Dual Plugging
dualplugging.htm-31
©
Copyright, 2011, R. Fleischer
>>>>The
following is my personal opinion on this
subject:!!!<<<<
I HAVE tried to be fair, honest, and FULLY inform you here!
***A technical, somewhat scholarly, but very 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, ask for the papers on Dual Spark Plug
Ignition. He can e-mail these to you
(ask that he include the update from 2002). 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 more of
the theory and background.
http://rubberchickenracinggarage.com/Downloads/TomCutterDualPlugIgnition.pdf
Please refer to other
articles on this website, dealing with the pulse-air system and
ignition system, ETC.
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 would do it to my otherwise stock 1984 R100RT at valve
and top end time (where I would also increase the compression
ratio); unless
I
got so old that I give up riding two wheels first!....,
"I did not
think it necessarily warranted for all
riders." One would have thought that I had
posted about desecrating 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 (I think) PERSONAL OPINION comments
on dual-plugging and automatic advance
units....and setting
timing, etc.
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. At first glance,
faster burn time would seem to be counter-productive,
and tend to bring about pinging or detonation much easier.
That is not so, in practical usage, and is the reverse actually. Combustion
efficiency will
go up.
If
the engine had the original
spark timing, and dual-plugging was added, the engine would now
have its 'over-all' combustion event start too early
with regards to the position of the piston as it comes up on the
compression stroke. This would cause problems, and
thus the spark must be
retarded (several ways of doing this are possible...see
much later herein), particularly at the slowest rpm, to
compensate. It may also be needed in
the upper rpm area (one version of ATU modification). The
purpose of the Airhead Automatic Advance (Timing) Unit is to 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 mark is the maximum timing advance 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 version of the ignition
timing method with dual-plugs, and ARE for the other type. One method
basically uses
the stock ATU, but sets the
timing at approximately the OT mark at idle rpm. Versions of
that type
may, or may not, include expanding
the maximum
timing amount. The other method uses, more
or less, timing at the S mark, but has restrictors
of some sort added to the ATU,
so its maximum
advance is not as great as
with the stock unit, as far as total number of degrees of change (that
is,
from static or low rpm, to maximum
rpm).
Both types of timing have their advocates, and their
ARE differences in minor things, depending on which is used.
I will get into those things here,
before I get into the why dual plugging, etc., information.
In a
dual-plugged conversion with an unmodified ATU, or
in one in which the maximum range is slightly increased, the
timing is set near OT.
The spark MUST be retarded for the
initial static condition; that is,
with the engine starting or running at idle rpm. The maximum markings
(F or Z) are less important with this method, and may
not even be visible in one version of this timing modification
(depends on if the ears for the
timing limits are bent outward, or there is some other thing done
to expand the total range of advancing possible.
I will put some of the above in different words here, even expanding a
tad....so maybe one of these explanations will be clear to you:
For one type of timing 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. The Z or F markings do not get into the
center of
the window, at any rpm, but
are
at the bottom or below 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 (or otherwise modified). Thus, the F dot or
Z marking can be brought into the window NEAR the bottom. Due to how dual-plugging actually works dynamically, less than
the normal maximum
advance is perfectly OK, thus modifying for
extra range
of advance is not any actual necessity.
I have experimented, and it can help a small amount,
so I do modify the range, in this particular timing setup.
PLEASE NOTE!!......A
dual-plug installation has its biggest effect at lower rpm.
As engine rpm increases, a
retarding of the
spark has
less and less dynamic effect. Keep in mind that mixture burning is
a time-related event.
For an exercise/experiment, WITH a dual plug setup, and WITH
an unmodified
ATU: After setting
the OT mark in the center of the window at
idle rpm, then increase the rpm slowly, and try to
see
where the maximum timing mark (Z or F) ends up at.
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
normally
be retarded at low rpm. This is true for ALMOST 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 rpm. The timing unit is usually modified
with restrictors limiting maximum advance.
