Modifications For
Performance,
and in some instances, reliability; for
BMW Airhead Boxer Engines, Electricals, Suspension, ETC.
©
Copyright, 2012, R. Fleischer
mod4performance.htm-77
This section is devoted to what the title, above, says. This section will identify, sometimes in great detail, modifications to almost any part of the airhead motorcycle, and this can include electrics, suspension, motor, etc.....just about anything. NOTE!!...this is NOT the only place on this website that information on modifications are listed!
Please pay attention to this paragraph. Many will be looking at this article for engine performance increases, although that is hardly the only thing here. Before you make a possibly huge error, in $$$, labor, and results, DO understand that increasing the torque/horsepower potentially can reduce reliability or create a more peaky engine. In order to have faster acceleration, the moving parts, in particular the flywheel and clutch assembly, needs to be light. That leads to more vibration from a variety of other effects that are masked by heavier flywheel and clutch assemblies. This is quite noticeable between with the 1981+ bikes, just from the stock factory changes in 1981. That makes carburetion more critical for balance. If you increase compression ratio, you may have problems with gasoline octane. If you change camshafts you will find that the lower rpm areas are lacking in power, and must do something else to bring that power back up. You will find a need to use much higher rpm most of the time. Fuel economy will likely decrease a substantial amount in some instances. Just raising the compression ratio may IMPROVE mileage...a bit. As you 'hop up' the motorcycle more and more you will find that you have placed vastly more stress on some components, such as the valves. You may find that the engine needs a lot more maintenance, and even may be so unreliable as to be a real headache. Your engine may also be too close to disaster. As a general rule in 'hotrodding' engines, when you modify one thing, you need to modify other things in compensation. There is a limit to what modifications can be reasonably safely done to the Airhead engine. After a certain point is reached, the engine casting parts will move in relationship to each other, causing problems. Even well before that point... once you exceed a certain point in hop-up, you will be faced with a FAR less tractable engine, one that you must pay a lot more attention to, as far as rpm, gear selection, fuel grade, and increased maintenance.....to mention only a FEW things that will occur. Be cautious about modifications. Some modifications, such as dual-plugging and slight to moderate CR increase, may well be rather useful, others are possibly not. If you are after a 'sleeper' bike, one that will run away from other Airheads, and maybe even Oilheads; or, are considering racing an Airhead; remember that the bike world is full of even smaller displacement bikes that will outperform ANY hopped-up Airhead.
NOTE!!: The author is not promoting any of the following items. The author is not guaranteeing safety, longevity, nor anything else. Every sort of legal disclaimer is made! In SOME instances, pros and cons that I KNOW OF are detailed.
I. Electrical System (these items in blue, just below, are hyperlinks):
accessory
outlet
Bosch electronic
voltage regulator
Bosch mechanical
voltage regulator
Wehrle electronic voltage
regulator
Testing
voltage regulators
Diode
boards and grounding wires
GEN
lamp resistor
Dual plugging
Ignition system: Elsewhere's on this website are
several articles on the ignition system, the automatic advance,
etc. You are referred to those articles. HERE I will say
a bit about changes for increased power and
acceleration. Firstly, if you have close to 100 octane
fuel available, you will obtain better acceleration by using a faster
advance. In the earliest /5 bikes, BMW used an automatic advance
that peaked at only about 2000 rpm. That would be
better. If you have a pre-1979 airhead, you can fit the early /5
automatic advance, or simply use one or two lighter strength springs in a later
version (to 1978); or, simply remove some metal from both automatic advance
ears, until you obtain the advance you want. You could even tailor your
own advance curve. It is a lot more work to do this sort
of thing on a 1979 and later, as the canister has to be disassembled, and unless
you fashion a jig of some sort, you will be assembling and disassembling a
considerable amount. There are aftermarket ignitions
available, from the Dyna type to the Boyer....to very pricey crankshaft driven
types. All of these have
advantages and disadvantages. For some 1981+ bikes that
came with a single dual output coil, use of
TWO of the 6 volt Bosch coils that have a lightning bolt on the side of them, in
series connection, instead of that single coil with dual outputs, may be
advisable IF...IF...your bike has a modestly hopped-up engine. If you are
replacing an early grey-bodied single coil (dual output type) anyway, you can
use the later type, OR, these two mentioned 6v
coils. For use with pump grades of gasoline, on a
somewhat hotted up engine, it may be best, especially if the compression ratio
is 9.2 or higher, to use a dual-plug conversion. In fact, to
obtain the full advantage of a dual-plugged Airhead, one of the things to always
consider is raising the compression ratio, IF PRACTICLE.
