Modify a BMW airhead?

I've always liked single-banger motors. Would it be possible
to modify a BMW boxer motor such that the 2 cylinders'
power strokes were simultaneous instead of alternating?

In effect it would be equivalent to a thumper
but better balanced for less vibes.

SQ

 

*Anything* is possible. All you have to do is pull the camshaft out,
saw it in half, rotate the halves to achieve the firing order you
want, and weld them back together.

http://www.bikebandit.com/1973-bmw-r75-5/o/m295#sch324018

 

Xposting to reeky since it's pretty slow

Ought to be a piece of cake on a BMW oilhead,
as there's a separate cam for each head. That may
leave some interesting problems with the fuel injection,
but hey, how hard a hack could that be ?

http://tinyurl.com/3m3x3gr

Reeky eagerly awaits the results of your experiments.

 

I'm sure Sean would tell us ;-)

 

As I said though, if you could get the injection working,
an oilhead engine might run using absolutely stock cams,
as each oilhead cylinder runs off its own chain driven cam.
I believe the oilhead is also lost spark ignition.

Might even be something that could be cobbled up in the
garage with no machining or electronic trickery. Not sure
what's involved with the F.I. system.

 

It would be no problem at all to hook more than one injector up to fire
from the same signal as the other injector, you'd need to use a driver
circuit rather than just put the two injectors in parallel[1], but it
would be pretty trivial.

[1] which would probably work for a little while until the output
transistor in the EFI computer blew out.

 

Yesterday I phoned Shadbolt Cams Ltd in Vancouver.
They would need a cam billet because too much material for welding
would have to be added and then ground to fit in order to relocate
two of the lobes 180 degrees (that much heat would warp the camshaft).

However they do have a regrinding master pattern for the 1970 R75:
http://www.shadboltcams.com/cycles.html

The cam lobes are in pairs, but within each pair the lobes are not
quite parallel; they're a few degrees apart. I can't figure this out.
At first thought, why would the cylinders seem to have different
valve timing -- and how would this relate to ignition timing?
Are the cylinders not quite horizontal and 180 apart, or is it
(on 2nd thought, duh...) merely because the pushrods are at an angle.

The ignition fires at (or near) each TDC so there's a wasted spark
at the end of the exhaust stroke. This feature would give me ignition
on both cylinders at compression TDC (theoretically).

SQ

 

Simple slave setup should do it ?

No per-cylinder feedbacks to worry about I guess.

An 1100 oilhead ought to be about the same price
as an airhead these days.

Go for it Sean. We'll hold your coat for you while you
do battle with this project.

 

Even without doing anything to the injector timing at all it would
still probably run OK-ish- it's not as if the fuel spray has to be
timed exactly to the opening of the intake valve on a fuel injected
engine, check Wikipedia's entry for the various flavors of multi-port
fuel injection, esp. simultaneous mode.

 

Basically you want a big bang motor from a twin...you can do that with
a harley it's called a twingle.

 

He's wanting to make it either a twingle or a big bang motor. Twingle
would work well, but big bang? Not so well I suspect.

 

A horizontally opposed twingle would be challenging.

AFIK, Spagthorpe was the only manufacturer who ever
worked on this concept. Their Pariah enjoyed some
success, but blown head gaskets were always a problem
in the large common combustion chamber.
was

 

Wouldn't it double the power-pulse load on the crankshaft as both the
cylinders fired at once?

Or is there something I'm not seeing here?

 

So is this the BMW or Dnepr clutch ? Or some
unnatural combination of the two ?

 

Thanks for you generous offer. Just gimme a sec while I search
Craig's for a suitable test candidate.

Meanwhile I gotta figure out what to do with this R75/5 write-off
I just bought. There are two applicants for its motor:
the '78 Dnepr MT-10/36 and the R75/5 bare frame I also just bought.

If I choose the Dnepr I'll need an adapter (~$100) so that
the Soviet transmission will match up to the BMW motor (with some
futzing. Someone local on the Dneprheads blog did the same
conversion and is advising me on this).

However if I build up the BMW frame to a complete bike then it'll
cost me close to $2k to have the Dnepr sidecar fitted to it
(at Side Effects in Kamloops).

[...doing the math...]

Uh, looks like the Dnepr wins. The bare frame will have to gather dust
until some other bent Beemer shows up. Which it should, eventually,
and I can afford to wait.

ps. the bare frame came with a collection of loose parts. Mostly junk,
but there are some usable pieces including cylinder heads. There is
a R60/6 write-off available which could be upgraded to 750cc.

SQ

 

No I think (but can't prove you are correct) that is the real problem.

 

ah yes...sadly the web page is down so I can't research it, pitty that.