Resto-Mod Build Up – Sea Doo HX

OVERVIEW: Our customer was a die-hard HX lover (as many HX owners are). His very fast HX race boat had the normal compliment of aftermarket goodies… Factory Pipe, Novi carbs, Total Loss ignition, etc. Unfortunately, it also possessed the “less attractive” qualities that come with high rpms… heavy fuel consumption of 100+ octane fuel, and poor crankshaft life span. This customer wanted us to construct him a “daily driver” HX practice boat that offered great power delivery and good fuel range. He was not very interested in big peak speed numbers, but he did want a reliable 91-octane “Shredder” that had the strong torque and hard acceleration that HX owners value so highly.

The Objectives – The key to this build-up is that we did not need to seek exceptionally high peak speeds, but we did want to get the maximum in strong acceleration. The big choice that we had to make was whether to use the stock pipe, or use the Factory Pipe Products race pipe. There is no doubt that the FPP pipe makes a huge increase in overall power, however much of that power is generated by a huge increase in peak rpms ( from oem6800 to FPP7600+). Normally it’s not a big deal to rev a 68mm stroked engine to 7600rpm. However the SD 720 engine has a very long heavy crankshaft whose bearings do not stand up very well (long term) to the loads of 7500+rpm operation. In addition, those same high rpms net a big increase in fuel consumption… not a good feature for a practice boat.

With all this considered, we opted to configure this HX “Shredder” around the stock exhaust system. While the stock pipe does not allow high rpms, it does offer a very strong and linear acceleration curve. The objective would be to produce the biggest possible increases in torque and response along with modest increases in rpms.

The following text will outline those tuning choices, and how they contributed to the goals of the build-up. At the bottom, you’ll find the complete price list.

The Finished Boat – Our finished HX “Shredder” is an absolute joy to ride. It starts and idles just like a stocker. It travels long 5mph zones with no loading up of coughing…. And it offers strong and instant acceleration anytime at any speed. One our original goals was to have this boat run the speeds of a stock HX with an aftermarket pipe on it…..mission accomplished. Our stock-piped HX consistently runs 54-55mph while only turning 7050-7100rpms. In addition, it has the ability to cruise smoothly at any rpm from planning-speed up to peak-speed (unlike “piped” boats that tend to have a “hit” that makes mid-range cruising difficult). Along with all of that, our “HX Shredder” can deliver over 40 miles of hard riding on one tank of fuel…. Much better than the high rpm HXs. This “daily driver” HX is everything our customer was looking for… and more.

Crankshaft – As mentioned above, the real weak link of the Sea Doo 720 engines is the crankshaft’s poor resilience to high rpm operation. Understanding this weakness is the key to keeping our HX reliable.

The SD 718cc engine first appeared in 1995 in the lightweight and great handling HX and XP hulls. Given the light weight of these hulls, it was a given that operation in rough water would see the engines flashing up to peak rpms when the pump was unloaded, as well as experience a very heavy load when the pump “reconnected” with the water. SD engineers took two measures to “protect” the crank bearings from these loads. The first was to build in an uncommonly low rev 6950rpm limiter into the ignition. This would limit the “unloaded peak” thus reducing the loads that the crank would be subjected to upon reconnecting the pump to the water. The second measure was to add very heavy “flywheels” to both ends of the crankshaft. The front flywheel is the ignition rotor that contains heavy magnets and a large steel starter gear. The 5.5 pound rear flywheel is referred to as a “PTO”. PTO is short for “power take off”, a term established by the snowmobile industry that uses this same style of rear flywheel. In the snowmobile world, a heavy PTO is needed to avoid rpm drops when their centrifical clutch engages the drivetrain (often at rpms above 6000rpm). Since PWCs have no such clutch, a heavy PTO merely slows the rate of acceleration at all speeds. The only advantage of the added rotating mass of the ignition flywheel and PTO is that they keep the HX crankshaft (and crank bearings) from being instantly decelerated to low rpms when the pump reconnects to the water. This added weight is great for crank bearing life…but very bad for quick acceleration.

