Sea Doo 720 – Recreational Engine Mods

This literature gives background information on all of our “recreational use” modification kits for the Sea Doo 720 engine models, as well as some recommendations for the best bolt on parts compatible with those kits. We strongly recommend that you read this entire document before deciding which modifications are right for you and the kind of riding you intend to do. As new information becomes available, we will post that data to our 720 Updates Document. Our Sea Doo 720 recreational kits are as follows:

SWIFT KIT – This modification is intended as an affordable and “user friendly” modification that nets the maximum amount of acceleration increase for the money spent. In most cases, this will include a modification to the stock head and carb(s). Average speed increase of 2 – 3 mph on 2-seaters and 1 – 2.5 mph on 3 seaters

SLEEPER KIT – Intended for the average recreational rider, this very strong accelerating engine kit also consists of modification to existing pieces. The engine compartment of your finished Sleeper will look virtually stock. The Sleeper will start and idle like a stocker, have the sound level of a stocker, and run on 92 octane fuel under normal riding conditions. This kit is applicable to all 720 models (not IJSBA limited legal).

Average speed increase of 4 – 5 mph on 2-seaters and 3 – 4 mph on 3-seaters.

The Various 720 Models – No Sea Doo engine has appeared in more different hull platforms than the 720 engine. Since Sea Doo attaches similar model names to different year machines with different engine sizes, many folks (our technicians included) have found the Sea Doo model designation system to be a bit confusing. To clarify all this, the following is the list of 720 models we are offering kits for, and the way we refer to those models in house.

Dual Carb Models

• HX ————– dual 38 carbs, Stainless steel prop, and the same “sport ” style hull
• 95 XP ——— dual 38 carbs, Stainless steel prop, and the shortest 2 seater 720 hull
• 96 GTi ——– dual 38 carbs, Stainless steel prop, and the 1st generation 3 seater hull
• Sportster — single motor w/ dual 38 carbs, Stainless steel prop, and 14.5′ jet boat hull
• Speedster — dual motors w/ dual 38 carbs, Stainless steel props, and 14.5′ jet boat hull

Single Carb Models

• 97 SP —- single 40 carb, Aluminum prop, and the shortest 2 seater 720 hull
• 97 GS —- single 40 carb, stainless steel prop, and the larger 2 seater 720 hull
• 97 GSi — the GS model with adjustable trim and a gauge package
• 97 GTS — single 40 carb, stainless steel prop, and the 1st generation 3 seater hull
• 97 GTi — single 40 carb, stainless steel prop, and the 2nd generation 3 seater hull
• 98 GTi — single 40 carb, stainless steel prop, and the 2nd generation 3 seater hull

About our Group K Modifications for the 720’s – The 720 engine platform, from Sea Doo, has been a very popular platform for performance modification for some time. In the past, the majority of the performance parts and modifications available have been geared more towards the needs of closed course racers, not recreational owners. These race-oriented modifications often entailed large carbs (with very high fuel consumption rates) and high revving power bands that significantly shortened engine life. Stock 720 motors typically run in the 6750 – 6850 rpm range (different models use different pitch props). 720 Race motors that rev 7500+ rpm are common fare, however these high rpms can take a swift toll on many internal moving engine parts (the crankshaft bearings in particular). We at Group K will develop our own racing specification engines for the 720’s as well. However we recognize that there is a large number of recreational owners seeking increased performance that “does not” come along with heavy fuel consumption and sky-high rpms. Our recreational “Swift Kits” and Sleeper Kits” for the 720 have been developed to produce the maximum increase in the bottom end and mid range acceleration, while maintaining a (more crankshaft friendly) peak rpm of no more than 7100 rpm. Our testing shows that a 720 that operates under this rpm peak can still yield the good “long term reliability” that recreational owners seek.

The performance increase offered by these kits is by no means “small or casual”. These kits deliver big increases in acceleration and throttle response through the entire power band. In most cases, your Sleeper modified 720 will be an easy match for the performance of the 785cc version that uses your same hull.

Pump Condition and RPMs – Virtually all of our other website documents outline specific rpm/ peak water speed data…this document will not. Surprisingly, the reason for this is connected primarily with the variables of pump condition. During the testing of our first 720 (XP) prototype, we discovered this variable. Unbeknown to us, our test boat had a significantly “grooved” wear ring, and slightly damaged impeller edges. Had this pump been on a 785cc Sea Doo, we would have experienced “major” cavitation all nearly all engine speeds. However, for the weaker 720 motor, this compromised pump acted as a “limited slip clutch” that allowed the engine of the stock 720 to come off the bottom end easily, and accelerate through the range quickly. Even at peak rpm, this “harmed” pump allowed “higher than normal” rpm because of the ongoing slippage.

