Dual Plugging BMW Airhead Motorcycles
(and information on head milling, barrel shaving, CR, etc.)
© Copyright, 2012, R. Fleischer
>>>> The following article is my personal opinion. I ave tried to be fair, honest, & fully inform you <<<<
A technical, somewhat scholarly, worthwhile information on the merits of dual-plugging, with a somewhat different viewpoint than mine, can be had from Oak Okleshen, at firstname.lastname@example.org. 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 & had a professional background of engineering work in combustion chamber investigations ....and was involved heavily with the original Airhead dual-plugging projects.
Here is a URL for Tom Cutter's rather old article on dual plugging. While it has some errors & things that need updating, this may be useful if you are interested in more of the theory & background: http://rubberchickenracinggarage.com/Downloads/TomCutterDualPlugIgnition.pdf
Please refer to other articles on MY website, dealing with the pulse-air system and ignition system, ETC. :
Here is link to a somewhat out of date article on dual plugging. I disagree with some of it. Note that one of my disagreements is the recommended use (by me) of Hylomar ...I have not recommended that in a long time. Other things too. Worth your time to read the whole article. NOTE, in particular, that the owner made a LOT of $$$ changes besides just dual-plugging. He lists the wrong URL for my website and article in several places in his article, including one place at the bottom of the article....those had not been fixed as of 01/27/2017 when I sent Scot an E-mail on this.
I have responded many times to questions on & off the Airheads Internet Mailing LIST about dual-plugging. I have also have had many E-mail's regarding automatic advance units, whether this is for dual plugging or not. I've made comments on setting timing & details regarding the automatic advance units & modifications to those units as made by others. I also 'admitted' that although I have had dual plugging on two of my own Airheads, & likely would do it to my otherwise stock 1984 R100RT at valve & top end time (where I would also increase the compression ratio); "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 WILL do is to make some hopefully definitive comments on dual-plugging and automatic advance units, setting timing, etc. in this article.
In my estimation, dual plugging is, or can be, a low to moderate cost solution to possible problems; 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 possibly more complete. Combustion efficiency of more complete combustion, could increase performance. That is a simplified statement. Dual-plugging may also reduce exhaust value and seat problems (see 1., just below).
SO.........why should you dual plug?
PART I. Dual plugging DOES:
1. Reduce spot heat in the combustion chamber, likely due to more even distribution of heat, & thus is probably easier on valves & valve seats, especially in the R100 models. This is likely a more major benefit than often discussed. A good argument is that dual-plugging decreases engine heating problems. Another is that under heavy throttle the valve seats likely will not tend to warp towards the spark area. It is possible that warping towards the exhaust port would be lessened too.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.
4. OFTEN allows the use of regular, versus previous use of premium gasoline, saving fuel $.
5. Allows modifications for higher compression ratios & thus more performance for the later low compression models, with possible further INcreased mileage and power.
6. Will allow QUITE high CR if one wants to use premium fuels & high profile pistons.
7. Looks neat.
8. May pay for itself over the long term in fuel mileage, wear & tear on starter motors, batteries, etc. As an example, just for fuel usage, if you get 40 mpg, & ride the bike for 80K over some years, you will use 2,000 gallons of fuel. If you can save 20 cents per gallon by not using premium fuel, that is $400.00 savings. That means the dual-plugging, assuming head was already off for such as a top end job, may paid for a fair amount of its costs for the extra head machining for the second spark plug (and the ignition changes) just in fuel use.
9. In case of coil failure, you have an extra coil to fire both cylinders.
10. In almost every instance the engine will start easier, particularly in cold weather ....if installed properly with proper ignition parts. This increases life of the starter motor, battery, and probably a few other things.
11. See the mentioned article by Oak.
Dual plugging with the recommended ACCEL coils does NOT make the ignition work harder as some erroneously say.
PART II. Disadvantages, special concerns ...pros and cons ...
