new volvo penta sx duo prop

[Miss $tre$$]

I am looking for info on the new volvo sx duo prop drive system.<br /><br />I have just recently switched from an OMC cobra leg to this new volvo duoprop system. I did not change engines, my engine had just been rebuilt and tested out fine. The boat is a 1989 Bayliner, 2556 Cierra/Trophy command bridge.<br /><br />The old OMC developed 31 MPH at 4600 RPM. The new volvo leg with D3 props develops 27.3 MPH at 4600 RPM and with D4 props develops 28.1 MPH at 4700 RPM. Top RPM recommended is 4600 to 5000, so all of the above are within range. Also I am told that there is no cross over relationship between the two systems but just in case, the OMC prop was a 15 x 15 1/2.<br /><br />First I would like to know if it is true that the new volvo system is not expected to increase the performance, i.e. speed and fuel economy?<br /><br />Second The ratio on this new system can be either 1.78:1 or the 1.95:1. What if any impact does the ratio have on the speed and or performance when connected to the 350 Chev engine developing 260 HP?<br /><br />I would appreciate any info on the above data or even the system that anyone can provide. Thanks