849cc Big Bore Mods for the Kawasaki SXR800 & SXi 750
For the 2008 racing season the IJSBA is allowing 2cycle stand-up race crafts to have a maximum displacement of 850cc. For Kawasaki SXR owners, this means that pistons up to 85.4mm (3.4mm over stock SXR pistons) will be permitted.
The Wiseco Piston company has been the standard source for high performance big bore pistons for most watercraft. Wiseco normally manufactures and inventories big bore pistons up to any diameters that enable simple over-boring of the cylinder with no other significant machining modifications. However, big boring the Kawasaki SXR/SXi engines to diameters in the 850cc range does require other modifications to get the big-bore setup to “work right”. Given that, Wiseco will not be inventorying 850cc range SXR/SXi big bore pistons for conventional dealer and retail sales. However they are willing to make proprietary big bore SXR/SXi piston designs for specialty race shops like Group K. Given that, we have acquired specially designed SXR big bore pistons in diameters from 84.5, 85,0, and 85.4
While there is certainly some interest among closed course racers to use these pistons for racing applications, our greatest customer demand has been for a quick and reliable 91-octane freestyle engine package. The following text is about the package we developed for these customers.
849 Sleeper Package – 7000 rpm 54-55 mph
Patterned after our very popular Sleeper kit, this 91-octane big bore version offers a stock looking engine compartment, peak speeds that match many “high revving” race boats…. And low rpm acceleration that no mod race boat can come close to. The raw torque of this package offers the instant acceleration that free-riders demand, in a reliable low revving package. The 849 Sleeper offers the fuel consumption and linear power delivery of a stocker, It’s literally stocker on steroids.
849cc Sleeper engine Modification Set $1190.
849cc Sleeper Overview
“What is “linear power … and what does it do for me??
In short, linear power means the ability to maintain a steady cruising speed at any rpm in the range. Modified machines with racing exhausts on them allow you to ride below 30mph, or over 40mph ... but it is impossible to sustain a steady rpm between 30 and 40 mph. For rec riders and freestyle riders, having this "linear" power is a huge advantage in controlling and setting up maneuvers.
PWC exhausts are all designed to assist with the movement of fresh fuel mixture into the cylinder, while at the same time “scavenging” out the maximum amount of spent exhaust gases. Sadly, no pipe design can offer “excellent” scavenging at all rpms …they can only offer over excellent scavenging over a certain rpm range. Tuned racing pipes can offer much higher revs than a stock pipe, and still have a “reasonably” wide powerband. However all racing pipes have one annoying characteristic that comes with those higher RPMs. Racing pipes exhibit a very sudden surge of power (often called a “hit”) in the lower range of the powerband. This sharp increase in power is caused by the engine reaching the RPM where the scavenging goes from “just okay” to “incredibly excellent”.
On a dyno chart, this “hit” is the part of the dyno curve that shows the sharpest increase in horsepower output with each additional 100 rpm increment. However “on the water”, this “hit” is a part of the throttle range where the engine suddenly jumps past a 800-900rpm part of the rpm range. During on-water testing with a racing exhaust, a digital tach shows that even the most skilled throttle hand on earth cannot maintain a steady cruising speed anywhere within this “hit” range. Since it’s physically impossible to operate the machine steadily with in this rpm zone, our test riders call this range a “non-op” zone. It very literally is a part of the RPM range that you will never get to use because the engine cannot be held there steadily.
For the average racing Kawasaki 750/800 this “non-op” zone is roughly from 4800rpm to 5600rpm. “On the water” this rpm range accounts for roughly the 30mph-40mph speed range … where much of freestyle and recreational riding is done. Since most closed course racers are in a “all-on or all-off” throttle situation… a “non-op” zone at these mid-range speeds is no big deal. However for freestyle riders and high performance recreational riders …. a “non-op” zone in this speed range is a very, very big deal.
