electronic ignition question (aftermarket box ?)

I've got an '88 boxer with a funny BMW ignition design. The bike
uses Hall sensors in combination with a centrifugal mechanical
advance rather than just using fixed sensors feeding a black box
electronic advance.

The bike uses a wasted spark design, sending one spark
to both cylinders on every revolution. This is what Harley
calls "dual fire". Full retard is 6 degrees, with ~25 degrees
advance and full advance at about 3000 RPM.

What I'm wondering is whether it's possible to buy just an
electronic advance unit and trigger the unit using the original
Hall pickup with the mechanical advance disabled.
I don't find anything on the market with a drop in sending
unit and suspect that the original unit could be adapted
fairly trivially, possibly with a swap of Hall transistors.

Any thoughts on aftermarket black boxes that might fit
the bill ? The mechanical advance is getting cranky and
I'd love to stop fighting with it.

TIA

 

I don't know about an aftermarket unit, but you might look into
an OEM unit from another make with similar sized cylinders and
wasted spark. After thinking about it a little, even an ignition
unit from a big (400-650 cc) single cylinder engine should work
OK, too, the ignition module doesn't care whether the coil
secondary is dual-ended or not.

The biggest problem I see is getting a good match to the advance
curve, I'm thinking modern 4-valve combustion chamber designs
will probably have more total advance and may kick in at lower
revs than you want. Also it's going to be difficult to get
advance curve data without measuring it yourself.

A smart fella might figure out a way to fit a microcontroller
between the Hall effect sensor and the stock ignition amplifier,
but that could turn out to be a lot of work...

 

Did you see this guy's page?

http://www.stephenbottcher.net/BMW/R100T.htm

Follow the links in the "March 2005" section here:

Install an electronic ignition with alternator pick-up (Made in Germany)
and 16 different advance curves (beancan is still installed, correctly
adjusted and can be activated on the road with just one simple cable
swap) . Here are the installation instructions in English.

 

?
I must disagree about the total advance.

Suzuki's GS-1100 TSCC 4-valve engine only used 32 degrees full advance
at 2350 RPM, compared to the Honda CB-750F that used up to 50 degrees
full advance.

Honda's F-series engines used the obsolete pentroof
combustion chamber. Perhaps Honda was copying what Ford did with their
4-cam Indy car engine in the early 1960's?

The four valve engine with pentroof combustion chamber has been around
ever since the 1910 Miller engine.

Pentroof combustion chambers result in an engine that breathes well,
even better than a hemi-head engine. The fuel air mixture enjoys a
straight shot into the combustion chamber, yannow...

But the problem with the pentroof combustion chamber is slow burning,
wasted fuel and the need for lots of ignition advance. The famous
Offenhauser engine that was developed from the Miller design needed 45
degrees of advance at high RPM to "get 'er done".

Exactly what is it, that engineers are trying to "get done"?

Yes, it is true that that the ignition coils exhibit inductive
reactance and that means that it takes *time* for the magnetic field
in the primary windings
to collapse and it takes time for the voltage in the secondary
windings to build up high enough to jump the spark plug gap.

But the time required to collapse magnetic fields and build up voltage
is not the *only* reason for using a certain amount of ignition
advance, even though hot rodders like me thought so for decades.

The reason that later model 4-valve engines use a lot less advance is
that there is more swirl and turbulence in the combustion chamber,
allowing a leaner mixture to light off and burn rapidly.

This means that a Suzuki TSCC engine at full advance would be lighting
off the charge *later* and combustion would be completed *earlier*,
transferring less heat to the metal parts of the engine and using more
of the heat to expand the combustion gasses and push the pistons down.

The Honda CB-750F was a notoriously hot running engine, since it took
as much as 100 degrees of crankshaft rotation to complete the burning
of fuel.

Contrast that with the GS-750/1100/1150 and the GSXR motors that take
only about 64 degrees of crankshaft rotation to burn the fuel.

Kevin Cameron wrote of the Honda NR-500 oval-piston GP motor that ran
up to 20,000 RPM and stated that it only needed 23 degrees of ignition
advance.

With that kind of full advance requirement, who needs an advance curve
at all?

Maybe the curve went to full advance at 10K RPM?

I dunno. Irimajiri doesn't answer my calls...

 

Have you tried new springs, lubing and all that? My '78 predates
electronic ignition but replaced it MANY years ago because it was
my least favorite tune up item, with first a Martek, then Dyna III
both utilizing the stock advance and later Boyer with electronic advance.
Got rid of the mechanical advance altogether and never missed them.
The Boyer has been very dependable and would probably fit but
I doubt it's made anymore. Maybe used?

 

BMW doesn't offer any beancan parts. They only sell new
units at maybe $400 a pop. Motobins sells a spring that
they say is for beancans and I've got some on order.

The plastic bushings can sometimes be a problem as
well. Looks like AFM may offer parts that would work,
but I haven't tracked down a retail source. Beancans
are a real PITA to work on and are supposedly not
user serviceable. That's why an electronic unit looks
attractive.

Boyer is a good thought. Ever check the advance
curve against the timing marks ? Do they match
to stock and at what RPMs do you see full advance ?

There's also a new German unit "omega", but I think
this locks you into either the stock or the Motorrad Elektric
alternators, as it bolts to the front of the stator.

Thanks

 

Only problem with this is that it limits your ability to do
add an aftermarket charging system because it bolts to
stator plate. There are a couple of trick charging systems
that I think this unit precludes. It's definitely a candidate
though.

 

I don't know what you mean by the advance curve. I always set
the timing to be a bit advanced at 4,000 rpm. The timing dot on
the flywheel (I painted it white) is a bit high in the viewing hole.
I learned to ignored the idle timing, I didn't care what the performance
was at idle and you set it for one or the other if I recall right, but's it's
been so many years since I had to fool with it I don't remember even
the Boyer having a separate adjustment. I set the idle speed with the
Bings afterwards. She always ran like a raped ape, got many compliments
from folks who know.

 

What I was curious about was the RPM at which you hit full advance
and whether the mark for fully retarded was lined up at idle. I gather
from what you're saying it's probably pretty close and the engine's
happy. Urban legend from a pretty knowledgeable shop has it that the
Boyers may advance a little more slowly than some early mechanical
devices.

Thanks

 

There are several different ignition systems available in Germany for
Airhead BMWs but it seems that most of the sheddies inventing them have
little appetite for selling them outside Germany.

 

For me the stock and electronic advances were pretty close as far as full
on and fully retarded went. The triple timing marks were always visible in
the view port at about the center. It's possible that the Boyer advanced
more slowly but the stock return springs were pretty light and I had swapped
them out for heavier duty ones pretty early on. Since the first two electronic
ignitions used the mechanical advance I didn't notice a difference with the
Boyer's electronic one.

I never suffered the pinging some guys had and I never had mine dual
plugged. Same with the '84 RS heads switched to 10 years ago, it still runs
great on low octane gas. Some of the dual plugging business was hype,
there were some combustion chamber designs that benefited but not
most of them. I think it was the early years that had higher compression,
higher octane in mind.

Those were great motors and easy to work on. Which reminds me, I need
to put a battery in her pretty soon!