1. Cost for the conversion and parts. If you have the heads off for a valve job, or to replace pushrod tube rubbers, IMO that is a good time to think about dual-plugging; whether or not you also consider raising compression ratio by skimming/milling the head, or other means, including turning a bit off the cylinder base. If the head is warped, sometimes both head and base are done anyway.
2. The automatic advance system MAY (or may NOT) be a bit more of a compromise than the stock system. Mostly a nerdy point. In practice it is NOT very important. Both popular methods of dealing with this are fully explained in this article, well below.
3. The BARELY off-idle performance MAY suffer a rather minor stumble, often never noticed by the rider anyway. It is easily & cheaply fixed, typically by just changing the idle pilot jet size. Use of idle RPM of 1050+- will help. Modifying the ATU for decreased maximum advance, see later herein, might negate any slight need.
4. You have to be careful not to over-torque the lower plugs if 1/2 inch reach types are used & you must use the proper length of plug. See 6.
5. You have twice as many spark 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 or a spacer to the head & 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. You can also use smaller diameter spark plugs such as 10 or 12 mm, instead of the stock 14 mm. Some prefer that, saying there is less chance of the heat stress cracking to the hole. I've never had a problem with 1/2 inch reach 14 mm plugs for the lower plugs, but I LIKE the idea of the welded material (sleeve) & machining. MY preference is to make an aluminum spacer & WELD it to the head, & do both the head & the welded spacer drilling & tapping for threads at the same time. This will allow the same size of spark plugs at all four places, with no need to consider anything else (except, maybe, one heat range hotter at the lower spark plugs).
I will explain it a bit differently, here, so that it is more fully understood, as to your options:
If you have installed a welded OR UNWELDED washer (typically 1/4" thick); and, it is NOT threaded, and are using a 3/4" reach spark plug, then use torque value of 12-14 ftlbs. If you have welded up the area, by adding material, or welded the washer to the area, and the threading covers the entire hole/washer/weld, then use my recommended 14-15 ftlbs. Use an antiseize compound on the threads during installation of the spark plug.
7. The lower plug TENDS to carbon-up some; ...especially noticeable if the engine is older & burning some oil ....compared to the top plug. This is easily seen from LOOKING at the two plugs. A one step hotter heat range plug may be required, advisable, or useful, at the lower plug position. Inspection & comparing of the plugs after a few minutes at heavy throttle & high rpm, will probably tell the best story. You should compare to the other cylinder's plugs too, before making a decision.
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). But, with the extra coil, if one coil ever did fail (VERY RARE), you have the other coil to enable continuing on your way, with both cylinders firing ....an advantage over stock. The coils must be properly connected for this.
9. For the 1981 & later models, you ONLY need the two dual output Accel coils, the spark plugs, & 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 that is not totally necessary. See prior comments ...and later comments. Emerald Island makes a version of their canister ignition for use with dual-plugging, that is, the timing is not a ATU in the canister, and the MODULE they supply does the advancing, which is tailored for dual plugging. However, purchasing one increases your costs by a fair amount.
10. 1981+: Do NOT use a Dyna booster with a 140404 coil set. The 140404 coils come 2 in a set, for that ONE part number. Accel lists them for use with CDI ignitions ...they ARE the correct coils for your airhead of 1981 & later. DO NOT use Dyna 3 ohm dual tower coils (green) ...they are 3 ohms each, & ignition output will suffer if those coils are used with the Bosch electronic ignition.
11.POINTS Airheads (all Airheads up through 1980): there was an ACCEL points amplifier (booster), that was YELLOW in color. It is good & reliable ...but no longer made. DO NOT use the old BLACK Accel points amplifier (booster). For other Accel amps & for wiring:
12. For the points models (1970-1980), due to what is now available for a points booster, you have some choices to make, slightly complicated, & there are details to know about regarding coils & boosters. One way to go, that will work OK! ...is to use the Dyna points booster & change the BMW/Bosch coils to the Dyna coils that have primary coil resistances of ~1.5 ohms that will not cause the booster to fail from the higher primary current of the Accel coils. See (13) that follows.