This type of timing has a very small possible advantage in that
the
closed throttle butterfly position on the CV carburetors is less closed
than on the other type of dual-plug timing, and
hence not quite so
critical, as to carburetor
adjustment for idle
rpm balance and idle mixture. Note that this is a
small effect, because this other type of timing
ALSO effects the butterfly setting some. You
also seldom have to change the idle mixture
jet. However, for engineering reasons,
I personally prefer the
other
method, of setting timing to near OT, and
NOT using restrictors
(In fact, I sometimes ADD some timing range). Any of the
methods of timing, restrictors or not, etc., is acceptable
to me.
I'll explain that just a bit more here:
Because the
rise in effective
pressure in the cylinder happens more quickly due to two flames
being started, if nothing else
was changed
(such as no change in the ignition
timing setting) after conversion to dual plugging, you
would see a much
higher idle rpm, no matter the
ignition timing method. To
bring the idle
down to proper rpm, particularly with the non-modified
ATU installation, the tuner will adjust the
carburetor butterflies more CLOSED. This,
in
itself, makes adjustments a bit more critical, as the butterfly
lower edge interacts with the
carburetor idle ports closer. Carburetors with poorly fitted
or
backwards (wrongly) fitted butterfly valves possibly 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 idle port(s), 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 off-idle throttle openings. 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.
SO...with the SEEMING complexity (not really!) of the previous statements, why the heck should you dual plug?
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. This is likely a more major benefit
than often discussed. 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 heating
problems. Example: under heavy throttle use the valve seats
likely will not
tend to warp towards the spark area.
2. Likely better performance, particularly
in the later, LOW compression ratio models.
3. 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 $$.
4. Allows modifications for 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.
7. Looks neat.
8. May pay for itself over the long long term in fuel mileage,
wear and tear on starter motors, batteries, etc.
9. In case of coil failure, you have an
extra coil, to fire both cylinders.
10. In almost every instance the engine
will start far easier, particularly in cold weather....if
installed properly with proper ignition parts.
11. See the mentioned article by Oak.
12.
Dual plugging with the recommended ACCEL coils does NOT!! make
the ignition work harder!!!!
There
are, of course, some possible 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; whether or not you
also consider raising compression ratio.
(2) The automatic advance system MAY be more of a compromise
than the stock system. Mostly a nerdy point.
(3) The BARELY off-idle performance MAY suffer a tad, although
that IS easily & cheaply curable, typically by changing the
idle pilot jet size.
(4) You have to be careful not to
over-torque the lower plugs (if 1/2 inch reach types are used) and
to use the proper length of plug.
(5) You have twice as many
plugs to change when that is needed.
(6) 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 an ALUMINUM 1/4" sleeve spacer.
(7) 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 high rpm, will
probably tell the
story.
(8) 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 140404 kit seem to
last forever).
(9) For the 1981 and later models, you
need only the two dual output 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 totally
necessary. See prior
comments!
(10) Do NOT use a Dyna booster with the 140404 coils (these come
two in a set, with that part number; and note that Accel lists
them for use with
CDI ignitions...they ARE the correct
coils for your airhead of 1981 and later).
(11) Years ago, for POINTS Airheads (all up through 1980), there was an ACCEL points
amplifier (booster), that was
YELLOW. It
is good and
reliable...but no longer made. DO NOT use the old
BLACK Accel points amplifier (booster).
(12) For the earlier points models (1970-1980), due to what is now available
for a points booster, you have some choices to make, and they are
complicated, and there are a lot of details to know about regarding
coils and boosters. One way to go, that will work OK!...is to
use the Dyna
points booster, and change the BMW/Bosch coils to the Dyna
coils, that have resistance (1.5 ohms approximately) that will not
cause the booster
to fail from the higher
primary current of the Accel coils.
Points amplifiers, sometimes
call points boosters, are made by a number of
manufacturer's. They will GREATLY increase points life.
Numerous makers.
Accell and Dyna are
two of the popular makers.
NOTE: http://www.qkits.com/ www.apogeekits.com
and maybe others...
Velleman is probably the
actual maker of a number of kits sold by others, using the model
number K2543, this is a kit.
