[HOME]
PREFACE - During the later part of 1993 and early 94, numerous performance shops nationwide started selling Yamaha stroker cranks and components. By late spring, thousands of Yamahas with these stroker parts were run for the first time. In short order, many stroker owners found that their $2000 - $6000 stroker was no faster than their limited boat. The phones of Group K were besieged by unhappy stroker owners looking for someone who would "fix" their slow strokers. These owners told us they had been "blown off" by the folks that sold them their stroker parts. Our testing, in 1993, led us to believe that there is no "reasonably priced" way to make a stroker run significantly faster than our 55 mph 701 Hammer kits. It was not practical to describe the technical reasons to each caller over the phone. Instead of wasting our technician's time on these lengthy and fruitless phone calls, we produced the following pamphlet that could be mailed. The sellers of stroker engines did not like the pamphlet (a lot), however to date none of these shops has offered to resolve slow stroker motors from other shops.
ADA of Phoenix AZ, has linked their site to this document in an effort to discredit the information. Their respondent document "The Truth About Strokers" makes many weak and half true observations. It's context is often adolescent and vaguely worded. While most of the points in their document don't dignify an intellectual response...a few do. Our responses to those points are at the end of our article here. Happy Reading. Harry Klemm
ABOUT YAMAHA STROKER ENGINES by the technicians of GROUP K
In 1993 the IJSBA set the displacement limit for
all modified classes at 785cc. The largest displacement Yamaha engine at the
time was the 81mm bore x 68mm stroke 701cc Waveblaster motor. Almost immediately
several performance shops started building versions of this 701 engine with an
increased stroke. The most widely used has been the 73mm stroke (5mm over stock)
that has resulted in the 82mm x 73mm (770.6cc). All these shops assumed that an
increased stroke motor (also called a "stroker") would obviously be
much more powerful and offer a significant competitive advantage.
The unfortunate reality, however, has been that the strokers are only
sometimes better than a well prepared 701cc standard stroke engine. This is not
because the builders of these 770cc stroker engines are incompetent or
inexperienced. It's because the 701cc Yamaha engines lend themselves very badly
to this modification.
At Group K we had many happy 701cc Hammer kit customers who requesting that
we build them a 770 stroker. So we tested at length with our own 770 stroker
prototype, in hopes of coming up with a competitive engine package to sell.
After months of testing, we realized that the 770 stroker concept was wrought
with many technical problems. We felt that it would be virtually impossible to
build production units for sale to our customers that would be significantly
better than their current 701cc Hammer kits. The following is an accounting of
the problems we encountered. Some problems were matters of excessive expense,
and others were a matter of defying the laws of physics...at Group K we avoid
both of these.
THE THEORETICAL ADVANTAGES - From a theoretical standpoint, the
displacement increase of the 770 stroker engine should have one fundamental
advantage. That is, increased torque output through out the entire rpm range.
This theoretically happens because of a larger volume of gases being involved
with each power stroke, as well as a slightly longer power stroke. Understand
that with regards to a two cycle engine, the "actual power stroke"
refers only to the distance from top dead center to the opening of the exhaust
port.
Along with these advantages, each engine builder understands that there is
one premier disadvantage of a stroker...reduced peak rpm abilities. It is the
hope of each engine builder that the trade off of increased torque vs. decreased
rpm ability will eventually favor the stroker engine. Unfortunately for the 701
Yamaha engine, other pre existing design problems make the positive side of that
trade off very difficult to realize. Understand that for any engine package to
be considered truly "better", it must offer a significant power output
advantage along with a maintenance cost that owners can afford to live with.
Engines with short fuses do not qualify.
REALITY - This is much easier for the layman to grasp. In the
real world, the only thing that matters is how many rpm can a strong
accelerating engine turn a given prop pitch and nozzle diameter. In the August
1994 issue of Watercraft World Magazine, Group K prepared a 701cc Super Jet as a
Limited, and then a "105 octane Hammer kit". Using the stock nozzle
and a Skat Trak 10-18 variable pitch prop, the Limited ran 49.3mph @ 7000 rpm
and the Hammer engine kit (at $1732) ran 54.5 mph @ 7300 rpm. Our tests
confirmed that Wave Blasters with these same engine setups will net the same
peak rpms. However, the Blasters equipped with these kits and the stock 15-18
Blaster prop, runs about a half mph slower (a result of the Blaster's additional
hull drag). The next steeper prop for both machines is an 11-19 pitch. The
Hammer kit and the Limited both lose an unacceptable amount of low end with the
11-19 pitch prop.
