This document is being created in autumn 2000. As other beneficial modifications become available, we will be updating this document on our website http://www.groupk.com/
The Stock Boats – While the engine and driveline of these machines is basically the same as the 1999 XLL, there have been a number of detail changes. Of these the most notable are:
A) 2000 cylinder head dome design is changed.
B) 2000 ignition curve is altered to give better resistance to full throttle detonation.
C) 2000 has new carb calibration with no accelerator pump on the center carburetor.
Overall the stock 2000 XL handles and rides the same as the 99 version. The various updates, made by Yamaha, have yielded cleaner carburetion and a very slight (about 1mph) increase in speed.
The GPR 1200 (new for 2000) has quickly established itself as the most reliable “rough water” muscle boat in the industry. It is also the heaviest 2seater in the industry. This added weight is a great benefit in rough water handling, however it also represents an ongoing difficulty area of pump “hook up”. While much of this hook up problem can be minimized with various bolt-on parts, the presence of some slight cavitation will likely always be part and parcel of GPR/XL ownership.
Swift Kit – This easy to install kit is intended to offer the maximum benefit in performance for the minimum in cost and down time. Installing the Swift kit requires only basic tools, and basic mechanical skills. Requires 92octane pump gas.
Swift Kit -Cylinderhead mod, Ride Plate Mod, Solas Concord impeller.
Sleeper 1 Kit* – This kit is for the owner wanting slightly more acceleration and speed than the Swift kit, while retaining the stock electric drive exhaust valve system. Ideal for high performance recreational riding as well as local level competition.
Sleeper Kit 1 – Cylinder Porting, Cylinderhead mod, Carb Re-Jetting, Ride Plate Mod, Solas impeller.
Sleeper 2 Kit* -This kit offers the maximum overall power increase available on 92-octane pump gas. The Sleeper 2 GPR is fast enough for local level closed course racing, and among the most potent “grudge racing” platforms around. The XL Sleeper 2 is ideally suited for the aggressive rough water rider (an XL1200 Sleeper 2 won the ‘99 BP 6-hour Endurance Challenge).
Sleeper 2 Kit- Cylinder Porting, Head mod, Carb Re-Jetting, Riva Exhaust Valves, Ride Plate Mod, modified Solas impeller.
*Note: Since the Sleeper kits entail removal of the cylinders and carb bank, the disassembly and reassembly should be performed by individuals with very good mechanical skills.
58 - 59 mph @ 7150 rpm
62 - 63 mph @ 7150 rpm
Group K Swift Kit (92 octane)
61 - 62 mph @ 7200 rpm
65 - 66 mph @7200 rpm
Group K Sleeper 1 (92 octane)
62 - 63 mph @ 7300 rpm
68 - 69 mph @ 7300 rpm
Group K Sleeper 2 (92 octane)
63 – 64 mph @ 7350 rpm
69 - 71 mph @ 7250 rpm
About Triple Pipes – Factory Pipe Products manufactures a replacement exhaust system for the GPR/XL that employs the use of three individual expansion chamber bodies. While the price tag of this exhaust system is considerable, so is the horsepower increase ($2500 for 50 hp more). If this system could be installed as an individual modification, it would be a very practical option for many GPR/XL owners. Unfortunately, getting the triple pipes to work well is a bit more complex than that. The added rpms of the triple-pipes require an ignition curve that has much more retard than the stock unit can offer. To date, the only solution to these collective problems is to install a programmable MSD total loss ignition. While these ignitions can yield excellent performance, they do not have a charging system of any kind. The battery energy used to make sparks is totally lost without being replenished (hence the term “total-loss”). Group K has been involved in the testing of these “triple-pipe / total-loss platforms. While they yield phenomenal performance, the long term reliability (and the complexities of the ignition) are not yet what we consider “recreational friendly”. As further development of this platform advances, we will be posting a “triple pipe” document to our website.
About Cavitation - The majority of GPR/XL owners will at one time or another experience “cavitation”. The basic definition of cavitation is, “Whenever air or air pockets are introduced into the pressurized water column within the pump”. While this definition is excessively simple, the factors that can cause cavitation are everything but simple. The term “cavitation” is in reference to the actual air “cavities” within the pressurized waters entering the pump. As these air pockets make their way to the exit nozzle, the momentary absence of thrust is felt by the rider as a loss of acceleration (or a “slipping clutch” effect). While most instances of cavitation are caused by the prop “spinning” to create air cavities between the blades, cavitation can also be caused when aerated water is admitted into the pump before reaching the prop.
