4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Sea Doo 720 – Hammer Racing Engine Kits

By Group K

This document is intended to be a supplement to our “Sea Doo 720 Recreational Kits” document. We strongly recommend that the recreational document be read in addition to the information here. As some of this technical info is time sensitive (and therefore outdate-able), we will be posting technical updates to a separate “720’s Updates” document on this website.

Objectives of the Hammer Kits – The two most popular high performance 720 models are the SPX/XP 720 runabout, and the HX 720 sport models. Since these machines are by no means top performing “muscle boats”, they are most commonly used for recreational riding. The HX is much more commonly used in sanctioned IJSBA racing than the XP, because it is clearly the best “platform” in the competitive “sport” classification (even though the sport class allows 785cc displacements).

Since the larger 785cc SPX/XP models are so affordable and available, we have not encountered any XP 720 owners that are looking to build a heavily modified 720 boat. Given this, our Hammer kits have been aimed primarily toward the HX 720 model. Besides a large contingent of HX owners seeking big power increases, these machines are often prepared to conform with the IJSBA Super stock “sport 785” class.

Our 720 Hammer Kits are built in two different versions based on the octane of the fuel that they can reliably operate on.

The 92 octane Hammer kit is essentially our Sleeper modification kit, along with the Factory Pipe Products exhaust system, and a steeper impeller. This makes the 92H kit an affordable, and relatively simple modification.

The overall performance of the 92H is much better than that of our Sleeper kit, however the peak rpms are much higher as well. The 92H is ideal for the high performance recreational rider that might occasionally race a local closed course event. The strong acceleration and good peaks speeds of this 92H kit allows the HX to be a top level competitor in local level IJSBA 785 Sport competition (note: we recommend a 50/50 mix or race or “avgas” fuel during “racing” use).

Getting this 720 kit to deliver exceptional performance is easy. Getting it to deliver that performance (reliably) on 92 octane pump gas is everything but easy. The specifications required to obtain pump gas safe operation on this kit are somewhat crucial. We set up all these crucial specifications when we modify your 720 parts. However the 92H kit must have these particular modifications, and only these modifications, in order to operate safely on pump gas. Substitutions in carburetion, exhaust, etc. would induce variables that would make it impossible for us to assure pump gas compatibility.

The 105+ octane Hammer kit is a very potent and serious closed-course racing machine. This kit nets the maximum amount of acceleration that the hull (and pump) can deliver. The 105H produces the brutal overall power that is required for competitive national level closed course competition. This increased performance (and rpm) does require some additional maintenance considerations…but for the dead serious HX racer, the 105 Hammer is it.

The 105H utilizes more extensive modifications than the 92H (44 carbs, case porting, lightweight ignition, etc). All these additional modifications are discussed below.

About Big Bores – One of the most popular modifications for 720 owners to consider, is to merely bore their cylinders to a larger displacement. For many engines, big boring does offer big dividends of overall horsepower…but this is not always the case. Our testing led us to believe that the 720 Sea Doo platform is an engine platform that gains very questionable benefits from big boring (given the parts currently available).

The 720 engine utilizes a two ring piston that uses an “L” section (aka dykes) ring as the top ring. This means that the top edge of the L ring is the edge that designates the moments of cylinder port opening and closing. The actual placement of this ring edge is significantly lower than the “top” edge of the “non-L ring” big bore pistons that are most commonly used. The result of using these, non L-ring, aftermarket big bore pistons is a significant reduction in port timing opening. This problem can be quickly resolved by simply raising the ports. However raising the ports makes for significant changes in transfer port angles that “some” engine builders believe to be “not ideal” (we are among those builders). In addition to this, the rings on these aftermarket pistons typically have much more ring tension against the bore diameter that the stock oem piston rings have. This additional tension makes for greater resistance against the bores that can sap away some acceleration ability. The oem stock L-ring utilizes the pressure of compression to drive it against the bores for improved sealing, and so their resistance is somewhat less.

