Mercruiser 2.5 120 hp engine RPM ??????

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stingray165ss

Seaman
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Jun 20, 2012
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Okay guys quick question here, I currently have a mercruiser 2.5 120 Hp engine turning a 14.25 X 18 SS prop. I can currently get around 4500 RPM with this prop. The plate on the engine says that maximum rpm is 3900-4300, so i am assuming i need to go up in pitch on my prop.The problem i am having is that Mercruiser support says that the max rpm on this engine is 4200-4600 which is correct? Did they manufacture two different 120 2.5 in the 80's? I just want to be sure i dont over rev my engine. any advie would be greatly appreciated.
 
achris

achris

More fish than mountain goat
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27,468
Re: Mercruiser 2.5 120 hp engine RPM ??????

Engine serial number? But yes, the engines got re-specified in 1985...
But if you're making 4500 and it's pre-85, no problem, and your engine will appreciate you for that... truly...
 
S

stingray165ss

Seaman
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Messages
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Re: Mercruiser 2.5 120 hp engine RPM ??????

Thank you for the reply, I will have to get the serial number this evening when I get home. So if it is pre 1985 it is ok to run 4200-4600. If it is post 1985 is it still ok to run 4200-4600.
 
thumpar

thumpar

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Re: Mercruiser 2.5 120 hp engine RPM ??????

I run a 19 pitch prop on my 83 120. I can almost touch 4500 if it is trimmed out on smooth water with a light load. After 6 years of owning it this has never been a problem.
 
S

stingray165ss

Seaman
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Messages
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Re: Mercruiser 2.5 120 hp engine RPM ??????

Thank you thumper I thought about switching to a 19 pitch. So I guess all the 2.5 are the same they just got re spec. Later on
 
Don S

Don S

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Re: Mercruiser 2.5 120 hp engine RPM ??????

All the same parts are used in the 120 engines from 83 to 86.
In the engine service manual #3, the specs for the 120 up to 85 was 3900 to 4300
In the engine service manual #10 the specs for the 120 from 85 to 88 was 4200 to 4600.

My guess is that they found the lower rpm was lugging the engine too much and raised it. There for, I would use the later spec without hesitation.
 
S

stingray165ss

Seaman
Joined
Jun 20, 2012
Messages
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Re: Mercruiser 2.5 120 hp engine RPM ??????

Thanks guys, I really appreciate it. This is a great forum with tons of knowledge.
 
achris

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Mercruiser 2.5 120 hp engine RPM ??????

Don S said:
All the same parts are used in the 120 engines from 83 to 86.
In the engine service manual #3, the specs for the 120 up to 85 was 3900 to 4300
In the engine service manual #10 the specs for the 120 from 85 to 88 was 4200 to 4600.

My guess is that they found the lower rpm was lugging the engine too much and raised it. There for, I would use the later spec without hesitation.
Click to expand...

I wish I was as articulate as that.... :D
 
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