In short, some level of cavitation will always exist in any high performance pwc in the same way that some wheel spin will always exist for any high performance automobile. Through the use of certain parts and modifications, cavitation can be minimized to a manageable level. However, at a certain point, the operator will be required to use some skill and finesse to maintain that minimum …as you would with a high performance car.
While cavitation is a phenomenon that involves mostly air in the pump, the strong physical effects of cavitation can result in very real wear and physical damage to parts of the pump (like the very real wear and tear aspects of high performance car wheel spin). Prudent throttle management will result in much longer wear life of all the parts involved. However no matter how prudent the user is, eventual maintenance will still be imminent. That is, occasional prop repair/replacement will always be a part of owning a GPR/XL that is run hard.
Cavitation is a bit more acute on the GPR and XL because these are not only the most powerful machines in their class, but also the heaviest. While the added weight of these machines greatly improves rough water handling, it also compounds the cavitation problem because (as one engineer put it) “there is a lot more boat in front of these pumps”.
We mention this subject before all others because the power increases of our modifications are only felt when the pump can translate that power in to thrust. That means that pump mods and pump maintenance must always be done hand in hand with the engine performance mods. Our specific pump modifications for each kit will be outlined later in this document.
About Exhaust Valves – Another basic component that greatly affects the tuning and available performance gains from the GPR/XL motors is the electronically controlled exhaust valves. These Yamaha 1200s employ an exhaust “guillotine valve” that very literally changes the “effective” exhaust port height at different operating rpms. Unlike the pressure operated exhaust valves, employed on many Sea Doo models (that swiftly swing open within a 200 rpm range), the exhaust valves on these Yamahas are very gradually opened via electronically controlled cable drive. This electronic system allows engineers to precisely select a particular exhaust port height for every rpm across the range. By so doing, engineers can get very smooth and strong low rpm acceleration from an engine platform that has a very low compression ratio. It bears noting that the lower position settings of the exhaust valve can actually induce measurably higher “effective” cylinder compression. In the case of these particular Yamaha engines, the valve is maintained at a very low setting from 6200-6800 rpm. This low setting allows for “surge free” cruising and towing within this crucial part of the rpm range.
Like most pwcs, the GPR/XL responds very favorably to increases in compression ratio (by modifying the cylinder head). Getting an “increased compression” GPR/XL engine to deliver “detonation free” performance at full throttle (when the exhaust valves are all the way up) is really quite easy. However then that same engine is slowed down into the 6200-6800 rpm range, the lower exhaust valve position creates a much higher “effective” compression ratio. This higher “mid-range” compression ratio can easily become excessive enough to create a very significant “partial throttle” detonation risk. For this reason, the tuning of modifications on these engines becomes a balancing act between the performance you want at full throttle/full valve opening, and the temperatures you can control in the partial throttle/low valve opening 6200-6800 range.
At Group K, we gather this detonation risk information with an on-board detonation sensor that uses a microprocessor to show the degree of detonation at any given rpm or throttle position (since this instrument is $6000, sensitive to vibration, and not waterproof, it’s not very practical for private owners). With this data, we select staggered compression ratios and staggered carb settings that result in more equal operating temperatures and manageable detonation risk.
A kit is available from Riva Yamaha that converts the electronically driven exhaust valve system to a simpler pressure operated system (much like the Sea Doos). While top end power is the same with both valve systems, the Riva conversion kit can significantly reduce the 6200-6800 rpm detonation risk of a modified GPR/XL. However along with that reduced risk comes a less progressive, and more abrupt, kind of power delivery. For aggressive riders who intend to do primarily full throttle riding, the hard hitting mid-range is not a problem (in fact some guys prefer it). All in all, the Riva valves make for such a big reduction in mid range detonation risk that a slightly higher compression ratio can be safely used without incurring any increased detonation risk. This is the concept behind our Sleeper 2 kit. The Sleeper 2 kits also utilize impellers with a slightly steeper impeller pitch. The overall speed increase availed by the Riva valve/higher compression/steeper prop setup is not alot, but it is the very fastest 92 octane safe arrangement we have built.
