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Kawasaki Ultra 1350 – Outlaw Big Bore Kits

Group K Modifications

This document will outline the development process that resulted in the construction of 1350 Big Bore kit for the Kawasaki Ultra. The process is no short story. This document is intended as a supplement to our first web document for the Ultra 150, and it contains more detailed background information that is a result of our Ultra testing in the interim.

The Group K 1350 Outlaw Kit – In short, our Ultra 1350 kit can add 4-5 mph of peak speed to a stock machine, along with a huge dose of increased acceleration. The Ultra 1350 is the strongest accelerating pump gas pwc we have ever constructed. Along with this strong overall performance, the 1350 is a relatively easy installation that requires no lower end engine mods at all. Better yet, the 1350 is designed to operate with the stock exhaust, ignition, and the very fuel-efficient stock CV carbs (re-jetted). This is THE most powerful 92 octane modification available for the Ultra 150.

About the ’99 – ’00 Boats

The most significant “intended” difference among the ’99 – ’01 Ultras is the 2000 model introduction of altered ignition mapping that allowed for the use of less expensive spark plugs (more on that later). However there are several other areas of “unintended” manufacturing variables that have accounted for some very measurable variations in peak speed abilities among the production units.

It should be understood that Kawasaki, like all other pwc manufactures, try’s their very best to maintain the best possible level of overall performance consistency among their production machines. However the Ultra, like many other pwc muscle crafts, uses component designs that are pressed to the outer limits of their efficiency and design capabilities. This results in two problem areas: 1) Any component that is in less than optimum condition can significantly deteriorate performance, and 2) There is not a huge measure of performance ability “left on the table” using the stock components.

Production Variations – It goes without saying that there will always be some performance variations that result from production line manufacturing. As any machine relies on advanced design components to make it’s performance, the subtle variations in those components can affect performance of the whole unit. The following are the most common variations (we have proven so far) that can have an effect on the true speed ability of any particular Ultra.

Hull – In the same sense that elite athletes are often a product of random “good genes”, so too are particularly fast Ultras often a product of randomly good hulls. This does not say that any Ultra hulls are bad, but rather that some a measurably above par. We tend to think that this may be a function of how the Ultra hulls rely so much on high-speed lift in order to attain true peak speeds. We refer to the Ultra’s long, slow acceleration to peak speed as “crawl time” (literally the time it takes the boat to crawl up to peak rpm and peak radar speed). Our considerable on-water testing has led us to believe that this long crawl time is more a function of the hull than anything else. That is, the Ultra hull does not completely get up out of the water until speeds have gotten quite high. As the speeds slowly increase, the hull lifts slightly higher out of the water, which allows slightly more speed…and so on. To be sure, the goal of most modifications is to reduce this crawl time, however on very smooth water conditions it will always exist to some measure. The exceptionally fast hulls that we have seen not only go slightly faster, but also they typically have shorter crawl times as well.

The act of “hull truing” is something of a “black art” that we do not claim to do, nor have any expertise in. We can say that we have not observed any re-occurring hull shape characteristic of Ultra hulls that routinely offers better speeds when attended to. Just the same, there are plenty of folks who have attempted to do their own kind of “hull truing” to their Ultra in an effort to improve the “genes” of their hull. As expected, many of these folks have claimed to see “big” results from this work…maybe so. All that said, we have not yet seen any truing technique that has been able to consistently (in back to back radar tests) make slower Ultra hulls into faster ones. We will leave that quest to the “hull folks”.

Props – The Kawasaki parts books show only one part number for the ’99-’01 stock Ultra impeller. Referencing any pitch numbers to this prop is a complete waste of time. The stock Ultra prop has blades that are curved on three planes, most of which are not measurable with any of the normal pitch gauges. In addition, the stock Ultra prop has the thinnest blades of any OEM prop we have ever seen. The 3-D blade contours and thin blade material is another example of design pushed to the outer limits to optimize a part. If the thin blades of the Ultra prop are subjected to even the slightest rock damage, overall speed and performance suffer heavily.

