4597 Calle Del Media

Ft. Mohave, AZ 86426

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Sea Doo 951 – Y2K Engine Mods

Group K Modifications

Overview – Since the introduction of the Rotax 951 engine in Summer 1997, Group K has been conducting on-going testing in an effort to produce an effective “Sleeper” modification kit that offers the performance “and” overall reliability that our customers seek…it has not been easy.

This document will outline the all our 951 modifications, including the new 92 octane Y2K 951 Sleeper modification format. This Sleeper kit is very much the product of our long testing program involving different models, and different years of manufacture. To completely understand why our Y2K 951 Sleeper kit is put together the way it is, it is important to understand the lessons of our previous testing. At the end of this document (below the price list) is the outline of those tests, and the deductions we drew from them.

The 951 Group K Kits
  • Quick Kit – This modification is for the recreational user interested in a simple yet effective modification that offers a significant increase in acceleration along with a 1-2 mph increase in peak water speed.

    Inexpensive, Easy…and Quick.
  • Y2K Spec Sleeper – This kit yields the best in overall acceleration and peak speed that can obtained reliably on 92 octane fuel. This kit permits (and mandates) the use of the stock pipe, carbs, ignition, and a 99/Y2K flame arrestor. The Sleeper offers a big increase in overall acceleration and a 3 – 5 mph increase in peak speed (depending on model hull).
  • 100 Octane Hammer Kit – This no holds barred modification for the 951 offers the best in sheer acceleration and peak speeds for the 951. The Hammer kit is well suited for closed course competition as well as median length endurance racing. Hammer equipped machines (XPL’s in particular) will require routine maintenance that requires good technical skills.


Stock OEM

Swift Kit
92 Octane

Y2K Sleeper Kit
92 Octane

Hammer Kit
100+ Octane

98 XPL

61-62 mph

63-64 mph

64-65** mph

67-68+ mph

99 – Y2K XPL

60-61 mph

62-63 mph

64-65** mph


98 – 99 GSX

60-61 mph

62-63 mph

64-66** mph


98 – Y2K GTX

57-58 mph

58-59 mph

60-61** mph


99 – Y2K LRV

50-51 mph

51-52 mph

No Data



58-59 mph

61-62 mph

63-64** mph

**Requires the use of Solas Concord Impeller

Compression – Increasing compression ratio is always a very popular first modification, and the 951’s respond well to “subtle” increases in this area. The use of compression gauges (to establish a measurement in “psi”) has been a very popular way for owners to compare compression. As the “Compression” document on our website points out, measuring compression in this way is, at best, a “ball park” measurement with limited meaning. It the case of the 951 Sea Doo, indicated compression measurement is even more worthless than it has been on earlier pwc’s. The primary reason for this is the many variables that affect electric starter cranking speed.

Accurate comparative indicated compression measurements must be taken at cranking rpm speeds that are as close together as possible. For pwc’s with smaller displacement individual cylinders (which in this case is “all” pwc’s), it’s not too difficult for the starter to maintain a relatively narrow range of starter cranking speeds over a very wide range of indicated compression numbers. However the starter in the 951s has a lot more work to do than any other pwc starter. If a very high compression head is installed, a compression gauge will not be capable of showing an accurate measurement of the increase because the extra load of compression significantly slows the starter cranking speed. On the 951s, we have seen cylinder heads of several cc’s difference show indicated numbers only a few psi apart, while the accurate difference would have been 20 – 30 psi. All this said, “relative” indicated compression measurements might be a useful diagnostic tool on an otherwise unchanged 951 motor. However discussing various 951 compression ratios in terms of indicated psi is (in our experience) a total waste of time.

All this said, the compression ratio increase of our 951 kits is “very” conservative. Our testing showed that very strong overall acceleration can easily be accomplished without big increases in compression ratios.

All of our 951 head modifications include altering the squish angle of the stock head. he Sleeper head mod is unique to those done by other shops in the sense that it has a “staggered” compression ratio. That is, the front and rear cylinder head domes are cut to slightly different compression ratios. This staggered arrangement helps to equalize the operating temperatures of the two cylinders, and helps eliminate detonation.

Included in our head modification is a cooling system upgrade. The stock 951 head casting can “land lock” air pockets in the water jacketing around the combustion chambers. If this air pocketing happens during high-speed operation (on a modified motor), it can cause localized over heating that can easily lead to detonation. Our cooling upgrade completely eliminates this air pocketing. (Please note that our technicians are not permitted to discuss the specification information of our cylinder head modifications or cooling upgrades).

About “O” Ring Heads – There are many aftermarket heads available for the 951 engines. Many are called “O” ring type heads because they utilize a large rubber “O” ring, instead of the stock steel head gasket, for sealing between the cylinder and the cylinder head (as is done on the smaller Sea Doo engines). These heads are typically crafted from high quality billet aluminum, and often have more water jacket volume than the stock part. While many owners have had good results with these heads, they can represent two difficulty areas that should be kept in mind.