13. A problem can possibly occur if you use coils that draw considerably more amperes than the points boosters/amplifiers are rated for. Many have used them in overloaded condition, if they are kept reasonably cool. The Velleman seems to hold up. You can install a higher powered 'transistor', that has a higher current rating, & more heat-sinking. Due to the DWELL angle of points, the AVERAGE current through the points is much lower than the maximum. Thus most boosters hold up OK. Where the problem is more likely to occur is if you have the ignition ON, and the engine is NOT running, and the points happen to be closed, and you stay in the engine not-running mode for too long (minutes of time+). The problem is rare, anyway.
I prefer to increase the heat sinking of a slightly oversize transistor in the points booster. If the transistor has a maximum current rating at least 50% over the actual CLOSED POINTS current draw, and the heat sinking is decent, there WILL NOT likely ever be problems. The Dyna booster is sealed. NOT SO the Velleman's, which are kits.
Points amplifiers, sometimes called points boosters,are made by a number of manufacturer's. Use of them will GREATLY increase points life. Accell & Dyna were/are two of the popular makers. See http://www.qkits.com/ www.apogeekits.com & maybe others. Velleman is probably the actual maker of many kits sold by others, probably all using model number K2543.
http://www.vellemanusa.com Click on PRODUCTS. Or, to see just the ignition, enter it as a search term: ignition. As noted, you can likely put a higher powered transistor into the kit, and run lower ohm, even higher performance coils.
It is possible to design & make your own points amplifier; or, perhaps there are other such kits on the market, ...just be sure that the booster amplifier will handle, perhaps, 8 amperes or so. The 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. Accel makes coils with various primary resistances.
14. With a stock points plate, with dual-plugging, & the ATU not restricted in amount of advance, you must retard the ignition. You may not have enough adjustment range on OLD POINTS Airheads; you may have to do some hand-filing of the points plate holes. Restricting the amount of advance usually means some sleeves over the ATU advance pins for any of the ATU models. Use of thick heat-shrink tubing often works very well. For modifying the points plate, see: http://www.rubberchickenracinggarage.com/Downloads/TomCutterDualPlugIgnition.pdf
15. Dual-plugging leaves the motorcycle no longer stock, nor stock-looking.
16. You could contact Ted Porter at his Beemershop.com; or, OAK at email@example.com ....either or both ....for the latest information on how to incorporate a dual plug conversion into your POINTS bike; if keeping the points (or, not!). 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 & later canister electronic ignition, whether the BMW/Bosch version; or, the Emerald Island canister and module type. You can even use the Acell coils I have mentioned. This makes a loverly setup. Dyna also makes coils with low primary resistance.
17. If you are 'hopping up' a pre-1979 Airhead, and plan to change the camshaft to such as 'the 336'; you might consider using the flat nose cam and installing the BMW electronic ignition....or the Alpha ignition (Emerald Island).
A concern, if you have ATE brakes with the master cylinder under the fuel tank, is that the ATE master cylinder is located where the later module is mounted. There is no problem leaving the ATE master cylinder where it is (although the on-bars type of a later model is a better conversion) and mounting such as the stock or Alpha or Emerald Island ignition module on the right side of the backbone; similar to how BMW did this on the 1985 and later bikes.
A salvage yard or other source from an owner who has done a different conversion would be much cheaper than a new 1981+ canister.
18. For the Alpha ignition ....which is actually a product by Jeff Lee's Emerald Island company... see
http://beemershop.com on the West Coast; or, Motorrad Elektrik
www.motoelekt.com in the East Coast area.