It it is rated at 4 amperes,
but with the heat sink that
comes with it, I think it will handle MORE, if placed in a
relatively cool place
on the motorcycle.
http://www.vellemanusa.com
A problem can occur if you have coils that draw more amperes than
the points boosters/amplifiers are rated for. Many have used them
in this somewhat overloaded condition, if they are kept
reasonably cool. The Velleman seems to hold up.
(13) With the stock points plate, with dual-plugging, and the ATU not restricted in amount of
advance, 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.
You could contact Ted Porter at his Beemershop.com;
or, OAK at
askoak@aol.com ....either or
both....for the latest
information on
how to incorporate a dual plug conversion into your
POINTS bike. HOWEVER, for the 1979-1980 models,
which use the canister POINTS
ignition driven by a flat face engine
camshaft nose, you COULD also change to the
1981 and later canister electronic ignition system (canister and
module), and that WOULD be a NICE
conversion with Accel coils (granted, Dyna also makes coils with low
primary resistance too). This is a
direct swap, as far as the canisters are concerned; but you need
to get the cable plug, and a few minor other things, including
the module. A concern
if you have the early ATE brakes, is that the ATE master cylinder
is located where the later module is mounted. If you wanted to spend money,
you could do this canister conversion to electronic canister
type. A salvage yard
would be much cheaper than a new 1981+ canister. You would not
mount the module where
the early front 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.).
Still, the trick setup for a 1979-1980 points bike is to convert to 1981 or
later canister,
put the module where the disc brake master cylinder is
(just like BMW did in 1981+), and use a 1 mm smaller MC on the
handlebars for added braking power.
It is possible to design and make your own points amplifier, or,
perhaps there are such kits on the market, from some-such as Velleman...just
be sure that the booster amplifier will handle 8 amperes or so
(average current will be closer to half that though), if you are using
the Accel or certain
Dyna coils with under 1 ohm primary winding.
DETAILS:
When you dual plug a head, bottom plug 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, perhaps 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. 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. 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. This depends on
original plugs that were used on a specific engine/year. Note AGAIN, that
in
most instances of dual plugging I have seen, 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 step increase in heat range
value. This is done, if needed, after a spark plug
inspection after a hard
hot run on the highway.
SPARKPLUGS.HTM
In dual-plugging installations, 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. Some do the smaller plug because they say the
smaller thread plugs are 'easier'
to find at
the local auto-parts
store (NGK brand, for instance). Properly done 14 mm
is OK, if the lower plug drilling/tapping is done
correctly. I see
no problem
with 14mm....and that is what I have
used. I don't have a problem with using 12 mm either! There MAY
be a problem with 12 mm plugs with obtaining
the correct type, size and heat range. The
extra spark plug hole must be drilled
squarely at the correct
place, and someone who has done many of these
is the best person
to go to for the drilling and threading.
Some folks will weld some aluminum to the partly
machined second plug hole area, 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 (sometimes one heat
range hotter for the lower plug though). I have NO
objections to
this if the welding is done properly and machined
properly. You now
also don't have the potential error of over-torqueing
the lower plugs...unless terribly ham-fisted.
For the more common machining method, with NO welding, it is
true that the lower plug fits in a 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 such 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.
I disagree somewhat, and like a bit less,
not even going
quite to 16 ftlbs. This is without antiseize.
SOME spark plug manufacturers and Airheads LIST guru's have stated that 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 settings for spark plugs with the threads treated
with
antiseize compound are: about 13 to 14 ftlbs for the top plugs
and about 10-11 (MAX) ftlbs for the bottom 1/2 inch 14 mm plugs.
Tighten slowly and
carefully, and be sure the fresh crush washer does crush some.
If you do use an old already used
washer be sure to tighten with a torque wrench, unless your
'feel' is good.
DO
NOT use resistor plugs at any position!
For those with the BMW electronic ignition (1981 and later models), be
sure you use the nominal 5000
ohm spark plug caps, such as the NGK LB05F.
Do NOT USE
resistor plugs. I also 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
three?