First and foremost is that the elimination of the head gasket itself incurs a 2.3cc reduction in combustion chamber volume. This means a 47cc domed head, used with no head gasket, is actually the equivalent of a (higher compression) 44.7cc dome being used “with” a head gasket. It’s very important that all dome volume terminology’s be defined as “with gasket” or without” gasket. Having 2.3cc too little volume can easily lead to detonation and swift engine damage.

Besides the issues of volume, we have experienced some sealing difficulties with “O” ring heads on 951 engines. It should be understood that an “O” ring can easily seal water away from the cylinder bores, however no rubber “O” ring on earth is strong enough to seal back the pressures of compression. “O” ring setups depend on relatively broad, and perfectly flat, metal-to-metal mating surfaces to effectively seal compression. The absolute flatness of these two surfaces is fundamental to maintaining a lasting long-term seal. Herein lies the problem with some “O” ring setups.

The popular 718cc and 782cc Sea Doo engines use only “O” rings for head sealing. However the cast components of those engines are not nearly as large as the 951’s, and so subject to considerably less sealing surface “movement”, compared to the 951 top end. If the top sealing surface of the 951 cylinder and the sealing surface of the “O” ring head are both lapped to assure the perfect flatness of both surfaces, the “O” ring head can seal well. However as time goes on, and repeated temperature deflection of both surfaces takes place, there becomes an increasing risk of a breach at the “O” ring. For owners who intend to remove their cylinder head periodically to inspect and dress the sealing surfaces, the “O” ring head can be a very practical choice. However we consider the stock 951 head gasket able to offer a better long term seal than any arrangement using an “O” ring. For this reason, all our 951 modifications are setup to retain the stock head gasket. We will prepare kits for 951 owners wishing to utilize an “O” ring head, however we will take no responsibility for the lasting seal of the head surface joint.

Flame Arrestors – While the installation of aftermarket flame arrestors has been a very popular modification for many recreational pwc’s, we do not use or recommend them on our Swift Kit or Y2K Sleeper Kit. The technical reasoning for this choice is related entirely to the stock carbs used on the 951 engines. The carbs used on all 951s are the first generation of a carburetor that is designed as much for emissions friendliness as it is for performance. Among the most notable design features that make this difference, is the significantly increased distance between the butterfly and the “booster venturi” style fuel atomizer (aka “bombsight” atomizer). We cannot claim to know all the effects of this design departure, however we know a lot about one particular effect called “fuel delivery signal”.

In short, signal is the amount of vacuum within the inlet tract that helps to draw fuel from the metering circuits in the carburetor. The air restriction of the stock flame arrestor serves greatly to keep that vacuum very high. This high vacuum actually helps the quickness of throttle response at all engine speeds. When a free breathing flame arrestor is installed, this vacuum (aka signal) is reduced. The reduced signal means a slightly leaner fuel mixture through the entire range. This is normally not a dangerous issue for most pwc engines. However the emissions-conscious stock 951 carbs have an exceptionally weak signal at ¾ throttle opening. This means that a more open arrestor will cause the 70 – 80% throttle range become “a lot” leaner than the rest of the range. There is no jetting that can solve this problem (we tried). The “only” effective solution is to leave the stock flame arrestor (with the bolt on air horns) intact. Despite the restrictive appearance of the stock arrestor, it can easily pass enough air volume for excellent performance up to the 7140 rev limiter. The stock arrestor has received an undeserved reputation as “choking” the 951 into a bad rich condition. In truth, the rich condition is a correctable jetting issue, not an air access issue.

The stock arrestor cannot admit enough air to satisfy the higher revving Hammer kit, so we do utilize an aftermarket arrestor on the stock carbs. The ¾ throttle lean condition is still there, however the added octane in the fuel keeps that lean condition from inducing any detonation damage. In addition to that, it is presumed that not many Hammer owners will spend any great deal of time “cruising” at ¾ throttle.

Carburetors – While the stock 951 carbs are not the ideal choice for all high output applications, they can still offer excellent results (using the stock arrestor) on pump gas arrangements, and very impressive results on the high octane Hammer arrangement. For 951 owners who prefer to escape the limitations of the weak signal issues of the stock carbs, there are plenty of aftermarket carb arrangements available. Our favorite choice for racing carbs on the 951’s are the 44mm Novi XR Maxflow carbs. These carbs are specially modified “BN” series Mikunis that come with all the manifolding, linkage, etc.

About Carb Throat Size – While the stock 951 carbs are denoted as “46mm”, the actual minor diameter at the main venturi is slightly under 42mm. With respect to pwc carburetors, there has often been a mindset of “bigger must be better”. In the real world (and the 951 world) this is simply not the case. When Novi Performance conducted dyno tests of their 44mm XR carbs vs. the XR 46mm (on a stock 951 engine), they obtained the same horsepower at peak rpm…with an 8 horsepower increase at 5000 rpm. Our own “on water” carb tests consistently confirmed that our Hammer Kit 951s yielded better overall power with slightly smaller throat carbs.

Other Inlet Parts – For all our kits we recommend the use of the stock reed cages and petals. The Novi carb kit comes along with its own manifold and reed stuffers. Since reed stuffers of a sort, are built onto the stock inlet manifolds, we recommend no replacements.

