Kawasaki SXR 800-SXI 750 – 849cc Big Bore Mods

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.