I bought a new Ranger bass boat with a 115 HP 6 cylinder "Tower of Power" in 1989...on the title listed as a 1989 engine. Folks in the know on here said that Merc. did the change over in 1988.......sooooo I guess I had an engine built in '88 and marketed in '89, not an unusual process. The engine was "Direct Charged" and was Merc's intermediate solution to the OMC Loopers which were known for their fuel economy over their previous "cross flow" conventional scavenging engines....which loved to stop by the gas station....BTDT
Out comes the new Loop Charged Mercs in 1989, 3 and 4 cylinder, 60 HP Jet 3 cyl to 125 HP 4 cyl. Later on I bought a 115 hp one of these engines. I remember the '89 was very fuel efficient as is the current 2002 115. I can't recall that fuel was an issue worth mentioning with either. Do I have a preference? Well, it's hard to beat a 2 stroke 6 cylinder engine of any type for smoothness of idle.....like a well oiled Singer Sewing Machine. But the current 4 cylinder, which runs on 2 cylinders at low speeds is very smooth....as long as you tilt the back of the engine up 10-15*. Otherwise it wiggles a bit.
To answer your question, I'd say noise would be the big difference as the newer engines had a foam insulated one piece, plastic cover, very effective noise canceller, where as the inline 6 had a wrap around cowl with a plastic bag of loose fiberglass for sound deadening. Course when it was making the most noise, it was going the fastest, which it did very well and most of the sound was way behind the boat.
Also the lower unit was beefier, I don't recall the '89 being thermostatically controlled, and the improved trim setup did a better job of allowing the engine to raise up when hitting an obstruction, reducing damage to the LU.....the trim ram was separate and just pushed up against the mid section rather than being a single cylinder for trim and tilt, locking the midsection to the hydraulic pressure of the TT unit at the time of impact.....as I recall on the '89. I recall an earlier Merc 850 I had with the same trim and one day I ran over a concrete anchor to a marina. Rear of the boat raised and fell as I passed over it.....don't recall any noticeable movement of the engine. Upon loading the boat on the trailer to go home, I noticed that the lower part of the skeg was missing...just sheared right off.