The stock SXR pipe is not a true expansion chamber… it is (what technicians refer to as) a “stub-pipe”. The big advantage of a stub-pipe design is that it is very compact, and has a very smooth “linear” power delivery. By “linear, we mean that the stub pipe allows steady cruising at any rpm you desire… from initial planeing speeds to peak speed. Unlike the racing pipe, the stock “stub” pipe has NO “non-op” zone. The down side of a stub-pipe design is that it only allows the engine to rev to a relatively low peak rpm (compared to racing pipes). In truth, stub pipes can make great power … they just can’t make great “high rpm” power.
In the case of our 849 SXR, we set out to make a stock-pipe kit that had so much overall torque that it could pull a much steeper prop to the same acceleration rates and peak speeds as a high revving race-pipe setup. However, unlike the race-pipe boat, the Group K 849 Sleeper has absolutely NO “non-op” zone. The 849 Sleeper can be held steadily at any rpm in the full range. For free-ride and short buoy-course riding, this linear power delivery is a huge advantage when setting up for a turn or jumping off a wave. The linear power of the 849 Sleeper has no sudden “hit” in the power range that suddenly upsets your positioning during a maneuver.
Every one of our 849 test riders was a devout “high-rpm” rider … and all were very skeptical of an SXR with a stock pipe. After one quick ride (and radar run) on our 849 Sleeper, every one of them was a “stub-pipe” believer. They all said that the linear power delivery allowed them to pull off quick maneuvers with a lot more precision because the engine had “a very electric” power delivery and instant throttle response.
Why is the average piston speed of the 849 Sleeper important??
One of the biggest factors that engine builders use to predict engine reliability is “average piston speed”. In short, the peak rpm and stoke length are plugged into a formula to obtain the average piston speed in “feet per minute” (it’s a finer measurement than mph). Here are The numbers:
Stock OEM peak rpm
Piston speed @ stock RPMs
Piston peed @ 6800 rpm
Piston peed @ 7000 rpm
Piston peed @ 7200 rpm
Piston peed @ 7300 rpm
Piston peed @ 7400 rpm
Piston peed @ 7500 rpm
Piston peed @ 7600 rpm
Piston peed @ 7700 rpm
4000+ fpm – Completely unpredictable life span of crankshaft components
3700 fpm – Crank life can predictably be 20-35 hours
3500 fpm – Crank life can predictably be a full season of use
3300 fpm – Crank life is predictably 2-3 seasons of use
3100 fpm – Production unit range, predictably 4-5 seasons of use
It is common knowledge, among stand up racers, that modified SuperJets have considerably better crankshaft life than modified SXRs …. Average piston speed is the reason why. One of the best features of the 849 Sleeper is that it delivers the water-speeds of a high revving setup, but yields the significantly lower piston speeds that improve crank life. It’s true that the slightly increased weight of the 849 Sleeper pistons does slightly increase loads on the connecting rods. However that load increase is nowhere near the load increases subjected by the extra 400-700 rpms of the higher revving race pipe setups.
How does the 849 Sleeper maintain the fuel range of a stock SXR??
As mentioned above, one function of any pwc exhaust system is to assist with cylinder scavenging. Ideally, the fresh fuel/air mixture coming in from the transfer ports pushes out the spent exhaust gases from the previous combustion cycle. A well designed exhaust creates a negative pressure wave at exactly the right moment to literally help “suck” out those spent exhaust gases. Racing exhausts create a negative pressure wave that is so strong that a considerable amount of fresh incoming fuel mixture also gets sucked out along with the spent exhaust gases. This exhausting of unspent fuel results in a much greater rate of total fuel consumption. Adding to that loss of unspent fuels, the racing-pipe setups are pulling through these unspent fuels 500-700 more times per minute (because of higher operating rpms). The end result is that a race pipe setup goes through a tank of fuel at roughly twice the rate of a stock pipe setup.
