4cyl 120HP safe cruising RPMs and MAX RPM's?

[Fordiesel69]

I chose the general forum as my question is not really a techincal / repair related.

I have been reading past topics of the 2.5L and 3.0L 4cyl's likeing to throw push rods / breaking rockers. So I guess this stems from over reving and or an incorrect prop. So to my question, what is a good safe cruising RPM, and what would be acceptable MAX rpm for short bursts?

I had mine to 4100 for a few sec and really did not like the sound it made. It sounded like it was really being strained.

FYI, it is a merc 120HP.

 

[windsors03cobra]

Re: 4cyl 120HP safe cruising RPMs and MAX RPM's?

I have a like motor with a couple more jugs on one end, I like to cruise at 3 grand and have a WOT of about 4500 of which I only use short bursts.
The engine is older than I am and I am not trying to tear it up when it works fine. I know I have no speed boat and cruise it lovingly. :redface:

 

[scoutabout]

Re: 4cyl 120HP safe cruising RPMs and MAX RPM's?

4400-4800 WOT RPM are the numbers I've most often seen for the Mercruiser 3.0.

Rather than sounding strained I thought my 1996 SeaRay 3.0 sounded terrific wound out.

 

[QC]

Re: 4cyl 120HP safe cruising RPMs and MAX RPM's?

Based on all of my reading, operations manual info and this forum, I believe that Cruise RPM is somewhat arbitrary. Many say 80%, many say best fuel economy, many say 3/4 throttle (I don't believe any of them have measured anything, so I am not clear what that really is).

Depending on the hull weight etc. 3000 RPM could easily be too slow to be on an efficient plane. Depends on a lot of factors.

What is not arbitrary is wide open throttle RPM range. You must be within that window or all operating points could be bad. Annnnd, I believe that your 2.5 has a a range of 4000 - 4600. Yes, some 3.0's were rated higher, but most then were 4200 - 4600, I also found some 120s rated between 3900 - 4300, but I can't confirm the '73 or '74 if that's what you have.

I am going to move this to the I/O section. Agree, not maintenance, but this section is also non-engine and it's hard to make this thread meet that . . .

 

[Fordiesel69]

Re: 4cyl 120HP safe cruising RPMs and MAX RPM's?

Those who have had pushrods come off, what RPM were you at?

 

[Howard Sterndrive]

Re: 4cyl 120HP safe cruising RPMs and MAX RPM's?

I have been reading past topics of the 2.5L and 3.0L 4cyl's likeing to throw push rods / breaking rockers.

been around a lot of these motors and never heard of that.

 

[QC]

Re: 4cyl 120HP safe cruising RPMs and MAX RPM's?

Yeah, me either, ^^^^^ and my BIL has a 3.0 that has had the snot run out of it since '84 when I bought the boat new. A LOT of time at 4600 RPM. Head has never been off and maybe not the valve cover

 

[elkhunter338]

Re: 4cyl 120HP safe cruising RPMs and MAX RPM's?

I run a 21 foot starcraft cuddy cabin model boat, weight is 4300 on the trailer axles so the boat is fairly heavy for a starcraft.
140hp that only has 140 hrs on it, replaced the old 140 in 2007. Both motors run the same.
Top speed with the prop I have on it is 3900 rpms @ 27 mph. I have tried other props and found I like this one.
I cruise at 2800 rpms 20 mph and found this seems to get me the best fuel econmony. Maybe I will push it up to 3,000 rpms on a nice day. I do not like to push it above 3,000 just sounds too high rpms for a in-line 4. Max. torque is around 2800 rpms which should give you the best fuel economy. Alot of times I cruise at this speed for 20-90 min with no rpm change. I can get more top end speed with a higher pitch prop but rpms also drop and no low end power. Shallower prop and I seem to burn alot more fuel if I cruise at 20 mph and 3200 rpms, but my top end is above 4,000.
I have ran this boat for the past 5 yrs in the ocean and probaly atleast 150+ hrs of run time or more. Many 20 mile trips one way out halibut fishing.
One day ran 40 miles each day 3 days in a row.

 

[woosterken]

Re: 4cyl 120HP safe cruising RPMs and MAX RPM's?

here are the numbers from the "pocket service guide" that came with my
1974 merc.120hp WOT 3900-4300
140hp WOT 4200-4600
165 hp WOT 3900-4300


hope this helps

woosterken