Compression – The GPR/XL responds very well to increases in compression … within limits. Our Swift Kits and Sleeper kits have been developed to run the highest compression ratios that we consider to be 92 octane pump-gas safe. We attain this higher compression by machining the stock cylinderhead (a much more affordable alternative to buying another head). After milling the stock head, we also recut the domes for a shape that better staves off detonation. The specification of the Swift and Sleeper modifications are different, however the Swift modified head can be re-cut to the Sleeper spec. It also bears noting that our GPR and XL head specifications are very different from one another and should not be interchanged.
Because of variations in starter cranking speeds, compression gauges, and measuring techniques, we do not specify the compression of our different arrangements in terms of “psi” pressure readings. Our technicians do not discuss any compression specifications of these kits in terms of psi or ratio … please don’t ask them to.
About “O” Ring Sealing – Some performance shops manufacture “billet” replacement cylinder heads known as “O” Ring Heads. These heads are manufactured to utilize a large diameter rubber “O” ring to seal between the cylinder and head (instead of the stock metal head gasket). This “O” ring system can offer great convenience to owners who regularly remove the head for the purpose of inspection or compression changes (in the case on changeable dome heads). The other big plus to the “O” ring setup is that it can significantly reduce squish clearance without having to machine the top surface of the cylinder. This “O” ring system is the primary means of head sealing on many stock Sea Doo and Polaris engines.
It should be understood that an “O” ring can easily seal water away from the cylinder bores, however no “O” ring on earth is strong enough to seal back the pressures of compression. “O” ring setups depend on a relatively broad metal-to-metal mating surface around the bore diameter to seal compression. The absolute flatness of the surfaces on the head and the three cylinders of the GPR/XL is fundamental to maintaining a lasting long-term seal. Herein lies the problem with some “O” ring head setups. The three cylinders must have perfectly flat (preferably lapped) surfaces on their top “head sealing” surface. These three cylinders must also have the exact same deck height to keep from flexing or distorting the one-piece head. The stock metal head gasket has the ability to more effectively accommodate all these possible surface variations. For this reason, all Group K kits are prepared to utilize the stock metal head gasket.
About Spark Plugs – Unlike most other pwcs, the GPR/XL has very high output ignition capable of firing a spark across a .044” plug gap. This larger plug gap is a big asset to the performance of this engine. The spark plugs used in the GPR/XL are a NGK BR8ES-11. These plugs are the same as the more common BR8ES in every way … but one. These “11” series plugs have a ground electrode with a different radius bend that allows for an accurate .044” gap. If you try to gap a conventional BR8ES to .044”, the ground post will be bent in a way that does not allow for consistent firing. If you have a GPR/XL, use the “11” series plugs gapped to .044”.
Carburetion – The GPR/XL uses the same 44mm “I” body Mikuni carbs as used on the GP 800. These carbs offer very good performance, as well a fuel delivery curve that helps satisfy EPA emissions mandates. Unlike the GP 800, the carbs of the GPR/XL have adjustment screws with “cover caps” pressed over the top of them. These caps are removed for our Sleeper 1 and 2 modifications, allowing for the adjustment of the high and low speed adjusters.
Some GPR/XLs appear to have carburetion that is somewhat over rich. In the case of other over rich pwcs, such a problem is commonly solved by the installation of a free-er breathing aftermarket arrestor. There is a commonly held belief that free breathing flame arrestors resolve rich conditions by accessing more air to go along with the fuel … this is not true.
On board fuel flow meter tests show that the installation of a free breathing arrestor reduces overall fuel flow between 5%-15% at various throttle settings. This happens because the inlet tract vacuum, (aka signal) that pulls fuel from the carb metering circuits, is much lower with a free breathing arrestor. If the net effect was an even 5% fuel reduction at all throttle settings, this mod would be very viable. Unfortunately, the emissions conscious “I” body carburetors employed on the GPR/XL can experience unusually serious lean conditions in the mid-range that cannot be corrected by jetting.
During testing with our kits, we found our best fuel flow results on our 92 octane kits by retaining the stock arrestor case, but removing the rubber air hose between the main arrestor body and the air inlet plenum chamber. This allowed for improved air access, and a level of signal reduction that could be corrected by jetting (for 92 octane). All our GPR/XL kits, to date, retain the stock carb throats with choke plates intact.