If the engine had the original spark timing and ATU, & dual-plugging was added, the engine will now have its combustion event start too early with regards to the position of the piston as it comes up on the compression stroke. This would or could cause problems, & thus the spark either must be or could be retarded. There are several ways of doing this ...see much later herein... to compensate. Spark retardation may be needed in the upper rpm area and/or lower rpm area. Some folks will retard at both high and low rpm. The purpose of any, even the stock Airhead Automatic Timing Advance unit (ATU) 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 this website on the various versions used in the airheads over the years) matches the needs of the stock engine rather well. The information on all these ignition items is in articles 28 to 31.
Adding dual-plugging IN EFFECT advances spark timing for the 'combustion event'. THUS, spark MAY need to be retarded (I will explain later why, in one method, this is not exactly so). In the two methods outlined in this article, one method does retard the spark (from S mark to approximately OT mark), the other method works differently. Both methods have advantages & disadvantages. For the OT mark type of low rpm (idle) spark timing, the carburetor adjustments get somewhat more critical (at idle), a bit more-so than with the other method; sometimes you need to change the idle jet to slightly richer. These are hardly big deals, not in the slightest. For the other method, there is more sensitivity to fuel octane in-so-far-as-pinging (pinking) is concerned ....and you must actually modify the advance maximum range, easy on the pre-1979 models, and a fair amount of work for the 1979 and later models, where the ATU is buried in the canister. For the aftermarket canisters, such as the Omega, or Emerald Island, the ATU is built into the electronics modules under the fuel tank, the ATU in the canister being done away with.
I will explain this differently, as some have a bit of trouble grasping these details:
In the stock engine, with one spark plug per cylinder, the more important ignition timing
is the maximum ignition advance timing, indicated by an advance mark (dot or line, F or Z). The idle RPM S marking
is of secondary importance. Depending on the year, the S may be just that letter
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
One method uses the stock ATU, but sets the timing at the OT mark at idle rpm. The OT mark is used because it just so happened to work out that way for proper timing. Sometimes the OT mark is put SLIGHTLY higher up the timing window a degree or three. Versions of this type may, or may not (usually), include expanding!! the maximum timing amount. The other method uses, more or less, setting timing at the stock S mark (at slow idle RPM), but has restrictors of some sort added to the ATU, so its maximum advance is not as great, as far as total number of degrees of change (that is, from static or low rpm, to maximum rpm). Both types of timing (and even some variations) have their advocates. It is easier, and perfectly OK, to use the simpler method: Idle timing at the OT mark (or, up to 3 degrees additional timing advance). You can increase the timing range if you desire, but not really needed. 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 canister type timing limits are bent outward, or there is some other thing done to expand the total range of advancing possible). For timing (idle rpm) at ~OT, the Z or F marks 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 slightly advantageous, so the mechanical stops for the ATU weights are modified, so the weights can move outward a bit more. One can also modify the weights for faster advance per unit of rpm, or both. Thus, the F dot or Z marking can/could be brought into the window NEAR the bottom at ~3000 rpm on an unmodified weights unit. As the weights are lightened, especially in the outer portion, THE RPM at which maximum advance is obtained, will rise. In some bikes, with high compression pistons, etc., 3500 or so may be advantageous, in order to use lesser fuel grades, and sometimes in order for premium fuels to work OK if the CR is very considerably increased, such as over 9.8:1.
Due to how dual-plugging actually works dynamically, less than the normal maximum advance point on the flywheel is also OK, thus, modifying for extra range of advance is not any actual necessity on most Airheads. I have experimented, and it helps only a small amount.ANY dual-plug conversion has its biggest effect at lower rpm, after all, mixture-burning is quite obviously a time-related event.
The OTHER type of timing, expanding here, 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 small possible advantage in that the closed throttle butterfly position on the CV carburetors are or will be SLIGHTLY less closed than on the other type of dual-plug timing, & hence a bit less critical as to carburetor adjustment for idle rpm balance & idle mixture. Note that this (and the other) is a small effect, because this other type of timing still affects the butterfly setting. You also seldom have to change the idle mixture jet, but that IS possible & EASY anyway.
