Safe cruising revs for older Mercruiser 120?

[Michael JD]

I've got a Mercruiser 120 I/O system on a 17' Bayliner, love it. Engine runs great. Run it mostly in salt water, has raw water cooling. Guy who works on boats like mine said it's a great engine, can run all day at 4200 rpm. Having had a tach in numerous cars, that sounds pretty high. Friend pointed out that, unlike a car at cruising speed, a boat engine at cruising speed is under a lot of load, and 4200 is a lot of revs with a lot of load. Boat planes ok at 3500, but engine does seem to have a sweet spot around 4200. Another guy said raw water cooling, while more corrosive, keeps the engine nice and cool. Water temp in Puget Sound is around 55 F. Opinions?

 

[fone_man]

Re: Safe cruising revs for older Mercruiser 120?

The man that works on boats is correct... Max RPM on 120/2.5L is 4200-4600. So you are OK... as for sweet spot @ 4200... This when your engine performs at it max... or "sweet spot"!!!