** For more info about “signal”, refer to our website document titled “Racing Carburetors 1999”
About Larger Carbs – Installing larger carbs has been a popular modification for many high performance pwcs. We have chosen to not utilize aftermarket carbs on our GPR/XL Swift and Sleeper kits for a few reasons. First and foremost, the location of the carbs underneath the stock exhaust pipe makes it virtually impossible to reach the adjuster screws of all three carbs. This means that the pipe body must be removed to make uniform carb adjustments (no small job). Beyond this, larger carbs will generally result in significant increases in fuel consumption, and an obvious reduction in fuel range. Since fuel range is a pivotal concern for many owners, we feel that the reduced range of larger carbs would be hard to justify in most riding applications.
Finally, we question the need for larger carbs on platforms using the 3-into-1 single pipe. A large part of our triple-pipe testing has been conducted with the stock 44mm “I” body carbs. Even on the triple pipe platform, these carbs offer an excellent balance of strong overall power and “reasonable” fuel range. Larger carbs did offer slightly better peak rpms. However those rpms came at a considerable cost of fuel range.
Cylinder Porting – The cylinders on the GPR/XL are unconventional by pwc standards. Instead of an aluminum cylinder with a thick iron sleeve, the aluminum cylinders of the GPR/XL have a thin layer of hard plating for the piston to reciprocate in. Yamaha calls this plating “Yamaha Ceramic Coating”, but it’s basically “Nicasil” (a nickel-silica plating). This thin, but super tough, bore diameter has a much lower friction coefficient than iron, thus allowing for much quicker piston acceleration. This thin plating also makes for a much more conductive-friendly path for the heat of the piston to get to the water-jacket. The material of this plating is so tough that it can stand up to the forces of several piston scoring “episodes” with virtually no damage to the bore at all.
The only down side of the plated bore is the inability to be “bored-oversize” after an engine failure. In situations of minor damage to the plating, re-plating of the cylinders is available from the aftermarket. In the event of a major failure, the cylinder must be replaced.
All this begs the question, “what about cylinder porting?” Plated cylinders, like those on the GPR/XL, can be ported. However special attention (and special cutters) must be used to assure that the plated liner is not compromised. The cylinder porting used in our Sleeper kit is not “radical”, so it does not represent a risk to the integrity of the plated liner. We would accent that the porting of plated cylinders should only be done by experienced technicians. At Group K, we have many years of experience performing cylinder porting on plated bore cylinders, like those on the GPR/XL.
In the past, cylinder porting has received an undeserved reputation as a modification that harms low end power and reduces engine life. This may be true for poorly executed port work done by folks with little experience. However quality cylinder porting can be tailored to create an accent toward any kind of horsepower need. Our cylinder porting modifications for the GPR/XL carry an accent towards strong middle to high range power to help drive the hull off the turns. At the same time, the excellent low-end acceleration is fully maintained. An unintended benefit that comes along with this porting is the added high range torque that increases the peak by 100-150 rpm.
Our cylinder porting is available in two different finish modes. Both the "Recreational" and "Competition" finish modes include all the same port timing, port shaping, and specification work. The Competition mode also includes additional exhaust port finishing and a transfer port "rough finish" that further improves fuel atomization. This additional finishing work accounts for 30% more shop time, yet contributes to only about 10% of the total performance gain (about .5 mph and a little bit of acceleration). The recreation finish is standard in the Sleeper kits, and the "Competition" finish is available as an option. The cost difference between the two, ...$700 vs. $450.
To assure that the modified cylinders will have the correct compression ratio and squish clearances, we modify the cylinders and cylinderhead as a matched set only. All ported cylinders are also honed.
About Pistons – As previously mentioned, the plated cylinders of the GPR/XL cannot be bored, but that does not mean that there are no oversize pistons. The cylinders on the GPR/XLs vary in diameter over a very narrow range (a few thousands of an inch). To accommodate this variations in diameters Yamaha has four different “standard” pistons for the GPR/XL (denoted as a0, b0, c0, d0). When replacing pistons, be sure that you are installing the correct piston for that cylinder.
