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Re: Dual plugging. Was "Surge Free" spark plugs

Hello Ben,

Ben Barkow <dr.ben@xxxxxxxxx> wrote:

> Nice post from Steve, thanks.
> The nature of posts is that it looks like I was trying to educate Steve
> specifically when in fact I have abundant respect for his knowledge...
> except maybe for spark plugs.

You flatter me. Honestly, I am very careful to disclose what I don't know, and
what I am not too certain about (just because I am not a fountain of
knowledge). That's why I state things such as "according to X" -- because I do
not have the knowledge to confirm or challenge his position.

> Gord Bush, who is the gold standard for bike and racing machinists in
> Canada, hasn't started doing dual plugs in Oilheads. I phoned him this
> morning and his reaction was what I expected: crazy... why would a
> 4-valve engine need a second plug... might arise from excessive leaness
> or pollution care... just enrich.

Yes. That is the easy answer. John Parker (Budds BMW and John Parker Racing)
confirmed this. In fact, years ago when Oilheads surging threads were
extremely common, I asserted this on my web site and I took a lot of heat from
Big List Experts for not knowing what I was talking about. Enriching the
fuel-air mixture will amlost certainly eliminate the surging (assuming the
bike is in an otherwise good state of tune and repair), but at the obvious
cost of increased fuel consumption.

In theory, one spark plug in the center of the head in a 4-valve cylinder is
the optimum placement for an ignition source. That's the theory, until you
think in three dimensional space + flame propagation. If you envision a
cylinder in three dimensions with the piston at top-dead-center (TDC) and a
somewhat domed cylinder head, having the single ignition source in the center
of that three dimensional space makes the most theoretical sense because the
flame front will theoretically travel as an expanding sphere throughout the
cylinder. By having the ignition source located on the upper wall, that being
the cylinder head, the flame front theoretically travels as a semi-sphere
starting at the top of the cylinder head and taking longer to travel
throughout the cylinder. That's the highly-simplified theory, because we know
that other factors are in play such as the "swirling" of the mix in the
cylinder, and so on.

That is why a projected tip spark plug may help -- It moves the ignition
source from the extreme top of the cylinder head to a location that is a
closer to the center of the three dimensional space of the cylinder at the
time of ignition (yeah, I know it doesn't happen exactly at TDC). And in a big
cylinder, and especially when there are only two cylinders in total trying to
average out all the slight irregularities to produce a smooth engine, little
changes can result in noticeable differences.

The second spark plug helps to produce a more though combustion, and in doing
so, it helps to produce more consistent bangs. To the rider, this consistency
results in an engine that idles smoother, accelerates smoother, and runs
smoother (i.e., no surging). The more thorough combustion results from having
multiple ignition sources that are separated spatially (unlike a dual
electrode plug that has two ignition sources very closely spaced).

The virtues of dual plugging, including in 4 valve engines, are well
documents, so this is not news. Rapid adpotion of dual plugging in the car and
motorcycle industry tells us it's a Good Thing. More significantly, you see
dual plugging being adopted by motorcycle makers who sport large-bore engines,
especially twins (Harley-Davidson and Ducati, just to name two aside from

If you have any doubts about how effective dual-plugging is on a boxer,
contact someone who has had the San Jose BMW Dual Plug upgrade. I have
dialoged with over a dozen of these folks, all of which have strongly positive
anecdotal evidence that says the upgrade is almost like getting a new engine.
Similarly, get one of these upgraded bikes at idle and disconnect the second
(upgraded) plugs. The SJBMW dual plug solution has the second set of plugs
always firing (i.e., they don't "kick in" at certain RPM or other conditions),
so if you disconnect them, you get an instant A/B comparison of what that
second set of plugs does for the Oilhead. And the difference, according to
owners, is "like night and day."

> I am sure he's happy to do the job esp. if he had a wrecked head to
> play with first.
> Like I said, if adding a second plug to a 4-valve head makes much
> difference, we must have chamber deficiencies or something.

Hopefully my earlier explanation helps to clarify things. In theory (and when
considering only specific elements), a single ignition source it the center of
a four-valve head is the optimal ignition source. But as the saying goes, "in
theory, there is no difference between theory and practice, but in practice,
there is a world of difference between practice and theory." When you take
that single ignition source theory and put it into practice, you get different
results. Also, if you take that single ignition source theory and you take
other factors into consideration such as flame propagation, dual-plugging
makes sense even theoretically.

I am reluctant to conclude that "chamber deficiencies or something" else is to
blame for why a single plugged Oilhead is susceptible to surging and rough
running, because in actuality, many things come into play, including (but not
limited to) the exact voltage (to two decimal places) that your specific
oxygen sensor reports at a known oxygen level. That's right! Your Oilhead's
oxygen sensor is an analog component that feeds a variable voltage to your
fuel management computer. Subtle differences here, when combined with the
exact smoothness of your intake and exhast ports (turbulence and swirling),
and a host of other factors, can result in meaningful differences in the
engine's feel (smoothness).

> I'm far from wanting to tear down my heads. While pretty easy on an
> Airhead but looks like a lotta hours on an Oilhead. But it would likely
> help my grunt-range, I'd bet. I think Gord charged c$175 for my two
> Airhead heads but he's been adding plugs to Airheads for a long time.
> Gprd Bush in Malton, Ontario (Toronto area): 905-677-8657
> How is the electrical side handled? Just a matter of hiding one or two
> more coils? Nothing much more?

This I don't know for certain. My understanding, according to what was told to
me by San Jose BMW in a phone interview, is that their solution "replaces some
BMW ignition components to accommodate the extra set of plugs". I am guessing
the SJBMW kit has two more coils, lines, plug caps plus a replacement module
into which the spark plug cables tie in to get their juice.

- -Steve Makohin
 '01 R1100S/ABS
 Oakville, Ontario, Canada