It would seem obvious that the 770 strokers would easily pull the 11-19,
right? Not exactly. During all of our months of 770 stroker testing, our
prototypes were never able to pull an 11-19 beyond 6900 rpm (51 - 52 mph). On
testing done with numerous "over the counter" 770 strokers, we have
found similar results. If 770 stroker engines could effectively pull an 11-19
into the 7300 rpm range, the makers of these engines would recommend that
prop...THEY DO NOT! In fact page 34 of the October 1994 Splash magazine states
that the Pro Tec 770 stoker engine (at $5750) will only turn a 10-18 prop to
7100 rpm. Being generous, that would drive a Super Jet hull to the 51 - 52 mph
range.
At Group K we are not surprised. Our best 770 prototypes never exceeded 7200
rpms and never pulled an 11-19 with good overall acceleration. However we are
surprised by the comparison of a $1732 / 7350 rpm engine package and a $5750 /
7100 rpm engine package.
It bears noting, that the owner of the Group K magazine boat used it to win
the Busch Cup novice vet championship. He got nearly every hole shot, and
reported never once being out powered by any Super Jet. Many of his regular
competitors had fuel injected 770 strokers.
The April and May '95 issues of Personal Watercraft Illustrated featured the
construction of a project 770 Blaster stroker. The finished machine, built from
over the counter modification parts, cost over $9000 to build...and ran only 52
mph. As many Blaster riders know, a good limited can run 52 mph. This Blaster
used a 17-23 prop, which would have put the peak rpm at about 6800. The editors
and technicians involved with this project prepared the machine correctly. They
were just forced to learn what many stroker owners already know...the results
don't always match the money and the hype.
In all fairness, many 770 strokers (our prototypes included) have the ability
to "leave harder" than a 701 Hammer. "Leaving hard" refers
to the acceleration ability from an (IJSBA type) standing start. This type of
start puts a tremendous load on the engine, since accelerating from low speed
and "coming up on plane" are happening at the same time. On small
Super Jet hulls this is not a big problem, however on the larger
"sport" and "runabout" hulls, it can be. After our months of
770 testing, we simply couldn't justify an additional $4000 - $6000 expense to
our customers just to make a race boat "leave a little harder".
THE THEORETICAL PROBLEMS OF THE 770cc STROKER
AVERAGE PISTON SPEED - For each
engine there is a theoretically "safe" maximum rpm. This theoretical
peak is measured in the average feet per minute that the piston travels. However
engineers and engine builders refer to this limit in "feet per minute of
average piston speed", not rpms. They have found that exceeding an average
piston speed of 4000 feet per minute, on high output racing engines, is very
risky business. There is a formula used to calculate the rpm that equals 4000
fpm. The primary variable of this formula is the engine's stroke. Longer stroke
engines have higher average piston speeds, and therefore, lower rpm thresholds.
When an average piston speed of 4000 fpm is exceeded, the risk of component
stress failures increases almost exponentially. Failures including, but not
limited to, broken crankshafts or connecting rods, separating pistons, breaking
rings, disintegrating lower end bearings etc. Operating an engine at 4000 fpm
stresses all the moving pieces to their limit. Race engines that are run at 4000
fpm revs have component replacement intervals rated in operating minutes...not
hours.
An added complication for PWC race engines is that they have direct
drivelines with no transmission device. For this reason, pwc race motors run
significantly higher compression ratios compared to most other high output
racing engines. These higher ratios are needed to maintain strong low speed
acceleration characteristics in the absence of a transmission. Unfortunately
these higher ratios also reduce the "safe" average piston speed by
200-300 fpm. Another complication is the relatively high percentage of "not
hooked up - non cooled operating time". When the pump of a race boat comes
out of the water, it not only permits much higher rpm, but it also ceases
delivering the badly needed coolant to the engine. The added stresses of
unloaded rpm, and the higher temperatures of intermittent coolant supply further
diminish the "safe" peak.
