Resto-Mod 1997 Sea Doo GSX 785

Exhaust System Modifications – One of the natural first choices for many high performance minded PWC owners is the installation of a high performance exhaust system. Since the 785-engined models were so often used for racing, there was a large abundance of different high-rpm racing exhausts made for them. Of all those exhausts, the ones made by Factory Pipe Products offer the biggest performance increases. However these exhausts were developed mostly for racing applications that required very high rpms. These high rpm pipes could often operate on the 92-octane fuel blends of 1996. However, today’s premium fuel blends have far less detonation resistance than 1996 fuels had. In short, the big power increases of the aftermarket pipes generate a temperature increase that cannot be supported by today’s 91-octane pump gas. In addition, the very high rpms of these racing pipes causes a big increase in fuel consumption… which is exactly what we did not want for this project.

As a result, the best option, for this build-up, would be to optimize the stock exhaust system. The best way to accomplish this is by modifying the plumbing system that injects water into the pipe at the head-pipe and waterbox. In short, we removed the regulator valve on the waterbox, and created a simple plumbing system that meters (through changeable jets) water to the head-pipe and waterbox. While the plumbing kit appears very simple, it is the result of many hours of on-water testing to determine the ideal orifice sizes for the best performance. This exhaust jetting kit nets a very noticeable increase in response and performance on a 785 motor with the “Sleeper” modifications already in place, however the gains on the “Limited” setup are more modest.

Cylinder Porting & Compression – Cylinder porting has often gotten a bad reputation as a modification that hurts low end power as well as reliability…. And that may be true for poorly executed porting. The porting modification used in our 785 Sleeper kits was developed for use in endurance racing engines that put a premium on long term reliability, and strong power at all rpms. Our Sleeper porting involves changing port heights and widths, while at the same time addressing numerous port shaping and mismatch problem areas. The result is a significant improvement in performance at all rpm ranges, along with improved high rpm ability.

Increasing compression ratio is also a very popular mod for 785 owners seeking to get an overall power increase. Like every other engine specification, there are limits to how much is “too much”. With compression, the limiting factor is the detonation resistance of today’s 91-octane fuel. Surprisingly, our tests showed that the whenever we used excessive compression, we experienced very heavy detonation at 6600rpm, and only trace detonation at wide-open throttle. This happens because the ignition curve has greater advance at 6600 than it has at 6900+ rpms, hence a higher detonation risk at 6600. It bears noting that our head modification specification is the same for both the Limited Kit and the Sleeper Kit, so the head mod does not need to be done a second time for customers upgrading from a Limited to a Sleeper.

Squish Clearance – One important part of getting the 785 SD engine to make it’s best power is properly setting up the squish clearance (this is the clearance between the piston crown and head dome at TDC). The base gasket surface deck height on the 785 cases varies so much that Sea Doo offers a range of different thickness base gaskets that allow technicians to setup the correct squish clearance for each particular motor. For this same reason, we prepare our Sleeper ported cylinders and milled heads as a matched pairs to assure that you have the correct compression ratio when the squish clearance is properly set. Our kits come with detailed step-by-step instructions on how to perform this squish clearance setup.

The net result of our Sleeper top end modifications is an engine that makes “a lot” more torque at all rpms. This added torque allows for the use of a much steeper impeller, so you get your speed gains from puling a steeper prop instead of turning sky-high rpms.

About Cylinder Boring – There are many PWC models that net significant gains in power from big-boring to a significantly larger piston size. Sadly, repeated tests have shown that the 785 Sea Doo engine does not react very well to this mod. However the 785 does respond well to having a very close clearanced fit between the piston and cylinder wall. Out project GSX had very worn bores, so we bored it to 1st oversize Sea Doo pistons. The oem Sea Doo pistons are a bit more expensive than the aftermarket versions, but we have found the performance and long-term wear of the oem pistons to be far better than any others.

It’s also important to note that anytime a 785 engine is bored, the Rave-valves “must” be clearanced to avoid the valves making contact with the new larger pistons. Valve clearancing is included with all Group K 785 cylinder boring.

Air Inlet Modifications – There are many different “free-er breathing” aftermarket flame arrestors available for the 785 Sea Doo models. While they all have good performance abilities, very few of them include a “carb to head” brace. Without this import brace, there is a very high risk that the unsupported weight of the carbs and filters will cause the inlet manifold to fracture.

Since our GSX was not intended as a high rpm race machine, we opted to modify the stock flame arrestor case (which does have the much needed carb-to-head brace). We simply removed the fine screens from the arrestor element, and added a few air access holes on our flame arrestor lid. This modified part matched the peak rpms of a set of aftermarket “pods”, and offers considerably better water protection.

Ignition System – The ignition system is the one area where the 1996 & 1997 models vary. The 96 model has a rev limiter at 7050 rpm, and the 97 model limiter is set at 7150. Our GSX was a 1997, and so we felt no need to further raise the rev limiter. If we had done a 1996 model, we would have installed a Micro-Touch rev module that offers an adjustable limiter. Beyond that, the ignition system of the GSX is an excellent setup that requires no modification.

Carburetor Modifications – Another modification included in our 785 Sea Doo Sleeper Kit is the “True-Boring” modification to the stock 40mm Mikuni carbs. While the stock carbs are denoted as 40mm, the actual minor diameter at the throat is about 37mm. With this mod, we make the stock carbs a full 40mm along the entire throat. These modified carbs offer easy staring, steady idling, and a good improvement in overall acceleration characteristic. The True-Bored carbs are a very popular modification because they net a nice overall improvement in power, with only a small increase in fuel consumption over stock carbs. The carb True-Boring is included in both the Limited and Sleeper Kits.

Increasing Fuel Economy – For our GSX project, we decided to test an option that could significantly increase fuel range while having a minimal impact on overall performance. The “option” would be the installation of a set of re-jetted dual 38mm Mikuni carbs that came stock on all the 720 models. Externally, the 720 carbs look cosmetically identical to the 785 carbs, and have all the identical cable connections. However internally the 720 carbs use a 38mm butterfly instead of the 40mm butterflies in the 785s. To be sure, we suspected that the overall performance of our project GSX might be harmed slightly by the smaller carbs (but we didn’t know by what margin). Since we were not constructing a high-rpm racing engine, a minor performance loss from the smaller carbs could likely be worth the fuel economy gains that might come along with them.

We fitted our GSX785 test boat with a digital fuel flow meter so we could measure the exact differences in fuel consumption while we were simultaneously evaluating performance differences of the two carb sets. The following chart shows the data: