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Every technician that has any pwc experience has
seen it. The engine whose rear piston has scored (seized) for no diagnosable
reason (while the other piston(s) look perfect). subsequent teardown inspections
out rule air leaks, fuel and coolant blocks, oil, ignition, etc. At Group K we
believe that "nothing" happens for no apparent reason. However in the
last decade of working on pwc engines, we have seen numerous seized rear pistons
that we could not account for. This document makes an effort to account for an
awful lot of the ones we've seen.
SOME IMPORTANT EXPERIENCES - Among Sea Doo race engine
builders, richer jetting in the rear cylinder (to avert piston seizure) has been
standard fare from the 580 cc days. We remember speaking with some of those
technicians about the apparent need for "uneven jetting". They all
told us that identical front and rear jetting in their Rotax's was a near
guaranteed rear piston seizure. We argued that none of our 650 Waverunner's or
Kawasaki SS twins had ever required uneven jetting. We felt certain that they
had some other technical problem that they were overlooking. We continued to
believe that, until the day we started testing our first Yamaha 701 Raider.
Our Sleeper kit development for the 701 Raider was relatively uneventful.
However when we installed the Coffman pipe that drove rpms to 7100 (from the
stock 6400), we began to experience a rash of scored rear pistons. We realized
that we were probably facing the same gremlin that our Sea Doo pals had been
living with for years. We were bound and determined that we would be the guys
that would solve the problem. We believed that we would show that uneven jetting
would not be needed. We then experimented with milder compression, retarded
timing, richer overall jetting, dual cooling with additional flow for the rear
cylinder...and plenty more. Three weeks, and a dozen seized rear pistons later,
we finally had a strong running format that could run wide open for almost a
full minute...before it seized piston number 13. We finally broke down and put
one size larger main jet in the rear carb. With this richer jet, the machine
still accelerated strong, and reached peak rpm quickly. We ran it full speed for
over 15 miles...we couldn't make it seize (or foul a plug). In fact, to our
surprise, later tear downs would show not even the slightest visual signs of
over richness or fouling. Shortly there after, we repeated this same frustrating
scenario with the (then new) Yamaha 1100 triples. Only the rear cylinder desired
richer jetting.
Armed with this experience, we revisited our earlier model stand up and
runabout dual carb engine kits. Remarkably, none showed the need for uneven
jetting. Even more remarkable was that an identically modified 701 Raider motor,
mounted in a stand up hull, did not require richer rear cylinder jetting. The
stand up, with the Raider motor could run wide open all day long on
"parallel" jetting. When that same motor, with it's parallel jetting,
was then installed into a Raider hull...immediate rear piston seizure.
As all this went on, we realized we were creating far more questions than
answers. For the meantime, we simply accepted our experiences at face value, and
used parallel jetting when we could...uneven jetting when we had to.
During the beginning of our development on the 782 Laydown Rave motors, we
immediately started to experience rear cylinder scoring (that was staved off by
richer rear carb jetting). At that same time, we were lucky enough to be having
a conversation about this problem with Ross Liberty of Factory Pipe Products.
They were in the process of developing their pipe for the Laydown Rave in their
newly completed dyno facility. This dyno, and it's instrumentation, can tell
"everything" that's happening while the engine is under load. Ross
mentioned that they too were chasing the "richer rear cylinder"
gremlin. He said that they had eliminated every imaginable variable, and no
matter what they did, the combustion chamber temperatures were never identical
under full load. He said they suspected something related to the crankshaft
twisting. He suspected that this twisting was affecting the "sameness"
of ignition timing. We suspect that he is correct.
CRANKSHAFT TORSIONING "The Gremlin" - This
suggestion, by Factory Pipe, is what we consider to be the true cause of the
countless scored pistons we saw during our own tests. To understand it, one must
first understand the difference between twisting and torsioning. Crankshaft
twisting refers to a crank that has been rotationally jolted so hard that one of
the press fit connections rotates out of index...and stays that way. When a
crankshaft "twists", there is large and immediate loss in power that
comes along with a noticeable vibration. When a crankshaft "torsions",
there is a momentary rotational springing action...nothing comes out of index,
there is no vibration, there is no noticeable power loss. A welded crank cannot
"twist", but it can still "torsion". The total amount of
"torsioning" that takes place depends on the length and rigidity of
the crankshaft. The longer and less beefy...the worse the torsioning. What we
now refer to it as "crankshaft torsioning" (not twisting)...works like
this:
The front cylinder on all pwcs is the cylinder closest to the ignition rotor.
The amount of torsioning that can take place between the ignition flywheel and
the front crank pin is nearly non-existent. However, structurally speaking, the
rear crank pin is much "farther away" from the ignition flywheel. When
the pump (at speed) suddenly hooks up on some smooth water, the front cylinder
and the ignition flywheel have enough rotational momentum to torsion the
crankshaft over it's entire length. As this happens, the flywheel and front
cylinder can actually get 2 to 3 degrees of rotation ahead of the rear cylinder.