An added fuel-range benefit to the 849 Sleeper is that it makes enough torque to pull a much steeper prop than a stock SXR. The end result is that the 849 Sleeper actually has a better “cruising” fuel range than a stock SXR, and an overall “regular use” fuel range that is virtually identical to a stock SXR.
How about using aftermarket CDI boxes on the 849 Sleeper??
The aftermarket CDI units available for the SXR are generally intended for setups that turn very high rpms, and have low compression ratios. These boxes offer a much more advanced ignition timing value at mid-range rpms (for better mid-range drive), and more retarded high rpm timing values (similar to the stock timing) to avoid detonation at high rpms.
The 849 Sleeper gets it’s strong overall power from “higher than average” compression ratios. On-water testing with the 849 Sleeper showed that installing an aftermarket CDI made a very small increase in mid-range acceleration, and no increase in peak rpms. Can the aftermarket CDI units be used on the 849 Sleeper? .. Yes. Are the aftermarket CDI units a big benefit for the money spent on an 849 Sleeper? …. None of our test riders thought so.
One modification that was a nice addition to our 849 Sleeper was the lightening of the stock ignition flywheel. Our mod lightens the stock flywheel by about .6 pounds. Since we remove the majority of this weight from the outer parameter of the ignition flywheel (where it hurts the inertia the worst), there is a very noticeable improvement in throttle response and acceleration. For freestyle riders, this all-gain, no-lose mod gives an added dose of the instant response that you need.
What about Exhaust Modifications for the 849 Sleeper??
Understandably, the 849 Sleeper has to process and pass more CFM than a stock engine, in order to make all this additional torque. The excessive amounts of water being fed into the interior of the stock exhaust system make for excessive back-pressure that is “unfriendly” to the 849 Sleeper. Sadly, just bypassing water away from the cooling system (as is often done to reduce water pressure to the pipe) is not nearly precise enough to net the best performance effect. For our 849 Sleeper, we developed a new exhaust plumbing configuration that brings more precisely admits cold water at two separate input points on the stock exhaust body. With this new exhaust plumbing arrangement, exhaust system back-pressure is greatly reduced, yet the exhaust pipe body components all remain very safely cooled.
The net performance result of this kit was a noticeable gain in overall acceleration, as well as a modest gain in peak rpms (80-100). The water input orifices of this kit are somewhat small, and so require a filter to keep them fro being blocked by debris. During our 849 Sleeper testing, we ran for many hours with no water filter at all ….and no obstruction issues. However 849 Sleeper owners who ride in debris laden waters would need to run the filter and occasionally back-flush it.
Since some owners ride exclusively in “debris heavy” waters, this exhaust plumbing mod is offered as an option. That said, all our test riders felt that the increases in acceleration and peak rpm makes this exhaust mod well worth the price.
It bears noting that there are a number of popular stinger tubes and reduction sleeves being used by stock class SXR racers. Most of these mods net a power increase by increasing backpressures on the stock SXR engine. However the 849 has very different needs, and can develop overheating issues if these parts are used. Given that, our exhaust mod is the only one we recommend.
Regarding the Group K exhaust mod, the proper installation of the fittings and block-offs into the exhaust body is a bit touchy (you only get one chance to do it right), and so we do it with machine mounted fixtures. This requires that we have your entire exhaust body for this upgrade.
Carburetion and Inlet
On the 849 Sleeper, we use the same carburetor and inlet manifold modifications as our standard bore Sleeper kit. Again, because the 849 Sleeper is not a high rpm package, the 40mm “true bored” stock carburetors are easily able to meet the full needs of the stock-pipe setup. We do consider the installation of aftermarket flame arrestors to be a mandatory item to get the best results from the 849 Sleeper.
Larger aftermarket carbs on the 849 Sleeper might possibly net some additional increases in acceleration … and most certainly significant increases in fuel consumption. Since maintaining low fuel consumption rates was one of our primary goals of this kit, we did no testing of larger carbs.