In the interest of gaining instant acceleration, top-level HX racing engines eliminate the 5.5lb. PTO altogether, and they routinely employ a very lightweight aluminum ignition flywheel. This big reduction in rotating mass makes for a huge increase in the quickness of acceleration. Sadly however, the bearings of the very long HX crank experience dramatically increased stress from the non-stop action of being instantly accelerated and decelerated during high-speed rough water riding.

For Our HX “Shredder”, we opted to retain the stock weight ignition flywheel, and just “reduce” the excessive weight of the stock PTO. Eliminating the PTO altogether was not an acceptable option because the heavy front flywheel might easily “twist” the crankshaft “out of true” when the pump re-connected during rough water riding. By reducing the PTO weight by 2 pounds, we retained plenty of the crankshaft protecting mass, while still gaining a very noticeable increase in the quickness of throttle response and acceleration. In addition to this, we were maintaining the stock pipe so that our HX “Shredder” would enjoy the added crankshaft protection of peak rpms no more than 7100.

Cylinder – In an effort to get the maximum peak power from the stock 718cc engine, SD engineers selected cylinder port timings that are much more radical that that of Yamaha 701cc or Kawasaki 750cc PWC engines. While these radical port timings are very suitable for racing setups that use the high revving FPP pipe, we found the porting to be a bigger harm to torque than a help for top end. As a result, we aimed our cylinder porting modification toward maximizing bottom end and mid-range abilities, with increased rpms as a secondary consideration. In the end, our cylinder porting still gained us a small amount of added rpms, along with the big gain in bottom end and mid-range that we were seeking.

Head – The next place to go looking for our much needed torque and acceleration was compression. The stock head is a very good part that offers great cooling and sealing characteristics. It can also be economically modified to offer all the compression we would need, and then some. Contrary to what many folks think, there is no such thing as a standardized “maximum pump gas safe” compression ratio. The maximum safe (pump gas) compression ratio changes when there are changes in peak rpms, pump hook-up, and ignition advance. Our HX would be maintaining stock ignition timing, however it would be experiencing a greater detonation risk from the added hook-up of the Solas prop we would be using. All that said, a battery of on-water tests quickly indicated the maximum safe compression ratio for our HX project.

Big Boring & Head Surface Sealing – Big boring is often the first modification folks consider when seeking more torque. The Sea Doo 718cc engine has enough sleeve thickness to accommodate “some” big boring (about 2-3mm oversize). Unfortunately, the sleeves of the HX cylinders have an “O” ring groove cut into their top flanges. When the bore diameter is significantly increased, the amount of sealing surface area between this “O” ring groove and the bore diameter gets precariously narrow. This narrow sealing area can make it very difficult (if not impossible) to maintain a lasting seal between the cylinder and head. In addition to that, significantly larger bore pistons would further increase loads on the connecting rod bearings that we are trying to protect. Given the reliability goals of this project HX, we considered big boring to be a mod that stood to create more problems than it was solving. That said, the 718cc engine does respond very well to having fresh close fitting bores, so we bored This HX .010” oversize to OEM SD pistons. While there are many aftermarket pistons available for the 718cc Sea Doo engine, we have found none that match the quality and durability of the OEM Sea Doo pistons, and we use them whenever possible.

Exhaust System – Most full-out racing HX’s employ the Factory Pipe Products exhaust pipe. But for all the reasons mentioned above, we opted to retain the entire stock exhaust system for this project. The stock pipe offered the good torque and great long term reliability we were seeking. It bears noting that while the stock ignition rev limiter has a max of 6950 rpm, the stock pipe will actually allow peak rpms between 7050-7100rpm (depending on prevailing air density). Given this, we chose to bypass the stock rev limiter with the Micro-Touch rev module. This module does not replace any existing component, but rather just patches into the existing electrics to remove the ignition limiter. Even with the Micro-Touch module in place, the design of the pipe keeps the engine from flashing up beyond 7100rpm (a very safe limit for the 718cc motor).