Either of the two basic methods of timing, restrictors or not, etc., is acceptable. Just why preferences seem to be modestly to strongly held, is a good question.
I'll explain things a bit more here (as if I hadn't been verbose enough already
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. To bring the idle down to proper rpm, you would adjust the carburetor butterflies to a more closed position (or, lower the slide on slide carbs). This, in itself, makes carburetor adjustments a bit more critical (or, just say, more sensitive), as the butterfly (or on non-CV carburetors, the slide closer to the ports) angled-edge interacts with the carburetor idle ports more.
CV Carburetors with poorly fitted or backwards (wrongly fitted) butterfly valves possibly will have real problems. It will behoove you to check the butterfly valve versus the photos in my BING CV articles, and be sure yours were installed correctly. Do check that you have no vacuum leaks or other carburetor problems.
Keep in mind that much of what I have discussed about timing, ATU, idle ports and idle jets, etc., is really verging on NERDY, and, for the most-part, differences are hardly noticeable to many, if not most riders.
When you dual-plug a cylinder head, bottom plug machining is probably more often done for 14 mm 1/2 inch thread length and the top plug a 14 mm 3/4 inch thread length. 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 large change; colder plugs will usually be needed. In most instances of dual-plugging I have seen, as mileage gets large & more oil are being burned (typically from valve guides and/or rings), the lower plugs may need to have a step increased in heat range value, otherwise they carbon-up & might even misfire. Whether or not to go to a hotter lower plug is determined after a spark plug inspection after a highway ride at various throttle and rpm settings. Due to the position of the second plug in the combustion chamber, I have seen instances of even quite well-burning engines, with near zero oil use, require a one-step hotter lower spark plug.
In dual-plugging installations, the top plugs always 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 from the threaded hole. 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 mostly used. I don't have a problem with using 12 mm either! There MAY be a problem with 10 mm and 12 mm plugs with obtaining the correct type, size and heat range. No matter the size of the threads, 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, such as a thick special sleeve-washer, to area for the lower spark plug hole area, and then drill and tap through both to match the stock type 3/4 inch reach projected nose type 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, as noted). I vastly prefer this method. An additional advantage is that you don't have the potential error of easily over-torqueing the otherwise smaller and/or shorter lower plugs, nor, the error problem of installing a longer reach plug. If you just use a removable 1/4" thick aluminum spacer, that will work OK, and, yes, the spark plug will run a very slight bit hotter (you still may need a one step hotter rated spark plug) ...but, this does not fix the potential problem of a thinner material in the head itself ...or losing the spacer and installing a now too long spark plug .....so, best to do the welding and machining/threading.
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 HIGHLY disagree, and like less, not even going to 16 ftlbs. This is without antiseize, which, if used, will require even less torque. 14-15 footpounds on 3/4 reach plugs, with antiseize compound on the threads, is my preference.
SOME spark plug manufacturers & one Airheads LIST guru have stated that you should NOT use antiseize. I do NOT agree with them, although I understand their concerns. I believe 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 you can use NEW gaskets (crush washers), torque to JUST crush the gaskets ...BUT I HIGHLY recommend an ACCURATE torque wrench! My recommended settings for spark plugs with the threads treated with antiseize compound are: ~14-15 ftlbs for the top plugs (and same for 3/4" reach bottom plugs of 14 mm) and ~12 (MAX) ftlbs for the bottom 1/2 inch 14 mm plugs. If you re-use an old washer be sure to tighten with a torque wrench, unless your 'feel' is really good.
DO NOT use resistor plugs at any position!For those with the BMW electronic ignition (1981 & later models), be sure you use nominal 5000 ohm spark plug caps, such as the NGK LB05F. 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 & 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 & fresh heat-transfer compound applied every year or three? (unless you have the latest riveted type, most of which are mounted on the right side of the backbone, vertically).