While there is only one prop part number offered by Kawasaki for the Ultra, we have observed a wide variation of peak rpms from “identically appearing” stock props. During our big bore testing, we were in possession of two OEM props that pulled the same engine to rpm peaks 250 rpm apart. Oddly enough, the radar speeds of the two props was very close, but the hook up and acceleration and hook up characteristics were different. We have seen situations in the past where subtle changes in hub contour, and blade “root pitch”, have made rpm variations on this scale for other machines. The best that we can presume is that these props come from different vendors whose different manufacturing techniques make for props that are dimensionally close to blueprint, yet are “seen” very differently by the water passing through the pump. Whatever the case, this variable has made it very difficult for us to cite Ultra rpm numbers for machines using “stock” impellers…shimmed or not. Since the 1350 kit makes too much power for either of the stock prop variations we have seen, this problem is resolved by simply going to the Solas 16-20 Dynafly

Pump Bearings – Since the Ultra pump utilizes a tapered diameter prop that is shimmed closely to a similarly tapered housing, the Ultra relies heavily on excellent condition pump bearings to maintain optimum impeller to housing clearance. If there is any part of a high output Ultra that might be considered a “weak link”, the front pump bearing and seal would be it. Even though the front bearing in the pump is a heavy-duty high quality sealed bearing, it can still allow some prop movement when subjected to the staggering loads of a high output Ultra operated at full loads. To date there is no way to remedy the wear that this abused bearing eventually takes. While solutions are being sought, the only current remedy (for Ultra owners wishing to get optimum performance) is to install a fresh bearing and seal regularly. In the case of high output Ultras that are operated in the most abusive way, replacement intervals of 15 – 20 hours are common. If a machine is operated with a worn bearing, the prop can move slightly forward to make physical contact with the impeller housing…resulting in sudden and significant speed loss. In many cases, a whining sound can be heard coming from the rear of the machine, indicating the failed bearing.

It bears noting that a newly installed bearing seems to go through a break in period, of sorts, that results in an eventual gain of 1 mph or more. We don’t know exactly why it happens…we just know that it does.

Modified Speeds – Given all the above variables, it’s hard to predict “exact’ peak speeds for any particular Ultra (stock or modified). There is no shortage of individuals that claim to have modified 70+-mph pump gas Ultras. We have prepared Ultras with our stage 1 kits that have run 69 mph…and many others identically prepared that ran 66-67mph. Based on all the variables that we have seen during our testing, we are inclined to believe that these “exceptionally faster” machines are much more the exception than the rule. This being the case, our text here, regarding various components, will tend to speak more in terms of mph increases rather than just peak speeds.

High Performance Options – Our perspectives on the areas of limitation. For the money spent, our (standard bore) Stage 1 kit still offers the biggest overall performance increase that you can apply to an Ultra (basically a head mod and pump shimming). In addition, the Stage 1 modification has proven to be completely detonation free on 92-octane gasoline.

While many individuals have sought more speed via other additional modifications, none have been able to completely stave off the damaging detonation that accompanies virtually all of them. The text below will outline our experiences with the more popular modifications, and our perspective of their performance verses detonation-risk prospects.

During all our testing, we utilize a detonation sensor that uses a microprocessor to determine what percentile of the total combustion cycles are detonation strikes. Most pwc engines can operate safely with 5 – 8% detonation at any given rpm. Anything over 15% for more than 4 – 5 seconds will result in immediate piston scoring.

The end goal here is to do any modifications you like, so long as it does not increase detonation risk. Unfortunately, most of the mods that improve performance will also significantly increase detonation risk. Selecting mods that minimize “deto” risk has been the primary goal of our Ultra development, and was the eventual reason we chose to pursue a big bore configuration.

Octane – By far the biggest deciding factor for high performance Ultra owners is fuel octane. In most North American outlets, 92 octane is the highest available at the pump. Since most Ultra owners don’t want to deal with the costs or logistics of getting higher-octane race fuels, 92 octane fuel is the limiting factor. More accurately, the potential detonation risks of 92 octane are the limiting factor. With 100+ octane fuels, a lean carburetion setting will cause a loss of peak performance…but no detonation and no piston scoring. With 92 octane, any significant lean condition will quickly result in detonation. This detonation results in swift overheating in the combustion chamber that leads to equally swift piston scoring. For owners willing to use 100+ octane fuels, we have developed a 100+ version of the 1350 kit (more on that later).

Stock Ignition – In an effort to get the maximum power out of the Ultra 150 engine, Kawasaki engineers developed an ignition that has multiple timing curves. Not only do all three cylinders have different timing curves, but the ignition “mapping” delivered to all cylinders varies based on throttle position, a function dictated by the throttle position sensor (or TPS). When the TPS “sees” a swift opening of the throttle, the ignition delivers very advanced timing for a brief amount of time. As peak rpms are achieved, the timing switches to a slightly more retarded mapping. However the timing required for detonation-free partial throttle cruising is even more retarded than the full rpm mapping. When the TPS “sees” the throttle slowly lifted from wide open to partial, the ignition selects a much more retarded mapping to stave off detonation at mid range cruising speeds.

All in all, this ignition works wonderfully to allow strong overall power along with high detonation resistance. However it must be kept in mind that the Kawasaki engineers laid out this ignition mapping to be optimum with stock compression, and the fuel mixture of the stock carbs. This means that “liberal” increases in compression can easily result in increased detonation risk. Our Ultra Stage 1 (stock bore) modification utilizes different compression ratios among the 3 cylinders (called “staggered” compression) to accommodate the different deto risks that are induced primarily by the stock ignition mapping. To date there is no way to modify the mapping of the stock ignition, and there doesn’t appear to be any mods available anytime soon. For now, the best tack to take is to simply work within the limits of the stock ignition mapping.

Other 92 Octane Non Options

Heat Sensor Advance – The Ultra has a heat sensor that can advance the entire mapping by about 3 degrees when it “sees” exceptionally high air temperatures in the engine compartment. The intent of this sensor is to maintain good performance in areas with poor air density caused by high air temperatures. Installing a resistor in place of this sensor “fools” the ignition into seeing high temps all the time, hence offering the more advanced timing all the time. Our on-water deto tests showed that this mod can be safe “sometimes” on an otherwise stock machine. However when combined with higher compression heads (ever our staggered ones) the deto risk is dangerously high. We consider this to be a 100+ octane mod only.

TPS Removal – There is no benefit to removing the TPS from the stock carbs. However many aftermarket carburetor sets do not accommodate the stock TPS, hence they require that you tie it to the side (locked in the full throttle position). By doing so, you will be getting full throttle ignition mapping at all engine speeds and throttle settings. As previously mentioned, this mapping offers a very dangerous deto risk at partial throttle. We consider any arrangement that does not use the stock TPS to be a 100+ octane only setup.

Flame Arrestors – Like increased compression, aftermarket flame arrestors are a favorite bolt-on performance part. In the case of a pump gas compatible Ultra using the stock CV carbs, installing an aftermarket flame arrestor is not a good idea. This is not a function of the flame arrestors being poorly designed, but more a case of the carb design not accommodating the change. The stock CV carbs have a conventional throttle butterfly just like all other pwc’s. However the CV carbs also have a vacuum operated slide that is driven by inlet tract vacuum. A free-er breathing aftermarket arrestor allows air to enter the carbs more easily, thus reducing the inlet tract vacuum that lifts the slides. This means that at most throttle positions, the vacuum slide (and its fuel-metering needle) are considerably lower in the carb bore than they should be. The result is some very serious lean conditions in the mid range (particularly at 6200 rpm). No carb jetting to date has been able to attend to these lean conditions.

It may be possible to maintain an “acceptable” deto risk on an Ultra that is “completely stock” except for aftermarket arrestors. However, again, when used in combination with higher compression heads, the deto risk (especially at part throttle) would be unacceptably high.

