4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Yamaha GP800 – High Performance Mods

Single Pipe Modification Kits

The Swift Kit – This kit is a small collection of the most effective bolt on parts for your GP 800, along with modification of your stock cylinder head. The GP 800 Swift Kit makes for a big increase in overall acceleration, and a 1.5 – 2 mph increase in peak water speed. This is the ideal kit for the owner who is looking for an easy and quick way to bolt on a significant improvement in power and handling. (92-octane compatible).

Sleeper Kit – The Group K GP 800 Sleeper Kit allows your stock looking, and sounding, GP8 to accelerate and top end right along side a GP 1200. The huge increase in overall torque allows the GP8 to bolt out of high-speed turns with no bogging at all. The 3.5 – 4 mph increase of the Sleeper kit puts this lightweight on a par with most of the top muscle-boats. The 92 octane compatible Sleeper Kit includes all the same bolt on parts as the Swift kit, along with Sleeper cylinder porting and cylinder-head modification (carburetors need not be removed).


Peak Speed

Peak RPM



57-58 mph



Swift Kit

59-60 mph


Solas “K” with Stage-1 Re-Pitch

Sleeper Kit

61-62 mph


Solas “K” with Stage-2 Re-Pitch

About Dual Pipes – Not long after the release of the ’98 GP 800’s, Factory Pipe Products released their dual pipe exhaust system for this new machine. The dual pipes are certainly an effective modification, however some GP8 owners consider them a bit too costly or too complex. As of this writing, we have not conducted any extensive testing with the FPP dual pipes. All of our testing to date, for our Swift Kit and Sleeper Kit, has been “only” with the stock GP 800 exhaust system. As we complete our testing with the “dual-pipe” platforms, we will be posting that information to a separate document.

About Stock GP 800 RPM’s – Establishing the “stock” performance data for the GP 800 has been more difficult than with any other machine we have worked with. Part of the reason for this difficulty has been the variations in the “stock” machines provided to magazines and after-marketers by Yamaha themselves. It often happens that manufacturers must supply “pre-production” machines to publications for testing, simply because the true production-run machines may still be a couple of months away. After magazine testing, these same pre-production machines are used for dealer demonstrations and ultimately offered to aftermarket manufacturers throughout the industry as test units. There is no disagreement about the peak speed of the production and pre-production GP 800’s…every one we ever saw radared in the high 57 to low 58 mph range. However there does seem to be a huge (and unexplainable) difference in the peak operating rpms of these machines.

The ’98 and ’99 pre-production GP 800’s tested at the Watercraft World magazine “Dream Demo” both had peak rpms between 7050 – 7100 rpm (as Yamaha representatives said they would). However every other “pre-production” GP 800, that we gained access to, had a peak rpm between 7400 – 7450 rpm (but they all ran 57 – 58 mph). In addition to these inconsistencies, the “stock” GP 800 used in the Personal Watercraft Illustrated magazine test of the FPP pipes had a peak rpm of 6980 @ 58.4 mph (Dec. 98, page 46). This is a solid 100 rpm’s under “target” with no apparent loss of radar speed.

We do not care to speculate on the cause (or reasons) for the broad variations in the peak rpms of these machines. All we care about is “what is actual stock production rpms.” We finally obtained a showroom production ’99 GP 800 for our testing. To our surprise, the peak rpms increased noticeably during the break-in process. On our first tank of fuel, we saw peaks of 7180 rpm. The second tank yielded 7250 rpm, and the third tank attained our eventual sustained peak of 7320 rpm. Subsequent tests of other stock GP 800’s have shown similar peak rpm results.

We bring up all this information because we suspect there has been a whole lot of testing of aftermarket components that has been done on GP 800’s that do not have “production” rpm peaks. Given this, you may see rpm data from various aftermarket manufacturers that may seem conflicting. We will not make any attempt to sort out this conflicting data. Instead, we will focus only on the rpm data gathered from our test units that we know to be 100% “showroom production”.

About Compression – Both of our GP 800 kits include modification of the stock cylinder-head. Increasing the compression of pwc’s has always been an easy way to get a good overall performance increases. The GP 800 also benefits from increased compression…within limits. During our testing for both of our GP8 kits, we found that the 800 engine does not react well to “big” increases in compression ratio (92 octane pump gas). Exceeding the optimum ratio resulted in a peak rpm loss, and slight overheating. With respect to the GP 800 compression, our testing indicated that it is very easy to get too much of a good thing. (Note: please do not call our technicians for compression specifications, as they are not permitted to give out any of that information).

It bears noting that the specification and dome design of our two kits is very different. That is, If you have the head modified for the Swift Kit, that head will have to be re-cut again if you later decide to upgrade to the Sleeper Kit.