Exhaust Systems – Our Swift Kits, and Y2K Sleeper Kits, are both setup to be used with a 100% stock exhaust system. We conducted tests with altered plumbing styles of the stock pipe, however found no significant benefits.

The Hammer Kit utilizes the Coffmans exhaust system. This single bodied exhaust pipe is accompanied by a water injection system that greatly improves acceleration characteristics. It bears noting that the Hammer Kit, with this pipe, is intended to rev into the 7300 – 7400 rpm range. Since the stock 951 rev limiter will only permit 7140 rpm, an aftermarket rev limit module will be required. As of this writing, there are no rev modules available for any 1999 or 2000 951 models. For owners of these year models, there are two options. The first is the installation of an MSD “total loss” ignition. These ignitions have no charging system. This means that all the voltage they draw from the battery is totally lost without being replenished (hence the term total loss). This means that operating range is a function of the energy available from the battery. Typically these ignitions are used exclusively for racing applications.

The second option (for now) is to “back date” the ignition to a ’98 version that can accept the Micro Touch rev module. Unfortunately this back date process requires changing the coil, MPEM, stator, and rotor. The cost of these parts makes this conversion a very expensive proposition. At such time that a 99/Y2K rev module is made available, we will update this page with that data.

Cylinder Porting – The Group K cylinder porting for the Sleeper and Hammer kits is by no means “radical”, but it does yield an impressive increase in overall power. The Sleeper porting specifications are biased toward strong overall acceleration that peaks at rpms very close to the stock rev limiter. With this strong bias toward lower range power, the Sleeper ported cylinder can easily pull a much steeper prop, resulting in the peak speed increases.

Our Sleeper cylinder porting is available in two different finish modes. The recreation finish (used in the Sleeper kit) offers all the fundamental dimension and port shaping work, without the more costly fine finish work. 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 cost difference…$120.

The Hammer cylinder porting has a spec arrangement that is bias more towards the higher rpm output facilitated by the Coffman exhaust system. Please note that Sleeper ported cylinders can be upgraded to Hammer Specifications, However Hammer ported cylinders cannot be modified to allow for 92 octane use in a Sleeper (stock pipe) format. Which-ever specification or finish you choose, we require the cylinders with the complete Rave valve assemblies mounted in them. All ported cylinders are also honed before return shipment.

About Big Bores – Increasing the displacement of engines by “big boring” to a larger piston size is a very popular modification for many pwc’s. In the case of the 951 engines, we consider larger bore setups to have a few technical disadvantages that (for us) gives big boring a very questionable overall benefit. The two most glaring of these disadvantages is head gasket sealing, and detonation risk. We would presume that anyone attempting a big bore setup would do so on a high rpm “aftermarket exhaust” type arrangement. A larger bore diameter not only makes for a narrower sealing surface around the bore diameter, it also makes for less rigidity in the cylinder block itself. The added temperatures of larger pistons, and the added potential “movement” of the gasket surfaces (from less rigidity) would further complicate the job of maintaining a lasting head surface seal.

Added to this, is the higher detonation risk that can so often accompanies larger diameter combustion chambers. When the a combustion chamber charge is ignited, there is a strong pressure wave that permeates throughout the entire combustion chamber. If the “end gases” at the outer edge of the bore diameter are extremely unstable, they can be detonated by this pressure wave (hence the term “detonation”). Larger diameter combustion chambers do not always have a greater tendency to experience the detonation of unstable “end gases”. However they do have that tendency where higher rpms and compression ratios are applied. Again, it’s presumed that such a setup would be run at higher rpms (using an aftermarket exhaust). Even if race gas were used, it would require a very conservative compression ratio to control detonation. In the end, we believe that a low compression big-bore setup would become questionably more effective than a higher compression standard bore setup that might have the ability to be more resistant to detonation.

Even with these issues not withstanding, the long term wear issues of increased bore wear from piston rocking and increased connecting rod/bearing wear from the larger impact loads, are issues that would never be far out of the reliability picture. Since we consider high rpm 951’s to already be dealing with their fair share of longevity difficulties, we have chosen to construct all out kits with only oem overbore diameters.

Lower End – The crank bearings on the 951 engines have ball bearing separators (aka races) that are made from a Teflon plastic. With heavy high-rpm use, it can happen that these plastic separators break apart and exit an otherwise “good running” engine. If this happens, the crankshaft will eventually begin to eccentrically “run out” for lack of uniform support. This “run out” can cause the engine to slow, vibrate, air-leak, and eventually quit. Since we have seen these bearing cages fail over a wide range of usage hours, so it is impossible to accurately assess their life span. For this reason, we consider the installation of steel caged crank bearings to be absolutely mandatory for any Hammer Kit 951, and highly recommended for all others.

The stock 951 connecting rods are plenty strong enough to contend with the rpms and loads of the Swift Kit and Sleeper Kit. These two kits further benefit the lower end by virtue of the water ingestion protection offered by the stock ‘99 flame arrestor. However the rpms and horsepower loads of the Hammer Kit are enough to represent a reliability risk related to the stock connecting rods. To deal with this reliability problem, crank builders have retro fitted high rpm 951 cranks with Honda 500 motocrosser connecting rods (Please note: we don’t get these parts or do the work, so please don’t call us for specifics on this procedure). Since the Honda rods are slightly shorter than the stock 951 rods, the top crankcase must be machined to assure correct cylinder deck height. Group K will perform this machining operation. While there are some 951 owners that swear they have had no crank difficulties on their 7200+ rpm engines, we still consider this connecting rod/front bearing upgrade to be mandatory for any high rpm 951 platform.

Air Leaks – Another area that requires frequent monitoring on 7200+ rpm 951’s is air leaks. While air leaks can develop in many areas, the most sensitive areas are the front crank seal, and the inlet manifold joint surfaces. We recommend periodical pressure testing of the crankcase every 8 – 10 racing hours on 7200+ rpm platforms. Pressure test kits are available from Watercraft connection in Oregon at (503-232-2062).

Case Porting – Case porting is a popular modification for many racing engines. On the 951’s case porting is a “detail” preparation that can offer noticeable overall performance benefits on high rpm platforms like the Hammer Kit. However on lower rpm pump gas arrangements case porting might offer some questionable benefits for the costs involved. If you do opt for case porting, note that your base gaskets will need to be trimmed and fitted by hand to assure that they do not protrude into the port passages.

Impellers – The stock 951 impellers are known as a “swirl” blade design prop. Swirl design props have additional blade surface area that allows them to have exceptional “hook-up” characteristics on initial take-off and in high speed rough water conditions (much as wider tires on a car can offer better traction). Unfortunately, this additional blade surface area also puts added drag on the engine, particularly when accelerating from high-speed turns. The best (currently available) alternative to the stock impeller are the Solas “Concord” series impellers. The Solas Concords are a swirl variation that utilizes considerably less blade area with less blade overlap. Despite the reduced blade overlap, the Concord hooks up very well in most conditions. However the biggest asset of the Concord, over stock props, is the big improvement in acceleration off the turns, and an increase in peak speed (about 1 mph) over a stock impeller turning exactly the same peak rpm.

The Swift Kit is intended to be run with the 15/20 Solas Concord. The Sleeper Kit creates a great deal more overall torque than the Swift kit, and so has the ability to pull the 15/20 closer to the rev limiter. Sleeper kit owners can choose between the Solas Concord 15/20, or the 16/21.The 16/21 is best suited for very lightweight owners (130 pounds or less), or for owners interested in improved “partial throttle” cruising range. For GTX and LRV Sleeper Kit owners, we recommend an un-pitched stock impeller, or the Solas 15/20.

Since the Hammer Kit is a high rpm biased arrangement it does not require a prop that is steeper than the Solas 16/21. The additional mph of the Hammer Kit comes from the sheer rpms. It bears noting that the stock prop can be used on the Hammer Kit. However as rpms escalate into the7300 – 7400 rpm range, the stock impeller becomes less efficient (creating fewer mph per rpm increase). For XPL/GSX Hammer owners interested in retaining the stock pump case, the Solas 16/21’ is the best impeller choice.

About Y2k and Later Pumps – The ’98 – ’99 951 models are equipped with a pump whose vanes are made of brass. Virtually all of the aftermarket impellers for the 951’s were tested for use on this “brass pump”. The Y2k and later 951 models are equipped with a similarly designed pump whose vanes are made of a composite plastic. These “plastic” pumps work very well, however they create an important side effect. For reasons we cannot explain, the plastic pump bodies appear to “load the motor down” about 200 – 250 rpm more that the brass pump. That is, a “plastic pump” Y2k model will rev about 250 rpm less than an identical ‘99 model with the identical prop. As a result, most Y2k and later 951’s do not work well with “off the shelf” aftermarket props. Since the Solas 15/20 Concord is the mildest pitch (currently) available, it must be significantly re-pitched in order to offer the correct rpm in a Y2k and later boat. Group K has done the testing with the plastic pump 951’s to obtain the correct pitch specifications for the new plastic pumps. These “plastic pump” spec Solas Concord impellers are available from Group K. It bears noting that the Solas “Dynafly” series of props are correctly pitched to work with the plastic pumps, and therefore are much too low in pitch for the ’98/’99 models. While the Dynafly props have great hook up and acceleration on ‘00/’01 models, they are about 2 mph slower than a correctly pitched Concord on smooth water conditions.

Pumps – For The Swift Kit and Sleeper Kit, the Stock pump case offers excellent hook-up and peak speed abilities. However Hammer Kit owners that ride very aggressively in very rough water conditions may wish to opt for the Skat Trak all stainless steel “D” pump. This pump comes in 6-vane and 12-vain versions, and utilizes smaller diameter swirl props that are not compatible with the stock pump. The Skat Trak pumps offer a big improvement in rough water hook-up, and a slight increase in peak water speed. The only disadvantage of the Skat pump (besides the price tag) is the added wear a tear on the soft drive couplers (especially on the XPL models). Alignment of the driveline (using the special made Sea Doo tool) can certainly help to improve the life of this coupler. However the instant hook-up loads of these Skat pumps can eventually wear the soft coupler out. We strongly recommend that every Sleeper equipped 951 have the driveline alignment inspected. For XP Hammer kits in particular, we would consider regular driveline inspection to be absolutely mandatory…regardless of which pump is used.

98 vs 99+ Couplers – The 1998 XPL models utilize a 4 lobe rubber cushion between the metal drive couplers. The ’99 models utilize a 5 lobe cushion made of much more durable rubber. To convert a ’98 to the ’99 cushions, the drive couplers must also be upgraded. While this upgrade is a bit costly, we would consider it mandatory for any ’98 owner planning to install the higher rpm Hammer kit.

Handling – Since handling characteristics are so subjective from one rider to the next, we normally do not make recommendations. It has become common practice for XPL and GSX owners to install aftermarket replacement “sponsons”. These sponsons certainly improve the high speed turning stability of these machines, but they can cause a significant reduction in smooth water peak speed. While the stock Sea Doo sponsons do not make heavy contact with the water, they do provide some “lift” that helps to increase peak speed. When an owner installs a set of aftermarket sponsons, that lift (and the speed that came from them) no longer exists. Just the same, most XPL/GSX owners consider the gain in turning stability to be worth the 1 mph loss in speed.

The 951 Testing Data

Goals – Like all of our Sleeper kits, it has been our intent to get the maximum possible increase in overall speed and acceleration by modifying the fewest possible parts. It sometimes happens that significant changes in power (or peak rpms) creates a “side effect” need for other bolt-on parts or modifications (props, rev limiter mods, etc) Whenever possible, we try to minimize the need for these “side effect” mods.

Like most of our kits, we conducted much of our durability testing/performance evaluation within the year round Region 1 endurance racing circuit. The references to competitions and comparisons are from that genre.

97.5 GSXL – While these hulls did not possess the outstanding rough water handling of the 785cc GSX hulled boats, they were a great hull for recreational “grudge racing”. Given this, there was a great interest in the GSXL Sleeper kit by grudge racing owners.

The main difference between the 97.5 motors and all later versions were the very radical port timings used on the 97.5 (more excessive port timing than any other pwc made to that time). This excessive port timing made for very poor low range acceleration, and very questionable high rpm benefits. For our Sleeper modification, we attended to these problems by slightly altering the port timings for better low range power. This approach netted a big increase in acceleration while still maintaining very good high rpm abilities.

While the 97.5 GSXL Sleepers had great peak speed abilities, they also had some problem areas. First and foremost was a collection of problems related to (what would later be diagnosed as) water ingestion into the flame arrestor. These problems included crank seal failure, big end rod bearing failure, piston scoring, etc. While it is very difficult to specifically trace water ingestion as the sole culprit in each of these problems, water ingestion was certainly a factor in many of the failures we encountered during our 97.5 GSXL testing.

Along with these difficulties, we began to experience difficulties with an unsolvable mid-range lean condition in the 46mm “I” bodied Mikuni carbs. As we would later learn, these carbs are literally designed to have a partial throttle lean condition in an effort to satisfy emissions requirements. The larger bore modifications and free-er breathing flame arrestors we attempted to utilize dramatically compounded the problem area of “weak signal” that had been designed into these carbs. (for more information about “signal” please refer to our web document “Racing Carburetors 1999”). While the performance of these kits was very good, the “random” reliability (presumably attributed to water ingestion) caused us to suspend sales of the 97.5 Sleeper kits.

98 XPL – The 98 XPL hull was a quantum leap improvement in rough water handling over the 97.5 GSXL. Along with that, the ’98 Rotax engine featured new cylinders that had much more conservative port timing in an effort to yield better low range acceleration. The ’98 also featured a new design jet pump (for which there would no aftermarket impellers for some time). By this time, the electronics after-marketers had developed a rev limiter eliminator for the 951 ignition. Since no impellers were available, and rev modules were, there was a tendency among ’98 owners to gain the additional speed they sought by merely revving the engines higher. Our XPL Type 2 kit was fashioned exactly in this way. The stock pump and impeller seemed to deliver good speed increases as rpms were escalated (about 7200 rpm) outside of it’s stock 6900rpm range (uncommon for most swirl props). Not having to use a steeper impeller assured very strong low range acceleration. One side effect problem caused by these added rpms is additional driveline wear. In truth, the initial bulk of this driveline wear was a result of engines that were badly out of alignment with the hull-mounted drivelines. Re-shimming the engines made a huge difference in driveline life, however the problems were not over. The XPL model has a sub-shaft mounted in the hull between the engine crankshaft and the pump drive shaft. The “soft couplers” on either end of this sub-shaft do not like the effects of high rpm…“at all”. Perfect engine alignment can significantly reduce the wear on these soft couplers. However on virtually all our high rpm XPLs, coupler failure is just a matter of time.

