown coils of 1.5 ohms. On
that model, the coils primaries were wired in SERIES. For
the THREE wire sensor, the coils are independently driven by the
Dyna module. When either the red or white sensor units are
grounded, one of the coils will have high voltage
developed. Those red/white wires should, if an
ohmmeter is placed on them one at a time, show >1 megohm, in
relation to the black wire (and ground). Be sure the power
is off for this. The black wire supplies the power to
the sensors. It is probably best for you to NOT use
an ohmmeter on those leads. The Dyna uses UGS304T Hall
Effect devices. Watch
out for broken wires at the epoxy junction.
Do
NOT overtighten. 1000 ohm
spark plug caps should be OK. see: www.dynaonline.com/english/instruct/index.htm
Revisions: