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This document will provide technical
background and performance information about our various closed course and
offshore racing engine kits for Rotax 782cc and 787cc engines. It will also
outline why we considered some modifications to be effective...and others not.
Since the development of racing engines is an on going process, there may be
some changes to this information as time goes on. However, we expect that those
changes will be relatively few. During 1997, we will maintain a document on our
web site that will carry all the latest updates regarding these engine kits.
The information in the document will focus mainly on the XP model with
occasional references to the heavier GSX. We have not pursued race engine
development in the GTX or 97 XP hulls. While both of these machines have many
good competitive qualities (particularly in endurance racing), their length and
weight make them unlikely 785 class contenders. If you have not already read our
document "Group K engine mods for Sea Doo 782/800 Laydown Rave
Engines", we urge you to do so. It carries much basic information that is
of interest. This document should be considered an update supplement to the
first one.
ABOUT BMEP - Through out this document we will make references
to bmep, rather than "horsepower" or "torque".
"Torque" infers only low range power, and" horsepower infers only
high rpm power. We consider bmep to be a more descriptive overall term. BMEP is
the abbreviation for "brake mean effective pressure". It is a pivotal
part of the following equation...BMEP x RPM = Horsepower. There is no on-board
meter that can indicate the bmep number. You figure out the bmep number by first
learning the rpm and horsepower numbers from a dyno. By applying those numbers
into the formula, you figure the bmep numbers. Engine builders use the term bmep
because it describes the sheer efficiency of an engine "at a particular
rpm". In physical terms, bmep is literally the pressure, or force, that
pushes the piston downward. Bmep should not be confused with "compression
ratio pressure"...they are not the same thing. Increases in bmep can be
made by a better flame arrestor, a better pipe, porting, ignition timing, etc.
Any modification that increases the horsepower output, without increasing rpm,
is said to have more bmep. Knowing an engine's exact bmep number is not as
important as knowing how the overall bmep is affected by various modifications.
THE XP PLATFORM - The average box stock XP 782 will run high
56's to low 57's (mph) @ 6680 - 6750 rpm. These performance numbers have allowed
stock XPs to virtually dominate local level 785 closed course events. While the
rpm numbers of these engines is not very high (compared to modified Yamahas and
Kawasakis @ about 7300 & 7500 respectively), the XPs bmep numbers through
the entire powerband are extremely impressive. In a perfect world, engine
builders would like to build Laydown Rave race engines that keep all those stock
bmep numbers, and carry that same bmep trend up into the 7300 - 7500 rpm range.
To date, no one (including us) has done that. The closest thing to that type of
engine (that we have seen) is our own Sleeper kit, which increases rpm to 7050.
At the same time, the Sleeper kit can slightly increases bmep numbers through
the entire range. However "ALL" the modifications we tested on the XP
(stockers and Sleepers) that drove rpms beyond 7100, caused varying losses of
bmep in the bottom end and mid range. This is a bad trade off that often makes
many modifications for the 782 rave engine appear to make more power on the
dyno, yet slower laps around a track.
Despite this obvious bad trade off, race engine builders seem to be forced to
spin the 785 Rave motor into the 7300 - 7600 rpm range in order to get
significant peak horsepower numbers. Getting the engine to turn those rpms is
not difficult. However, getting a 7500 rpm engine to have strong bottom end and
midrange...that is difficult. Furthermore, the overall bmep numbers that come
with 7500 rpms are often so low that there is a questionable increase in total
horsepower. Many riders have experienced this when installing a high rev pipe
that must be used with a milder pitch prop. The milder pitch is needed because
the bmep numbers throughout the range have been reduced...not increased.
THE CURRENT "IJSBA TOUR" ENGINE FORMATS - While there
were several different 785 Sea Doo team efforts, most of them gravitated toward
the same modification format. This is an overview of that format. The top
priorities appeared to be rpms in the 7500 - 7800 range, compression in the 200+
psi range, 46+ mm carburetors, and the reduction of rotating weight.
Running rpms consistently over 7500 would require an exhaust pipe that, among
other things, is much shorter in overall length. These pipes typically cause a
measurable loss in the low range bmep numbers that are so badly needed for
strong starts. In virtually every case, ECWI systems (electronically controlled
water injection) was used to bolster up the low end power range (ecwi
dramatically improves acceleration in the 3500 - 6500 rpm range only). The ecwi
acts as a very effective "band aid" to mask poor low speed power.
Unfortunately if a component of this ecwi system failed during a race, low speed
power was immediately affected. Despite all these advances with the exhaust
system, racers were still clamoring for even more acceleration. The remaining
places to go for this acceleration were the reduction of rotating mass, and
increased compression.