Our Type 1 and Type 2 Sleeper engine kits for the ’98 XPL performed excellent in every way, except they were still very susceptible to “random” failures of internal moving parts (a result of water ingestion that was still unrecognized) In addition to this, the mid-range “signal” problems of the stock carbs were still a glaring obstacle for which we still had no effective solution. We did notice that this mid-range lean condition was “almost” non-existent when the stock flame arrestor was in place, however the stock arrestor was very “unfriendly” to the higher rpm direction that the 951 platform was moving towards. As we would later learn, the higher rpms, by themselves, had an additional negative effect on the engine signal to the inlet tracts. This effect made the stock carbs work even worse, and made for new “lean spots” at other isolated mid-range rpms. For that reason, all Group K Type 2 kits were mandated to remove the stock carbs and use aftermarket racing-carbs. The installation of aftermarket carbs completely eliminated all the “lean spot” nightmares, and yielded additional improvements in peak rpms. Unfortunately the higher operating rpms of our Type 2 mods made the internal moving parts “even more” sensitive to the destructive effects of water ingestion. During the ’98 endurance racing season, there were plenty of creative technicians (besides us) working to solve the engine reliability issues of the high rpm 951’s. While some technicians experienced “windows” of success, no one came up with a lasting solution for all the difficulty areas. In addition to all this, it had become clear that 951’s turning over 7200 rpm were very strained to run reliably on 92 octane pump gas. Increasing the octane (by mixing in percentages of race fuel) became common practice. As rpms escalated beyond 7200, so too did the percentages of race fuel need for reliable operation. Late in the 1998 season, Coffmans introduced their aftermarket exhaust for the 951 platform. The Coffman system could significantly increase the peak rpms of virtually any XPL, ours included. With the Coffman pipe in place, it was common for 951s to rev into the 7300 – 7500 rpm range. At these rpms, the stock pump/prop was well outside it’s window of design efficiency. It was possible to spin the stock prop into these higher rpms, however there were questionable speed increases coming along with those rpms. Most owners of these high rpm machines were forced to change over to the all stainless steel Skat Trak “D” pump. This expensive but effective pump allowed the high rpms of the Coffman piped XPL’s to get the added speed that should rightfully come along with the added rpms. While the added rpms of the Coffman pipe and Skat Trak pump served greatly to increase the XPL speed potential, there were equal increases in the rate of driveline and coupler failures. This mandated the need for driveline component replacement/re-alignment on a very regular basis. To avoid failures, the owners of these high rpm 951s simply have to replace these parts periodically (roughly every 6 – 10 hours). From our perspective, the maintenance intervals and expense of the fastest 1998 951 platforms was completely out of control. To be sure these high rpm machines were exceptionally fast. However most of our 951 customers were understandably frightened by the high maintenance that these machines required. From our perspective, these high speed/high maintenance 951’s were impractical for all but a select few owners. Our conclusions from our 98XPL prototype testing were:

Any 951 that routinely revs beyond 7200 has poor prospects of operating reliably on 92 octane pump gas. All 951 stand to benefit from having the driveline alignment inspected (especially higher revving 951s) All XPLs routinely revving beyond 7200 rpm should have the driveline cushions regularly inspected. The stock 951 pump and impeller loses considerable “rpm to mph” efficiency at rpms above 7300. The added hook-up loads of the “D” pump can significantly increase soft coupler deterioration. A free-er breathing arrestor on the stock 951 carbs (of a 92 octane arrangement) can induce unsolvable (and potentially lethal) partial throttle lean conditions (a result of reduced inlet tract “signal”). Free-er breathing flame arrestors are virtually mandatory to supply the air volume needed for effective 7300+ rpm operation. Free-er breathing arrestors typically represent an increased risk of water ingestion.

Water ingestion is much more damaging as peak rpms are escalated over stock. The ’99 951’s were virtually identical to the 98’s, except for two very important changes. The most visible change was a redesigned flame arrestor air inlet system that dramatically reduced the likelihood of water ingestion into the engine. While this new inlet looked a bit ungainly…It worked great. Unfortunately this air intake cannot satisfy the air intake needs of the 7300+ rpm engine setups The new 99’s also came with a new ignition system that was electronically more complex than the 98’s, therefore making rev limit changes virtually impossible. As of this writing (12/99), there are still no commercially available rev modules available for any of the ’99 951 models. This ignition redesign meant that ‘99 951owners had two choices when seeking higher performance:

  1. Work within the perimeters of the stock 7140 rpm rev limiter, or
  2. Install a 98 ignition that allowed the use of the already available rev modules. Most top level endurance competitors chose to use the ’98 ignition and rev module (not an affordable conversion). Doing this allowed them to maintain the high rpm components they had used in the ’98 season. Unfortunately some driveline issues still persisted. Even though the ’99 cushions were superior to the ‘98’s, the high rpms could still take their toll. In time, some 951 endurance competitors solved the driveline problems by simply switching to the GTX hull (which has no sub-shaft). The GTX hull is not quite as fast as the XPL hull, however the simpler driveline allowed for the use of slightly higher rpm (some turn up to 7600) to recover that lost speed. Here again, we saw the direction of the fast endurance 951 platforms continue to move toward very high rpms, very high costs, and very high maintenance.