If you plan to stay with points: Points bikes
can have an amplifier or booster added to the points to reduce electrical wear
on the points. Dyna makes those and there are Velleman kits sold too. Pay attention to coil primary
resistances when using points. Typically you do not change the coil(s). If the primary resistance is too low
(from changing to wrong type of coil),
points can burn up rapidly, or if using a booster (points amplifier) the booster
might not handle the current.
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.....and the mentioned Velleman kits, and there are
others...see below:
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.
II. Clutch:
Firstly, the clutches built before 1981 require a heavy amount
of hand pressure, that is sometimes complained about by those with less muscular
hands. A pulley/small chain affair, an EZ-clutch conversion, is
available from such as:
Craig Vechorik; Benchmark Works, 662-325-2103. This is a very simple
device that fits at the rear of the transmission, easy to install. It
roughly halves the effort at the bar lever.
CC products may still be selling a heavy duty clutch that is
quite different, as far as the friction disc goes, than the stock item. CC
Products may still have various types of lightened flywheels and heavyt duty
clutch parts available. Luftmeister, now long gone, used to
have such clutch parts, and others. It is possible that Matt Capri at
South Bay Triumph still has some of those items for sale.
The 1981 up clutch and flywheel are quite a bit lighter than earlier models, and
it is difficult to approach the weight of these later components by modifying the
older design. Nearly a pound can be taken off the pressure
ring on the earlier design. The early clutches in particular need
beefing up if a large increase in horsepower is going to be had. Hard
surfacing of the early parts is a good idea. Generally the later parts
already are hardened. The later clutch
parts will fit (including the carrier) but you have to change the input shaft of
the transmission (or, shorten its spline length). The lighter the flywheel/carrier/clutch assembly, the
more vibration the engine will produce, but the faster you can increase rpm,
and
the better the shifting of the gearbox. As the assembly gets lighter,
the carburetors become more critical to adjust for engine
smoothness. Perhaps that is best said that a small amount of
carburetor UNbalance, is seen more easily with SOME additional vibration. The big advantage of a lighter assembly is
more rapid acceleration. As noted above, some various types of special clutch discs were
once available from Luftmeister. Also note that those clutches tended to
be rather grabby, and a finer touch with throttle and clutch hands is necessary
for a smooth take-off. Those pulling trailers may well be advised to
beef up their clutches some....although most get along with the stock parts just
fine. Other sources can be such as Southland Clutch, that advertises in
Airmail....101 E. 18th St., National City, CA 91950 (619) 477-2105.
dan@southlandclutch.com
They can machine your plate, etc., and provide a stronger clutch if you need it.
For the more technically inclined...here is some information. Pressure can be increased in various ways to handle increased power or heavy 'slipping clutch' use. The following paragraph on INcreasing effective clutch pressure was taken from a posting by Jim Roche, somewhat edited by me here, and I tend to agree with it.
1970-1980: THREE methods, and in all
cases I recommend hard surfacing:
1. Bring spring closer to the pressure plate. Place a
.035"-.065" hard steel donut ring shim between the flywheel and the
spring base. Such large transmission and differential 'shims' are
available from such as RingPower heavy equipment shops; or, see any competent
local mechanic's junk box of old shimming items. For this modification,
the flywheel need not be removed, and the weight added is small.
2. Bring pressure ring and pressure plate closer to the spring.
Remove the flywheel (you were going to lighten it anyway, right??) and machine
off .040"to .085" of its face surface. This is the
face area where the 6 clutch bolts hold the clutch assembly to the
flywheel.
3. Bring the pressure plate alone closer to the spring by installing a
THICKER driven disc between the pressure plate and the pressure ring.
Install a 3 or 4 'wing' metallic 'competition' type clutch disc, which is
thicker than the original. Its extra thickness means that
washers, about .040" thick, must be placed between the pressure ring and
the flywheel face. Some very slight interference with the
transmission case might be seen for a short while, or you can relieve it.
1981+ clutches:
These later parts are already hardened. Some improvement can be had by
machining the fulcrum ring contact point .030"- .050" to the
outside...which increases spring pressure considerably. Hard surfacing is
a good idea for modified parts.
