RESTO Seadoo Direct Injection Question

[mostlyharmless]

I have a question about a rebuild I am doing on a 2002 GTX-DI.
From most of what read on forums and the internet, even after a top end rebuild, the tested cylinder psi achieved is about 130-135.
Is there a reason that the desired cylinder psi is 155 on most every other 2 stroke Rotax engine, yet the direct injection set up seems to have the reduced cylinder psi when brand new and after a rebuild?
The reason I ask is that when I will do the bore/hone, there is a new process on my CNC at work that will, bore through, offset away from the bit and return to the initial cutting plane, then return to the bore center coordinates and do a synchronization of spindle and feed rate to create a cross hatch spring pass pattern that the manual says eliminates the honing process and creates a more consistent size and surface finish than achieved with the earlier process. I am hoping that using this new process, I will be able to size the cylinder to get the best compression possible (155 psi)?
If I achieve that type of cylinder pressure however, will I have to somehow modify/adjust the fuel and air pressures of 107 psi for a 130-135 psi, to something higher to more evenly feed a 155 psi cylinder?
I would guess that the new volume of the cylinder after being bored oversize, as well as any increased psi would change the requirements needed of the fuel/air system to correctly feed the engine.
Any insight would be greatly appreciated. I also would like to thank the members here for their advice and input with my questions since I became a member here. The assistance given has been invaluable.

 

[rotaxrule1]

I am still fairly new to the 951 DI platform as I have only owned my 2002 RX DI for a year or so. My understanding of the 951 is that BRP reduced the compression on the later models to reduce the stress on the engine and crank as they are already pretty tapped out for horsepower and have been known for shorter service life than the 800 and others. But yes, in my opinion, you definitely would have to come up with a way to increase air and fuel pressure to compensate for the added cylinder pressure.

 

[etemplet]

I don't keep the DI's I build long enough to check the cylinder pressure after break-in and I've never tested a fresh DI. No matter what you do to the bore you are not going to increase the pressure. Just do a good top end and don't concern yourself with that. If you want to raise the pressure I got a 46cc R&D cylinder head for your and I'll sell it to you cheap. LOL Personally I don't see where stressing the engine is gonna change much if anything. Heavy as these skis are 5hp isn't going to make a difference and you won't get that much. Check the squish and set it to a minimum is about the best you can do.

To your question, If you bump the compression at all you won't have to do anything to the fuel pressure. Just not enough difference. You're piston to bore clearance with WSM is gonna be about (.0045") nothing much you can do there is going to raise compression other than squish. A picture of the bores when you get them done would be interesting.

 

[mostlyharmless]

If the lower compression numbers will place less stress on the engine, then I am all for that. I am waiting on the 2nd piston for the 787 engine I am in the middle of rebuilding. In the case of the 787 the manual states a .001 wall clearance between the piston and cylinder. I was going to micrometer each piston and take the highest measures of each. Then bore .002 above that. Then set the rings inside the bores with a fixture matched to the bores -.005 for clearance, but with an oversized flange on the end to locate on the top of the cylinder for squareness. I was going to locate the rings .625 into the bores and adjust the end gap per the manual.
I am going with a WSM rebuilt crankshaft assembly mainly because they use Bearings manufactured in either America or Japan that are equal to or better then OEM. If anyone has had experience with WSM products, and has an opinion about the quality of the engine internals they produce, please advise me as to your thoughts.
I looked everywhere for a new BRD replacement, but they are discontinued as an OEM new replacement.
I would like to thank you both for your advice with my questions. Once I try the new bore process on a setup part and verify the measurements, I will post photos and measurements here. Again, thank you for your time with my questions.

 

[etemplet]

You'll seize that engine running the bore that tight. The Skirt of the piston is the size factored into the cylinder bore clearance. WSM (.0045") if I remember correctly. OEM pistons are alittle tighter (.0035" maybe) but none of that is going to matter from a performance perspective. When you insert the rings the gap is going to exceed the recommended gap on WSM pistons. (Maybe .018" - .021") What you are trying to do is futile as far as performance is concerned and dangerous from a reliability perspective. Following the manufacturer's recommendations is your best chance of success. Good Luck !

 

[mostlyharmless]

I was able to locate OEM pistons. Is the manuals dimentions of .004 to .005 for clearance on a diameter or per side. The manual shows measurement on one side of the piston with feeler gage stock while it is inside the cylinder. In the picture it looks like it is .004-.005 per side.

 

[etemplet]

Total clearance is gonna be (.0045") With a feeler gauge on the piston skirt (bottom) you can only measure one side and the piston will move over in the bore. Micrometer measurement of the bore and piston are most accurate. Good Luck !

 

[mostlyharmless]

I did some in depth reading on the principals of cylinder surface finish and the results that are required for proper ring break in and continued lubrication. The process I was going to use achieves the correct sizing and eliminates out of round and taper, however the finish would be too fine for the lubrication requirements and would fail in short order. Once I have both pistons in hand, I will use a local shop that specializes in 2 stroke engines. I will keep the group posted.
I would like to thank everyone for their help and advice.

 

[motobub52]

I'm working on the same machine is there a way to test the cps before I put it back in and the encoder ring bolts look like they have been messed with how can I check aliment is correct I can't find either answer in the manual

 

[etemplet]

Back in the day after we bored a cylinder we would use emery paper to make cross-hatch lines in the cylinder bore (I still do that). That was new at the time. Before that cylinders were simply bored and installed. The information available today builds on those practices and now people believe engines won't work properly if you don't do this or that. When we bore a cylinder those cross-hatches get in there by moving the hone in and out quickly a couple of times and they are not more than a scratch in the surface finish. Often what we read are opinions and those opinions proliferate and of course people make money by telling you you can't do this or that with the "You'll shoot your eye out kid" mentality. The thought process concerning repairs is very much different that it was in 1960's and 70's. Good Luck !!