Sleeper R75/5

sleeper_R75/5.htm-83

Description follows:

 This is a posed shot, we did NOT ride in those jeans and shoes, with no helmets!!
I THINK this was taken at the Prineville BMWMOA National Rally in 1980

This is obviously a 'toaster' R75/5.    First, some identification points.  This is a stock-looking bike.  The fairing is an Avon, the bike was delivered to the dealership by Butler and Smith, the distributors, WITH that fairing, and  the white round item in the windshield area is a rear facing audio 'horn'; there is a hidden tiny HAM radio on this bike.  The antenna is there, but you will have to guess where it is!  The hard-to-see items at the ends of the bars are a type of cold-weather 'Hippo-hands'.   The leather bags are by Bates.   I purchased this bike brand new, and it was torn down when brand-new, by myself, and essentially 'blueprinted'.  Some modifications were made...well, a LOT of modifications...some of which did not happen until many years later.  In the beginning, whilst blueprinting the bike, many carb changes were made.  Alternator modified, wheel bearings setup modified, CAMSHAFT changed to one I personally ground after I designed it.  Rockers were modified for increased valve lift.   Flywheel lightened.   Transmission modified for fast shifting....and on and on it went...but it remained looking very stock.  

  I worked part time at a dealership as a mechanic at that time.  It is a 1972 model.  The passenger is my X-wife, Jean....in a much happier time for the two of us.  We are wearing the light-weight windbreaker jackets of the Flying AAA Ranch, where we sometimes went in our Cessna C-182A (1957), and I went, later, in my 1967 C-182 airplane, for week or weekend retreats/vacations.  I had been many times to Baja with that bike, and those airplanes.

What is so unusual about this bike, is what it became; and, how stock it looked ALWAYS.  After we were divorced, and with OVER 250,000 miles on this bike, I decided it was time for a TOTAL overhaul.  It became very much more.  I had never originally intended to do other than restore it to showroom fresh, keeping the previously done modifications, primarily for handling and higher rpm operation.  

Then...I was given a very unique supercharger, and the juices began to flow.

 I modified the bike very extensively; yet  it remained looking exactly as you see it, above.  Well, NOT exactly.  If you looked VERY carefully, you would see what appeared to be a stock but, somehow taller, starter motor area cover.  You would see some beefing-up at the steering head.   The rear brake rod and lever might look a tad different...the pedal too.  Very subtle changes.

1.  The carburetors were sold, and a pair of identical carburetors, but with broken internal main jet castings, hence the carbs were obtained for nothing, were installed.  Before installation, the carburetors were completely sealed from the outside air, and were totally bored out, and very much modified, so that they were, mechanically, simply straight tubes.  The intake ports and spigots, etc., were also much modified, as were the heads internally.  The throttle and 'choke cables were original, NOT functioning, and total dummies. These cables led to an area under the fuel tanks, where they were simply chopped off and tied to the backbone area, doing nothing.. There were NO butterflies, choke parts, floats, diaphragms, or anything else in these carburetors. 

2.  The engine was disassembled for overhaul, and a few other things done besides a typical overhaul....such as... the oiling system was modified, including modifications to the rod and main bearings, the oiling distribution at the main and cam area modified, the oil pump modified, the oil filter area modified, the engine fully balanced, special pistons installed, special rings, much MUCH head work.  The old offset high lift rockers I'd previously installed were removed and custom aluminum roller rockers installed.  Pushrods and lifters were modified. The  custom made camshaft with very long intake duration and higher lift was retained from my original work.  

Yes, the engine was no longer a "750", the displacement WAS increased, but LOOKED stock.  This was a very tricky thing to do to the cylinders, which, when stock, are alloy castings, with iron cylinders.   It was tricky to increase the displacement, due to how the studs fit the engine on the early engines.

  A very long list of internal modifications could be listed here, but I won't.   I mentioned the oil pump was modified as I originally anticipated very high rpm and was worried about cavitation.  An oil pan modification was made, and the oil pickup modified too.  It all SEEMED to look stock, from the outside.   The heads had a type of squish chamber of my own design, single plugs, and the compression ratio was lowered, from the head work, to about 8.3 or 8.4:1.  The squish area was very tight, however.  The exhaust valve was originally stock, the intake valve came from a Lincoln V-8.  Special valve seats were installed, the heads ported from a 'guesstimate'.  NO, I never flow-tested the heads.

  The ignition was, initially, the stock points type (modified some), the long dwell early BMW automatic advance was used.  An ignition points amplifier was designed and built.  Points current....from memory here....was about half an ampere, and the points amplifier was designed with a circuit such that multiple triggerings from one points opening or points bounce that is,  would be unlikely.  Later, I designed and built a capacitive discharge ignition, that fired surface gap racing plugs.  That ignition had a very special set of racing coils.  The final change to the ignition was to make my own pickup that was crankshaft triggered (end of alternator), and the CDI ignition was modified so that MULTIPLE sparks occurred at each cylinder's triggering.   This proved to be VERY good.  I did a LOT of work to make an electronic circuit that controlled the ignition advance.   At one time I tried a vacuum advance unit, was not satisfied.  I ended up with a circuit with a knob that could be turned to change the advance curve and total amount.  Once I found the correct amount on a dyno, I sealed the control.