When we found this problem, we installed a new stock prop and liner insert. While the peak radar numbers remained the same, there was a loss of 100 – 120 peak rpm, and a noticeable loss of bottom end acceleration (as a result of the increased loads of the additional pump hook up). In addition, the “slight” prop slippage at initial takeoff was completely eliminated.

We realize that the majority of 720’s out there are older ones that have high prospects of mild impeller and wear ring damage. We strongly urge any 720 owner (in search of more power) to first inspect the condition of their prop and liner before spending money on any performance modifications (ours included). If you install performance parts on a “harmed pump” 720, you may experience increased rpm…but not much increased performance.

Along this same line, we also strongly recommend that all 720 pumps be removed and resealed (where the pump case meets the hull. Any slight leak in this area creates exactly the same “cavitation” symptom that a damaged prop would cause. If you are not familiar with the resealing process, have it done by a qualified dealer.

For these reasons, quoting and comparing the peak rpm numbers of various 720 units would only be relevant where a resealed pump with a prop and liner in perfect “unharmed” condition. That is to say…all you have to do is eat a few rocks or have a poor pump seal, and your rpms will be higher than our quotes.

About Exhaust Systems – Perhaps the biggest difference that sets apart the Group K recreational kits from most others is that we prefer to retain the stock exhaust system. The aftermarket exhausts available for the 720 engine certainly can offer increases in peak power, however they usually accomplish that by significantly increasing peak rpm. In many cases these higher rpms need to be complimented with much higher compression, and large racing carbs. The net result is an gas guzzling engine that runs much hotter, at high rpms that will prematurely wear internal moving engine parts.

During our testing we found that the stock exhaust system, when matched with a small collection of conservative modifications, was capable of delivering a surprisingly broad and controllable powerband. By themselves, none of our other modifications would offer a “big” power increase…but collectively they yield very big increase in acceleration along with a “livable” increase in peak rpm.

Our modifications can certainly work very well when used together with an aftermarket exhaust pipe. However the peak rpm and horsepower output would require 100+ octane fuel…the 92 octane pump gas (that most recreational owners prefer to use) would result in rapid overheating and eminent engine failure.

About Compression – There is no shortage of folks that are willing to offer their “opinion” of what is the maximum “safe compression ratio” for a 720. In truth, the maximum safe compression ratio of any pwc engine is a function of the peak rpm of your particular engine combination. Since our kit modifications can rely on the “torque oriented” design of the stock pipe for strong low-end power, we do not need to employ excessively high (heat causing) compression ratios to get good overall power.

The specifications of our “Swift” kit and “Sleeper” kit cylinder head modifications are quite different (as are the prices). The Swift kit modification is safe for any machine that employs a stock exhaust pipe and stock cylinders (stock porting that is). The head modification used for the Sleeper kit has dome dimension and head volume specs that are specifically intended to work together with the ported and decked cylinders of this kit. We cannot use the same spec head on both kits, however we can easily upgrade the less expensive Swift kit head to the Sleeper specification. (Note: our phone technicians cannot offer information about these head mods as it is our policy to not give out specification information on mods that we do)

About Carburetion – The 720 engines come stock with two different carburetor arrangements:

1. Dual BN 38-34 Mikunis and;
2. Single 40-38 Mikuni. In each of these cases, we bore the stock carb(s) for the Swift kit and Sleeper kit. These intake systems have very different characteristics, and so we will describe them separately.

Dual Carbs – There are many 44mm and 46mm aftermarket carburetor sets available for the 720 engine. However the port opening in the stock crankcases is 39mm (equivalent). We question the performance benefits of carbs that are so much larger than the port openings in the crankcases. What is not questionable, is the big increases in fuel consumption that can come along with these oversized carb throats.

The butterfly diameter of the stock dual carbs is 38mm, however the actual minor diameter at the center of the venturi is 34mm. For both of our kits, we bore these stock dual carbs to a true 40mm, and fit them with 40mm butterflies. This new bore diameter, so closely matching the diameter of the rest of the stock inlet passage, allows for maximum inlet flow. More importantly, the 40mm throat carbs have much better inlet tract air speed than oversized 44 and 46 carbs can. This higher inlet tract air speed allows for easier low speed tuning, and very strong low speed throttle response. For easy cold starting, we also replace the choke butterflies with a primer kit.

Along with this carb mod we “strongly” recommend the all aluminum R&D flame arrestor. Besides the excellent performance of this arrestor, it best accommodates the stock cylinder head/carburetor brace plate. Maintaining this brace is absolutely mandatory. Without it in place, the stock inlet manifold will develop fractures in a matter of operating minutes (this applies to stock boats as well as modified’s).