While all these theoretical effects can be argued differently, on-water performance is the bottom line. During our tests, the 750/760cc big bores that we tried on our HX Hammer kits ran very well. However they never offered a “huge” performance increase over an identically prepared 720cc version. Given the extra wear and reliability risks of the big bore setups, we could never justify the costs of the big bore upgrade during our tests. This is a situation that may change with the introduction of aftermarket “L” ring pistons made in the same deck dimensions as the stockers. But until then, we stand firm on our oem piston recommendations. We did find a very significant performance improvement by boring worn 720cc cylinders to a new fresh, oem oversize piston. But this cylinder boring dealt with issues of fit, not displacement.

About the Factory Pipe Products Exhaust System – The FPP pipe is generally accepted as the best aftermarket exhaust system for the 720 platform. These pipes were the first produced by FPP that used ECWI and a mandatory component instead of a low speed acceleration “option. The ECWI system injects a measured amount of water into the interior of the pipe body between 3000 and 6500 rpm. This injection system is actuated by a microprocessor built into the cdi module that is supplied with each pipe. The injected water changes the internal sonic exhaust waves in a way that “fools” the engine into believing that the tuned length of the pipe is much longer than it actually is (hence a great improvement in low end power). After 6500 rpm this supplemental water source is completely shut off, allowing for strong high rpm output. If the ECWI system on this 720 pipe is disconnected, the low-end performance becomes so poor that the machine becomes very difficult, if not impossible, to ride.

FPP conducted all their discovery testing for this pipe on a dead stock 720 engine (flame arrestor and all), and many 720 owners have installed these pipes on stock machines with excellent results. Unfortunately these stock 720s, with the FPP pipe fitted, seemed to be “very” intolerant of “any” additional modifications. For some time, most performance shops (Factory Pipe included) had not been able to obtain reliable “pump gas” operation from HXs when “any” other modifications were added along with the pipe. The big reliability problem was that of mild detonation. Ordinarily, dealing with this sort of detonation is not a difficult task. Unfortunately the 720 engine is very easily sent in to detonation by incompatible modifications, and it tolerates very little detonation before scoring a piston. In addition, the 720 is a somewhat small displacement engine that is pushing against the loads of a somewhat large hull.

Despite all the difficulties involved, we were able to get very strong and very reliable pump gas performance with our modifications in concert with the FPP exhaust pipe. We used one of the recently available (and very expensive) microprocessor detonation sensors to detect and resolve the detonation risk points of this arrangement. We found that there were three key areas to avoiding the detonation difficulties that so many other shops had experienced. These are compression, carburetion, and rpm management (all discussed below).

Water Boxes – The issue of waterbox mufflers is not only an issue of performance, but also an issue of politics…particularly in recreational riding areas. The 92H kits are designed and developed to be run with a completely stock waterbox muffler along with all the stock water input plumbing. In truth, the stock 720 waterboxes are very well designed, and offer “close to optimal” results in stock fashion. The sound level of a 92H is virtually the same as a stocker.

The IJSBA Super Stock rule does permit the replacement of the stock waterbox. We have not seen any aftermarket water boxes that net “big” increases in overall performance, however we have seen plenty that have offered a huge increase in overall sound level. We intent to test further in this area. However for now, we will recommend exclusively the stock 720 waterbox muffler.

About Compression – Perhaps the biggest lesson we learned in the development of our 92 H kit was that the ECWI system on the FPP pipe allows for the use of “much” milder (than normal) compression ratios, without any loss of acceleration. In addition, we found that this reduction in compression did not degrade peak speed ability by any measurable amount (providing you used the right impeller). Within this compression reduction, we also found the need to “stagger” the compression as well. Staggered compression refers to an engine whose cylinders are intentional setup with different compression ratios. In the case of the 720, we found the rear cylinder was prone to a level of detonation that could not be resolved by richer jetting. However by giving the rear cylinder a lower compression ratio than the front cylinder, we were easily able to manage the remaining detonation risk with jetting. This staggered compression has no mechanically detrimental effects on any other part of the engine. Along with the reduced and staggered compression, we also do a special reshaping of the cylinder head domes that further helps stave off detonation.