 The alternator was changed to the late 1974, 280 watt type, and an electronic regulator of my own design was incorporated.  The battery was a 11 cell nickel-cadmium wet-cell type from a corporate jet, designed to look relatively stock.  Before the capacitive discharge ignition was installed, the spark plugs were experimented with for some time on a dyno, and the stock TYPE of projected-nose plug was decided-upon, but a rather cold rating was used. I tried surface gap plugs, and went back to the projected nose types.   The gap was only a bit more than stock, around 0.035" as I remember. Spark plug caps of 1000 ohms were used, with stock wires.   I had access to a dyno (Axtel's) so a lot of testing was done for these things, and many others.  Much time was spent on setting up the ignition and the ""carburetion"".   The exhaust system was designed by myself, and was simply a modified version of the stock system, with added crossover, and modified muffler innards.  It was relatively QUIET!...but was about 7 inches LONGER than stock, but hardly anyone ever noticed that.  This was done, in conjunction with a short (no room for longer) intake system intake horn, to obtain good torque characteristics over the rpm range (which I redlined, guessing at 7800).

3.  I made TWO versions of this motor.  In the first version, I removed the starter motor, removed the alternator, and made an adjustable diameter v-belt pulley that fit in place of the alternator rotor.  I mounted the special almost unobtainium modified-vane type supercharger in place of the starter motor.  It was slightly higher than would fit under the starter cover, so I heli-arc welded higher sides on the starter cover, then machined that area, so the engine looked stock to a reasonable look-see.   Due to constraints on pulley sizes that would fit, THAT did not work out for the boost pressure I wanted, so the blower was reversed, alternator reinstalled, and a much modified flywheel and ring gear was used to drive the blower.  Try as I did, that setup would whine a lot.   An adjustable mount that was far more complicated than can be easily described, was made to adjust the engagement of the blower gear and the 'starter' gear.  The starter, being gone, meant the engine was kick-started.  Starting would have been VERY difficult (and WAS, initially!), due to the long fuel path and very low amount of intake flow during kickstarting, so a simple fuel injector was modified and installed into the airbox, used ONLY for starting.  It operated every time the starter button was depressed.  It was a 1968 VW gasoline car electric  injector, the same type I used at the intake of the supercharger.

4.  The airbox was the stock clam-shell, with a few minor changes, but fully insulated, and the MAIN fuel system injectors were very simply modified ones from a 1968 VW fuel injection motor.  NO major attempt at cooling the intake charge with an intercooler was made, but careful attention to internal wall insulation, and a rather clever, I think, method of using the air from the timing chest (flow greatly increased, too) to keep temperatures controlled low enough, was done.  ALL was done so the appearance was stock...unless one had an identical bike next to it, to compare, back and forth. 

Almost no one ever saw this bike as 'modified'.   That had been my intention.

5.  The blower initially, before the injection was installed, sucked air from a modified carburetor, a rather flat carburetor, from an industrial engine....gravity fed from the stock fuel tank.  There was a pop-off backfire valve, that doubled as a over-boost release valve, crudely made but effective, in the right side clamshell area, that exited from a hole, that I told people, if they asked, that it was an extra engine oil breather outlet.   The entire intake system was fully closed, and sealed from the air normally coming from the timing chest/alternator area.   It was THIS carburetor that the SINGLE cable from the slightly modified throttle twist grip area, operated.  The type and model of carburetor, and how I did it, and kept it not tall at all, was also interesting.    This system worked, but had problems, so I removed it and used the VW injectors again, this time at the intake of the blower.  The injector was fed by a very common but pressure-regulated car fuel pump, and the electrical signal for the injector was a small flat industrial type potentiometer, hidden INSIDE the modified stock-looking throttle assembly at the handlebars.  Turning the throttle caused the potentiometer to change its position.   I did a lot of experimenting with this method, and it ended up working outrageously well.   There was major hassles along the way getting the fuel delivery to be SMALL enough, for idle; and, to have a butterfly air control.  I ended up using a butterfly from a much larger American car carburetor, and connected it in the intake area of the blower, and controlled it from the stock throttle cable.   It was VERY tricky to coordinate the injector pot and that cable-to-butterfly.   I finally found a very interesting way of doing it, with selectable pins holes on the stock top gear of the throttle assembly.

6.  The brakes looked stock, but had very high friction linings, cam modifications too.  The over-all leverage was rather increased.   The suspension looked stock, but the rear shocks were opened, played with, and the front forks rather modified internally.   The stock tires were always used.  