One nice advantage to having the stock electronic ignition (1981 & later) is that the EXISTING system need only have the heads modified for the lower spark plugs, and you need only two aftermarket Accel coils added, & spark plug wires/caps. FOR THESE, DO NOT add a Dyna Booster or other Dyna or Boyer. The Accel 140404 kit (of 2 coils) which were designed by Accel for capacitor discharge ignitions happen to be perfect for our usage, work great, 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 bit of ingenuity to mount the two Accel coils.The ACCEL coils ARE my favorites ....they seem to be exceptionally reliable (unless you STUPIDLY operate the system withOUT spark plugs or the caps otherwise not grounded).
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. 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 (perhaps the cam was not lubricated?), and/or a badly stretched timing chain. With the stock points plate, with dual-plugging, & the ATU not restricted in amount of advance, you must retard the ignition & you may not have enough adjustment area ....so you may have to do some hand-filing of the mounting holes....this depends on the particular timing unit setup, of the two I've mentioned. A sketch of this is in the obsolete Tom Cutter article, the link is well above.
Use ONE coil's dual 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. Because of the possibility of lower plug fouling in SOME instances, I ALSO recommend that setup. You also get more reliability if something fails. You will get the same good 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.
DO NOT use ONE dual output coil for both plugs on one cylinder. EACH dual-output coil should supply electricity to BOTH cylinders.The reasoning behind the coil connections recommendations are that one cylinder is on the compression stroke when the spark occurs & the other cylinder is not under compression. A spark will jump a spark plug gap at a lower pressure in a cylinder MUCH 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.
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 (sometimes 9.8) is the absolute upper limit for today's premium 91 octane (USA ratings) gasoline's. You could also use 9.5:1 pistons, & even shave the head a tad to clean it up & make sure it is flat. Only those who use premium gasoline's & keep their engines well tuned ...and fairly clean of carbon deposits ...should use 9.6 or above. Proper milling, even a minimal cleanup milling of the head is a good idea, especially the R100 models which often are a slight bit warped. Some have shaved the cylinder base, & a few have done that AND cleaned the head surface a bit. I DO like BOTH the head being milled AND the base being turned a bit on a lathe. If the head is milled too much for whatever reason, you could have a secondary milling (or cylinder) operation so the cylinder fits into the head properly.
I like about 8.8 to 9:3 CR. With that, you can often run regular grade gasoline even at sea level. Any higher CR MAY mean using a more expensive grades of gasoline at sea level. Some will find that they can NOT use compression ratios much over 9:1, even on 91 octane premium ....as carbon deposits & variations in engines will cause pinging. If your timing chain & sprockets are worn, you may require premium gasoline as a fair amount of wear will retard the camshaft, requiring higher octane. The R50/5, R60/5, and R60/6 models are even more marginal.... and just dual-plugging those models is often THE answer to annoying & engine damaging pinging, even when premium fuel was used in the past to eliminate pinging (pinking).
If you have modified the engine's camshaft, etc., you may be able to go over 10:1 CR.
OCTANE as used by me in this article is the R+M divided by 2, as used in the USA.
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. http://bmwmotorcycletech.info/sparkplugs.htm
The 1978 (probably January and later) and later flywheels have the ignition markings re-marked (compared to earlier flywheels), for 3 degrees additional. Flywheels from 1981 have the name changed to Clutch Carrier. The remarked flywheels and clutch carriers cause these later models to run a tad hotter, & this was done for emissions reasons. BMW also began to jet the carbs leaner & 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 power. 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. Some foreign shipped models still came with high compression pistons. Some decent power increase (about 10 real horsepower!) is possible by converting a R100 engine from 32 mm to 40 mm carburetors (the adapters need changing too) AND dual-plugging AND higher CR. ...and the large bore exhaust system helps too.