PWC engine builders understand that the 4000 fpm average piston speeds that
work for tuners in other motor sports has been pared down to 3500 fpm
neighborhood for their PWC race engines. They also understand that 3500 fpm is
the high stress limit. If they want any kind of reliability, they must choose a
lower limit. The following is a listing of the average piston speeds of the 68mm
std stroke Yamaha 701cc engine vs. the 73mm stroke 770cc aftermarket engines.
------------------------------7600 rpm -------7400 rpm ----------7100 rpm
-----------6800 rpm
68mm stroke --------------3389 fpm-------- 3300 fpm ---------3166 fpm
-----------3032 fpm
73mm stroke --------------3640 fpm --------3544 fpm ---------3400
fpm----------- 3257 fpm
It's easy to see that the 68mm stoke engine has a much bigger margin of
safety away from the 3500 fpm limit. This bigger margin would not be a big deal
if the 770 strokers could pull an 11-19 or 12-20 prop. The ability to pull the
steeper props might eliminate the need for turning allot of revs. However as
previously noted, over the counter 770's cannot pull these props.
CONNECTING ROD LENGTH - The connecting rod is the primary piece
responsible for converting the reciprocating motion of the piston into the
rotational motion of the crankshaft. There will always be a certain amount of
energy lost in this motion conversion, but additional rod length can help to
minimize that loss. When a high output engine has the stroke increased, rod
length is an important issue that shouldn't be ignored.
Stroke increases are accomplished by moving the crank pin location away from
the center of the crankshaft. A side effect of this modification is that the
connecting rod angle at 90° btdc and 90° atdc becomes steeper. This steeper
connecting rod angle means that the rod will be pushing the piston harder
against the cylinder bore on both the downward power stroke and the upward
compression stroke. This extra piston pressure against the cylinder wall results
in more rapid piston and bore wear, not to mention additional heat from the
surface friction.
An easy way to reduce these problems is to simply use a longer connecting
rod. The longer rod can significantly reduce the connecting rod angle, and the
wear that comes with it. Unfortunately, none of the 770 strokers currently being
built for sale are fitted with longer rods. Installing longer rods on the 770's
would require the use of a very thick spacer plate between the cases and the
cylinder. Most engine builders avoid using a spacer plate like this because it
significantly increases the crankcase volume. Increasing the crankcase volume
reduces the crank case compression ratio. High crank case compression ratios are
needed to rapidly deliver the raw gases, in the lower end, through the transfer
ports into the combustion chamber.
MAXIMUM PISTON SPEED - Another disadvantage of using the ,
relatively short, stock 701 rods on a 770 stroker is the terrific increase in
the "actual maximum" piston speed. Calculating the average piston
speed, as shown above, is an effective reference tool for engine builders.
However it only indicates an "average speed" not the "actual
speed". In reality, the piston is stopped at bottom dead center before it
accelerates rapidly upward. The piston then accelerates to it's maximum speed,
which takes place when the crank pin is 90° from bdc. As the piston passes the
90° position and approaches tdc, it begins decelerating rapidly until it comes
to a full stop at tdc. While the average piston speed may be calculated at 4000
fpm, the actual maximum piston speed at 90° past tdc is about 10,000 fpm (or
about 120 mph).
Coincidentally, this maximum piston speed and the maximum rod angle take
place at exactly the same moment. This particular moment, where piston speed is
at it's highest, is the worst possible moment for the additional friction of a
steep rod angle. A longer connecting rod would not only reduce the rod angle,
but at the same time, it would actually result in a slower "maximum piston
speed" at the moment of 90° btdc and 90° atdc. This reduced "maximum
piston speed" would not only result in less surface friction between the
piston and cylinder bore, but it would also result in significantly longer
piston ring life and ring sealing.
It bears noting that the 74mm stroke Kawasaki 750 engines are equipped with
significantly longer rods than the stock Yamaha 701 rods. These longer rods in
the Kawasaki are one of the design features that permits the 750 Kawasaki race
engines to rev reliably into the 7500 rpm range (that's 3637 fpm average piston
speed).