We also tested the removal of the rubber air inlet tubes from the stock flame arrestor cover. The net result was a slight leaning out throughout the entire range that offered a “slight” performance increase. However this mod resulted in occasional detonation strikes at full throttle, and significant deto strikes at 6200 (note: choke plate removal has a similar effect). For normal 92-octane use, we recommend that all 150 owners, using the stock CV carbs, retain the “entire” stock flame arrestor…this follows for our 1350 kit as well.

Cylinder Porting – Despite the fact that the stock Ultra cylinder castings offer very good port timing and port shape, there can be some power gains to be had by slightly altering the porting. Creating a port layout that can slightly improve high-speed ability is very do-able. Among other details, porting for better high rpm power would require increasing the exhaust port size. Enlarging the exhaust port area also results in a slight reduction of compression ratio, and so the heads must be modified to recover this lost compression. Unfortunately, by the time you increase the compression enough to recover the loss in low speed acceleration from the porting, you will have a compression ratio that results in a very high detonation risk (particularly at mid-range rpms). These detonation issues might ordinarily be attended to by altering the ignition curve…but that is not an option for Ultra owners (yet).

About Triple Pipes – While triple pipes have been in use on professional race versions of the Ultra for some time, all those applications have been “race gas only” setups. More recently, Coffmans has released their triple pipes for the Ultra with claims of pump gas compatibility. As of this writing, we have not had a chance to test the Coffman triples and we know of no reliable pump-gas Ultras using any style of triple pipes. While we do not doubt the performance abilities of triple pipes on the Ultra, we do have serious reservations about their ability to operate reliably on pump gas. Whatever the case, there are a few other technical items worth considering before choosing to install triple pipes, and the high rpms (7700+) that they can offer.

Ignition – While the mapping of the stock ignition may not be ideal for the triple pipes, it seems to work fairly well. Unfortunately the pipes have the ability to rev far beyond the 7400 rpm limit of the stock ignition. Since there are no Ultra rev limiter mods available to date, the only remaining way to eliminate the limiter is to install a total loss ignition. Because of the cramped space underneath the stock ignition cover, the difficulty of installing such an ignition is…very considerable.

Oil Injection – Whether or not you eliminate the stock rev limiter, the stock oil injector pump will certainly be under heavier strain from the added rpms. Normally, oil injector pumps are just removed from higher revving engines but the Ultra has a very uncommon injection system that complicates removal. The Ultra injection pump has 5 outputs, one for each carburetor, and one for each of the two balancer bearings. The oil delivered to each balancer bearing fills a small reservoir area that bleeds a 50% supply of oil to the front and rear cylinders (the other 50% is supplied to the front and rear carb fittings). The 5th output of the injection pump delivers a 100% supply of oil to the center carb. If the injector pump is removed, the oil reservoir areas for the balancer bearings would need to be sealed off, and then have their oil changed periodically (read; big pain in the butt). We are informed by knowledgeable sources that removing the balancers altogether is “not a good idea”.

Balancers – In addition to the oil injection issues, the balancer bearings themselves are an item for concern in a high rpm Ultra platform. Virtually all counter-balancers used in pwc and motorcycle applications consist of a counter-weight (or weights) mounted on a shaft supported by bearings on both ends. Having these bearing at the shaft ends divides the loads of the eccentric weight in a way that minimizes abuse to the bearings.

In an effort to keep the lower end of the Ultra more compact, Kawasaki engineers opted to utilize two smaller (and separate) counterweights, each supported by a single stub shaft and single bearing. These counterweight bearings do not get the leverage benefit of supporting the counterweights from both sides, and so can endure some significant abuse when spun to high rpms. For Ultra engines that operate within a “close to stock” rpm range, these counterweight bearings have not been a wear issue of any consequence. However if an Ultra engine were to be spun 700-900 rpm faster that the intended rpm peak, it’s realistic to presume that these bearings will require more regular replacement (requiring splitting the cases). All in all, this is a kind of maintenance that (we believe) most recreational riders may hesitate to undertake.