The only modification we made to the exhaust system was “the way” water was being admitted to the pipe interior. On the front of the stock head-pipe, there is a 90’ brass fitting that admits water into the interior of the pipe. This water is admitted to cool the exhaust gases enough to avert burning the rubber exhaust hoses further down the system. We replaced this stock fitting with a slightly smaller fitting that has a “fan spray” head on it. Admitting the water in a fan spray allowed us to cool the interior exhaust gases with a smaller quantity of water. Having a smaller quantity of water being injected allowed for stronger throttle response and slightly higher peak rpm. This mod would not be a big asset by itself. But in combination with all our other modifications, it made a very noticeable advantage.

Cooling System – The stock HX cooling system works excellent in every way, and most full out racing HX’s retain it. Our HX maintained an “all stock” cooling system.

Carburetors and Inlet – Most full-out racing HX’s use a billet intake manifold fitted with 44-48mm carbs. With the larger carbs and inlet manifold passages, the inlet ports in the crankcase are also enlarged to allow the full potential of the big carbs to be realized. Unfortunately, the big carbs used on race boats also have “very” heavy fuel consumption that would not be acceptable for this project.

The stock HX uses Mikuni 38mm SBN carburetors. While these carbs work well on a stock machine, they are a bit undersized to create the kind of acceleration we wanted for this project. We found ourselves stuck between the desire for good fuel range and the desire for bigger carbs. The ideal solution was to obtain a pair of stock carbs from a 782cc 96XP. These carbs are very commonly available on Ebay and at PWC salvage shops nationwide for under $200. The 96XP carbs are identical to the stock HX carbs in appearance and fitment. The only difference is that they use 40mm butterflies and throats instead of 38mm butterflies and throats. The 40mm carbs bolt up perfectly to the 718`cc HX manifold, and even have the same cable attachment points for the throttle and oil injection cables. The stock HX inlet manifold has inlet runners that are 39.7 – 40mm in size, so it does not offer any restriction for the 40mm carbs. Given that inlet tracts of the stock 718cc manifold were just barely big enough to accommodate our new 40mm carbs, we saw little advantage in removing the choke plates. It bears noting that leaving the choke plates in helps slightly to increase inlet signal (a problem area for the 718cc engines). This increased signal helps the engine to carburate better, particularly at lower rpms.

The performance result of adding the 40mm carbs was very noticeable. There was a clear increase in the authority of acceleration, as well as slightly increased peak rpms. The jetting of the 40s did need to be changed to accommodate the fuel demands of the 718cc HX, but once the jetting was done, the HX started, idled, and accelerated perfectly.

Air Inlet – The stock HX Flame arrestor offers excellent protection from water entering the engine, however it also offers unacceptable air restriction. It has been common practice for HX owners to remove this stock flame arrestor and just install a couple of aftermarket individual “Pod” type arrestors. The Pod arrestors do not offer near the protection from water-ingestion, but they offer very good performance.

On top of this, the removal of the stock arrestor also means the removal of the carburetor support brace (attached to the cylinder head). Without this brace, the lightweight inlet manifold has to support the entire weight of the carbs and pod arrestors. In less than 30 operating hours, the loads of the unsupported carbs will break the inlet manifold. The best solutions are to (A) modify the stock flame arrestor case to improve it’s airflow, or (B) fabricate a brace plate for the stock carbs… so we prototyped both setups.

No one makes a carb brace for the 718cc engines, so we modified an R&D brace that was designed for the 785 engines. With this brace, the carbs and pods were well supported, thus taking the loads off the inlet manifold.

The stock OEM flame arrestor screen element has multiple coarse and fine screens. We removed all but one coarse screen. In addition, we drilled several ½” holes in the lower arrestor case in areas where there was a low risk of increasing water exposure. These mods made a huge improvement in the performance of the stock arrestor on our HX, and offered a level of water protection that the pods could not match. In the end, the supported pods allowed a bit stronger acceleration and a 40-50 rpm advantage over our modified stock flame arrestor case. Our customer decided he wanted to have both. He installs the modified OEM arrestor when he goes surf riding (where the rpms are not important, but water protection is), and he installs his pods with support bracket when he does buoy riding at the local lake.