As this 2 to 3 degree torsioning rotation takes place, the front cylinder is
still getting perfectly timed ignition sparks. However the rear cylinder is
lagging slightly behind when it gets it's ignition spark. This means that the
rear cylinder is firing 2 to 3 degrees more advanced than the front cylinder.
Furthermore, this advanced timing is happening at the worst imaginable time...at
high loads and high rpms. As any engine builder can tell you, running 3 degrees
too much high rpm advance on a race engine is a guaranteed way to seize (or
hole) a piston.
While we don't have any iron clad proof that these presumed effects of
crankshaft torsioning are an absolute fact, we feel that have enough hands on
experiences and supporting information to consider it "a very probable
truth". Until some one with more insight and experience can come up with a
more probable truth...we'll consider crankshaft torsioning to be a reality.
WHY IS THIS SUCH A PROBLEM "ALL OF A SUDDEN"? -
Because late model pwcs have more power, more hull weight, and better turning
abilities than ever before. All these features load the drive train harder, and
increase the likelihood of crankshaft torsioning. Remember, the first machines
to consistently experience this were the 580 Sea Doo's. While those old 580s may
not be considered muscle boats, their engines generated lots of power per cc,
the pumps hooked up great, and the hull could hold turns at full speed. The
early runabouts from the other makers could do none of those things...hence they
never loaded the crank hard enough to induce torsioning. In this same vane, high
output stand-up boats are so lightweight, and so hard to keep hooked up, that
crank torsioning could barely take place. That's why our old "parallel
jetted" 701 Raider engine lived in the standup hull, yet seized the rear
piston in the runabout hull. The weight and constant hook up of the larger
Raider hull loaded the crank harder than any stand up hull ever could.
The apparent effects of crankshaft torsioning are likely among the reasons
that Rotax made such a quantum leap in crankshaft "beefiness" for
their 782 cc Laydown Rave engines.
OTHER SUPPORTING INFORMATION - Perhaps foremost in this area is
the current trends among the boat makers themselves. The '96 ZXi 1100 ignition
automatically retards the timing to the rear cylinder after engine temperatures
increase beyond a specified level. The '96 Yamaha Blaster II ignition fires 2
degrees retarded at all times, and the rear combustion chamber has considerably
less compression than the front. All the 1997 701 models, from Yamaha, also have
this "staggered" compression arrangement.
We also suspect that crankshaft torsioning was a consideration for the new
reed valve 950 Rotax engine. The rotary valve Rotax crankshafts are, by far, the
longest of the 2 cylinder pwc cranks. This length is needed to accommodate the
rotary valve diameter and related hardware. A 950 rotary valve twin would need
even more length, if the necessary larger rotary valve disc were used. A 950
rotary valve engine with 135 hp, and the large GSX hull, would have certainly
experienced unprecedented crank torsioning. The reed valve design would result
in a much shorter and stiffer crank, not to mention reduced lower end total
length and weight.
WHAT CAN YOU DO ABOUT TORSIONING - In a nut shell, anything
that will reduce the combustion chamber temperature of the rear cylinder. For
many engines, running slightly richer rear cylinder jetting is enough. Others,
with a more serious temperature problem need the richer jetting "and"
lower rear cylinder compression. Our testing, earlier this year, showed lower
rear cylinder compression to offer no appreciable loss in overall power, along
with a significant reduction in rear cylinder combustion chamber temperatures.
As a result, many 1996 Group K engine sets have been prepared with slightly
staggered compression ratios. (It bears noting that crank torsioning is a much
bigger issue for modified pump gas engines than engines running on race gas. Our
testing showed that many engines kits would overheat the rear cylinder on pump
gas, yet have much more even temperatures on 110 octane race gas. However $4 -
$5 a gallon gasoline is a very expensive solution.)
Of course, slightly retarding the ignition timing of the rear cylinder would
seem to be the smartest solution. However the electronics of such a device are
somewhat expensive and complex for an aftermarket approach. We are made to
understand that MSD (the Texas ignition makers) manufactured a few Yamaha total
loss racing ignitions, for one of the larger teams, that had an additional
"plus or minus 6 degree" adjustment plate for the rear cylinder
pickup. We suspect these plates were used to slightly retard the rear cylinder
firing of some "tour" engines that were dancing on the edge of the
reliability envelope.
In fourth coming year models, some boat makers may choose to drive the
ignition off the back of the crankshaft. A design like this could be effective,
but it would turn the rear area of the engine into "a very busy place"
However pwc engine compartments, as a whole, are certain to become "busier
places" anyway.
In time, we believe that all the boat makers will come up with their own
special way of dealing this problem on their stock boats. With each new years
machines making more power and more hook up, the phenomenon of crankshaft
torsioning on high performance pwcs will have to be dealt with somehow.