One nice advantage
to having the stock electronic ignition (1981 and up) is that the
EXISTING system need only have two aftermarket Accell coils
added, and
spark plug wires/caps. DO NOT add a Dyna Booster!!!! The Accel 140404 coils which were designed by Accell for capacitor discharge
ignitions happen to be perfect for
our usage, work great 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 seem to be exceptionally reliable.
If you are using points with a points amplifier, you can use 1000 ohm caps,
instead of the 5000 ohm caps;
for a small improvement in marginal
conditions. For points models, do not eliminate the
resistance caps entirely.
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, and the ATU not restricted in amount of
advance, 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.
For the ACCEL COILS (or other dual output tower coils) when using the
BMW canister
and module (and, usually for
most points setups too), use ONE coil's outputs for the TOP
plugs, the other coil's
outputs for the bottom plugs!
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 or with most anything in the ignition being marginal.
Other wiring works similarly, so
to make this quite clear, you will get the same results with
using one coil for
either both top
plugs, or both bottom plugs;
or, ONE COIL for one cylinder's top spark plug AND that same coil
for the other cylinder's bottom plug. The idea
is
that EACH dual-output coil should supply electricity to BOTH cylinders.
Do NOT use one coil
for both top and
bottom plugs on any one cylinder! The reasoning
behind the coil connections is that one cylinder is on the
compression stroke
when the spark occurs, and the other cylinder is not under
compression, and a spark will jump a lower pressure cylinder
easier.
Thus, since the two plugs under discussion (in different
cylinders) are both firing, it is best for the plug that NEEDS a
lot of
electricity (the cylinder under compression pressure) to get as
much spark as it can. Sort of simplified explanation.
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
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 9.6 or above. Properly milling, even
a minimal cleanup milling of the head on
both surfaces is a good
idea, especially the R100 models which often are a slight bit
warped. Some have shaved the
cylinder base, probably more
of a hassle. I like about 8.8 to 9:1 CR.
You can often run regular grade gasoline with that CR!
Higher usually means the more expensive
premium grades of gasoline.
Some will find that they can NOT use compression ratios
much over 9:1, even on 91 octane premium....as
carbon deposits and variations in engines will
cause pinging. If
your timing chain and sprockets are worn, you may require premium
gasoline with higher compression ratios; as a fair
amount of wear here will retard the camshaft.
A CR of about 9.1, is usually usable and OK on 87 octane fuel at higher altitudes. Some have no problem with 85
octane at high altitudes. I think
that 9.1 or 9.2 from
milling (or 9.5 pistons, milling cleanup only, if using piston
change for the CR) is a MAXIMUM figure for any
consideration of
'regular' grade fuel, and you may well
want to consider about 8.8 to 9.1. 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; and
usually you can use
regular grade of gasoline's.
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.
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. Some
decent power increase is possible by converting a R100 engine
from 32 mm to 40 mm carburetors (the adapters need changing too)
and
dual-plugging with higher CR. 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:: I will ASSUME...to keep this from becoming a
discussion that covers every
possible ignition modification, that approximately
the OT mark ignition point setting
is being used. This is
not the only possibility for the ignition timing setup, but it is
the
one I decided to show details of here.
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 fully proven to me, 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 with the stock
engine. That is one of the reasons
that various timing methods are used, as discussed much earlier
in this article.
But, on a practical basis,
for the method that uses timing near OT, the ideal or
near ideal situation, in MY opinion, is to STRETCH
the advance curve
a SMALL AMOUNT, which increases the maximum number of degrees the
ATU works through. With the ignition set for idle being
~the OT mark,
you are RETARDED at lowest rpm, and a bit 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. I have found a bit better over-all
performance
using this method, versus the timing at ~S mark, and restricting
the maximum advance. The difference is small.
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,
note that this is for the OT timing version.
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
9.2+. 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 more labor 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 but a
small amount 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 for a CR of 9.5 and
over 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
probably get. In
the long run, perhaps trying for these minute improvements may not be worth it, and the stock advance is certainly adequate!
In the
other methods of timing that I discussed much earlier in this
article, the pins in the canister must be
restricted, etc.