Head Sealing – During our testing we found the head gasket sealing surfaces of the GP8 to be somewhat “sensitive. That is, very subtle surface irregularities could result in breeches of compression or water. We consider the stock head gasket to be a very important part of maintaining a “lasting” compression and water jacket seal. For this reason we will not set up any of our kits to utilize an “O” ring type head that omits the use of the head gasket. For owners that do choose these heads, we recommend that the seal be inspected regularly.

About Cylinder Porting – The cylinders on the GP 800 are unconventional by pwc standards. Instead of an aluminum cylinder with a thick iron sleeve, the aluminum cylinders of the GP8 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 GP8, 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 using the right equipment. This porting also includes the matching of the exhaust valves in the cylinder, so the cylinder must be sent complete with the exhaust valves and shafts.

That said, the porting that we do for the Sleeper kit does make for a very significant increase in mid-range horsepower that is key to pulling the steeper pitched prop for that kit. This porting, like the Sleeper kit itself, is developed to be effective enough for local closed course racing, yet reliable enough for endurance racing. The porting mods used in our Sleeper kit are the result of many months of performance and wear evaluation testing. We confidently claim that Group K cylinder porting will yield a wider power band and longer piston life than any other porting modification available anywhere. All the GP 800 Sleeper Kit cylinders and heads are typically prepared as matched pairs, to assure that you have the proper squish clearance and compression ratio for the quality of gasoline that you intend to use. These cylinder and head specifications play the key role in determining the amount of overall power that the engine can make, as well as the quality of fuel that you can use (92 octane unleaded in this case).

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, the “Competition” finish is available as an option. The cost difference…$370. vs $450.

To assure that the modified cylinders will have the correct compression ratio and squish clearances, we modify the cylinders and cylinder head as a matched set only. All ported cylinders are also honed.

About Exhaust Systems – As previously mentioned, this document will cover only engine platforms that utilize the stock GP 800 exhaust system. This stock pipe is a fully water jacketed system that induces interior cooling water at the stinger tip (unlike most other systems that induce interior cooling water up in the head pipe). The end result of this design is “surprisingly” wide power band, along with excellent overall silencing ability. Both of our kits for the GP8 utilize all the stock exhaust components with the stock plumbing.

While the stock exhaust system may seem excessively large and heavy, that mass does serve a purpose. The weight of the exhaust system helps to actually “dampen” the high frequency vibration of the engine assembly. This underlines the importance to regularly inspect the bolts that secure the exhaust system to it’s mount points on the engine.

About Inlet Systems – The stock carbs on the GP8 are 44mm Mikuni “I” Body carbs. Yamaha engineers chose these new “I” body type carbs more for their emissions friendliness than their performance ability. These 44’s are not bad carburetors, however they can respond badly to many “standard” types of performance modifications. The installation of an aftermarket flame arrestor does increase cfm (cubic feet per minute of air) and help overall performance. However the free breathing flame arrestor also causes a slight reduction in the inlet tract vacuum needed to draw fuel from the carburetor jet circuits. Most of this lost fuel delivery can be recouped with richer carb adjustments. Removing the choke plates (in addition to the aftermarket arrestor) further weakens the inlet tract vacuum as it additionally increases cfm. This added vacuum loss creates a mid-range lean condition that cannot be adjusted or jetted away. For this reason, our stock pipe kits utilize the stock carburetors with the “choke plates in”.

The afore mentioned loss of the inlet tract vacuum is referred to as a loss of “signal”. While this inlet “signal” itself cannot be recovered, it is possible to modify the stock carburetors in a way that allows them to be more sensitive to the remaining signal that exists in the inlet tract. Such a modification is performed by Novi Performance (makers of racing carbs). This carb modification, by itself, does not result in a genuine power increase. However with this mod, the carbs will have enough “signal strength” to avoid the unsolvable mid-range lean condition that might take place with the choke butterflies and shafts removed. An added side effect of this modification is improved throttle response and improved ease of tuning. The stronger signal of this modification greatly improves the carburetor’s ability to “self adjust” the fuel mixture for changes in altitude and air temperature. This means less time spent manually fine tuning your carburetors for air density changes at your various ride spots.

This affordable Novi carb modification is not a standard component of our kits. However it is available as an option for customers who want to get that extra bit of acceleration and fuel mixture compensating ability for their GP8. This modification is available through Group K or directly from Novi (east coat customers may choose to go direct to Novi to save freight costs).

For additional background info about these carbs, please read our website document “Racing Carburetors 1999”

About Ignitions – The stock ignitions on the GP8 have a limiter at around 7450 rpm (+/- 50 rpm). For recreational level GP800’s, this is a good limiting device that does not hamper peak water-speed performance in any way. The peak rpm of our GP8 kits lies just below this limiter (we accomplish this by varied impeller pitches).