HINT!....Early models had the clutch actuating lever at the rear of the
transmission held to the two bosses of the transmission cover by a PIN, that
used a single C clip. That C-clip fit on the pin at the INside of the
lower boss. If the clip came loose, the pin could come upwards, and come
out of the lower boss, and the next clutch application could, and often did,
break off a transmission boss ear...necessitating a transmission overhaul....or
some inert gas welding at a minimum. A cure is to remove the old pin
and clip and install these parts:
#23-13-1-241-484 pin, that has a flange, and won't fly out.
This is used with a clip that is part 51-23-1-864-963. For the full
details, see my clutch article.
LIGHTENED FLYWHEELS: CLICK HERE!
III. Carburetion, valve gear, cylinder heads, and camshaft:
Valves.htm
Quietingpads.htm
If you are trying to get better fuel mileage, and only that, I suggest a very carefully done tuneup (valve adjustment, ignition timing, carburetor synchronization after checking all adjustments including fuel level, needle and jets, etc). Worn needles and needle jets WILL CAUSE poor fuel mileage. I do not suggest hopping up the engine, or, only a wee small thing or two, as you might be forced to use high test gasoline exclusively, which will offset fuel mileage gains. One thing you SHOULD look at is the rear drive ratio. This can affect fuel mileage by as much as 10 mpg. If you had an R80 engine, you might consider changing to a lower ratio. Some R80 bikes had as high a numerical ratio as 3.36:1 on the rear drive. Going to 3.09 will help mileage, as will, moreso, 3.00, and 2.91. Think this through carefully, as to cost, and effect. If you lower the ratio too much, you may find 5th gear is not all that usable. HOWEVER....the engine will withstand lower cruising rpm to a fair extent....and use of light throttle at, say, 3000-3500 rpm is completely OK.
If you go to dual-plugging, and raise the compression ratio to 9.0:1 (approximately)(if on a later bike with a lower compression), and in some instances to 9.2:1, you can use 87 or 89 octane fuel if the combustion chamber is smooth and clean, and if carburetion, ignition, etc., are all in very proper condition. This WILL increase fuel mileage a few mpg.
Mostly, people want MORE horsepower, so will deal mostly with that here:
For high rpm operation....well into and above normal redline rpm,
you want to rework the valve gear. You can start by using the '336' type
camshaft. This is a TIMING, not lift, cam, and is a very GOOD
camshaft for sporty to modest racing performance. The combination of the
336 camshaft and a lightened clutch will give you less grunt coming off the
line, and some of the usual minor hopping up feeling/problems. That is
helped considerably by a higher compression ratio. I do NOT recommend that
cam...or any cam other than the stock one....unless you have at least 9.2, and
preferably higher. Going to high
ratio rockers is risky, you MUST know what you are doing. You can lighten
the valve system by using later pushrods (or even aftermarket ones), and
elongating the slit in the followers. Use the biggest intake valves that
you can fit PROPERLY....probably 45 mm, and grind the inside of the seat.
Remove the unused unneeded area of threads on the valve adjusters. Clean
up and polish the rocker arms. Use the later needle bearing rocker arms,
etc. (Use the 1985+ type parts, and modify them). Other lightening
techniques can be utilized, just be careful. For a true race engine, very
hopped up, you will want to go to more exotic valve gear parts...spring buckets
to keep the spring cool, lightweight spring retainers and keepers, etc.
Note that if you modify to a different camshaft from stock, use different
pistons or heads, or any modifications of these types at all, YOU are responsible to
ensure that there is no interference between the various parts during engine
operation. There is even a 4 valve head available (Krauser). They
are tricky to get working correctly, but will perform very well, with a large
increase in power.
Some other suggestions:
Use the 40 mm exhaust system, dual crossover, and if for the street, modify the mufflers slightly by drilling a 3/4 inch hole from the outlet end with a LONG extended drill, through ...way inside baffle. Drill two each 3/8" angular holes, one on each side of the muffler OUTlet, about 3/4 inch or 1 inch forward of the outlet tip. For a race machine, the exhaust system should not be those heavy and even modified, stock-type mufflers. Some would recommend a traditional taper megaphone, with a reverse taper cone. Use a 2 into 2 system. There is the old Axtell systems....6° taper megaphones, ending in 45° reverse taper cone, the hole of which is half again as large as the entrance. This is not at all a bad idea for a 8000 rpm engine with a fair amount of hopping-up. As always, the top end is the secret to power, and a dyno is recommended.
Probably the 1977 40 mm exhaust heads and pistons would be a good starting place....but you CAN use any of the heads....and modify them. For the larger engines like the 800, 900, or 1000 cc engines, you need the appropriate carburetors and the proper INternal size of intake adapter (that's the screw-in metal adapter that screws into the intake port of the head).