Single Carb – Contrary to what most owners would think, the single carb machines are not at a big disadvantage with respect to performance. With the carb boring we do for the single carb kits, that difference becomes even smaller yet. The stock single carb on the 720 motors has a 40mm butterfly, and a 37mm minor diameter at the venturi. The stock inlet manifold on these machines has an inside diameter of 43mm. For these single carb machines, we bore the stock carb to a true 40mm. This modified single carb offers seamless operation, and impressive acceleration. The single carb modification does not include a primer for starting, because all the stock single carb 720’s employ an accelerator pump that performs exactly the same function. (Note: We prototyped and tested a 43mm carburetor, but it had worse fuel range than the 40, and performed no better than the 40)

Unlike the dual carb machines, we recommend the stock flame arrestor box on the single carb machines. The slight restriction of the stock flame arrestor element (even with the bored carb) offers a much needed “inlet charge slowing” that improves fuel flow through the carburetor. All of our tests with less restrictive flame arrestors resulted in less efficient fuel delivery. This is a phenomenon we have not observed on any other machine we have tested with, but we found ourselves unable to argue with the readings from all our fuel flow instruments. The stock single carb flame arrestor box is a bit large. However even with the stock screen element in place, it has plenty of air access ability. This arrestor box also perfectly accommodates all the stock carb braces. Like the dual carb setups, we consider the use of this stock carb bracing to be absolutely mandatory.

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 Sleeper kit, 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 Sleeper 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” 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 Sleeper 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 Ignitions & RPMs – The Sea Doo service manuals specify the rev limiter of the 720 engines to be at 7000 (plus or minus 50 rpm). This same limit spec is given for the 785 motors, which we have consistently seen at 7050 rpm. Unfortunately the 720 ignitions don’t seem to run at the high limit of the specification. We have seen most stock 720 ignitions stop cold at 6920 – 6950 rpm. Since the Swift kits increase rpms up to the low 6900’s, this is not a problem for customers using only the Swift kit modification. However Sleeper kit customers will need to take some measures to keep from “bumping” the limiter of their stock ignition. Sleeper kit customers can accomplish this in one of two ways:

1. Installing a Micro Touch rev limiter module that allows higher rpms, or
2. Install the slightly steeper pitch Solas “X” pitch impeller.

Micro Touch CDI Module – Customers interested in the best peak water speed abilities will get the results they are looking for by maintaining the stock impeller and installing this CDI module. The Micro Touch module is a small device that plugs into the stock electrics. This module does not increase the rpms by itself, but rather it allows the Sleeper modified 720 motor to rev to it’s actual peak (about 7000 rpm). These modules come in two basic styles, based on the stock electrics of your particular machine. The earlier 720’s Have all their ignition electrics housed in a grey plastic housing. The module for this style ignition is relatively inexpensive ($99). The later style 720 ignitions are driven by an exposed black electrical box (called an MPEM). These later 720 ignitions require a rev limiter module that is more complex, and more expensive ($225).

Steeper Impeller – Some 720 Sleeper kit owners may want to have a speed increase, but still retain their stock rev limiters (particularly those with the later ignition requiring the more expensive rev module). For these customers, we recommend the Solas “X” pitch impeller. This prop has a slightly milder low rpm pitch (that improves takeoff), and a slightly taller high rpm pitch that will increase speeds while still holding the 720 Sleeper under the stock rev limit. This prop combination is generally about 1mph slower than the “stock prop/hi-rev module” combination, however many 720 owners consider 1 mph a small price to pay for the improved fuel range and long term engine life that comes with the lower operating rpm. While the pitch of the Solas X is virtually ideal for all the 2 seater 720’s, it may be a little steep for the 3 seater models. For the 3 seater owners, Group K will be offering these X props with a slight pitch adjustment to accommodate the larger (and heavier) boats.

About Rotating Mass – The 720 engine has a relatively large and heavy ignition flywheel attached to the front of the crankshaft, and an additional flywheel (called a “PTO”) mounted onto the rear of the crankshaft. For folks building race engines, the removal (or reduction) of these two “external” flywheels is among the first modifications made. Significantly reducing this heavy rotating weight helps a race engine to accelerate into the high rpm range much more quickly. Unfortunately, this rapid engine acceleration (and deceleration) can take a toll on all the bearings on the crankshaft as well. These crank problems are not a function of heavy loads when the boat is accelerating, but rather a result of “out of water” high rpm flashes, and the heavy loads on the crank when the pump reconnects with the water. While the quick acceleration of lighter flywheels may sound beneficial to a racer, those benefits are seldom worth the crankshaft reliability loss for recreational applications.