CYLINDER PORT LAYOUT - When the stroke of an existing engine is
increased, the port timings automatically become more advanced. That is, the
port timings are actually increased by sheer virtue of the changed connecting
rod geometry. This automatic increase in port timing can be a benefit on
cylinders with uncommonly mild port timings (like the 633cc Yamaha cylinders).
It just means that you have to do that much less grinding to arrive at the
proper port timing. However on cylinders that already have very aggressive port
timings, this automatic increase in port timing can easily put the stock port
heights beyond the desired range. This is the unfortunate case with the 701
Blaster and Wave Raider cylinders. A 770 stroker with a stock 701 Yamaha
cylinder has a more radical exhaust port timing than the 7300 rpm Group K 105
octane Hammer kit. That is to say, it has allot more port timing than it needs.
If the exhaust port on the 701 cylinder could somehow be lowered a few
millimeters, it would end up with the same port timing as the Hammer kit
cylinder. However the real benefit of this lowered exhaust port, would be a much
desired increase in the length of the power stroke. (remember that the longer
power stroke is one of the theoretical objectives of building a stroker in the
first place) Since lowering the exhaust port of the 701 (or Raider) cylinder is
not do-able, the stroker engine builder must choose between having a shorter
than optimum power stroke, or starting out with the much milder ported 633
cylinder. While the bored out 633 cylinder can be cut to ideal port timings,
engine builders who choose this cylinder will unfortunately get their fair share
of problems as well. The 633 cylinder casting has very little spare material
around the exhaust port. So little, in fact, that the exhaust port cannot even
be made as wide as a stock 701 exhaust port. This is much less than the optimum
width that a 770cc race motor would demand.
DECK HEIGHT - If dealing with the port layout problems wasn't
enough, there is the added problem of the piston that now pokes out of the top
of the cylinder at tdc. To say the least, this is not a desirable arrangement.
The only option is to put a spacer or thicker base gasket between the crankcases
and the cylinder. Unfortunately this will again increase the port timings and
shorten the power stroke. A spacer that is thick enough to get the piston crown
back down into the cylinder, is unfortunately also thick enough to put the port
timings of the mild 633 cylinder at the raged edge of acceptable. This same
spacer (between 1.5 - 2.5 mm) will put the port timings of a stock 701 cylinder
into the twilight zone. Under other high output circumstances, port timings this
radical might be acceptable. However we must remember that the theoretical
advantage of a stroker is increased overall torque. It's difficult to accomplish
that with road racing style port timings.
Perhaps the most effective way to deal with all of these problems would be to
make a new 770 cylinder from scratch. Such a cylinder could have ports located
at the right height, as well as the additional deck height to accommodate the
added piston stroke and some longer connecting rods. Unfortunately a special
purpose cylinder like this would further increase the price of an already too
expensive engine.
It was at this point in our 770 stroker development project that we realized
the 770 design had allot of problems...big problems. Furthermore, all the
available solutions had equally big problems of their own. From this point we
realized that any "over the counter" 770 stroker engine, made by Group
K or any one else, was going to contain a collection of very poor technical
choices. It appeared unlikely to us that such a race engine could be
reliable...much less competitive. We have not yet seen any over the counter
770's that can run 55 mph in a Super Jet or Blaster hull. All the over the
counter 770's we have seen are slower, less reliable, and much more expensive
than our existing 701 Hammer kits. As a result we do not (and will not) sell any
770 engines or components.
ABOUT THE NEW 1996 753 cc RAIDER MOTORS - These new 84 x 68mm
top ends offer an interesting option for the 785 class racers. It's very likely
that this new top end can be adapted to most existing 701cc lower ends. However,
more importantly, with a stroke increase of only 2mm they will yield a 775cc
displacement. This 775, bored to the 84.5mm 1st oversize piston will yield a
limit pushing 784.7cc. A Yamaha engine with only 2mm of stroke increase would
not experience many of the technical problems outlined in this pamphlet. This
shorter stroke will make the 84 x 70mm layout would suffer from fewer design
problems and be potentially more reliable than any of the current 82 x 73mm or
82 x 74mm strokers. While this is speculation now, only testing during the
winter of '95 will tell for sure.
