Sea Doo 951 GTX Di – High Performance Mods

Our Technical Choices for Speed Increase – The 951 Di utilizes a direct injection fuel system designed by an Australian firm named “Orbital” (hence the term Orbital direct injection). The Orbital system itself is relatively simple, however it utilizes a special computerized fuel mapping matrix that is everything but simple. Sea Doo has heavily encrypted the programming of the Di electronics in an effort to make it “very” difficult to alter the fuel mapping matrix in any way … and they did a darn good job. Unlike many automotive applications, there is no “chip” that can be plugged in to a Sea Doo Di to offer better fuel mapping.

For a while we toyed with the prospects of modifying the fuel mapping, but finally chose not to for two reasons. First and foremost, virtually all shop diagnostics for the Di are performed with a computer link to the on-board electronics. If a dealer plugs into your Di for normal diagnostics, and sees that values have been reprogrammed, most technicians will immediately decline to perform any further work at all to your machine (the technician fearing getting into an electrical rats-nest that is not oem). Furthermore, if notation of the modified electrics is input into the Sea Doo data base, you could be denied all services at any Sea Doo dealer.

Given all these bad scenarios, we focused on effective modifications that allowed our customers to retain all stock oem electrics and fuel mapping.

The Ride, and Ride Plate – The first thing we noticed about our “Big Di” was that it had a very strong “nose down” ride attitude in the water. That is, the hull/ride-plate had been designed in a way to keep the nose planted to the water at all times. This “nose down” ride attitude creates two big problems. The first is that the hull offers a very “punishing” ride when ridden at high speed through rough water. The rider would be exposed to a very rough (and often very wet) ride.

The second problem caused by the nose-down attitude is sheer speed loss. Since the nose is being driven downward so strongly, a large portion of the hull is making contact with the water (especially on glass water). This large “water contact patch” scrubs off a significant amount of speed, and gives the feeling that the boat is “steering on the nose”.

To address all these problems, we built a special fixture that allowed us to modify the stock ride plate in a way that lifted to nose of the Di off the water at higher speeds. We tested many prototypes before finalizing on one specification.

Our final specification of the modified ride plate made a huge difference in the overall feel of the boat. At high speeds, the nose rode noticeably higher, and allowed for a 2-3 mph speed increase on glass water. In the rough water, the boat was more controllable and much less punishing. One test rider described the Di (with the modified plate) as “ a dirt bike wheeling over the rough stuff”, thus letting the center of the hull take the rough water punishment and giving the rider “a lot” less abuse.

While the ride plate modification lifted the nose considerably at high speed, the effect was much less at lower speeds. When the throttle was released from high speed, the nose of the hull dropped gradually, allowing for normal low speed steering. All out test riders agreed that of all our mods, this mod made the single biggest overall improvement in the boat.

One unexpected benefit from the ride plate modification was increased fuel range. Reducing the “wetted” surface of the hull with the ride plate modification made a big reduction on the “load” on the engine. That is, the engine was not having to work near as hard to turn peak rpms, because to wasn’t having to “push” near as much hull through the water. These fuel range gains were greatest while cruising at higher speeds in smooth water, and less pronounced during lower speed use.

About Ride Plate Installation – Sea Doo seals the stock ride plate to the hull (making it a bit of a job to remove). We strongly recommend to reseal the ride plate to the hull after modification. The best sealer we have found is “Permatex Clear Silicone” (aka Industrial Clear). This silicone is usually found at auto parts stores in both tube and cartridge form. It requires 24 hours for full curing.

Increasing Power – Besides making the choice to not modify the fuel mapping, we also made the choice to maintain our modified Di as an 87 octane safe platform. We tested with many of the top end modifications that had netted good results on the carbureted version of the 951, but the Di reacted quite differently because it had a different set of needs. In the end, the most effective modification was increasing the compression ratio and reshaping the combustion chambers to stave off detonation. The combustion chamber design that worked best on our Di was considerably different from what worked best on the carbureted 951s we have developed in past years. However (again) our testing showed us that the Di engine had very different needs from the older carbureted engines. Our “Di Specification” head mod gives the 951 Di much stronger acceleration through the entire rpm range, and slightly increases peak speed as well.

About Cylinder Head Installation – The Di head can be removed with basic tools. The head is sealed to the cylinder with a multi layer steel head gasket that can easily be reused with good long term results. When re-using this head gasket, we recommend to thoroughly clean both the head and cylinder surface, and then apply a very thin coat of 3Bond 1211 Gasket Sealer to both sides of the head gasket. This gasket sealer is available from Group K with your order.

Hooking Up the Power – Once we had increased the engine performance, we began to experience more and more impeller cavitation. To address this problem, we opted to install a Solas 15/20 Concord impeller. The concord has about the same blade surface area as the stock prop, but has a co- nsiderably smaller hub. This smaller hub design allows the Solas to literally, process more water per revolution than the stock prop. The end result is better hook up along with better peak water speed. Unfortunately however, the pitch of the 15/20 was clearly too steep for the 830lb Di (filled with gas and oil). Since Solas does not offer a milder pitch of this design, we custom pitched our 15/20 to suit our modified Di. It bears noting that the stock impeller holds the Di under 6700 rpms. Even with the ride plate and head mod, the Di could not reach to 6900s that we found ideal for most 951s. With that, we reset the pitch of the Solas Concord to allow peak rpms of 6900. This offered even stronger acceleration from a dead stop, and a slight addition to peak speed as well.

About Prop Installation – The prop is mounting in the pump assembly (which is removed from the rear of the boat. Replacing the impeller requires a stable bench mounted vise, and a pump-shaft tool ($11). The impeller is surrounded by a replaceable black plastic “wear ring”. If this wear ring has full diameter grooves or other damage, your pump performance will be seriously compromised. In addition, the interior pump bearing oil must be changed. Many customers ship us their entire pump for the prop change with oil change. We also inspect the wear ring for replacement, and we can provide replacements when needed.

About the OPAS System – In 2002, the Big Di was the first 951 offered with the Sea Doo Off Power Assisted Steering system (called OPAS). This system is intended to offer novice operators “throttle-off” steering control. While the intent and the concept was certainly good, many of the OPAS parts proved to be extremely fragile, and prone to malfunction. While some Di owners have removed their OPAS systems in the interest of gaining speed (there is a slight speed increase) we removed ours to forever eliminate the prospect of having to “limp” home with a broken or malfunctioning OPAS (it happened to us often). For owners that will routinely have novice operators on their Di, it might be worth the effort to keep the system intact and operating. That said, “all” of our experienced test riders were much happier with the steering and performance of the Di with the OPAS system removed. One test rider remarked that it felt like the day he got his training wheels off his bicycle … we tend to agree.