The 92 Octane Group K 1350 Big Bore

From a sheer technical standpoint, a big bore modification offers good overall power potential, without amplifying any of the detonation risk or reliability issues of other modifications. A big bore can deliver strong overall power (especially acceleration) without the need for high compression ratios or high rpms.

About the Cylinders – The Nickasil plating on the stock bores offers several very good technical features, however we opted to install steel sleeves into the stock cylinder casting rather than run a larger “plated” bore. It bears noting that engines that are set up with plated bores will typically carry a very tall piston deck height. The deck height is the distance from the center of the wrist pin to the top of the piston face. This tall deck height allows for very little piston “rocking” within the bore. This reduced piston rocking helps to keep the piston crown from “tending” toward the empty open area of the exhaust port on the upward stroke. Some degree of “tending” will always take place on any high output two cycle. However if excessive “tending” takes place in a plated bore, there will be significant wear on both the plating above the exhaust port and the piston rings and piston exhaust face.

When big boring, the piston crown height is not reduced, but the bore to crown-height ratio is increased…serving to net a similar effect. That is, a larger bore diameter piston has a slightly greater risk of “rocking”, and “tending” the exhaust crown of the piston into the exhaust port. There is an expected amount of normal long-term cylinder wear that takes place in most engines from this piston rocking. This wear is dealt with by way of overbore piston sizes on bore-able cylinders, or cylinder replacement on plated bore engines.

We have serious reservations about the long-term cylinder wear prospects of a plated bore big-bore Ultra cylinder. While Nickasil plating is certainly tough stuff, it is not indestructible nor impervious to normal wear…it can be worn off or chipped. For these reasons, we chose to construct our 1350 big bore with relatively thin walled steel sleeves that allow for two additional over bore sizes (.020” and .040”). The installation of these sleeves makes for a slightly higher initial cost. However the long-term wear (and maintenance) prospects are much better.

From a more specific perspective, The Ultra engine does a very nice job of accommodating the dimensions of a big bore upgrade. The parent casting of the Ultra cylinders have plenty of extra mass. The cylinder can easily be bored to accept a sleeve, without creating any dangerously thin or weak areas in the part. In addition, the head gasket-sealing surface still allows for plenty of safe sealing area around the new larger bore. The best “up-side” of big boring on the Ultra is that it requires no modification at all to the lower end of the engine…a 1350 big bore simply bolts on to the stock lower end.

The 1350 kit includes cylinder porting that is designed to deliver strong and linear acceleration from the very bottom, all the way to peak 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 less than 10% of the total performance gain (about .5 mph and a little bit of acceleration). The Recreation finish is standard in the 1350 kits, and the “Competition” finish is available as an option. The cost difference between the two is $220.

All 1350 cylinders are chamfered, sized, and finish honed to the diameters of the supplied pistons. To assure that the modified cylinders will have the correct compression ratio and squish clearances for the octane specified, we modify the cylinders and cylinder heads as matched sets only.

About Cylinder Heads – Whenever possible, we prefer to have completely stock heads for conversion to the 1350. Group K Stage 1 modified heads can also be converted to 1350 heads. Heads that have already had significant dome modification will need to be inspected by our in house technicians to confirm that they have ample material for a safe 1350 upgrade.

For big bore owners that prefer aftermarket cylinder heads, we recommend the R&D cast individual heads. These heads can be used with, or without, the optional “girdle” kit. This additional girdling hardware uses full-length studs to help secure the cylinders to the cases, thus reducing the loads on the mount points of the cylinder casting. The girdle kit is only mandated for owners constructing 100+ octane 1350 kits.

Whichever head is chosen, we mandate that our big bore head gaskets be used with them. There are many “O” ring heads on the market that utilize no head gasket at all. We have not been impressed by these heads ability to maintain a “lasting” head-gasket seal on standard bore engines. Sealing the head surfaces on the 1350 is more challenging yet. Given this, we will only use heads for our 1350s that allow for the use of a stock thickness head gasket.