After the earliest /5 (they had advance
stopping at approximately 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. A fast advance, high
compression, and high octane gasoline, produces a lot more power
output.
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. Don't
push your
luck....you really need an engine in very proper state of tune,
and internally clean of carbon.
***To help 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, AND 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 rare these days. Full synthetic
oils like Spectro's, BMW's, and some diesel type oils (that have
ZDDP!!), see my oils
article!....are very nice. Mobil 1 in
15W50 and its V-twin oil in 20W50, both work quite well in airheads....and have very low deposits
produced.
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 best coils I know of for that are made
by Accel, and the kit of TWO of the coils is Accel #140404 for yellow coils and
yellow wires; and if
you want black coils and black wires, the Accel kit number is 140404K.
Both the yellow and the black coils are 0.7 ohms. The coils may be
shown in Accel's literature as for CDI type ignition; do not be confused, these
ARE the correct coils for your 1981+ Airhead utilizing the stock
BMW Bosche electronic ignition. There IS an Accel number 140404KS,
that is a single black dual tower coil; not used on a dual-plug conversion
unless for some reason you need to replace one of the above coils. I don't
have a number for a replacement YELLOW coil.
If the Accel-supplied ignition wires happen to be resistor
types, then discard them 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!
For POINTS operation, there is information just below.
NOTE:
Please contact OAK, at askoak@aol.com,
or see the Airheads LIST archives for the discussion in late May and
early June, 2009, for detailed
information on the use of various coils
and Dyna boosters....as it is a bit complicated, and the information I
have, just below, is not a complete
telling of the story!
DYNA:
The 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...but the best installation
is a double booster setup and low resistance coils. The Dyna
booster is not rated to
handle the coil current of the later BMW low resistance coils
(nor the Accel coils mentioned in this article). Many have
used the
booster however. There are other boosters from other
companies available.
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 D35-2. Dyna has had TWO types of installations for
single
plug bikes. 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
Points amplifiers, sometimes
call points boosters, are made by a number of
manufacturer's. They will GREATLY increase points life.
Numerous makers.
Accell and Dyna are
two of the popular makers.
NOTE: http://www.qkits.com/ www.apogeekits.com
and maybe others...
Velleman is probably the
actual maker of a number of kits sold by others, using the model
number K2543, this is a kit.
It it is rated at 4 amperes,
but with the heat sink that
comes with it, I think it will handle MORE, if placed in a
relatively cool place
on the motorcycle.
http://www.vellemanusa.com
A problem can occur if you have coils that draw more amperes than
the points boosters/amplifiers are rated for.
Many have used them
in this somewhat overloaded condition, if they are kept
reasonably cool. The Velleman seems to hold up.
Boyer-Bransden makes a MicroDigital ignition for airheads, and they NOW have
one that has an ignition advance curve (have not seen it) that is specifically
for dual-plugged bikes.
See my ignition.htm
article for information on the Boyer, where to buy it.
Revisions:
01-28-2003: lots of revisions, mostly to clarify
meanings. Nothing substantial in intent and tech numbers.
01-28-2003: clarify details in an area now identified as Introduction:
04-09-2003: add official .htm title plus
clarifications and emphasis items here and there.
09-28-2003: more clarifications, fix some typos, add notes
on points plate
02/26/2004: add REFERENCE to articles and contacts for Tom
and Oak
03/22/2006: Edit entire article and add note on COILS
05/27/2006: Fix typos on spark plug
numbers. Update rest of article, adding emphasis.
03/27/2007: Revise information on spark plugs, and remove
special spark plug information and refer by hyperlink to my spark
plug article
01/11/2008: Revise entire article, for clarity and
for specific recommendations, etc.
06/02/2009: Revise article for better clarity
10/26/2009: Review. Clarify a few details. No
substantive changes.
12/16/2010: fix hyperlink to Tom Cutter's dual plugging
article...now is on his site.
06/08/2011: Explain a bit more in depth about why the coils
are each connected to both cylinders.
12/30/2011: Clarify Accel coil numbers and description.
© Copyright, 2011, R. Fleischer