120 Lehman RPM

Author Message   norm fredrick New member Username: nfredrick Post Number: 1 Registered: 09-2008 Posted on Monday, September 22, 2008 - 11:02 am:    I know I've seen many references to 1750 rpm for these engines. We have been running our 1982 36 ft Albin with 24 " prop at 2200 Rpms and the engine runs at about 185 degrees. Is this too fast for this engine for crusing ? Thanks Norm   Fred Frog Member Username: lilypad Post Number: 30 Registered: 01-2008 Posted on Monday, September 22, 2008 - 11:52 am:    The short answer is yes. The longer answer is that the prop diameter (24") is only half of the equation. The pitch also plays a role. You should be able at full throttle to get 2500 RPM (+or- 50) out of your engine. The governor should allow you to turn 2650 with no load on the engine. If you turn faster or slower at full throttle in gear then your prop is over or under pitched. Many people play with pitch to improve economy and this may or may not affect engine longevity. Assuming that your boat is set up "normal", 2200 is above the 80% load, which is generally considered the maximum you should routinely run your diesel engine. 2000RPM is 80%, 2250RPM is 90%. Your fuel economy will jump dramatically if you back down to 1750 RPM from 2200. I don't have my chart from my engine in front of me, but I will guesstimate almost half the fuel at 1750 compared to 2200. My 120 Lehman burns 1.9 GPH at 1750 per the FloScan gauge. 70% load is the "magic" 1750 RPM. This is often quoted as the optimum speed for longevity because the engine is "loaded", but not overloaded. It is working and developing good heat energy to burn off carbon deposits and keeping the oil hot without allowing acids to build up. 185 degrees is probably OK since most gauges are not exact instruments anyway. The thermostat should be a nominal 180 degree thermostat and somewhere close to that is fine. The manual advises 175-190 is the normal operating temp. K   norm fredrick New member Username: nfredrick Post Number: 2 Registered: 09-2008 Posted on Monday, September 22, 2008 - 12:43 pm:    thanks

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