About Carburetion – The stock “CV” type carbs of the Ultra may not the optimum carb for racing applications, but on the 92-octane 1350, they can deliver impressive performance that is surprisingly close to that of many racing carbs.

It might seem natural to presume that the 1350 would require much larger than stock carburetion. However, remember, the 1350 is still “exhaling” through a virtually stock exhaust pipe, and is not designed to turn particularly high rpms. These factors make the cfm (cubic feet per minute) capabilities of the CV carbs a very acceptable “no compromises” choice for high performance recreational riding. To be sure, the CV carbs need to be re-jetted to accommodate the changed fuel demands of the 1350. However after jetting, the CVs offered impressive overall performance along with “not far from stock” fuel consumption.

For 1350 owners who choose to utilize the CV carbs, the use of the entire stock flame arrestor is considered to be absolutely mandatory. We consider the inlet tract vacuum offered by this arrestor to be fundamental for the correct metering of the CV carbs. Failing to use the entire stock flame arrestor can result in severe (and lethal) lean conditions that cannot be addressed with jetting. Our testing showed that there were no performance benefits of modified flame arrestors that matched the huge reliability risks.

Your stock carb rack must be sent with the cylinders and heads for the 92-octane 1350 modification. We will perform re-jetting along with overall inspection of the carbs. Note that carbs with chokes removed CAN NOT be used on the 1350. The presence of the chokes significantly affects the inlet tract vacuum, and therefore the carb calibration.

As of this writing, we have not completed test evaluation of any non-stock carb arrangements. For the meantime we would consider any 1350 utilizing larger aftermarket carbs to be a 100+ octane arrangement. If further testing shows that any aftermarket carbs can be pump gas safe, we will post that data here.

About Reeds – All of our 1350 testing was conducted using the stock reed cages and petals. We believe these stock parts offer an excellent balance of performance and reliability. It should be noted that any aftermarket reeds will have a significantly different fuel demand, and require different carburetor calibration to maintain 92-octane compatibility. Since correct carb calibration is such a lengthy process, we will not be calibrating any 92-octane platforms for use with any aftermarket reeds.

About Air Access – When revving your Ultra out of the water, you can visually see the top seat surface being sucked down slightly by the engine compartment vacuum created by each grab at the throttle. As previously mentioned, the restriction of the flame arrestor is fundamental to proper metering of the CV carbs, however the restriction of air admitted to the engine compartment can have a negative impact on the performance of the 1350. It must be kept in mind that any modifications that can access additional air to the engine compartment can also offer the potential to introduce water as well. Given this, any air access mods must be done in a way that minimizes the potential for water intrusion. It simply is not possible to create a completely watertight air inlet that offers “zero restriction”. The more important priority would be to simply minimize the level of air restriction that exists. Failure to make any air access modifications can inflict a loss of 40 – 80 rpm to a 1350 Big Bore. Recommendations for air access modifications are provided with the 1350 kit.

About Ignitions – The mapping of the ’99 ignitions is slightly more advanced than the 2000 – 2001’s. This slightly increased advance does little to improve power, but serves greatly to increase the partial throttle detonation risk on the 92 octane 1350s. For ’99 owners who wish to run their 1350 on 92 octane, we strongly recommend to get the 2000 cdi box that contains the updated mapping. This update will also permit the safe use of the lesser expensive BR8ES spark plugs.

About Exhaust Systems – The stock Ultra exhaust system may not be the ultimate design for racing, but it performs as well on the 1350 as any single pipe we have seen. As of this writing, we have not found any modifications to the stock exhaust that net a significant overall improvement on the 1350. If any such modifications come forward in the future, we will post that data and pricing here.

About 100+ Octane Versions of the 1350 – Since 100+ octane fuel offers a huge margin of safety against detonation and overheating, any 92 octane 1350 can be quickly converted to a 100+ octane version. Our testing showed that merely removing the rubber air inlet tubes offered a slight leaning out of fuel metering across the board that nets a noticeable improvement in overall performance using 100+ octane. For this 100+ octane setup, we also replaced the engine compartment temp sensor with a 500-ohm resistor. Between the timing advance of the resistor and the leaning of the arrestor lid mod, the 1350 Ultra can run 1 – 2 mph faster and gain a significant margin of acceleration. When additional compression was applied to this setup, there was a good gain in acceleration, but a nominal gain in peak speed. Group K can offer higher compression spec 1350 heads, however they will only be made by request.