Fuel System – While conducting detonation tests on our HX, we encountered an unusual problem. As long as we ran the boat in a straight line, it ran detonation free at peak rpm. However as soon as we put the boat into a hard turn, we got heavy detonation strikes on both cylinders. After inspecting several things, we realized we had under 1/3 tank of fuel. We filled up the tank, and the detonation in the turns completely disappeared.

We realized that we needed to re-define the term “pump gas safe”. As long as our HX had a full tank of fuel, it was totally “pump gas safe”. However as soon as the tank got under half full, the occasional air in the fuel line, from rough water riding, could easily create piston-killing detonation in mere moments. For any closed course race boat, it is impossible to keep air from entering the fuel pickup tube… the only thing you can do is eliminate those air bubbles before they reach the carb… so we did.

The solution was to install a pulse-pump fuel-air separator. With the separator mounted, we were able to run the tank to less than a half gallon at peak rpm with no detonation or surging at all. The only down side of the separator is that when you run out of gas…. you are “out of gas”… there is no notice. Just the same, we figured this inconvenience is better than a scored piston. The “off engine” mounting of the fuel pump and separator canister was a simple task. For any modified closed course or freestyle “pump-gas” boats (like our HX), we strongly recommend a fuel/air separator to avoid damaging a piston when fuel levels get low.

Ignition – For the many reasons noted above in the “Crankshaft” section, we opted to retain a completely stock ignition system with the exception of adding the Micro-Touch Rev Module. Truth be known, the ignition flywheel is a bit heavier than what we would like, but we cannot argue with it’s crank bearing protecting qualities… so we kept it stock.

Tiny Tach – Like all older Sea Doo watercrafts, the HX has no digital tachometer, and no hour-meter. We consider both of these items to be of great importance for the fine tuning and monitoring of a high performance PWC (for all the obvious reasons).

The Tiny-Tach is a waterproof and very durable tachometer/hour meter that is easily installed on any pwc. The hour meter registers any time that current is moving through the spark plug wires. The digital tach readout (which reads the voltage pulses from a plug wire) offers rpm data that is updated every two seconds. This means that meaningful tachometer testing can only be done on long runs of smooth water were rpms can be sustained long enough for accurate measurement. For our initial testing, we mounted the Tiny-Tach on the hood panel where it was easily seen during full speed runs. After our detail tach testing was done, we moved the tachometer to the interior of the engine compartment where it served nicely as an hour meter in the relative protection from water spray.

Pump & Impeller – All HX’s come stock with a stainless steel impeller. These stock props work fine on a stock machine, however they do not offer near the efficiency that a good aftermarket impeller offers.

After much testing, the aftermarket impeller we had the best results with is a re-pitched Solas 14/22 Concord. The Solas Concord has a better blade design, and a smaller hub contour that actually allows it to process more water (per revolution) than the stock impeller. “Out of the box”, this Solas prop is too steep for the HX. However with the repitching, it offers excellent hook-up at all speeds, as well as great peak speed abilities. We pitch this prop to run a projected target peak rpm of 7050 (meaning a rev limiter module would be needed with it). Our tests indicated that steeper pitching (for lower peak rpms) makes for poor low range acceleration, and increased detonation risk. Our repitched Solas impeller is prepared to deliver the target rpm range using the stock pump case and stock nozzle diameters.

Wear Ring – The wear ring is a removable, Teflon plastic, liner that the impeller spins within. In a perfect world, this ring has no grooves or gouges on its surface, and it fits closely to the impeller’s outside diameter. However, if the wear ring is gouged or grooved (as can happen from digestion of rocks or branches), there can be considerable cavitation and pump pressure loss. In addition to the cavitation, a damaged wear ring reduces pump efficiency in a way that can cause the engine to run a higher than normal rpms (for any given water speed). The end result is reduced pump thrust, acceleration, hook-up and fuel range. If your wear ring is damaged in anyway, it’s very cost effective to replace it… and then stay away from rocks.

Handling Parts – For recreational riding, a stock-piped HX 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. These top-loader scoop-grates can cause a smooth water loss of 2-3 mph, but that is no big deal for owners who ride in big chop. Our HX customer used the stock scoop grate for most of his lake riding, and then installed the Jet Dynamics grate when he went surf riding.