The 105H kit obviously utilizes a much higher compression ratio than the 92H, however it also requires “some” stagger in the compression and dome re-shaping. Compared to the 720 racing engine kits offered by other builders, ours tend to run a slightly more conservative compression ratios. The use of excessive compression on the 720 race engines can make for extreme overheating at high rpms, as well as a noticeable reduction of peak rpm ability. Our testing led us to find that all the low end power requirements of a racing HX can easily be met with the use of range oriented porting, strong signal carburetion, and a well chosen lightweight ignition rotor. We contend that with the use of these components, sky high compression ratios are simply not needed for a top flight racer.

About Cylinder Porting – Cylinder porting has received a bad reputation as a modification that nets only top end power at the expense of low end power. This may be true for poorly executed porting. However professionally done cylinder porting can be tailored to produce increases in any area of the rpm range. In the case of the 720 engine, the weakest part of the power range is the bottom end and mid range. We specifically aimed our porting modification, for the 720 Hammer kits, to net the biggest performance increases in these lower rpm areas. This porting also accesses an increase in peak rpm ability, but we consider that top end increase to be a side effect of all the flow improvements, not a result of high rpm oriented specifications.

The port chamfers on the stock 720 cylinders are nearly non existent, and can cause the rings on the 720 to wear prematurely. When we do the porting, we also chamfer all the port windows to assure better long term wear for the rings and piston.

Our cylinder porting is available in two different finish modes. The “competition finish” (used in the 105H kit) includes all the port shaping, specification work, and passageway final surface finishing. This final finishing work accounts for about 30% of the total time (and cost) yet contributes to only about 10% of the total performance gain. The “recreation finish” (used in the 92H kits) offers all the dimension and port shaping work, without the more costly finish work. The cost difference…$400 vs $280.

As we receive cylinders for porting, we also inspect the bore diameters of all cylinders. If we find cylinders out of clearance specification, we can bore your cylinders to the next oversize (you may provide pistons or buy them from us). In any case, every set of cylinders (bored or not) receives a honed finish after porting.

About Carburetors – The 92H uses stock 720 carbs that are “true bored” to 40mm, and then re-jetted. While the basic carb modification is the same as used on our Sleeper kits, the jetting is very different to meet the greater fuel demands of the pipe and the higher rpms. This underlines one of the slight disadvantages of the FPP exhaust system, that is…fuel consumption. One of the jobs of a well designed exhaust system is to improve scavenging. Scavenging is the act of processing fresh mixture into the cylinder during the transfer cycle. The FPP pipe scavenges so well that it can cause some unburned fuel to be sent out the exhaust port during this cycle. The result is having to jet up to accommodate for this lost fuel. That richer jetting can result in a noticeably shortened fuel range (if the engine is run hard). Our 720 Hammer kits are very fast, but they are not nearly as fuel efficient as the stock piped Sleeper kits.

Another side effect of this heavier fuel consumption is the need for stronger fuel pump capabilities. This problem can be solved in one of two ways. For folks that might have another un-used Mikuni carb at hand, you can simply install a second fuel pump on to the rear carb, and install a pulse fitting into the rear crankcase to drive that added fuel pump. The other option would be to purchase a 15 gph fuel pump kit. This kit comes with a block off plate that will eliminate the stock pump on the front carb. One of these two systems “must” be in place to feed the 92H a sufficient supply of fuel.

The 105H kit utilizes a dual 44mm Novi “XR” carburetor kit. This kit includes spigot mounts, fuel pump, air separator, and hardware (an R&D manifold plate is also required). While 46mm carbs are often used by other builders, we find that the 44’s offer “no comprise” performance along with much stronger low end acceleration, and easier tuning than the 46’s.

It bears noting that the stock inlet ports in the stock 720 crankcases are 38/39mm. Given this, the inlet ports in the crankcase, and the new R&D manifold, must be increased in area to really maximize the benefits of the larger carbs. The lower end of the engine must be disassembled in order to perform this “case porting”. We consider the installation of the larger carbs, without case porting, to be a very questionably worthwhile modification.