About the Pumps and Props – The stock Ultra pump is still a very effective unit within the rpm range that the 1350 operates. However the Ultra pump does have some “sensitive” areas that require attendance in order to get the best performance and efficiency.

The first and foremost upgrade required for the pump is reducing the impeller to wear-ring clearances to a minimum. This is a modification that we address for our other Ultra kits as well (called “pump shimming). Since the manufacturing tolerances of the pump components vary, it seldom happens that merely removing a shim from the pump can have ideal clearance. The various parts involved are commonly out of round, and/or not concentric with one another. When we shim pumps in-house, we machine the parts of the pump to address these concentricity and out-of-roundness issues.

The stock Ultra impeller is designed to deliver the maximum in speed and hook up for the output of the stock Ultra. One of the features that allows this prop to work so well is the uncommonly thin impeller blades. Thinner blades make for more thrust because they displace less of the water passing through, and they move more easily through the water that is present. The down side of these very thin blades is that they are very easily damaged (even by small debris), and that they begin to flex significantly against the loads of big power increases. This flexing results in very noticeable cavitation, particularly at low hull speeds.

The strong low end and mid range of the 1350 generates a torque that seriously flexes the stock Ultra impeller. The best solution is to install an aftermarket impeller with thicker blades. As of this writing, more impeller makers are taking a serious interest in designs for the stock Ultra pump. To date, the best choice for the 1350 is the Solas Dynafly 16 – 20. This impeller has the same basic blade design as the stock prop, but uses much thicker blades for less flexing and more durability. This Solas prop, in a properly shimmed pump, allows the 1350 to bolt out of the sharpest apex turns with incredible authority, and virtually no cavitation. It is a perfect match for the power of the 1350.

About Handling Hardware – The stock Ultra ride plate, scoop grate, and sponsons do a very good job of allowing the hull to deliver the best in smooth water peak speeds. However many owners (especially those of modified Ultras) have a great interest in improving the rough water and/or high-speed turning of their machines. Unlike so many other pwc’s, there are no aftermarket handling parts for the Ultra that assure better rough-water abilities/turning abilities without causing serious losses in smooth water speed. For lack of a better way to put it, the Ultra hull is much more speed “sensitive” to handling mods.

Unfortunately, the added power of the 1350 nearly mandates some improvements. In particular, it needs the hook up benefits of a more aggressive scoop grate. As of this writing, the Worx scoop grate appears to offer the best in hook up with the least loss in peak speed.

With regards to ride plates and sponsons, there is no clear cut “best”. With the installation of every ride plate or sponson, we have seen speed losses that run anywhere from 1 – 4mph. However we are made to understand that there are some combinations of a particular plate with a particular sponson (when used together) that can net decent turning along with a very tiny speed loss. Since handling characteristics are so subjective, we do not test all the possible combinations. We only know that nothing we have seen (to date) runs faster on smooth water than the stock ride plate and sponsons. If a better combination comes forward, we will post that data here.

One simple handling/speed related modification we recommend for our 1350s is to shorten the threaded end of the trim cable by about 5mm. This allows the nose of the hull to lift slightly higher out of the water, thus slightly improving smooth water peak speed. There is easily enough trim adjustment remaining, after this mod, to allow for aggressive “nose down” turning.

About Measuring Speeds – Since it’s introduction in 1999, the Ultra 150 has been the fastest production pwc on the market. There has never been any mistake about the intent of the engineers priorities when it came to the Ultra…maintain good smooth water peak speed abilities even if it requires compromising high speed rough water abilities. This set of priorities is shared by the majority of Ultra owners.

When talking about comparative speeds of Ultra mods, it’s important to understand the various means of measuring speeds. All of the info below presumes that speed measurement is taking place on very smooth water conditions.