FINAL THOUGHTS AND THE FINAL NUMBERS - The obvious question
here is " How do 82 x 73mm Yamaha 770 stroker motors dominate the pro stand
up and sport classes, if they only turn a 10-18 prop at 7100 rpm as claimed
(theoretically 52 mph?) The answer is...the winning pro boats either turn allot
more than 7100 rpm, and/or they are using a very different prop. Having an
uncommonly talented and lightweight rider certainly wouldn't hurt. However,
rider talents aside, there is no doubt that the winning pro 770 strokers on the
IJSBA tour are probably capable of 55 - 57 mph. To do so, they would need to
turn an 11-19 prop at about 7300 rpm or a 10-18 at about 7500 rpm. In either
case, any 770 stroker capable of this output would likely require frequent
replacement of all internal moving parts (crankshafts, pistons, rings, etc.)
While this kind of maintenance might be affordable for a fully sponsored pro
racer, it would obviously be out of the question for a reliability minded local
level racer or weekend warrior.
At Group K we intent to continue development of our existing 701cc Hammer
kits. We expect that we will be able slightly increase the rpms into the 7400
range. The resulting speeds (in Super Jet and Blaster hulls) should be in the
55-56 mph range. The Wave Raider hulls will likely run in the 62 mph range.
These performance numbers may not be quite as good as the fully sponsored IJSBA
national championship 770's. However, the 701 Hammer kit performance is still
much better than the output numbers of any over the counter 770's being sold to
the public. Even if new cylinders and longer rods are produced for the new
generation of 770's, they will be unable to hold a candle to the price value of
a 701 Hammer. That is...unless the laws of physics change in the near future.
"Some Additional Truth That ADA of Phoenix Should Consider"
Linking onto someone else's site, for the purpose
of slamming it (as ADA has done), is really "asking for it"...in this
case we are only too happy to oblige.
Item 4 of the ADA document discusses the limits of average piston speed. The
ADA technicians should understand that engineers use average piston speed as a
barometer for "all" the stress loads taking place with "all"
the moving parts...not just the piston itself. While metallurgical advances in
the last 20 years have certainly yielded better pistons, there has been little
change in the physics of "skidding" rod bearings and excessive
connecting rod angle. The 4000 fpm limit, cited in the 1973 Gordon Jennings
book, IS NOT the "safe " maximum limit that ADA claims. 4000 fpm is
the maximum piston speed at which the imminent failure of moving parts can still
be "predicted". That is, you know that the crank will fail after so
many operating minutes, and that the pistons will break if not replaced after
every "x" number of operating minutes. All the road racing engines
Gordon worked with were maintained in this fashion. Frequently exceeding 4000
fpm means a crank could last 2 hours or 2 minutes. ADA should also understand
that while Gordon Jennings is very intelligent and experienced, the road racing
engines he worked with never EVER "flashed-unloaded" to rpms well in
excess of the engine's peak horsepower output (as all pwc race engines do
constantly). Had this been the case, Gordon would have also recommended a buffer
zone for anyone wishing to have "predictable" reliability of moving
engine parts.
With a perspective toward the bigger picture, we would like to remind the
technicians of ADA what the owners of high performance PWC (on planet Earth) are
actually interested in. They want to go as fast as possible, for the least
possible cost, and have the maximum possible reliability. Furthermore they want
an accurate assessment of "exactly" how much speed they will get for
"exactly" how much money. All of the Group K literature, and
development work, is based on meeting these needs. We urge the web site builders
at ADA to wake up to this reality. They would better spend their time posting
specific price and performance information about "their" engines on
their website. We think it's pretty silly for them to launch a website by taking
pot shots at a company that sells reliable engines at 1/4 the price of their
own.
For folks that don't already know, a complete ADA stroker motor sells for
about $8000. We look forward to the ADA site showing comparisons between their
$8000 stroker engines and the racing strokers sold by other shops (Riva,
Pro-Tec, Power Factory, etc.)
As a footnote, we too encourage all Group K Hammer owners to go to ADA and
take the Hammer challenge outlined in their website. However if you want your
Hammer "upgraded" to an ADA stroker, you should bring about $8000 with
you and prepare to have your boat run 2 - 3 mph faster...what a bargain!