Sea Doo 785 – Recreational Engine Mods

All speeds and rpms are on glass water @ 500 ft. alt, 15% humidity, 100′ temp, 200 lb. rider, hot engine.

ABOUT THE STOCK BOATS – While The same driveline is used in all these machines, the “on water” abilities of the hulls vary greatly. As a result, identical modifications to all models will not yield identical performance changes. These overviews of the stock machines will help you to understand why.

95/96XP (aka 97/98 SPX) – Lightest, best turning, quickest, fastest. CONS : Very “skittish” at high speed on smooth water, hull can “hook” a turn sharp enough to lose rider, difficult to control at high speed in rough water, difficult to hook up in rough water, short fuel range.

GSX – Excellent turning control in smooth and rough water, excellent hook up at high speed in rough water, best fuel range. CONS: Extra weight and length make it slower that the 96 XP models.

97 XP – The most comfortable at high speed in rough water, very controllable “lean in” type turning. CONS: The “nose heaviness” and extra water contact surface area give this hull the weakest low speed acceleration and acceleration from turns, the long driveshaft system can develop weaknesses if used with high rpm engine arrangements.

GTX – Most stable and controllable at high speeds in rough water, good stability makes this the best machine for non-athletic riders. CONS: Long water contact surface reduces peak waterspeed ability, heavy weight of machine has the pump at the limit of it’s ability, frequent cavitation, pump limitations preclude the effective use of high rpm “racing type” engine arrangements.

ABOUT THE STOCK ENGINES – Sea Doo watercrafts have been fitted with several different Rotax engine designs. Of all these designs, the 782cc “laydown rave” style engines appear to be far and away the most durable (and reliable) of them all. This improved durability of the Laydown Rave engine (aka LR engine) allows for significant increases in horsepower…with nominal losses in reliability.

Virtually every pwc engine is designed so that the cylinders mount on from above, and the piston movement is completely vertical. The Rotax LR cylinders mount onto the side of the cases at about a 45° angle that makes them appear to be “laying down” on their side…hence the term laydown. These cylinders are fitted with a full time active valve that offers low exhaust port timing at low rpms, and higher exhaust port timings at high rpm. Sea Doo’s trade name for this valve is “Rave valve”…hence the term Rave. While valved cylinders have been common fare in high performance two cycle motorcycles for two decades, the Sea Doo is currently the only pwc that uses them in mainstream recreational boats.

ENGINE SPECIFICATION…A “MEDLEY” – From a specification standpoint, the LR motor has radical exhaust port and rotary valve timings that would normally be associated with a “poor low end” high rpm racing engine. However as most LR owners agree, the opposite is true. Every pwc engine builder has his own opinion about how this can happen…and why Rotax has chosen this specification approach. Our development work has led us believe that these specifications can only work this well when they are part of a specification “medley”. The port specs used on this engine would not work well on any other pwc engine…nor would the exhaust pipe specs, nor any other part. However these specifications, used collectively, are able yield a very exceptional end result.

While doing our LR engine development, we found that the engine responded poorly to “significant” single specification modifications. For example, most other pwc engines we’ve worked with responded very well to significant increases in compression alone. Not so, for the LR motor. Likewise, other pwc engines have offered very substantial overall increases in torque and rpm with somewhat dramatic port timing changes. Again…not so for the LR motor. After numerous modification tests, we found that the LR motor responded best to a collection of subtle specification changes. Once we figured out the need to accommodate this “specification medley”, the forward progress of development moved much quicker.

MODIFICATION OBJECTIVES AND LIMITATIONS – The aftermarket is full of shops that specialize in preparing Rotax engines for closed course racing. However virtually all the LR owners that contacted us seemed to be interested in endurance racing engine mods. The big difference is that endurance engines must operate at full rpm for extended periods of time. Furthermore they have to be stone reliable and good on gas consumption (relatively speaking). Making the modified engines safe to run, on 92 octane pump gas, is a very high priority. However, running a race engine wide open for 100 miles on pump gas is a tall order. In the end, we were unable to avoid the reality that while our Sleeper kit operates fine on 92 octane for normal use, the temperatures of extended full throttle use would mandate a 50% mixing of avgas or racing gas along with the normal 92 octane unleaded.

For the performance shops that prepare closed course racing machines, violent acceleration (for the starts and corners) is a priority that is equal to or greater than peak speed ability. Furthermore, they never need to sustain peak water speed for more than a few seconds. While we hoped to also accommodate these needs, sustained peak speed was “our” biggest priority. Our objective, from a horsepower standpoint, is to net the biggest possible increase in peak water speed without sacrificing “any” low end power. Maintaining and improving the low end power was not very hard, However overcoming the high temperatures of sustained peak rpm operation was a problem. Our test boats exhibited the performance losses associated with overheating (on 92 octane) whenever we exceeded 170 psi compression and/or 7100 rpm. Whatever we would accomplish would have to be done within these limits. Later tests on 110 octane fuel allowed safe operation with higher rpms, but not higher compression.