Stock On-Board Speedometer – Typically, the stock speedo shows much higher than true speeds. In addition, the paddle wheel input for the speedo is greatly affected by parts that change the movement of water across the rear of the hull (ride-plate mods, etc). On pure glass the stock speedo can show momentary speed “spikes” that are not actually taking place. Given all this, we do not consider any data shown by the stock speedo to be dependably consistent or accurate.

GPS – There are many good GPS units that are a good basic speed-measuring tool. However here too, we have seen many GPS units record a “spike” measurement that never actually happened. We have also seen numerous situations of “chord-ing”. That is, the GPS will calculate speeds on a series of straight “chord” lines rather than measure the true distance covered in the course of a large arc path. When this happens, you will see an initial high number on your GPS, and then there will be a slow dropping of shown speed that is not actually taking place. GPS chord-ing and “speed spikes” can also be caused by a GPS that loses or gains tracking from a satellite while in the middle of a speed run. All it all, GPS readings can be useful information, but they are not as flawless as real-time radar.

Radar – While radar guns are expensive and unfriendly to getting wet, they do offer the most accurate level of exact speed measurement. With respect to the Ultra, radar gun readings do have one very big flaw. Most Ultras tend to take a significant distance to “crawl” up to their true peak speed. In most cases, an Ultra will drive off the end of a radar beam before it finishes that very long “crawl time.

Side By Side Operation – It bears noting that 1 mph equates to roughly one boat length in 7 seconds. While side by side operation (on totally smooth water) offers no exact number, it does offer the most accurate and most visible example of speed changes. Radar testing often strains to repeat accurately when there is a difference of only .2 or .3 mph. However side by side operation allows that margin to be seen much more accurately.

About Tachometers – The stock tachometer can have an error range of about 200 rpms. We urge 1350 owners to get a more accurate digital tachometer. The two best options are the “Tiny Tach” ($55), and the PET 2100 ($308). Since the Tiny Tach updates every 2.5 seconds, it can only yield accurate data on long smooth-water passes. The PET 2100 updates twice a second, and so offers very accurate data in less time. Regardless of update time, any pwc will always yield it’s highest rpm numbers on smooth water.

About the Parts – Since the cylinders and heads will be getting a major machining reworking during the 1350 modification, we do not require “pristine” condition cylinders or heads to build the 1350. That is, unusable cylinders with badly damaged bores, and heads with debris damaged domes work as well as brand new pieces. If you are unsure if the damaged parts you have can be safely used for a 1350, we will gladly inspect and confirm your parts at no charge.

92 Octane – Group K 1350 Big Bore

Group K Price

Group K 1350 “Outlaw” Big Bore Modification (92 octane)

Includes: Sleeving, Boring, and Recreational Finish Porting, Cylinder Head Modification, Cooling Upgrade, Big Bore Head Gaskets, Base Gaskets, and Jetting of CV Carbs


Optional “Competition” Porting Finish

(Parts Required for Modification: Heads, Cylinders, and Carb Bank)


Group K 1350 “Outlaw” Big Bore Modification (100 Octane without Carb Jetting)

Includes: Sleeving, Boring, and Recreational Finish Porting, Cylinder Head Modification, Cooling Upgrade, Big Bore Head Gaskets, & Base Gaskets


Optional “Competition” Porting Finish

(Parts Required for Modification: Heads, & Cylinders)


Individual Piston Assembly (Piston, Ring, Wrist Pin & Clips) (85.0, 85.5, 86.0mm)

(NOTE: Replacement Pistons will only be provided to 1350 Owners)


Big Bore Head Gaskets


Install Sleeve, Bore, and Recreational Finish Port 1 Cylinder (No Piston)


Install Sleeve, Bore, and Competition Finish Port 1 Cylinder (No Piston)


Tiny Tach


PET 2100DX Digital Tachometer


Pump Shimming


Solas 16-20 Dynafly Impeller


Worx Scoop Grate



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 !!