About Head Gasket Sealing
One of the design problems that comes with any Big Bore kit is sealing the head gasket surface. This head gasket sealing issue is a two fold problem. The first issue is the sheer reduction of sealing area between the head studs and the new larger bore diameters. The second issue is the tendency of the top cylinder surface to deflect slightly and “move around” from the constantly varying temperatures of operation. The big boring removes a considerable amount of sheer mass from the cylinder casting, and thereby further increases the likelihood of gasket surface “movement”. With all these issues at hand, there is not nearly enough surface sealing area to allow for any kind of effective “O” ring sealing setup. For our 849 Sleeper we opted to use a stock metal head gasket bored out to the larger bore diameters. These gaskets along with a thin application of 3-Bond 1211 sealer) offer excellent long tem sealing. Unlike “O” ring setups, our metal gaskets have the ability to maintain a good seal on the slightly deflecting surfaces of a heated engine. In addition, every 849 Sleeper cylinder has it’s top surface decked and then lapped to assure a perfectly flat sealing surface.
About the Cylinder Big Boring
For many years, it was a very popular modification to bore the 80mm 750 Kawasaki cylinder to 82-83mm. This mod worked well, however 83mm was arguably the absolute limit that the 750 cylinder could be bored to. The stock 82mm bore SXR 800 cylinder is a slightly beefier casting than the old 750s, and the cast-in sleeves can easily accommodate bore diameters of the 849 Sleeper. Since the peak rpms of the 849 Sleeper are so low, the oem sleeve material has very good long term wear prospects in this application. However with the additional stresses and loads of a high rpm racing setup, the stock sleeve material would be at much greater risk.
Can the 849 Mod be done to 750s Also??
In short … yes. However, for 750 owners, the situation is somewhat different because the 750 cylinder sleeve casting will not accommodate the 849 bore diameters. To convert a 750 cylinder to 849 requires re-sleeving. Unfortunately, the stock 750 sleeve is not a straight diameter sleeve that can be slipped out by simply heating up the cylinder casting. Instead, the irregular outside diameter of the stock cast-in sleeve must be complete bored out of the aluminum cylinder casting. Once this is done, we fit in high-nickel content sleeve sleeves, and bore them to the 849 diameters. In truth, the quality of the material of these sleeves is much better than the material quality of the stock cast-in 750/800 sleeves. For anyone wanting to prepare a high rpm 849 racing engine, we would strongly recommend a re-sleeved cylinder so that you can have these higher quality sleeve bores.
The large top flanges on these aftermarket sleeves encroach slightly into the threaded head-bolt holes around the bore diameter. Given that, the sleeves must be precisely machined to clear the head studs without removing too much from the valuable head gasket sealing surface around them. This whole process adds to the cost of the sleeved cylinder, but it is crucial for the sleeved cylinders to maintain a good long-term head gasket seal.
About the Cylinder Studs
The removal of the sleeve material for the big boring (whether 750 or 800) slightly reduces the overall rigidity of the cylinder casting. Most importantly, it reduces the amount of material between the cylinder-head studs, and the bore diameters. The cylinder casting mass in this area becomes so thin that merely installing the studs into the cylinder creates a high spot on the bore diameter at each stud location. In fact, the actual installation torque applied to the studs makes a difference in how much bore diameter deflection there is. If a cylinder is merely bored and sized to 849 without the studs torqued in … and then assembled, there is a high likelihood that it will experience a non-stop series of piston seizures caused by these cylinder-stud high-spots.
To eliminate the effects of this problem, we mandate that your top cylinder studs be furnished to us with every 849 Kit. During the final preparation of each Group K 849 cylinder, we will torque in the cylinder studs, and then do the final cylinder sizing to eliminate the high spots in the bore. Note that we will not produce any 849 kit or cylinder unless we get the top studs for this operation. If a stud is broken off while removing it from a cylinder, we will machine out the broken stud and install a thread insert for $35 per broken stud.