While the plating on the GPR/XL cylinders cannot be bored, they can be honed. This means that if you experience some kind of piston scoring, a small amount of material can be honed out of the GPR/XL cylinder to offer a good clearance fit to a slightly larger Yamaha piston. Please note that there is only one piston ring size to cover all these piston diameters.
About Rev Limiters - As mentioned above, our kits utilize the stock ignition system. This ignition has a rpm limiter that offers very “unfriendly” ignition firing at about 7390 rpm. It often happens that a modified machine running close to the rev limiter can have speed difficulties in rough water. When the prop comes “un-hooked” at high water-speeds, the engine immediately bumps the rev limiter. If the pump reconnects with the water while the ignition is “on the limiter”, the rider can get a sensation that feels like “landing with the brakes on”. In truth, the rider is experiencing the moment that it takes for the engine loading to pull the engine down away from the limiter. The 99 XLL had an additional “limiter” whereby the cdi box also signals the exhaust valve servo to “drop the valves” when the rev limit is reached. This gave the ’99 XLL an uncommonly strong “landing with the brakes on” feeling. The 2000 model GPR/XL models do not have this secondary limiter.
Pro Tec offers a rev limiter modification for the GPR/XL models. While this mod is not required for any of our kits to perform at optimum, it may be a benefit for owners whose Sleeper kits are bumping the rev limiter while running at very high speed in rough water conditions.
Exhaust System – The stock exhaust system of the USA-version GPR/XL is the only pwc (to date) to employ a catalytic converter for the purpose of reduced emissions. While this stock exhaust pipe is by no means perfect, it does allow for significant gains in overall performance without any side effect difficulties. Given this, Group K has chosen to design all our kits so that they can be used in conjunction with the stock cat-con.
The hose between the waterbox-muffler and the hull exit, on the GPR/XL, has a large attached plenum chamber to improve silencing at certain operating rpms. This entire hose can be replaced with a “low restriction” waterbox hose kit. The installation of the hose kit on the GPR/XL does not make a big difference in speed or sound level. However the hose kit does make a very significant change in exhaust back-pressure. This back-pressure has a significant effect on mid-range engine temperatures, and detonation risk. The benefit is so significant that we recommend this exhaust hose as a mandatory upgrade for all our GPR/XL modification sets.
Riva Yamaha manufactures a cat-con replacement unit that they call a “Stinger”. The Riva Stinger allows the engine to generate a great deal more torque in the lower and mid range (as well as allowing easier access to the carb adjuster screws). However along with the added torque, it stifles the ability to make strong horsepower much beyond 7000 rpm. The thinking behind this tuning approach would be to generate enough additional full range torque so that the engine can pull a steeper prop to make up for the lost rpms.
It bears noting that all our testing with the Riva Stinger was done on a Sleeper GPR that had Sleeper ported cylinders. Since the stock ported machines with the Stinger were no faster than our Sleeper 1, we were only interested in the possible added benefits (of the Stinger) to our Sleeper Kits. Our (stock-pipe) testing with the Sleeper kit showed that our porting offered a significant increase in acceleration and torque, however that increase also brought with it an increased mid-range detonation risk. That risk was abated with jetting and staggered compression between cylinders. Installing the Stinger along with the Sleeper ported cylinders further increased the mid-range detonation risk to a level that we could not manage or completely abate (on pump gas). Regardless of the octane used, our Sleeper/Stinger tests showed this torque vs. rpm trade off (of the Stinger) is fairly even on both ends. That is, there is little gain in peak speed going with the strong torque/steep impeller setup.
Since we did no in depth testing with the “Stinger/stock porting” arrangement, we have no data on the detonation risks with varying compression ratios, impeller pitches, etc. We recommend owners to refer to Riva for that data.
All in all, the Riva Stinger/steeper prop combination may have some potential for certain applications that may be uncovered in time. However for the meantime, our testing showed no combination with our Sleeper Kits where it offered better overall results than a slightly higher revving platform using the stock exhaust system.
GPR/XL owners should note that all of Riva’s advertising, for their Stinger pipe, prominently specifies the Stinger “for racing use only”. The modification and/or removal of emissions devices is perfectly legal for racing machinery. However it is a different story for machines used at public waterways. To date, there has been little or no inspection enforcement of the emissions components on pwcs. However it is realistic to believe that such enforcement will commence soon at many environmentally active venues … particularly National Parks. Even the most mechanically “un-inclined” park ranger will be able to differentiate between a stock cat-con and a Riva Stinger. As the law reads, there are significant fines for tampering with any emissions devices. We urge owners who plan to install a Stinger and operate at public venues to strongly consider these issues.