About Fuel/Air Separators – It’s a common occurrence for a pwc to offer poor performance (or surging) when the fuel level becomes very low. This change in performance takes place because the fuel mixture is becoming very lean from the air bubbles occasionally entering the fuel pickup (and then the carbs). High speed turning can cause this same effect, as the fuel is pushed to one side of the tank, away from the pickup tubes. For stock horsepower machines, this momentary lean condition is not much more than an inconvenience. However as a machine becomes more heavily modified, these occasional lean conditions can result in momentary detonation, and light piston scoring. This risk is particularly high under the heavy loads of high speed cornering. To avoid damaging the engine in this way, the Novi 44mm carbs (of the 105H kit) have a air separator system built into the supplied “fuel rail”. Unfortunately, this fuel rail cannot be installed on the stock bodied carbs of the 92H kit. For situations like this, Novi offers an in-line air separator that can be installed along with the 92H carbs. The only catch is that this separator can only be used in a fuel system that employs the remote 15 gph fuel-pump. It will not work on a dual (carb mounted) pump system. 92H owners who may want to add one of these separators later on, should plan ahead by choosing the 15 gph remote fuel pump arrangement over the dual pumps.

Other Inlet Modifications – For the 92H kits, we utilize the aluminum R&D power Plenum flame arrestor. This arrestor offers excellent air-flow, and a very strong cylinder head brace attachment. We consider this brace attachment to be absolutely mandatory to the long-term survival of the, somewhat fragile, stock rotary valve cover plate. As additional insurance against valve cover plate fracturing, we strongly recommend the use of a “brace plate”. This brace plate mounts over the cover and provides much needed extra support and strength. The R&D rotary valve cover used on the 105H kit is beefy enough, on it’s own, to operate safely without this additional brace.

The 44 Novi carbs of the 105H kit can accommodate individual “pod” type arrestors, or a one-piece “plenum” type. While the pods are often more convenient, we find the common plenum of the one-piece arrestor to offer slightly better throttle response.

About Impeller/Pump Choices – For the 92H HX, we use a Solas Xo impeller, re-pitched to keep the engine operating at a peak of 7300 – 7400 rpm. Our tests indicated that lower peak rpms make for an increased detonation risk, while higher peak rpms make for an “undesirable” reliability risk. Our re-pitched Solas Xo impeller is prepared to deliver the ideal rpm range using the stock pump case and stock nozzle diameters. For recreational riding, the 92H can hook up and accelerate very well with the stock scoop grate and ride plate. More aggressive riders may want to use a “top loader” type scoop grate for extremely aggressive rough water riding. However these scoop grates can put a lot of additional load on the engine, thus increasing the risk of detonation. For this reason, we recommend a 20% mix of race gas for 92H owners who intend to do aggressive rough water riding with a top loader type scoop grate.

105H owners have a broader range of impeller and pump setup choices. Since most 105H owners intent to do closed course racing, they tend to opt for the much better hook up of a Skat Trak swirl impeller, over the slightly better peak-speed abilities of a Solas prop. For 105H owners that want the very best in hook up, the Skat Trak “C-75” stainless steel pump is the last word in national caliber hook up. While these stainless steel pumps offer unequaled hook up, the benefit/dollar ratio would likely be appreciated by only the most serious racers.

There is some disagreement over the best peak rpm range for top flight racing 720’s. Our tests show that an rpm peak below 7500 rpm yields the best in overall performance and “racing level” longevity. Consistently exceeding this 7500 rpm range is certainly do-able, however we believe that these loads take an uncommonly high toll on the crankshaft bearings, in exchange for a relatively small benefit.

Ignitions – Both of our Hammer kits utilize most of the stock electrical system components. The Micro Touch CDI module (supplied with the FPP pipe) is an addition to the stock electrics, not a replacement item for any of the standard parts. Some HX race boat owners have chosen to install a “total loss” ignition system on their machine. “Total loss” means that the ignition is driven solely by energy from the battery, with no charging system to replenish the voltage that is lost. We would certainly concede that these total loss ignitions can be very effective. However we do not recommend them because we have experienced “random” reliability with them.