About Cylinder Porting
When the SXR cylinder is bored to the 849 diameter, all the port heights and widths are altered. The stock SXR porting dimensions don’t do very well to feed the 781cc diameters … and that problem is further compounded when bored to 849. As a result, all 849 Sleeper cylinders receive full cylinder porting to the specifications we developed during our testing. For owners of existing (stock bore) Group K Sleeper Kits, there is some additional port-work needed. We will perform the needed port-work to Group K cylinders at no charge. All other cylinders will be billed for porting.
There are 4 different porting configurations of the 750 cylinder, and one configuration of the 800 (which is different from all those 750 configurations). Sadly, none of these configurations have the correct port heights or widths when the cylinder is sleeved to 849cc. Given that, every “sleeved” 849 cylinder receives full porting to the specs we developed for the 849 Sleeper.
Since the 849 has to move more net gases, it’s very important to remove any flow interruptions between the exhaust ports of the cylinder and the exhaust manifold. Given this, we match the exhaust manifold on each 849 Sleeper setup.
About Pump Mods and Components
Since the SXR as been around for several years already, there is much available in the way of pump and handling mods (props, pump stuffers, bored nozzles, plates, grates, etc.). We tested with many different combinations of these parts just to have an idea of what mods were compatible with the 849 …. Not to evaluate these parts, nor to their measure their exact impact on performance. In many cases the claimed performance merits were either “very subjective” or accompanied by compromises of one kind or another.
A perfect example of this is the use of “set-back” impellers and pump stuffers. Both of these mods improve pump “hook-up (by a margin that can’t really be defined in numbers). However at the same time they can increase the load on the engine and thereby reduce operating rpms and speeds by margins that can clearly be defined in numbers. The number of pump/plate/grate/stuffer combinations is more than anyone can test … us included. For the sake of our 849 Sleeper testing we went with two basic setups. First a completely stock recreational setup that used a Worx plate, Skat-Trak C-75 Swirl 9/17, and an 80mm taper-bored exit nozzle. The second setup was a more closed-course oriented setup of TBM plate and grate, set-back Solas Dyna-Fly, pump stuffer, and 80mm nozzle. While both machines turned 7000rpm, the recreation boat ran about 2mph faster on glass. All our stated peak speed data is with the recreational setup. Please know that the other parts you may be using can affect smooth-water peak-speeds.
For most high performance recreational applications, the stock SXR pump assembly works fine. For owners that want to “fine-tune” the stock pump, we recommend pump blueprinting. This pump blueprinting does not increase smooth water peak speeds, however it can make a noticeable improvement in rough water hook-up ability.
SXR 849 Sleeper Modification – includes cylinder big- boring, cylinder porting, exhaust manifold matching, top stud installation, head mod, carb “true-boring and jetting, cooling upgrade, 3-Bond 1211 sealer, head gasket, Pistons, rings, pins & clips
849 Upgrade (for existing SXR Sleeper owners)
Optional 849 Sleeper Exhaust modification (purchased with 849 Kit)
849cc Re-Sleeved Cylinder Mod for 750/800 Cylinders – includes cylinder sleeve machining, sleeve installation, boring, porting, exhaust manifold matching, head stud installation, top surface decking & lapping, bore sizing, 3-Bond 1211 sealer head gasket, pistons, rings, pins & clips
85.0 -85.4 Piston (1 - pistons, rings, pins, clips)
85.5mm Head Gasket
Aftermarket Pod flame arrestors with adaptors
Optional Ignition flywheel lightening Modification ( - .6 lb.)
Skat-Trak C-75 9/17 Swirl Impeller
Solas Dyna-Fly 13/22 Impeller
Exit nozzle taper boring (80mm)
Worx Ride Plate
Worx Scoop Grate
Optional Pump Blueprinting
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 new 150-lb. UPS weight limit makes engine shipping practical and affordable.
All top-end “kit” orders that are 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.