99 XPL Prototype 99 – Rather than pursue the expensive high rpm approach (that requires the retro-fit of a complete ’98 ignition system), we chose to direct our development toward a 951 that could operate on 92 octane fuel within the confines of the stock 7140 rpm limiter. The approach was simple, just apply a steeper impeller (to gain mph) until the rev limiter no longer gets “bumped”…it seemed simple then anyway. Our Type 2 cylinder porting and head modification set was easily capable of spinning the stock impeller to the point of “bumping” the limiter (using a stock pipe), so we decided to stick with those specs. Since high rpms would not be sought, we decided to use the stock throat carburetors with jetting changes (and the choke plates still intact to help slightly strengthen signal). Even with this strengthened inlet signal, we chose to use the stock flame arrestor without the two bolt-on inlet horns. Then began the business of steepening the stock impeller (a processes we commonly perform in-house). Before long, we had found a pitch for the stock impeller that held the rpms slightly away from the limiter. Our peak water speeds were comfortably in the 64-65 mph range…exactly what our customers were interested in. However we encountered a very noticeable hesitation when the throttle was applied quickly from low speeds. Initially we presumed this hesitation was simply a minor carb calibration issue. After several weeks of on-water testing and jet changing (with fuel flow meters attached) we realized that this hesitation was not “calibration related”. This Type 2 modified top end had always offered excellent low range performance in the past, so we hesitated to increase the compression ratios. However after the jetting availed no solutions for our hesitation, we made the choice to slightly increase compression. The compression increase successfully reduced our hesitation, however our on-board detonation sensors were now showing dangerous levels of mid-range detonation where there previously was none. Another week of jetting showed that this mid-range detonation could not be resolved by richer mid-range jetting. With this, we went back to our previous compression ratio and decided to seek a solution to our hesitation by working with the impeller pitches and exit nozzle diameters. The basic idea was to reduce the leading pitch (the 16° end of a 16/21° impeller) to ease the load on the engine at low speeds. While this sounds great on paper, it has side effects in real life. We were able to reduce the hesitation slightly by reducing the leading pitch, however by doing so we had induced two new problems. On initial take off, we began to experience cavitation where there had been none before. Even though we had induced this slight cavitation, the hesitation still remained when making hard left hand turns. In addition to this, the high range pitch that was ideal before was now not steep enough (a function of less water being processed in by the milder leading pitch). To resolve this excess rpm, we had to resort to even steeper trailing pitches. For a couple of weeks we tested with various leading/trailing pitch and nozzle. At the end of all these weeks of hair splitting tests, we began to realize the greater problem we were dealing with. The 951 pump has the ability to take in and process the large volumes of water needed for excellent take off acceleration and good top speed. However if the low range output is softened by any margin, while the overall load against the motor is increased (as a steeper prop does), the initial loading of the pump will create a hesitation in acceleration that cannot be resolved. In short, we had created a kind of power delivery curve that the steepened prop and pump didn’t like. Throughout this entire battery of testing, we had been struggling to hold the high rpm output down, using excessive impeller pitch. It was clear that our Type 2 modified top end wanted to turn more rpms than we were ever going to let it turn (with the stock limiter). It was also clear that while this excessive pitching was succeeding in pulling the motor away from the limiter, it was not getting us any added mph because of how far we were departing from an “efficient” blade contour.

Along with all this, we were developing a brand new problem as well. The many weeks of fine tuning now had the engine operating on the brink of being overloaded at all rpms. This resulted in a new batch of midrange detonation strikes that we were unable to resolve with jetting. If we installed the stock flame arrestor (for the stronger signal effects needed to resolve this detonation) the peak rpm ability suffered badly and our hesitation became bigger.

We were now several months into this project, and we were out of “band aid’s” to resolve all of our side effect issues. We realized that an entirely different approach was needed.

Our conclusions from our 99 XPL prototype testing were:

  • Excessive impeller pitches that ideally suit peak rpms can sometimes cause unsolvable midrange hesitations. Additional impeller pitch does not necessarily result in higher speeds (where peak rpms remain the same) The loads of excessive impeller pitch can magnify the midrange “lean spot” effects of the stock 951 carburetors.
  • The installation of a stock arrestor can significantly impair the performance ability of an rpm biased engine arrangement (even where the stock pipe is being used).


99 XPL Prototype Y2K – It appeared that while our “99” prototype ran great at full throttle, it was simply generating more high range power than we needed for this setup to work. We knew we could run the same mph with a milder (and more efficiently shaped) pitch on the stock prop. We also knew that a lesser tendency toward high rpm output would reduce air inlet needs (allowing for the use of the stock arrestor). The added signal from the stock arrestor would easily eliminate all our mid-range “weak signal” lean spots, to say nothing of the added protection from water ingestion.

With this mindset, we prototyped a second head/cylinder layout that carried a port angle/area layout with even less bias toward high rpm output, and even more bias toward torque range. We understood that such a layout would not pull the steep high range pitches of the Prototype 99, however non-optimum blade contours of those excessive pitch angles were not offering added mph anyway.