For the 105H we recommend that the stock ignition Flywheel be replaced with a lightweight aluminum “charging” flywheel. This aluminum part has charging magnets that permit the machine to retain a functional charging system capability, yet still offers much quicker acceleration. Since varying levels of magnetic strength in the magnets can affect the ignition timing, we recommend that the timing be reconfirmed after the installation of any lightweight charging flywheel.

About Lower End Preparation – As mentioned in our 720 Sleeper document, any crankshaft that is suspected of being worn, should be rebuilt or replaced. This holds particularly true when constructing a higher revving Hammer kit. If you choose to get the crankshaft rebuilt, we recommend that “truing and welding” be a mandatory part of this process. Without the welding process, it is possible for the crank journals to rotate slightly out of index sometime during high rpm operation.

About Rotating Weight – Lightening a crankshaft, and it’s components, is a very popular way to improve engine acceleration. All 720’s come with a large flywheel (refereed to as the PTO) attached to the rear of the crankshaft. There is a great temptation to lighten or eliminate the majority of the PTO’s mass on 720 racing engines…”don’t do it”. Besides acting as a vibration dampening device, the pto also reduces the “crank twisting effects of a pump that is constantly coming in-and-out of the water at high rpm. That said, there is a significant advantage to replacing the stock pto with a “fluid balancer” replacement. This fluid balancer is slightly lighter than the stock pto, so it improves acceleration. More importantly, this balancer helps greatly to reduce high rpm vibration, and the destructive side effects of vibration. We strongly recommend this balancer for any 720 setup. The only down side to the fluid balancer is the installation process (or rather the removal of the stock pto). The stock pto is usually on “very” tight. We have yet to be able to remove a stock pto from an assembled engine. In each case, we have had to remove the crankshaft from the cases in order to effect a removal.

Another worthwhile measure that that can be taken against vibration is the balancing of the stock ignition flywheel. The stock flywheel does get some balancing at that factory, however that balancing is only intended to accommodate the lower rpms of a stock 720 engine. “Fine tuning” the balancing of the stock flywheel, for a higher revving 720, allows for smoother engine operation, and better crankshaft bearing life. This fine tuning can often be performed by shops that do balancing of automotive engine components.

Package / Upgrade / Parts

Group K Price

92 (octane) Hammer KIT MODIFICATIONS
Includes: Recreational Finish Cylinder Porting, Cylinder Head Modification, Carb Throat Boring and Jetting)

Parts Required: Cylinders, Cylinder Head, & Carburetors


Factory Pipe Products Exhaust Pipe with CDI Module & ECWI


15 GPH Remote Fuel Pump Kit


Dual Carb R&D Flame Arrestor


Solas “Xo” Impeller (with Pitch Adjustment)


Optional Novi In-Line Air Separator


Optional Rotary Valve Cover “Case Brace”


Optional Fluid Balancer


105 Octane – Hammer Kit Modifications

Group K Price

Hammer 105 Cylinder Porting


Cylinder Head Mill & Squish Cut


R&D Inlet Manifold Plate


Group K Case Porting


Novi Dual 44mm “XR” Carb Kit


Primer Kit


Factory Pipe Products Exhaust Pipe with CDI Module & ECWI


RAD Aluminum Charging Flywheel


Fluid Balancer


Solas “Xo” Impeller


Skat Trak Swirl Type Impeller (17/24° Pitch)


Skat Trak Stainless Steel “C75” Pump (with Impeller)

$950.00* exchanged

Rebuild Pricing

Group K Price

Engine Teardown, Spec Re-Assembly, and Pressure Test


Crankshaft Rebuild, Truing and Welding (includes new Bearings & Rod Kits)


Outer Crankshaft Seal Set


New stock OEM 720 Sea Doo Crankshaft


Cylinder Boring, Honing and Chamfer (per pair)


Sea Doo OEM Oversize Piston and Ring Set


*prices subject to change based on manufactures pricing
**NOTE: Group K will bill an additional $35.00 handling charge for engine assemblies via UPS


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 !!