After the earliest /5 (these had ignition advance stopping at approximately 2000 rpm), stock advance units (25 & 26 degrees available) go to full advance at about 3000 rpm. Modifying the ATU so that advancing quits at ~3600 will usually cure any last bit of pinging at sea level if you have a fairly high compression ratio or are using regular gasoline with moderately high compression ratio. This also works pretty well with single plug ignition, and even with the R50/5, R60/5, R60/6 who all have very mild camshaft timing, and have a tendency to ping. One must, of course, be sure that there are no other adverse combustion events occurring, such as unheard events at higher rpm.
Pinging (pinking) aside, best performance (overall acceleration from just above idle with an aggressive throttle), is obtained with a fast advance curve ...down around 2000 rpm for the maximum. A fast advance, high compression, and high octane gasoline, produces more usable power output ....if the gasoline octane allows.
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 proper state of tune, and internally clean of carbon.
To help keep your combustion chamber clean, I suggest using Chevron gasoline's or other Top Tier fuels ...or ...at slightly higher expense, adding Chevron Techron to your tank now & then. 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 a true 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! ....can be very nice. Spectro synthetic oils and Mobil 1 in 15W50 and its V-twin oil in 20W50, both work quite well in airheads ....and have very low deposits produced.
For stock electronic ignition airheads (1981+), the STOCK canister & STOCK module under the fuel tank, are all OK to drive two each 6 volt dual tower coils, primaries wired in series, for a dual-plug installation.
The best coils I know of for that are made by Accel. The KIT of TWO of the coils is Accel #140404 which provides yellow coils & yellow wires; and if you want black coils & black wires, the Accel kit number is 140404K. Both the yellow & the black coils have 0.7 ohms primary windings. 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 Bosch electronic ignition. There IS an Accel number 140404KS, that is a SINGLE black dual tower coil. It would be used on a dual-plug conversion if you need to replace one of the above coils. I don't have a number for a replacement YELLOW coil, ask Accel. I have NEVER seen an Accel coil fail, unless abused.
NOTE! The very last BMW Airheads twin tower coil was (as was all models of Airheads that came with ONE coil) 12 volt rated, & this last coil had a very low ohm primary winding (around 1/2 ohm), & the MODULE was improved to handle higher current and eliminated some spurious ignition problems. If you can afford it, use the very latest and last of the modules, which came on a heat sink in which the module was RIVETED onto it. Be sure to get the correct one. For all the details: http://bmwmotorcycletech.info/ignition.htm
If the Accel-supplied ignition wires (the kit of two coils comes with wires) happen to be resistor types, discard them. Use QUALITY silicon rubber insulated wires that have stranded copper wires cores. Use proper resistance type spark plug caps. For the electronic ignition use 5,000 ohm caps ONLY. Do NOT...NOT!!!....use resistor spark plugs with resistor caps! DO NOT!!!...NOT!!!...use resistor spark plugs and 1K or zero ohm caps. READ the spark plug article on this website ....there can be problems with Bosch's new numbering system & the plugs!
For POINTS operation, there is information just below.
NOTE: Consider contacting OAK, at firstname.lastname@example.org, 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 & Dyna boosters ....as it is a bit complicated, & the information just below is not a complete telling of the story! ...although probably more than adequate.
The Dyna III unit was for the stock 1970-1978 Bosch coils.
The Dyna Ignition Booster was for the 1970-1980 points models ....and can be used on dual plug installations with the proper coils.
The best installation on a points model Airhead, for dual-plugging, is is a double booster setup & moderately low resistance coils. This can be done with the Dyna Booster, or one of the Velleman kit boosters, etc. The Dyna booster is not rated to handle the coil current of the last version of BMW low resistance coils (nor the Accel coils mentioned in this article). Many have used the booster however, because the average current is lower during actual engine running.