IV: Transmission:
Modifications can be made to ensure a faster shifting transmission.
Removal of any kickstart mechanism is a good idea. Modification to prevent
pawl spring breakage is simple, and should be done. The 4 speed transmission is
not a good idea. Neither is the 1974 5 speed
transmission. 1981+ transmissions will fit earlier clutches if the
input shaft of the transmission is changed. The reverse is also
true. Modifications for racing can include using the later
stronger transmission case, ensuring you have the shift kit parts (see
transmission article) and undercutting and
possibly other modifications to the gears and gear dogs.
Special gear ratios are available for the transmission. Think carefully
before going that route.
V. Rear drive:
Select the ratio to match your engine and your expected speeds and rpm.
VI. Driveshaft:
Driveshaft
bolts
VII. Front and rear suspension:
Do a very carefully done fork bracing.... and install a billet top triple clamp (especially do THAT). Be SURE you get a billet top triple clamp that is properly made. Modify the front forks for improved valving. Using aftermarket springs, or modify the BMW heavy duty ones, by cutting at a calculated (HAH!) length, using a washer between the two pieces. Install a spacer inside one of the springs, NOT BOTH. I suggest that you cut at 50% of spring length. The spacer needs to take up a bit of the length of the one half. This will make a two-stage progressive spring, with twice the rate after the spacer'd section binds up. Obviously you can do 25% cut too, or anything you prefer. The 50% was suggested as the effect is large. I'd do 25% or 30% unless you had lots of springs.
If a /5 bike, braze up two of the holes. Go to
a REAL 10
weight suspension or fork fluid. Select from my
viscosity.htm article. Be SURE you spend a LOT of
time on stiction and alignment, see Randy Glass' article, posted on Duane
Ausherman's website now, instead of his own. (see URL.HTM
page).
Rear suspension: Twin shock Airheads can be modified for MONOSHOCK,
EASILY. The Monoshock rear end just bolts right up. I suggest
you modify the frame and make a curved plate with a slot, so that the single
spring/shock unit is adjustable; which, with the usual adjustments of the spring
perch and shock valving on most units, will offer you much improved performance.
Potentially it is possible to install a R100GS setup, with its transmission, Paralever, rear end, etc.
VIII. Brakes:
Upgrading the front drum brake is possible, by careful assembly and use of
appropriate friction materials. It is possible to install discs, at
some typically large expense....with wheel changes usually needed (but not
always). There are innumerable possibilities. Some
are costly and complex.
Example: To install a cast snowflake wheel on the front of any early
Airhead, you might naturally think of just grafting on a R90S dual disc brake
setup;....let us say you have a 1974 /6 you want to do that to. You will
find the TUBES spacing to be different! That means you need need triple
clamps, although the old tubes are OK. You will need 1975-980 dual disc
ATE Lowers, the 17 mm axle, etc. You CAN use the ATE calipers.
Finding an entire R90S front end, complete, is not likely.
Another example: You can convert a single disc ATE system to dual
disc by using 1979 parts...a right slider lower), the caliper, the disc.
You probably can use the existing MC.
Quite a number of Airhead owners have 'upgraded' their disc brake systems:
(1) An additional disc where only one was stock on that year and model.
This will improve braking SOME.
(2) Modified disc(s), such as some other than stock type of metal, perhaps
floating type of disc/carrier, and other changes. NOTE that drilled or slotted discs are not drilled or
slotted primarily for sweeping off rain water, as is commonly thought. The real
purpose is to reduce the gas that is produced from the resin binders in older
(especially) pads being heated. The gas goes to the pad surface, and thereby
makes a 'hydroplaning' type of layer. There are two other
purposes. Drilled discs help INcrease the coefficient of friction. The holes'
edges provide those edges to promote 'bite'. The latest high friction pads
minimize the effect somewhat. The other purpose of drilling (or slots or even
shallow grooves) is to help remove brake disc debris...they have NOTHING MUCH to
do with COOLING.
(3) Different master cylinder piston size, often 1 mm, rarely 2 mm smaller
than stock for the same number of discs.
(4) Different pad material. HH+ pads work very well on most discs.
If using cast iron aftermarket discs, consult the maker.
(5) Different caliper(s), such as more pistons, or staggered size pistons, etc.
I have seen a LOT of confusion over why an 'upgrade' brake caliper...to one with
more than one piston, where approximately the same total pistons area as the
stock one, has not improved braking.