Another technical benefit of this newer specification layout would be the reduced need for high compression. Since the bulk of overall power would now be generated by the sheer torque of the new port layout, high-risk compression ratios would no longer be needed for strong acceleration.

After only a few test outings, this new “Y2K” arrangement proved itself to be everything we had sought. Our Y2K XPL bolted away from a dead stop with no cavitation or hesitations. It also drove off of high-speed turns with impressive authority.

The only problem we noted with this new setup was a slight detonation on both cylinders when driving through sharp-apex full-speed turns on glass water (note: all these tests were performed with aftermarket sponsons that significantly increased the loads of high speed turning). Unchecked, this detonation had lethal abilities. A slightly richer high-speed mixture eliminated this detonation, but inflicted a significant loss in peak rpms.

Up to this point, we had conducted all of our Y2K prototype tests with the stock flame arrestor, less the two bolt-on inlet horns. Our fuel flow meter indicated that the full throttle detonation we were experiencing was likely a result of insufficient signal rather than insufficient fuel mixture. We installed the air horns and set our carbs back to the original leaner setting. The end result was a complete elimination of all detonation strikes, and a peak loss of only 10 – 20 rpm (easily worth it). Later tests confirmed that the removal of the air horns caused a loss in signal that would easily induce detonation over a wide range of rpms.

After completing the engine tuning, we revisited the pitching of the stock prop. We netted our best radar speeds (63 – 64 mph, 210 lbs. rider) with a pitch that allowed 7050 – 7090 rpm (nicely away from the limiter). While in the course of doing these pitch tests, we obtained some recently available Solas “Concord” series impellers. These impellers offer hook up that is very comparable to the stock “swirl” design prop of the stock 951’s. The Concord’s design advantage is that it has considerably less water contact surface area than the stock prop. The reduced drag from this design feature results in an additional 1 mph over a perfectly repitched stock impeller.

Our conclusions from our Y2K XPL prototype testing were:

  • The Y2K spec is far less impaired by the stock arrestor than are higher-rpm biased arrangements.
  • The use of the OEM bolt-on air horns can make a visible improvement in “signal” on the stock carbs.
  • The conservative compression ratio of the Y2K spec offers a significant safety margin against (92-octane) detonation that cannot be equaled by higher rpm biased arrangements.
  • The Y2K spec creates a power curve that offers “hesitation free” acceleration while using a moderately steepened stock impeller.
  • The Solas Concord impeller, on our Y2K Sleeper, offered a clear increase in acceleration and peak speed over a steepened stock “swirl” impeller.

Swift Kit

Group K Price

Cylinder Head Modification with Cooling Upgrade


Solas 15/20° Concord Series Impeller (’98/’99 Models)


Solas 15/20° Concord Series Impeller (’00/’01 Plastic Pump Spec)


Sleeper Kit

Group K Price

Y2K 951 “Sleeper” Engine Modification Kit
Includes: Recreational Finish Cylinder Porting and Decking, Head Modification, Cooling Upgrade, and Carburetor Re-Jetting

Parts Required for Modification: Head, Cylinders with Rave Assemblies, and Carb Set


Competition Port Finishing Option


Re-Pitching of Stock Impeller (XPL and GSX Models)


Solas 15/20° Concord Series Impeller


Solas 15/20° Concord Series Impeller (00/01 Plastic Pump Spec)


Re-pitching of your Solas Impeller


Odyssey Sponsons


Hammer 100 Octane Modifications – (’98 Ignition Models Only)

Group K Price

Hammer 100 Cylinder Competition Finish Porting


Hammer 100 Cylinder Head Modification with Cooling Upgrade


R&D Flame Arrestor (for Stock Carburetors)


Re-Jetting of Stock Carbs


Coffman Exhaust Pipe with Water Injection


Micro Touch Rev Module (’98 Models only)


Honda Connecting Rod Conversion


Solas 16-21° Concord Series Impeller (XPL and GSX Models)


Solas 15/20° Concord Series Impeller (’00/’01 Plastic Pump Spec)


Re-Pitching of your Solas Impeller


Odyssey Sponsons


Hammer 100 Options

Group K Price

Novi XR 44 Spigot Carb Kit


Case Porting (requires Cylinder and Top Case)


MSD Total Loss Ignition System


Skat Trak “D-75” Pump with Impeller – (exchange)


Skat Trak “D-75” Pump with Impeller – (purchase outright)


Skat Trak “D-75” Impeller


Rebuild Pricing

Group K Price

Engine Teardown, Spec Re-Assembly, and Pressure Test


Steel Caged front crankshaft Bearing (each)


Honda Connecting Rod Conversion (Performed to Your Good Crankshaft)


Factory Rebuild 951 Sea Doo Crankshaft (with Core or $200 Core Charge)


New Stock OEM 951 Sea Doo Crankshaft


Cylinder Boring, Honing and Chamfer (per pair)


Sea Doo OEM Oversize Piston and Ring (1 Cylinder)


*prices subject to change based on manufactures pricing
**NOTE: Group K will bill an additional $35.00 handling charge for engine assemblies via UPS


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 !!