Hello all!

I'm new to turbocharging and looking for help to select the right turbo to work on the Rotax 1503 engine found in the Sea-doo 260 RTX. The 260 Hp version comes stock with a supercharger that I believe makes about ten lbs of boost. (the same block is available in a 155 hp naturally aspirated engine, so 260 Hp/155 Hp = 1.677 or 68% more power. Sea level pressure is 14.7 PSI, so 14.7 PSI X 68% = 24.65 PSI Absolute - 14.7 = 9.96 PSI of boost.)

I'm planing to eventually use this in an aircraft and I'm looking for a turbo that produces 10 lbs of boost up to 20,000 ft. Spool up time doesn't matter at all as the engine can take five or ten seconds to spin up without complaint. The Borg Warner spreadsheet recommends a 64mm wheel with a 1.05 aspect ratio, but is there any penalty for going bigger in this context? Given the slow smooth throttle movement in this application, turbine waste gate chatter and boging shouldn't be an issue right?

 

This is a 2 stroke?

 

This is a 4 stroke running 91 octane gas in the supercharged versions. The engine works like this, here's a review of said engine, here's how the oil system works, and how the closed loop cooling system works.
(Sorry, the link system isn't working, so here are the videos directly):






Can anyone recommend the right turbo for this engine, or offer any advice on the process of choosing?

 

First point is absolute air pressure at 20,000 ft is 6.75 psi so the turbo pressure ratio needs to be 24.65/6.75 = 3.65
Second point is air volume flow into the compressor needs to be calculated considering the density at altitude.
Third point - if you use an air-air intercooler, the charge can be cooled to very low temperatures at 20,000 ft so you may not need as much MAP as assumed in point one.

 

All good points. I used BorgWarnerMatchBot with 10 psi boost and 20,000 ft to get the 64mm 1.05 aspect ratio numbers.
What turbo would you recommend I purchase?

 

I am not familiar with MatchBot but at first glance it seems to ask for boost pressure in psig so if you enter 10 psi @ 20,000 ft the MAP will only be 10 + 6.75 = 16.75 psia. Try entering 18 psig boost @ 20,000 ft to give you 24.64 psia and see what compressor it suggests.

 

Thanks for the tip! Doing as you suggest results in a larger turbine expansion ratio between 2.3 and 1.5 (link). I'm still having a rough time finding any store that lists turbos by their characteristics, much less allows for searching by specs. Where could I to find a store like this, preferably an online one with tech support available?