5.7 mercruser high rpm

[jeffwillb]

just had the lower unit replaced last winter and boat now has a high rpm (i think) RPM around 5500<br />and speed about 45 mph. With the old lower unit <br />RPM was about 5000 and the top end was 50 mph. Will running the 5.7 at 5500 hurt the engine?<br />Could the new lower unit have a different reading on the speedometer and Im really doing 50 mph or faster?<br />thanks

 

[flashback]

Re: 5.7 mercruser high rpm

during the outdrive exchange, someone may have swapped the prop with a different one... 5500 is too much for the 5.7...........good luck......

 

[L Christopher]

Re: 5.7 mercruser high rpm

I would guess the the max RPM range is 4,400 - 4,800 RPMs. Depending on gear ratio the prop would probably be 14 - 14.25 Diameter X 19p to 21p

 

[wvit100]

Re: 5.7 mercruser high rpm

The max WOT on my 1995 5.7 says 4200-4600.

 

[jeffwillb]

Re: 5.7 mercruser high rpm

contacted a chaparrel dealer and he said that the gear ratio is determined not only by the upper unit which is 1.62 but also the lower unit. I now suspect the repair guy put the wrong lower unit Does any one know how to tell the difference between one vs another? or how can I contact mercruser directly to get some answers

 

[Bondo]

Re: 5.7 mercruser high rpm

The overall ratio is determined by the gears in the upperunit......<br />All Lowerunits for Alpha1s,+ Mod.1s are 1.62/1....<br /><br />For a V8 at sea level, you Should have a 1.50/1....<br />Regardless, 5500rpms is Way to fast for a Merc.350...<br />A Larger Prop is the Answer....