While the stock limiter is unnoticeable on smooth water, the same cannot be said for rough water riding. As the modified GP8 travels across rough water at high speed, the engine will bump the rev limiter each time that the impeller comes completely out of the water. When this “bumping” takes place, the engine rpms become automatically reduced by the limiter. If the impeller “reconnects” with the water at this reduced rpm moment, the boat experiences a “landing with the brakes on” kind of feeling. This effect can be very distracting and annoying when trying to cover rough water at high speeds. To eliminate this effect, owners can install a Riva cdi box that offers an ignition curve identical to the stock curve (ironically called “Limited”)…but with an increased rpm limiter. During the testing of our kits, we tried the other ignition curves available in the Riva cdi box. None worked as well as the limited curve. It bears noting that the installation of this cdi box does not increase the actual rpm of these kits on smooth water…just the “out-of-water” limit.

About Handling – One of the biggest differences between the ’98 and ’99 GP 800’s is the hull change intended to reduce porposing. While the difference in porposing is significant, the dimension difference is somewhat subtle. It short, Yamaha engineer changed the radius on the back edge of the trim tab area of the hull on the ’99 models. Yamaha dealers have a service bulletin that outlines how to make this simple, but effective, modification on the older ’98 hulls.

To further reduce porpoising, we recommend the R&D ride plate. During our testing on the GP 1200’s, we found significant gains in peak speed and turning ability by cutting different “negative angles” on to the R&D ride plate. We re-visited these same tests with our GP 800 project, and found similar benefits. The specifications for our best cut to the 800 were different from the 1200, but the good net result is the same. We offer the R&D ride plates for the GP 800 with this machining modification already made, or we can modify your existing R&D plate.

For better hook-up, we recommend the R&D top loader scoop grate. This grate does cause a loss of about ½ mph, but the improvement in rough water hook-up and acceleration are well worth that small loss.

About Impellers & Pumps – The impeller pitch of the stock GP800 prop is much too mild to be of any use on a modified machine. Unfortunately, we found no “off the shelf” impellers that perfectly suited the wide powerband of our GP 800 kits. Part of the reason for this is a significant variation in the pitches of the blades on many aftermarket impellers. Our solution is to custom pitch the impellers for our GP 800 kits. The Solas “K” pitch impeller proved to be the most versatile choice. The “K” prop is well suited to meet the needs of both the Swift Kit and Sleeper Kit (using different pitch specifications). The slightly steeper Solas “H” pitch impeller can be suited for use on the more powerful Sleeper Kit, but not the Swift Kit. While the “H” runs about .4 mph slower than the “K”, the improved rough water hook up of this “H” prop would be the ideal selection for Sleeper kit owners who will be riding almost exclusively at high speed in rough water.

Another modification that can significantly improve rough water hook-up is “pump blueprinting”. This blueprinting removes all the interruptions and casting drafts in the pump vane case. The result is improved hook-up during high speed operation in rough water (note: blueprinting offers only a nominal increase in peak water speed on smooth water).

Hull Finishing – When you take delivery of any new GP 800, the bottom surface of the hull is very smooth and shiny. While this finish looks real nice, it is totally non-functional. The ideal bottom surface finish is a non-shine finish created by numerous full length front to rear “scratches”. The best way to accomplish this finish is with a piece of very coarse emery (40 – 60 grit is best). All the sanding strokes should be front to back (continuous) for the full length of the hull. The deep, full length, scratches should eventually eliminate any part of shiny surface. While this preparation may not look attractive, these scratches will act as thousands of small rudders that will make the hull track “a lot” straighter in all water conditions. These scratches will also allow for much better surface holding in high-speed turns. This sanding preparation also allows the GP 800 hull to run arrow straight at peak speed, along with much more responsive steering control.

Swift Kit Modifications

Group K Price

Cylinder Head Modification


Dual Carb R&D Flame Arrestor


Solas “K” Impeller with Re-Pitching


R&D “Pro Series” Ride Plate


Group K Ride Plate Modification – for R&D Ride Plate


Additional Swift Kit Options

Group K Price

Riva CDI Box


R&D “Pro Series” Grate Scoop Grate


Impeller Re-Pitching Adjustment – to your Solas “K” Prop


Pump Blueprinting (Send Vain Case and Impeller Housing)


Sleeper Kit Modifications

Group K Price

Yamaha GP 800 “SLEEPER” Engine Modification
Includes: Cylinder Head Modification & Recreational Finish Cylinder Porting


Mandatory Sleeper Kit Bolt On Parts

Group K Price

R&D Flame Arrestor with Adapters


R&D “Pro Series” Ride Plate


Group K Ride Plate Modification – for R&D Ride Plate


Solas “K” Impeller with Re-Pitching


Additional Sleeper Kit Options

Group K Price

Competition Finish Option for Porting


Optional Novi Carb “Signal” Modification with Primer Kit


Optional Riva CDI Box


R&D “Pro Series” Grate Scoop Grate


Solas “H” Impeller with Re-Pitching


Impeller Re-Pitching Adjustment – to your Solas Prop


Pump Blueprinting (send Vain Case & Impeller Housing)


*prices subject to change based on manufactures pricing


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 !!