The “D” Plate – While the stock exhaust system (with cat-con) works very well on stock and modified crafts, there have been cat-con units that have fractured or broken. Many owners have been reluctant to simply re-install another cat-con just like the one that broke. For many owners, the next best option is the installation of a “D” plate.
The European spec GPR/XL’s do not employ the use of a catalytic converter. In place of the converter Yamaha has mounted a stainless steel plate with a large “D” shaped hole that exposes roughly half of the exhaust body diameter. The “D” shaped hole is used because there is a significant amount of back-pressure needed to make the exhaust scavenge correctly. The engine cannot tell the difference between back-pressure created by the can-con, or back pressure created by the “D” plate, hence there is no performance difference between the two. Unfortunately, Yamaha has been reluctant to supply these “D” plate in great number to “outside USA” dealers.(they presume that many are headed to the US market).
Group K offers a duplicate of the European spec Yamaha “D” plate that can take the place of damaged cat-cons. From an EPA standpoint, removal of the cat-con is legal for machinery used for competition use only. However it bears noting that there is no external visual difference, and no standardized emissions measurement protocol to police the use of “D” plates. Just the same, we would recommend against the use of a “D” plate in any machine that is operated is “emissions restricted” venues such as various National Parks, reservoirs, etc.
About Lower End Preparation – The crankshaft and lower end components in the GPR/XL are some of the strongest parts around. That said, the sheer weight of these machines, and the loads incurred when the pump reconnects with the water (at speed), are enough to inflict crankshaft journal ” twisting” under very extreme conditions. We recommend crank truing and welding for any Sleeper 2 owners that will be consistently running their machines at very high speed in rough water conditions. This truing and welding is particularly important for rough water racers that will be using the Pro Tec limiter, and/or the Skat Trak E-75 Stainless Steel pump.
About Handling and Speed - While all owners would like to have the best of rough water handling and smooth water speed in one package, that seldom happens. For that reason, this section will attempt to make the best recommendations in both areas. One complication to these recommendations is the fact that the GPR hull (unlike any other machine we have tested) appears to run faster radar speeds on 3-4 inch wind chop than it does on glass water. While we have our own ideas about why this happens, we do not try to explain this trait … nor change it. That said, all the posted GPR/XL radar testing data in this document is smooth water only.
Like all pwcs, increased horsepower and speed abilities can have a significant impact on handling characteristics. The biggest advantage of the increased power of the Group K kits is that the machine “steers” much better with the extra thrust. That is, the extra thrust allows the rider to turn much more precisely, and the extra power itself offers a big improvement in handling control. At the same time, increased speed can sometimes create new handling problems that must be attended to. Such is the case on the XL.
XL Scoop-Grates - When the speed of the XL hull is increased, the nose of the hull rides slightly higher than stock. When the throttle is suddenly closed at high speed (as would be done setting up for a turn), there is a sudden weight shift from rear to front that drives the steering edges on the front of the hull into the water. When the XL is modified to run faster than stock, this weight shift is much more pronounced. It can happen that this weight shift drives so much of the steering edges into the water that the machine can engage a turn “much sharper” than the rider may have intended. This phenomenon makes it virtually impossible to predictably turn a more powerful XL with accuracy and confidence at high speeds. Fortunately there is a single bolt-on part that completely eliminates this problem.
The R&D Aqua-Vane scoop-grate makes a profound change in the “rear-to-forward” weight shift when decelerating from high speed. The effect of the R&D grate is so significant that our test riders were unable to induce “any” of the previous high-speed steering variations. At the same time, the R&D grate offers better pump ”hook-up”, and a peak speed increase of .3-.5 mph. Given all this, we consider the R&D intake grate to be an absolutely mandatory modification for every XL in existence. Group K will not perform modification work for any XL unless the R&D grate is also fitted.
