Packages for Rotax Laydown Rave Motors
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 Recreational Engine Mods for Sea Doo 782/800“, 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.
Package / Upgrade / Parts
100 Octane – S/S I
110 Octane – S/S II
Group K Price
“X” Denotes Mandatory – * Indicates prices subject to change per manufactures pricing.
100 Octane – S/S I
110 Octane – S/S II
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
- 100 octane / 7100 – 7300 peak
- 110 octane / 7300 – 7600 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.
ORDER INFORMATION: SEND ALL PARTS REQUIRED FOR MODIFICATION VIA UPS TO:
GROUP K • 4597 CALLE DEL MEDIA • FORT MOHAVE, AZ. 86426 • (928) 763-7600
GETTING THE WORK DONE – Most customers send GROUP K the parts needed for modification via UPS, and then do the engine assembly work themselves. We also do complete engine and pump assemblies for customers who want a finished unit ready for installation. The 150-lb. UPS weight limit makes engine shipping practical and affordable. NOTE: Group K will bill an additional $25.00 handling charge for complete engine assemblies. All orders prepaid with a cashiers check or money order will be returned freight free via ups ground service anywhere in the continental United States. All other orders will be billed to a visa/master card or sent freight collect cod cash. If you would like to pay additional for 3 day, 2 day, or 1 day return shipment, please specify your preference in a cover letter with your parts. Be sure to include your return address and day phone information in case we have any questions regarding your order. PACK YOUR PARTS CAREFULLY !!