7.  A LOT of effort was made to try to get good boost characteristics.   There are several types of mechanical superchargers, commonly seen are the vane and the centrifugal, and they have differing characteristics.  All pull a fair amount of horsepower from the engine, some WAY too much.  The type I used was a rather sophisticated modified vane type.  It was one of the very first ones ever made that had flexible vanes, and did not have too high a power drain, and it produced boost at a quite reasonable rpm, and had a rather nice boost pressure curve.  The vanes were exotic, and, as I noted, they flexed.

The supercharger had not only cleverly designed vanes, but a flexible diaphragm, that changed some clearances as rpm rose, to prevent damage and control boost characteristics.

Avon Green Spot race tires were my standard tires later on, as I got a mess of them free.

Now, you are probably asking, how about PERFORMANCE?

Well, let us start with the handling....with the modified front and rear shocks and changed springs and valving, and a top triple clamp that was actually a modified lower cast triple clamp, and a frame beef-up near the battery and steering head, and a beefy brace as part of the fender mount, ..... the handling was pretty good.  The short wheelbase, and the fairing, made for reasonable handling, and I refused to modify to the long wheelbase.  Actually handling was QUITE good.  Braking was about as good, for one hard stop, as dual Brembo discs on the later bikes.  I did not install discs, nor ventilate the brake backing plate, as I wanted it all to look stock.  The braking was considerably better than stock, to say the least, and QUITE adequate, for anything but a real road-race track.

Power:   YES!!!!....LOTS!    Amazingly, the engine idled quite well at about 1300 rpm.  The bike was probably a bit slower on a jackrabbit take-off.  After the rpm crossed about 2800, the boost came in, and it came in very smoothly.  The increase in power was VERY constant and VERY steadily increasing.   Sitting upright, in normal touring position, with full throttle and at about 4500-4800 rpm, the front wheel would start to rise, and continue to rise some, to a quite nasty height, to about 7500 or tad more rpm.  I only took it beyond 7500 a few times, once to just over 8000.  Generally, there was no reason for more than 7000 or so.    The bike was VERY quick, and I later incorporated a lower ratio rear drive, the stock had been 3.20.  The bike would reach redline rather easily.  The stock 4 speed gearbox was used; although modified for smoother shifting.  There was no problem with the clutch (I'd installed a stock BMW sport clutch spring).

The dyno showed about 84 corrected horsepower at the rear wheel at approximately sea level.  This type of power, meant, probably, approaching 100 at the flywheel, and I felt it could not safely be maintained....I was sure the engine would explode/melt-down;....so, full throttle runs lasting over maybe 20 seconds were ever performed.  There was never a serious mechanical problem, in normal (yes, very spirited!) riding.   I did have one of my earlier modified rocker arms break, and that did no serious damage.   Problems, little ones mostly, did occur, and these, such as oil leaks, were fixed, although I could NEVER fix leaks at the cylinder bases...the power was too great for the cases and cylinders. But, the was bike ridden extensively with no major problems.   NOT ONE PERSON ever noticed that it was supercharged.  Few ever asked about the gear drive whine, which I never tried to fix.

The use of this sort of power, even at part throttle at reasonable (?) speeds, was VERY hard on the exhaust valves.  The heads were removed at 15,000 mile intervals, and a valve job done.  Ultimately, a very pricey racing exhaust valve and seat was installed, that lasted longer.  Still, 25,000 miles was about it for valve jobs.

Oil:  Castrol R, 40SAE, just because I liked the smell!
Rear end:  3.00:1
Tires:  Back to Continental RB2 and K112, or Metzeler ME88's, after using up my available Avon Green Spots.
Front forks:  very progressive springs and two brazed up rod holes, a few other mods too, and I used Mil 5606A oil
Rear shocks:  Koni, internally modified to give stiffer shock action, slightly heavier spring.
Oil system pressure:  approximately 100 psi at just over idle (relief valve modified, plus MANY system mods).
Boost, high:  14.3 pounds at 7500 +- rpm (Lord Knows what it was at the 8000+ I hit once).
Boost, low: very approximately 1.9-2.4 psi at about 3500.  This was not overly consistent, and some boost was sometimes noted as low as 2500.
Boost calculations were normalized to 15 psi atmospheric, thus boost pressures noted are REAL, and above absolute atmospheric.
Sound level:  Almost inaudible blower sound.  Exhaust several Db louder, deeper, pleasing.  Gear drive whine was rather excessive.
Top speed:  unknown, never had it over about 125.
Drag strip:  never had it on one
Wheelies:  no problem.  It would wheelie in normal seating position, in any of the first 3 gears.   Sit too far back and whack the throttle at 5500, and things got WAY too exciting, WAY too fast.

Present location:  wouldn't you love to know!

How to identify from a stock /5 toaster if you see it:  UNlikely you would detect it as not stock, and, I am not about to tell you the pertinent details, or, who owns it now....he won't sell it back to me.  I hope to get it back though.

HINT:   look at the starter motor cover.....look a bit tall too you?