Sea Doo 785 – Racing Engine Mods

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.