seadoo 787 for freeestyle 300sx build

Hey everyone, I picked up a 300sx hull fairly cheap and wanted to build a freestyle/ potential backflip ski.I have a seadoo 787 and 717 laying around. compared to a 701 what motor can potentially make more low end power?. I was set on buying a 701 but kinda thought about this. am not worried about fitment, I have a sawzaw and fiberglass.
 
I think the seadoo motors are a little more built toward top end, and heavier, and have weird driveshaft couplings lol, there was a guy that crammed a 951 in a 300sx here somewhere, did a lot of cool stuff, removed counterbalancer to reduce rotating mass etc I don’t remember seeing a video of it flipping though
 
I think the seadoo motors are a little more built toward top end, and heavier, and have weird driveshaft couplings lol, there was a guy that crammed a 951 in a 300sx here somewhere, did a lot of cool stuff, removed counterbalancer to reduce rotating mass etc I don’t remember seeing a video of it flipping though
I do remember seeing that build in here. I would think the 951 would make it too heavy to flip. Id like to find numbers on a 787 vs 701 hp and tourqe on the low end.
 
I do remember seeing that build in here. I would think the 951 would make it too heavy to flip. Id like to find numbers on a 787 vs 701 hp and tourqe on the low end.
Might be tough to find apples to apples data on that, from what I could find the 787 is 82lbs without carbs or exhaust manifold, 701 short block closer to 60lbs I think. For that weight difference you could go with a big bore 66e if that guy ever makes those exhaust manifolds...
787 makes 110hp stock vs 73 for 62t/61x, I think the seadoo stock exhausts are quite a bit better than superjet to begin with though, there are definitely other advantages, rotary valves flow better, power valves should help broaden the power curve.
I'm sure you could drop the cylinder, and cut some domes to fix the height and raise compression to get more low end out of it. Programmable ignition might help. Even just adjusting power valves and playing with rotary valve timing might help a little. I haven't seen any 787 standup swap yet, somebody must have tried it, but probably the only way to know is to try. What do you plan to run for an exhaust?
 
Search on PWCtoday there is a thread where somebody put a 787 in an X2 that has a lot of info.

I'm not super familiar with the sea doo motors but here are the things that I think could be a problem for a 787:

1. It's a wierd shape. The 787 is designed to sit at an angle and the exhaust and carbs come out the same side of the motor. Not necessarily a deal breaker but it makes it trickier to fit in a place it was not designed to fit.

2. There are no readily available exhaust systems that fit in a stand up. The stock exhaust is probably really good, but it's also big and heavy and kind of the wrong shape

3. It has really high port timing. I'm not 100% sure on this but I thought I have read that they come stock with something like 200 degrees of exhaust port timing which is a hell of a lot and not ideal for bottom end power. Possibly not a deal breaker, but especially complicates the exhaust issue (you need a pipe that is designed to work with the porting of the motor) and typically high port motors don't have great bottom end. It also has decently high displacement and exhaust valves so maybe less of an issue that it at first appears, but be aware of it if you're not really confident in your tuning abilities.

4. it has a balance shaft (weight, size, and rotating mass), not ideal for fast response, lots of stuff rotating.

5. it has weird couplers for the drive shaft.

I love to see odd combinations and creativity so if you go into it aware of the challenges go for it, but if you're viewing it as easy/cheap big power for a freestyle ski it probably isn't.

I think the 720 avoids some of those issues, but not all. However it's also less displacement and less power, so it's a tradeoff.
 

97superjet

Site Supporter
Location
Minnesota
Search on PWCtoday there is a thread where somebody put a 787 in an X2 that has a lot of info.

I'm not super familiar with the sea doo motors but here are the things that I think could be a problem for a 787:

1. It's a wierd shape. The 787 is designed to sit at an angle and the exhaust and carbs come out the same side of the motor. Not necessarily a deal breaker but it makes it trickier to fit in a place it was not designed to fit.

2. There are no readily available exhaust systems that fit in a stand up. The stock exhaust is probably really good, but it's also big and heavy and kind of the wrong shape

3. It has really high port timing. I'm not 100% sure on this but I thought I have read that they come stock with something like 200 degrees of exhaust port timing which is a hell of a lot and not ideal for bottom end power. Possibly not a deal breaker, but especially complicates the exhaust issue (you need a pipe that is designed to work with the porting of the motor) and typically high port motors don't have great bottom end. It also has decently high displacement and exhaust valves so maybe less of an issue that it at first appears, but be aware of it if you're not really confident in your tuning abilities.