ROTATING MASS - If your engine has very poor low end power, you
can minimize your disadvantage by lightening up all the rotating parts in the
engine. This does not improve low rpm horsepower, it simply allows the engine to
get past your weak low rpm range in a shorter amount of time. The 782cc Rave has
plenty of rotating mass to pick at. To begin with, the stock ignition flywheel
is replaced with an ultra light aluminum total loss unit, and the pto at the
rear of the crank is cut down to nearly drive shaft diameter.
In time, however, these reductions were not enough. The still desperate
tuners eliminated the counter balance as well. They considered this necessary
anyway, since the 7700+ rpm's was destroying the balancer bearings and/or
breaking the balancer shaft. With the balancer removed, the entire boat vibrates
so violently that it's not ridable. To eliminate this vibration, the crankshafts
are then balanced with heavy metal plugs pressed into the flywheels. In truth
this does not actually balance the crank. You can't balance the cylinders of a
twin to act as counter weights for each other. You must select a "balance
factor" for the flywheels. This means that you can balance the crank to run
smoothly in one rpm range...but it will shake like hell in another range. This
regular and varying shaking requires that all nuts and bolts be checked
often...very often. The vibration of these non counter balanced motors would
make it nearly impossible for a rider to finish an average endurance race.
However in closed course there is enough time in between runs to keep the boat
together.
COMPRESSION - In an effort to escape the complications of
"no counter balancer vibration", some builders simply went for high
compression...very high. Normally, the engine builder's rule of thumb is
"run the lowest possible compression that yields acceptable low end power.
Unfortunately, the fundamental importance of launching quickly away from the
"IJSBA standing start" had left these builders with no "soft
compression" options. Cranking compression around 200 psi (valve up) became
common fare. The combination of 7600+ rpm, and this very high compression could
often cause meltdowns even when race gas was used.
INLET SYSTEMS - Right from the beginning, everyone building the
785 Rave motors appeared to be choosing a 46mm something (Buckshot, Novi,
Boswell, etc.) These carbs are very nicely crafted, and all look terrific on a
flow bench and a dynamometer. It was assumed that their huge cfm (cubic feet per
minute) abilities would be necessary to keep pace with the high rpms of these
engines. Unfortunately, along with the big cfm numbers comes low inlet tract air
speed at low throttle openings. The high inlet tract airspeed of the stock 40mm
carbs is, in large part, responsible for the strong and instant bottom end power
of a stock XP. High inlet tract air speeds deliver fuel more
"instantly" when the throttle is cracked open. Along with that, high
inlet airspeeds result in better fuel atomization. Reduced inlet tract air speed
can result in reduced bmep numbers in the low rpm range. With the larger 46mm
carbs, the lack of strong inlet tract air speed at low throttle openings can
make the low speed metering very temperamental. However, in the case of these
high revving race motors, the high compression and reductions in rotating weight
minimized the amount of low speed "stumbling" that these engines might
experience as a result of this poor airspeed.
THE GROUP K MODIFICATION SETS - All our racing kits are
intended to meet the specific needs of various levels of competition and budget.
They are as follows:
92 OCTANE SLEEPER, 7000 - 7050 rpm peak (stock prop, 85 - 87 mm
nozzle) 62 - 63 mph (96 XP)
This kit is ideally suited for offshore racing and entry level closed course
racing. Since the stock pipe is used, it offers improved bmep numbers throughout
the entire rpm range. The conservative peak rpm assures a combination of good
longevity along with reasonable fuel consumption (a plus in offshore racing).
The larger 87mm nozzle will offer strong closed course starts, but detract 1 mph
from smooth water peak speeds. Despite this, we consider this format to possess
the maximum safe peak rpm for reliable 92 octane pump gas operation.
100 OCTANE Super Stock I, 7150 - 7250 rpm (Solas Xo prop, 85 - 87mm
nozzle) 63 - 64 mph (96 XP)
While the peak speed is only slightly higher than the Sleeper kit, the
available increase in acceleration and standing start ability is monumental. The
larger 87mm nozzle on this format allows for outstanding overall starting and
acceleration with very competitive peak speeds. Using smaller nozzles can permit
improved speed and fuel range that might be needed for endurance racing. Besides
being the best dual application kit, it also offers the best in long term
reliability in a competitive closed course machine.
110 OCTANE Super Stock II, 7450 - 7550 rpm (Solas Xo prop, 85 - 87 mm
nozzle) 65 - 67 mph (96 XP)
This machine delivers impressive "IJSBA tour capable" starting,
acceleration, and peak speed abilities. The high rpms of this format require
regular maintenance, as well as regular "pre failure" replacement of
internal moving engine parts. With this in mind, we consider endurance racing of
this format to be extremely (if not prohibitively) expensive.
PRICES ("x" denotes mandatory) ------92 Sleeper --------100 oct
S/S I ------110 oct S/S II
Cylinder porting (competition finish)
-------------x--------------------x----------------------x-----400.00
Cylinder head
modification-------------------------x--------------------x----------------------
x------80.00
R&D flame arrestor----------------------------------x
--------------------x----------------------x----130.00*
Bore & rejet stock Mikuni carbs to 40 mm-------x---------------------x--------------------n/a
----180.00
Bore & rejet stock Mikuni carbs to 43 mm--------n/a-------------optional
------------------x-----390.00
Primer kit----------------------------------------------x--------------------x----------------------
x -----25.00
Dual fuel pump kit------------------------------not needed
----------optional-----------------x----- 94.00*
Case/inlet manifold porting (for 43mm carbs)----n/a --------------optional
----------------x--- 290.00
Engine teardown & re-assy for case porting
------------------------------------------------------300.00
Exhaust system plumbing kit-------------------------x ------------------n/a
------------------n/a----21.00
S/S pipe modification (to stock pipe) -------------n/a ------------------x
------------------n/a----180.00
FPP ECWI system w/cdi module------------ not needed ----------optional
--------------n/a----260.00*
FPP "Spec 2" pipe w/ ECWI & cdi module -----n/a------------------
n/a-------------------x-----850.00*
Jetinetics aluminum ring gear------------------optional
------------optional-----------------x------379.00*
PTO lighting modification------------------------optional
------------optional-----------------x-------60.00
R&D exit/steering nozzle kit---------------------optional
------------optional -----------optional---213.00*
Solas impeller---------------------------------------n/a
-------------"Xo" pitch-----------"Xo" pitch-210.00*
Pump
blueprinting-------------------------------optional-------------optional------------------x-----190.00
Worx scoopgrate--------------------------------optional-----------------x----------------------x-------98.00*
TOTALS (with no options)----------------------836.00 ------------1303.00 -----------------------3196.00
TOTALS (with all options)---------------------1776.00------------- 2999.00
----------------------3409.00
ABOUT THE GROUP K MODIFICATION FORMATS - The single component
of Group K formats, that separate them from all others, is the use of smaller
carburetor throats and inlet tracts. We consider this difference to be
fundamental to the performance of all our formats. Here is why.
The inlet port openings on the stock Rave crankcase each have the cross
sectional area equivalent to a 40mm circle. We suspect this is why we have
consistently seen such a positive result to our stock carb boring modification
(which makes the carbs a true 40mm). With this full length 40mm passage, the
inlet tract air speed (which promotes excellent bmep at all rpms) is at a
maximum for the cfm ability of the carb and port. Furthermore, these bored 40's
have given excellent overall results even on 7400 rpm race motors. We consider
their cfm abilities to be easily up to the job of feeding race motors at those
rpms. All our tests with 46mm carbs on stock cases (as so many Rave owners have
done) resulted in worse low speed response, along with no improvement in peak
rpm (over the bored 40's). This testing confirmed our original beliefs that a
46mm carb feeds a 40mm port questionably better than a 40mm carb can.
Our Super Stock II engine format utilizes the stock carbs bored to 43mm
instead of the more popular 46's. These 43mm carbs start and idle like a stock
boat, and offer seamless metering with violent acceleration at all rpms (up to
7600). Like most other engine builders, we enlarge the port openings on the
crankcases to allow for more inlet port area. However we believe that a good
inlet port passage shape is every bit as important as the increase in area.
After making the inlet ports as large as possible, with a passage shape that we
consider to be "good", the cross sectional area is equivalent to a
43mm diameter. We believe that further enlarging the port to a 46mm equivalent
would seriously compromise the efficiency of the port passages' shape, as well
as reduce inlet tract air speed. We contend that the end result of further port
enlargement (to a 46mm equivalent) would be a reduction in low range bmep
numbers, with no appreciable increase in the high range. We consider our 43mm
inlet to be the ideal balance of cfm and inlet tract air speed. No 46mm
arrangement we tested could match it's combined low and high range abilities.
It bears noting that the performance benefits of high inlet tract air speed
are very obvious during on water tests, yet difficult to assess during dyno
testing, and impossible to assess during flow bench testing. On the dyno and
flow bench, bigger almost always looks better. We suspect that the importance of
high inlet tract air speed has been inadvertently overlooked by many well
meaning engine builders who have done a lion share of their testing on a
dynamometer.