INLET SYSTEM – All the modification sets listed in this document utilize the stock 40 mm Mikuni carbs that have been modified by Group K. While these stock carbs are denoted as 40’s, they are actually 37.6 – 38 mm. Group K makes the carbs a true 40 mm and then re-jets them. We also use a primer instead of choke plates on all kits. (primer is included in the price of the Sleeper kit, but not the price of carb boring)

We have observed that many different arrangements of 44 and 46 Mikuni carbs are being offered throughout the aftermarket. On a heavily modified closed course race machine these large carbs may have some genuine merits. However on our recreational “specification medley” these larger carbs offered little advantage. We question the value of 44 &46 carbs on stock cased engines because the inlet openings in the crankcase itself are only 40 mm. We suspect this is why our 40mm modified stock carbs consistently offered better acceleration and throttle control than any other arrangement we tried. Page 38 of the December ’95 Splash magazine outlines a set of bored dual 44 carbs (@ $800) that offered a 50 rpm increase on a stock 1995 XP. Our carb boring and flame arrestor set (@ $330) offers an increase of 150 rpm on an otherwise totally stock ’96 XP. Even if some of the larger carb sets could offer comparable performance to our Group K modified 40’s, none of them can match the fuel consumption. For endurance competition (not to mention general recreational riding), fuel guzzling carbs are a serious competitive disadvantage. For customers dead set on larger carbs, we can bore the stock carbs to 43mm. However we would only recommend this for engines with case porting that enlarges the case opening to match. (The information and pricing for these mods are noted in our document “Sea Doo 785 Racing Engines”.)

FUEL SUPPLY – The fuel pump on the stock 38 Mikuni carbs is rated, by Mikuni, at 40 liters per hour (about 10 gallons). In truth, the pump on most new machines will do a little more than that. However, as time goes on, the “lph” ability of the pump can diminish slightly. This is not a problem for stock machines, but engines that operate at sustained rpms over 7100 can experience a fuel deficit that will cause them to literally “run out of gas” at full throttle. The end result of this fuel deficit is often a scored rear piston.

To attend to this problem, Mikuni offers a high volume fuel pump kit for the LR engines. We consider this pump to be mandatory for any engine that spins over 7100 rpm. For the lower revving Limited kit and Sleeper kit equipped engines, the auxiliary pump is not needed.

FLAME ARRESTORS – An aftermarket flame arrestor is a mandatory modification for all our engine sets. The arrestor that offered the best performance (per dollar) on our kits is the R & D arrestor. An honorable mention goes to the Evolution marine arrestor (about $100 more) which offered the same performance as the R&D along with a corrosion resistant anodized finish (important in salt water) and true air filtration. This filtration is a valuable feature for owners who transport their boats on long dusty drives.

CYLINDER HEADS – All our kits utilize a stock cylinder head that is modified. There are many aftermarket heads available for the LR engines. While many of them are very well made parts, none of them addressed any difficulties that we experienced during our testing.

A very important issue, related to the cylinder head of the LR engine, is the variations in deck height. All the LR cylinders we measured were very close in overall height specification (within .003″ from head to base gasket surface) However there is a variation in the actual deck height of the base gasket surface on the crankcases. To accommodate this variation, Rotax factory technicians use base gaskets of various thicknesses to obtain the correct “squish clearance (the distance between the piston crown and the cylinder head dome at top dead center). These base gaskets are available from your Sea Doo dealer in .004″ (.1mm) increments from .012″ -.036″ Each of these gaskets has a different number of holes stamped into the center area between the cylinders. Each hole represents .004” of thickness. The assembly instructions provided with our Sleeper kits offer an easy to follow procedure for finding the correct base gasket thickness for your particular engine.

CYLINDER PORTING – Making a significant improvement on the stock port layout is not at all easy. After much testing, our final port format does offer very noticeable gains in overall acceleration along with a slight gain in peak rpm. We suspect that the “more traditional yet unsuccessful” port formats that we tried in earlier tests did not yield better results because those specs disturbed too many other areas of the “specification medley”. It may happen that a “more traditional” port format could work well with a completely different design exhaust and inlet system. However, we had no interest in developing an entirely new specification medley around a port layout not compatible with the stock pipe or inlet systems.

Our cylinder porting is available in two different finish modes. The “competition finish” includes all the port shaping, specification work, and passageway final surface finishing. This final finishing work accounts for about 30% of the total time (and cost) yet contributes to only about 10% of the total performance gain. The “recreation finish” (used in the Sleeper kits) offers all the dimension and port shaping work, without the more costly finish work. The cost difference…$400 vs $280. Which ever finish you choose, we require the cylinders, along with the complete Rave valve assemblies mounted in them. All ported cylinders are also honed before return shipment.