XL Ride Plates - As of this writing, there are no commercially available replacement ride plates for the XL. Given that, Group K has developed an affordable modification for the stock ride plate that results in a significant improvement in peak speed and handling comfort. Our modification allows the nose of the hull to ride slightly higher in the water. This raised-nose attitude makes for slightly less “wetted” hull surface, and thereby less friction against the water … allowing for a measurable speed increase. At the same time, the raised nose allows the hull to “loft” over rough water (as a dirt bike would loft the front wheel over rough terrain). The result is a much less punishing ride from the large XL hull.
GPR Scoop Grates – The GPR hull does not experience the same rear to forward weight shift handling difficulties as the XL hull, and so scoop grate installation is not considered mandatory. For sheer smooth water peak speed, the stock GPR grate works best. Cutting the center bars out of the stock grate (a popular mod on other boats) only serves to slow the GPR down.
As water conditions become considerably rougher, the R&D scoop grate becomes a clear advantage over the stock grate. However the R&D grate is slightly slower than the stock grate on glass water conditions.
GPR Ride Plates – Like the XL hull, the GPR looses considerable peak speed ability on smooth water due to it’s “nose heavy” attitude in the water. Group K offers an affordable modification to the stock rideplate that offers a 1.5 to 2 mph increase on smooth water. While this mod is intended for the smooth water grudge racer, it still offers very good rough water handling when the trim is set one notch down. As of this writing, this ride plate mod offers the best performance per dollar increase we have seen. If other aftermarket plates of similar performance become available, we will post that data here.
GPR Trim Tabs – While aftermarket tabs are commonly used on the high output triple pipe racing GPRs, we saw no appreciable gains for the 66-71 mph single pipe platforms we tested with. If other, better performing aftermarket tabs of better performance become available, we will post that data here.
About The Pump – The pump on the GPR/XL utilizes a 155mm impeller (the same diameter used in earlier Yamaha 1200 models. However, since the GPR/XL prop mounts on a larger threaded shaft, and has a different “back-set”, the impellers from previous ‘97-99 GP1200s cannot be fitted to it. The 155mm pump is among the largest in the industry. However even with this large diameter pump, the GPR/XL experiences a considerable amount of cavitation. Part of this cavitation is a result of a sealing breech between the impeller housing and the pump shoe mounted directly ahead of that housing. On our GPR/XL test boats we took a two-fold approach to filling this gap. The first step is to install the R&D plug kit. This kit includes several rubber plugs that the owner silicones into place in all the casting cavities of the scoop grate and pump shoe. The second step is to epoxy the remaining gap between the impeller housing and the installed scoop grate (this is done from the rear of the machine while the pump is off). This epoxy filling is usually best accomplished in two or three layers (please do not call our technicians for information on the procedure of this epoxy work).
Another modification offered by Group K that helps high-speed rough-water hook up is pump blueprinting. This blueprinting removes all the surface interruptions and casting drafts from the vane body and impeller housing. While blueprinting does noticeably improve high-speed hook up, it has no effect on smooth water peak speed.
Impellers - Another part of the GPR/XL cavitation is caused by the varying clearance between the impeller and it’s housing. The outside diameter of the stock impeller can be welded and recut to offer very close clearance, however that will only make a small reduction in cavitation.
The best solution for the cavitation at the impeller is the installation of a Solas “Concord” design impeller. Our tests showed this design of prop to offer the best combination of smooth water peak speed along with greatly reduced cavitation. For the most part, all out XL kits utilize the 12/18 Concord while all the GPR kit utilize the 13/19 Concord. There is a steeper 14/20 pitch, however it is far too steep a pitch for any of our kits. Since the pitch “gap” between these different props is very significant, our Sleeper 2 packages utilize the same respective props along with our in-house repitching to allow for slightly better mph. The Sleeper 1 kits do not have the overall power to pull these steeper Sleeper 2 versions.
Even with all these problem areas addressed, a perfectly prepared GPR/XL pump will still experience “some” cavitation upon initial takeoff. This cavitation is simply a function of a slightly undersized pump that is trying to push a very large and heavy hull up on to plane. This further underlines the need for the operator to act with prudence when accelerating from lower speeds. Note: Any aftermarket impeller change will require an impeller tool and a driveshaft holding tool.