Dyna GREEN 3 ohm coils are sometimes seen on dual-plugging installations. I do NOT recommend them for the Bosch electronic ignition Airhead models. 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, but information may apply to your installation of dual-plugging, if using proper coils. On one type, the sensors have TWO wires, that was model D35-1, & 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
Makes a MicroDigital ignition for Airheads. They NOW have one version that has an ignition advance curve (have not seen it) that is specifically for dual-plugged bikes. See http://bmwmotorcycletech.info/ignition.htm for technical information on the Boyer, & where to buy it.
The pre-1979 ATU (Automatic Timing Unit) is mounted to the forward end of the engine camshaft by one nut (and a waverly washer). The ATU needs to be secure from rotating in relationship to the engine camshaft to which it is affixed, so the ATU has a D shape hole, which mates to a mating D shape on the forward end of the ENGINE CAMSHAFT. It is the TIP (threads and D shape) area of the engine camshaft and associated ATU that is under discussion here:
The forward part of the camshaft, that nose area, which has the threads and D shape, and a bit further aft, can be found bent at times. The amount of bending is NOT visible to the eyeball. There is a way to fix it, with a small brass or plastic covered dead-blow hammer. The bending may be only a fraction of a thousandth of an inch, to a few thousandths. The result is, as the ATU rotates, that the points do not typically have the exact same GAP (or, think of it resulting in the ignition timing as not being at the exact same place on the flywheel) for both ATU timing lobes. If you think about this, you will realize that a points gap difference between the two lobes of the ATU cam means a TIMING difference. The results are a dual-image when using a strobe lamp on the flywheel. This results in vibration, often at one or more narrow ranges of RPM. This type of problem is often WRONGLY interpreted as carburetors being out of synchronization. The Dyna dual-pickup electronic ignition will eliminate most double-timing, but the camshaft TIP is usually fixable. ONE OTHER cause for this sort of timing change is some advance unit WEAR, and sometimes slight irregularity in the grinding of the cam lobes. BMW at one time recommended 'stoning' those lobes to equality. That is a sneaky way of compensating for the lobe machining AND the bent tip area. DO NOT BOTHER TO STONE THE LOBES. But, a poor ATU needs fixing, as does a badly bent cam tip, unless the differences in timing are slight. If the dual timing images are over ~3/16" apart, you should consider fixing the problem, although MANY bikes are running relatively OK at a bit more difference. NOTE also, that a worn timing chain, worn chain guides, & wear on one or both timing sprockets, will ALSO cause double images & the same results. Quite noisy chains are also a giveaway. Of course, you can always spend $$$ and get a crankshaft triggered ignition setup. If your chain sprockets are worn, etc., fixing the ignition timing is still going to leave the engine camshaft lagging the crankshaft, power suffers. BTW, any wear on the camshaft bearing is usually not talked about, but can also create irregular timing, much of it due to the irregular load of the camshaft upon the timing chain, sprockets, etc.
Any brass hammer 'fix' must be done carefully, not knocking off the threaded cam tip. Use of a dial indicator on the smooth side surface of the cam tip will tell the story. Do NOT try the brass hammer fix unless you KNOW what you are doing, have a dial indicator & know how to use it!
If a cam tip is broken off, this usually happens NOT from the above brass hammer work, but from someone OVERTIGHTENING THE NUT. The Boyer ignition can be used, and was popular for this problem. Other methods include several ignitions that mount to the alternator area. Broken tips can often be center-punched (using a guide made on a lathe), and then screw threads can be put in a drilled hole. This enables the stock ATU to be used and no special ignition is needed, other than what the above article describes for any points ignition and dual-plugging. This is all touchy to do, but works well if done neatly. You may want to check with aftermarket Airhead tools suppliers. See the very beginning of the following article which goes deeper into the problem and fix: http://bmwmotorcycletech.info/cams.htm
© Copyright, 2012, R. Fleischer
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Last check/edit: Sunday, January 29, 2017