4. it has a balance shaft (weight, size, and rotating mass), not ideal for fast response, lots of stuff rotating.

5. it has weird couplers for the drive shaft.

I love to see odd combinations and creativity so if you go into it aware of the challenges go for it, but if you're viewing it as easy/cheap big power for a freestyle ski it probably isn't.

I think the 720 avoids some of those issues, but not all. However it's also less displacement and less power, so it's a tradeoff.

720 is fairly bulletproof, you sacrifice power valves and overall power going that route, but it sits normally in a hull
 
Little more power out of the Seadoo's 720 is 85 HP, Twin carb 701 is 73HP I think the single is 65HP.

Either way, a 701 with a pipe would be so much easier.

that was my point. similar hp once u mod a 701. modding a 720 in a stand up would be basically impossible.
 

Quinc

Buy a Superjet
Location
California
701 would be worth it just to be able to run a zeel and get the better timing. Or better yet go with a 760 and start at 90hp + zeel + bpipe + head + 46's should get you close to 110hp.
 
Location
dfw
The engine fitment will be easy after you made a 144-155 pump to fit. None of our engines make enough power without a good pipe so solving that problem is your second highest priority after the pump. Rotax 720 and 787 engines aren't ideal for standup use. You'll need to adapt a forward facing pipe. The little Yamaha is the easiest way out.
 

WFO Speedracer

A lifetime ban is like a lifetime warranty !
Location
Alabama
The guy putting the 787 in the X2 is me, it's a big bag of snakes but when I am done it will be very fast and damn near bulletproff, 787's are very stout 110 HP engines stock and they take mods REALLY well. I also had a 720 in an X2, also very fast but the engine bed plate hits the bottom of the ski and it is very hard to fit, as stated the exhaust points rearward so I buried a PJS resonator in the footwell and went straight out the back, even after tapping the pump for another cooling line and running it straight to the exhaust exit it was very loud, not the way to go if you want to stay friends at the lake..

The 787 required a hollow tubbie to be installed on the left side for clearance of the exhaust, the stock engine mounts were replaced with Superjet mounts then the rear engine mounting plate was flipped180 degrees and cut and bent to fit the angle of the Superjet engine mounts, the front onLy has one mount so an aluminum plate was glassed into the bottom of the hull and an Artic Cat mount is bolted to it, I glassed in a Seadoo pump tunnel so I was able to use my Seadoo alignment tool to set the engine in the proper place using an old RFI engine with a splined coupler , it went in with only a couple of shims., the exhaust exits out the right side in the rear, but it makes a 90 degree at the pipe exit and goes to two resonators mounted inside the ski behind the engine, then that makes another 90 degree turn into another resonator under the floor, it should be reasonably quiet.

A 300SX tank was used on this because it is the only tank that will almost fit , but to make that fit the floor the tank sits on had to be completely cut out, it didn't matter on this ski because the entire bottom has been reshaped with two part foam which allowed the floor to be moved down to accommodate the tank fairly easily. To build some strength back I used carbon fiber for the new floor.

Most here don't understand power, bottom end is not really a factor, the only thing that matters is how fast you can get an engine into it's usable powerband, to that end I am running a lightened flywheel, custom made PTO flywheel and total loss, 787's spin up fairly quickly already this one should do so much faster , also running a VE racing head and an R&D pipe , the R&D is more geared toward bottom end and midrange than top end power.

The build is taking a long , long time because I can only endure so much of this madness at any given time. My advice drop a 760 Yami in the 300SX graft in a pump tunnel from a Yamaha and go ride because a build like this is not only frustrating its pure madness. Lots of thinking goes into a build like this and so far everything is easily accessible and easily removed, starter, carbs , exhaust pipe , jet pump etc come off in minutes trust me I know I have had them off enough times.
 

Attachments

  • IMG_20200229_164750.jpg
    IMG_20200229_164750.jpg
    150.1 KB · Views: 18
  • IMG_20200229_164625.jpg
    IMG_20200229_164625.jpg
    99.7 KB · Views: 17
  • IMG_20200229_164531.jpg
    IMG_20200229_164531.jpg
    154.8 KB · Views: 18
  • IMG_20200229_164519.jpg
    IMG_20200229_164519.jpg
    135.3 KB · Views: 21
Last edited:
Top Bottom