COMPRESSION - With the strong overall bmep numbers offered by
our inlet choices, we found no need for high compression ratios to preserve
bottom end power. All our kits require no more than 170 psi (valve up) to
deliver strong starts and overall acceleration. All our tests with higher
compression ratios resulted in slight losses in peak rpm, along with increased
temperatures and decreased engine life. We hear alot about Rave motors that push
200+ psi. We can not explain why compression ratios this high hurts the
performance of our engine formats...we just know that on-water tests
consistently confirm it.
EXHAUST - The stock 782 Rave exhaust system offers unmatched
bmep numbers in the low and midrange rpms. Unfortunately, it is incapable of
delivering good bmep numbers beyond 7100.
The exhaust aftermarket is bustling with various pipes for this engine. While
all of these pipes have very different design approaches and performance merits,
we tended to categorize them by their octane computability with our basic
Sleeper kit. That is, 92 octane pump gas, 100 octane (avgas or 50/50 92 pump/110
octane race), and pure 110 octane race formats. As expressed elsewhere in our
Sea Doo documents, we have generally connected these octane categories to peak
rpm ability. In short they are:
92 octane / 6900 - 7100 peak All the aftermarket pipes we tested worked very well, in one way or another.
The common theme among them was they nearly all of them had been developed
around high revving limited (race gas) formats, or they yielded their best
performance when propped down to allow the higher rpms that normally mandate
race gas. We figure that if your going to have to run expensive race gas, use
the pipe that yields the highest rpm abilities...that would be the FPP Spec 2
pipe. We will grant that the 7400 - 7500+ rpm's allowed by this Spec2 pipe would
mandate periodical bearing maintenance teardowns. However that's no big deal for
someone building an all out race motor.
Unfortunately, most of the customers we talk to are not interested in race
gas formats. The overwhelming majority of customers are interested in the pipe
that offers the highest horsepower output, while running reliably on 92 octane
fuel. This is an area that we have focused much testing towards. However the
ever changing "quality" of 92 octane pump gas, has kept us from making
any such recommendation (to date).
Given the lack of progress on this front, we will focus our attention on the
best pipe combination that can survive (reliably) on 92 octane that has be
"octane enhanced" with either octane booster or various mixes of
aviation fuel. These would essentially qualify as S/S 1 type engine formats. We
will post the latest progress in this area on our "Sea Doo Updates"
document on our website.
IGNITION - All our kits utilize the stock stator and mpem
electrics. The S/S I and S/S II both get an adjustable rev module added to the
electrical box. This module plugs into the stock electrics to offer a variation
of rev limits and advance settings (although we found the stock timing to work
best on all our formats). If you are considering the ECWI for your S/S I kit,
you will need to get the module that is designed to interface with it. The FPP
Spec 2 pipe is supplied with the cdi module and ECWI.
The Jetinetics aluminum ring gear offers a very noticeable improvement in
overall acceleration and throttle response (and it's IJSBA super stock legal).
We recommend it for all applications.
BALANCER - We consider the removal of the stock counter
balancer to be a technically bad idea. We don't recommend it...and we won't
prepare any 785 Rave engines to operate without it. Our engine formats generate
excellent low range power with the balancer still in place.
PUMPS - Anyone who plans to race a 785 Rave ought to plan to
have a good selection of different diameter exit nozzles from 84mm to 87mm (in 1
mm increments). These nozzles will be your "gearing" so to speak. No
one diameter will be ideal for all competition applications.
While the Solas impellers we have chosen to recommend don't have the best
possible rough water hook up, they are darn close. The good overall performance
of these props, along with the well incremented pitches, made them an ideal
choice for our testing. Since the impeller makers always have something new, we
will constantly be watching for any new items that will suit these formats. For
now...this is it.
SCOOP GRATES - We recommend the top loader made by Worx Racing
(Anaheim, CA) It offers "good" rough water hook up that comes along
with a noticeable increase in peak speeds (on rough and smooth water). We
consider this grate appropriate for all recreational riding and racing
conditions.
ABOUT FUEL OCTANE RECOMMENDATIONS - We receive countless phone
calls from owners who want to engineer a way to buy a reliable race gas style
engine format that they can run on cheaper gasoline. If we though there was a
way to accomplish this...we would have already said so. We occasionally hear
from customers who have buddies that have run their race gas motors on pump gas
with no apparent difficulties. We think that's great...surprising, but great.
Despite all this, our fuel octane recommendations are
"non-negotiable". If a customer decides to play "cheaper
gas" Russian roulette with his or her engine...that's his or her business.
We will not change our recommendations.
100 octane / 7100 - 7300 peak
110 octane / 7300 - 7600 peak.