Sea Doo 951 – Y2K Engine Mods

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.