EXHAUST SYSTEMS – Many makers of exhaust systems intend for their pipes to be used with a small grouping of other modifications…we think that’s okay. However we are only interested in exhaust components that yield results with our modifications. We do no pipe testing on stock machines, nor testing of other companies specification medleys. During our testing, we got the best results with the Factory Pipe Products “Spec 1” pipe with ecwi. While this pipe delivered great overall power, the rpm range it liked to operate in (around 7400) was a much too high for recreational applications. Customers that want the “max” in bottom end power can also add the cdi controlled water injection to this system. The water injection dramatically improves acceleration in the 3500-6500 rpm range. To date we have not tested waterboxes.

IGNITION – We use the stock ignition timing on all our kits. The stock ignition system works well on all formats that utilize the stock pipe and the stock prop. For customers using the stock pipe who wish a higher rpm limit, we recommend the Micro Touch rev limiter module. This rev module replacement offers several, easy to change, rpm limit options. This micro touch unit retains the stock ignition curve (which we find to be very desirable) For all of our modification kits, we recommend against the use of any ignition device that advances ignition timing, or any part of the ignition curve. Additional ignition advance will only result in extra heat, not extra power. (Note: Using a high rev limiter module “will not” increase the “on water” peak operating rpm of a Sleeper kit)

PUMP SETUPS – Determining the “best” pump setup is a little difficult because different riders have different preferences. Despite this we can make some general recommendations. All the Sleeper kits offer the best overall performance with the stock impeller. For owners looking for a god replacement for their stock prop, the best choice is the Solas “X1” pitch. For customers looking for a rough water impeller, the best choice is the Skat Trak 17/22° or 17/23° swirl. These prop hook up great in rough water, however they do cause a slight loss of peak waterspeed in smooth water conditions. The GSX and GTX models usually work best with the stock exit nozzles.

’96 XP riders interested in sheer peak water speed can use the (slightly smaller diameter) R&D 85/88 mm nozzle set. Even on the lightweight XP hull, these nozzles will cause a slight low speed acceleration loss, but it’s worth a “smooth water” peak speed increase 1 – 2 mph.

The 97 XP is the only Sea Doo 785 model for which we bore the stock exit nozzle to a larger diameter. The large wetted surface of this machine (during takeoff and heavy turning) make for much slower acceleration. After much testing, we found that a combination straight/taper boring modification to the stock exit nozzle was the most effective fix for this problem. As a result, this nozzle boring mod is a mandatory part of the 97 XP Sleeper kits.

For the higher revving Hammer 100 octane kits, the stock prop has a little too much pitch on all the laydown rave machines. We recommend the Solas “Xo” prop for the GSX, and the “Xo” prop with the 85/88mm nozzle set for the XP/SPX models.

RPM’s & OCTANE REQUIREMENTS – Owners should understand the correlation between peak rpm and fuel quality needs. Big increases in peak rpm will result in significant operating temperature increases. The best way to control temperatures is to use an octane that matches your modified engine’s output. Here are some guidelines. Marina gas should not be used for sustained rpms over 7000. A 50/50 mix of 92 octane /110 octane (or avgas) must be used for sustained rpms over 7150. 100% race gas is required for sustained operation over 7300 rpm.

OCTANE OPTIONS – Not many folks want to drive a long distance to spend $5.00 a gallon on race fuel for their recreational pwc. However anyone that has paid an expensive bill for the repair of a seized piston has considered it. The repair cost of a seized piston is the financial basis we will use to look at the cost effectiveness of various octane options. The average repair bill for a seized piston is about $350 for a twin cylinder pwc.

By far the most cost effective octane option is a 50/50 mix of avgas. A gallon of 100-octane avgas costs about 50 cents more per gallon than 92 octane unleaded. Based on a 10 gallon tank of gas, you could buy 140 tanks full of 50/50 100 octane avgas/92 pump gas for the same money it costs to repair one seized piston.

Another option is a 25% mix of 110 octane race gas with 92 pump. With race gas at $5 per gallon, you could buy 28 tanks of this mix to equal the repair of a seized piston.

Lots of owners ride in areas where they need to buy fuel at outlets on the water. These folks are more interested in octane “boosters” that they can carry along. Unfortunately no octane booster can turn 92 octane into 96. At best, 3 oz of booster per gallon can normally make 87 into 92, or 92 into 93.5 – 94 (the increase offered by booster becomes less as base octane gets higher). Using more than 3 oz per gallon offers no further increases. In addition, this small increase comes at an expensive $1 per gallon (the 25% mix of race gas costs $1.25 per gallon).