Sometimes these modified systems have the unequal piston size calipers mounted
to their Airhead such that the stock LARGER piston, normally being the exit
section (tire rotating normal direction) is mounted backwards from what the
manufacturer intended. This, with the gassing, see item (2), results in less
braking. This type of problem can be somewhat minimized by being sure you are
using drilled discs. Staggered size pistons calipers are staggered in size
specifically to help with this gas problem. Very modern motorcycles with the
latest type of discs and calipers and especially pad compounds, are much less
susceptible to this problem of gas-hydroplaning, and may not even have drilled
discs. Beware of using the wrong pads!
NOTE: Some strange things you might run into include the REAR DRUM BRAKE fitment. PRE-1981 rear drives with drum brakes used WIDER shoes and narrower pivot pin; so the later shoes won't fit. A late model brake drum might need a ridge machined off due to wear from narrower shoes.
NOTE: Installation of 4 spot calipers from such as an Oilhead or K-bike is USUALLY rather simple, and results in VASTLY improved braking. If done in a DUAL disc setup, you can have truly modern performing brakes. You often can continue to use the stock master cylinder. The installation of dual discs and 4 spot calipers IS MY FAVORITE METHOD...as it gives a HUGE increase in REAL braking power, for the least amount of money.
IX. Tires/rims:
This is a complex subject, and individual recommendations can be made.
X. Frame:
Modifications will depend on use, and year of your frame. One of the
weak points is the subframe, and suitable beefups above the battery are a good
idea. Another beef-up is to remove the top cover over the starter motor, and make a
bridge/ladder/lattice type of strong adapter, and tie the top of the engine very
securely to the top frame tube. This is for an all-out race bike.
There are other modifications that can be made, some will result in vibration,
such as that tying the starter area to the top frame. Side braces will
help SOME.
XI. Carburetion and intake system:
see: Intake and
Exhaust tuning
The intake system can be modified, depending on your desired
performance level and usage. For an all-out motor for racing;
individually supported gauze intakes are desirable, but these do NOT work well
for street/touring, where the clamshell or square air-cleaners are far
better. The original paper filter elements are BETTER.
Slide carburetors will give vastly improved throttle feel, and often better acceleration...sometimes dramatically so, than CV carburetors.
If you use the Bing CV carbs, you can modify them, even going so far as to thin the butterflies, and many parts are available for the Bings for modifications, including various slides. Experience is needed...lots of it.
Definitely give a lot of thought to additional air, COLDER air, to the intake system. A very significant power boost can be had.
NOTE: Questions often arise as to whether or not RAM AIR would be helpful. This is not overly easy to accomplish on our airheads, but CAN be done. It seems relatively effective, as it gives COLD air at any speed, by modifying the intake snorkles. But, for a RAM AIR improvement, it is UNLikely to do anything, except at warp 10 speeds.
Keep in mind
the following:
Dynamic air pressure on a flat plate moving object (motorcycles are like that!) is relatively small at slow
speeds, and has no real effect of any consequence. At 68 mph, for
instance...let us say 100 feet per second...there is 12 pounds of pressure on a
SQUARE FOOT. A square foot is 144 square inches, thus the pressure is 12
divided by 144, or under a tenth of a pound. Pressure is
proportional to the SQUARE of speed. So, at 136 mph, one has 48 pounds per
square inch...or about 1/3 of a pound of pressure per square inch. This
gives only SOME barely noticeable 'boost'. Another thing to keep in mind is that
the horsepower to attain some higher speed, goes up ROUGHLY as the cube.
What all this means is that if you are not doing over maybe 120 mph, forget
trying to get RAM pressure, but it is always worthwhile to go for cold
air...which has a MUCH bigger effect, until speeds generally unattainable by a
motorcycle are reached. Try to AVOID increasing the intake path enclosed length in
order to get your colder air.
XII. Exhaust system:
Pulse air
system
XIII. Swapping parts such as heads, cylinders, etc:
various articles on this website dealing with engine internals.
XIV: Miscl.
Miscl.htm
Luftmeister side tanks
Install the later reed breather valve. You may want to
install a deeper oil pan with extended pickup, and for racing or VERY sporty
riding, suitably install oil surge baffles.
Depending on how far you want to go, you can modify for lightness all sorts of things. Like drilling and polishing those holes in the cam chain sprocket (yes, race folks go to that extent).
Use a later driveshaft housing, but install the earlier non-cush shaft.
Rev:
02/23/2005: update and release to web
01/17/2009: expand section VIII on brakes
04/21/2012: Update article
© Copyright, 2012, R. Fleischer