Pump Alternatives - To date there is “no” pump setup for the GPR/XL that can operate 100% cavitation free. However the “best’ option is the installation of a Skat Trak stainless steel “E-75 Magnum” 12 vane pump. This 155mm pump, originally developed for racing machines, offers much better hook up that any stock pump setup ever can. Part of the Magnum pump’s ability is offered by the 12-vane stainless-steel body. The other big design advantage is the “swirl” type impeller, which has a lot more blade surface area with which to process water. This swirl prop has the same type of effect as putting wider tires on a race car for better traction. As of this writing, Skat Trak does not have established pitches for single-pipe recreational formats. As that information comes forward, we will post it to this document.
The only down side to the Magnum pump is the price tag ($950 exchange with prop). However if you intend to operate your GPR/XL at high speed in very rough water, this pump is “the” way to go.
Some GPR/XL owners have experienced difficulties with the stock stainless steel impeller housing sleeve separating from it’s aluminum housing. The stock OEM Yamaha housing sleeve has a wall thickness of about .055”. In high performance applications where we experience this liner separation, we opt for a resleeve repair from Skat Trak rather than install another stock housing liner. The Skat Trak “wear ring repair” replaces the .055” stainless liner with a .100” wall liner. We consider this money well spent for any owner of a high output GPR/XLL. For more info on the Skat Trak pump modifications, visit www.skat-trak.com/, or call 909-795-2505.
About Teardown and Assembly – The teardown and assembly of our GPR/XL “Swift” kit can be done by owners with basic tools and basic mechanical skills. The Sleeper kits require the removal of the cylinders and the carburetor bank. Because of the added linkage hardware that operates the carbs and the exhaust valves of the GPR/XL, this teardown and re-assembly requires a higher level of mechanical ability and expertise. If you doubt your ability to correctly reassemble these components, we recommend that you have the job done by an experienced technician.
“Swift Kit” Spec Cylinder Head Modification 195.00
Group K Ride Plate Modification (GPR &XL) 60.00
R&D Scoop Grate (XL model – Mandatory : GPR Optional) 99.00
Optional “low restriction” Waterbox Exit Hose (XL) 45.00
Optional “low restriction” Waterbox Exit Hose Kit (GPR) 49.00
(send Cylinders w/ valve assemblies, head, carbs & ride plate)
Includes Recreational finish Sleeper Porting, Cylinder head modification,
and Carb adjuster cover removal, jetting & Inspection.
Optional Competition Porting Finish additional 250.00
“Low restriction” Waterbox Exit Hose (XL) (Mandatory) 39.00
“Low restriction” Waterbox Exit Hose Kit (GPR) (Mandatory) 42.00
Group K Ride Plate Modification (Mandatory) 60.00
R&D Scoop Grate (GPR & XL - Mandatory) 139.00
(send Cylinders w/ valve assemblies, head, carbs & ride plate)
Includes Recreational finish Sleeper Porting, Cylinder head modification,
and Carb adjuster cover removal, jetting & Inspection.
Riva Exhaust Valve Kit – (Mandatory for Sleeper 2) 660.00
Installation of Riva Valve Kit 50.00
Optional Competition Porting Finish additional 250.00
“Low restriction” Waterbox Exit Hose (XL) (Mandatory) 79.00
“Low restriction” Waterbox Exit Hose Kit (GPR) (Mandatory) 79.00
Optional Pro Tec Rev Limiter Modification 170.00
Group K Ride Plate Modification (Mandatory) 60.00
R&D Scoop Grate (XL model - Mandatory) 139.00
R&D Scoop Grate (GPR model – rough water) 139.00
Optional Pump Blueprinting (send vane body & impeller housing) 210.00
Driveshaft Holding Tool 39.00
Optional Skat Trak E-75 Magnum pump with Swirl Impeller (exchange) 950.00
Optional Skat Trak E-75 Magnum pump with Swirl Impeller (no exchange) 1315.00
Crankshaft Truing and Welding 260.00
Engine teardown and Reassembly (to access crankshaft) (no carbs) 450.00
Remove and reinstall Engine 650.00
**NOTE: Group K will bill an additional $35.00 handling charge for engine assemblies shipped via UPS
ORDER INFORMATION: SEND ALL PARTS REQUIRED FOR MODIFICATION VIA UPS TO:
GROUP K 4597 CALLE DEL MEDIA FORT MOHAVE, AZ. 86426 520-763-7600
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 new 150-lb. UPS weight limit makes engine shipping practical and affordable.
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 cashiers check (ups no long accepts cash for cods). 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.