Contrary to many other engine builders, we contend that the added flywheel mass is an advantage for recreational application 720 engines. With the stock flywheels in place, the engine cannot “free rev” as quickly when the pump comes out of the water at speed. More importantly, the flywheels maintain a great deal of momentum that helps to keep the revs from dropping off when the pump “reconnects” with the water. During this reconnection, the rotating mass of the flywheels also helps to reduce the loads on all the crankshaft bearings.

The main technical down side of these heavy flywheels would be a “supposed” reduction in acceleration. However the Sleeper kit 720’s can easily spin the impeller from a low speed take off. Having lighter flywheels on the crankshaft would not result in quicker take off…just more prop spinning.

About Pumps and Impellers – All the Sea Doo 720 models, except for the ’97 SP, come stock with a stainless steel impeller. During our prototyping, we tested many other aftermarket impellers in hopes of uncovering some hidden performance. After all was said and done, we found that the stock stainless steel impellers offered the best in overall acceleration and peak speed ability. However (as previously mentioned) those stock props do mandate the need for a rev limit module on machines equipped with the Sleeper kits. Please see the Ignitions and RPM’s segment above.

The only other aftermarket impeller that offered a specific functional advantage is the Skat Trak Swirl design props. The Swirl prop, used by many closed course racers, offers a huge improvement in rough water hook up and low end acceleration. Unfortunately these characteristics come along with a very noticeable loss in peak water speed ability. Just the same, some 720 3 seaters are used almost exclusively for towing skiers of floatables, where sheer peak speed is not an issue. On these machines a swirl prop can be a very good choice.

About 3 Seaters – The Sea Doo 3 seater hulls are not only heavy (for a 720cc machine), but they also have a lot of water contact surface area the serves to additionally “load” the motor. Along with this, the 1st generation hull 3-seater hull has a very “nose heavy” attitude on the water. During our testing with this hull, we modified the ride plate in a way that helps to significantly “lift” the nose of the hull off the water, without inducing any noticeable “porpoising”. The net result was a much smoother ride in rough water, and a 1mph increase in speed. Since the ride plate on these machines is cast as part of the pump-sealing ring, removing the ride plate mandates the removal of the pump. While this removal process is a bit tedious, the overall results of the ride plate modification are well worth the trouble.

Reliability Issues – 720’s that are new, or have relatively low hours, can easily accommodate the slightly increased loads of the Group K kits. However we realize that there are plenty of 720’s out there that have “lots” of time on the motors (150 or more hours). While these high hour engines “might” be able to continue to operate okay in stock form, the added loads of any performance increase can sometimes induce the failure of a time worn engine component. To be sure a normal rebuild, done by a qualified shop, can easily restore a 720 to full longevity strength. However owners with “median” hours 720’s can take some preventative measures. They are as follows.

Air Leaks – The lower end of the 720 motor must be airtight enough to hold 10 psi for about 10 minutes. Any leak that exists can allow raw air into the motor that will cause intermittent lean conditions, and possible piston scoring. The most common points of leak are at the rotary valve cover/inlet manifold, the crank seals, and the center case split (behind the starter). Also inspect the inlet manifold for any fractures. These leaks are easiest found with a pressure test kit – available from Watercraft Connection (503) 232-2026. We strongly recommend that any 720, with over 100 hours, be pressure tested before the installation of any performance equipment.

Crankshaft – The crankshaft in the 720’s may be unfairly characterized as a weak part…it is not. However it is a part that has longevity limits when subjected to high usage hours or high rpms. If you have any reservations about the condition of the bearings on your crankshaft, get it rebuilt…the investment will be well worth the money.

For customers who desire an engine rebuild, Group K will perform complete short block rebuilds assembled with your Sleeper modified parts. These rebuilds will include final pressure testing of your 720 engine.

About Strokers – For several years, the IJSBA competition rules permitted the use of stroker crankshafts in machines that used the 720 engines. The current IJSBA “Superstock” rules no longer allow the use of stroker crankshafts. Despite this many 720 recreational owners have considered the use of a “stroker” crank.

While these “stroker” engines certainly offered an increase in performance, that increase comes along with big losses in long term reliability (of the crankshaft in particular). We consider the longevity prospects of these “stroker” based engines to be so poor, that we will not construct any stroker 720 based engines, nor will we produce a Sleeper kit to be fitted on to a stroker.

Big Bores – Big bore kits for 720 based racing engines are quite common. However the long term reliability, in a recreational application, has yet to be proven. While the cylinders themselves have enough material to accommodate larger bore diameter pistons, we have concerns about the increased long term (and short term) wear experienced by the connecting rod bearings. We suspect that the key to the best reliability will be a result of keeping bore diameters close to OEM, and operating rpms to a minimum… our engine setups are aimed in that direction.