Puzzled around a 350 mpi. Someone tell me.

[QBhoy]

So recently I got egg on my face on another post about something or other to do with another owners Merc 350 mpi. He said his manual stated the top of the Rev range was 5200. I argued that it should be 5000. Turns out that the exception to the rule is with one type of this model of 350 mpi. The fresh water cooled version. It does indeed run to 5200.

Why is there a difference? Both quote the same power too. What’s going on. What’s the logic here. Maybe a simple answer ? Just been bugging me.

Cheers

 

[jimmbo]

Both are 300 horses, maybe one has horses that are tiny bit stronger. Maybe one of the numbers was a Typo, Maybe there was a slight change, perhaps the Timing Chain was retarded/advanced a degree or two, that could shift the power curve higher/lower, but not the peak HP. To get the answer would require speaking to the Engineers, and the Marketing Depts

 

[thumpar]

Does the Fresh cooled engine run at a different temp? Maybe a higher temp more like a car allows for higher operating RPM. I know raw cooled can't be run high because salt comes out of solution and water boils without pressure. I really don't know just throwing out ideas.

 

[andrewterri]

im guessing it has to do with the small amount of more power required to turn the impeller. From my understanding only fresh water cooled has the impeller to pick up water. Engine Internals would be the same so it much be an external factor

 

[thumpar]

im guessing it has to do with the small amount of more power required to turn the impeller. From my understanding only fresh water cooled has the impeller to pick up water. Engine Internals would be the same so it much be an external factor

They both have an impeller. The difference is the raw cooling just runs through the engine and the fresh cooling runs raw water through a heat exchanger with antifreeze mix running in the engine side.

 

[andrewterri]

Good to know, thanks thumpar.

 

[QBhoy]

Yeah it’s a mystery to me. You would imagine the most of the components would be the same for manufacturing costs. Especially the base engine...so why the different rpm range. 4600-5000 the norm. 4800-5200 the fresh water specs ?? Don’t get it.

 

[QBhoy]

AllDodge

saw your post in the other thread. Yeah see why you mean but I’m maybe meaning just specifically to this 350 mpi engine. Totally strange one.

 

[MalibuCorporate]

It’s different Valves that do it. The one which runs flat top valves will give you a higher compression and in turn will give you that rpm. If I can remember right it’s the hydrolic flat top valves.

 

[MalibuCorporate]

That’s why 305 SBC have a high Rpm when it comes to marine engines. Fuel injection means a lot now days too. They are getting the same out of V6 marine engines they used to get out of V8s

 

[MalibuCorporate]

When you say mpi your referring to multiport injection right

 

[MalibuCorporate]

It will change if your referring to fresh water and salt water. In that case no it’s not the valves. I could be mistaken. I’m not sure if it’s the salt water vs fresh water rpm your curious about

 

[MalibuCorporate]

Does the Fresh cooled engine run at a different temp? Maybe a higher temp more like a car allows for higher operating RPM. I know raw cooled can't be run high because salt comes out of solution and water boils without pressure. I really don't know just throwing out ideas.

I see hmmm yea I believe they do run at different temps. Hmmm that’s a good question I’m going to look into that

 

[Fun Times]

The 5200 RPM range for the 350 Mag stern drive package started with the late 2002/early 2003 model years when Merc went with the newer designed 2002–UP, start GM IAFM (GMEFI) intake manifold, 555 ECM, your favorite flat cap designed distributor ignition system., GEN+=Vortec, with roller lifters all working together to allow them to bump up the RPM to get closer to the 300-HP range.
Here's some of the differences as of 2002-08 MAY....There used to be an Merc engine ID chart that came out in 2005 online but it's no longer available which ought of been a better guide vs the following 2002 bulletin.
https://www.perfprotech.com/blog/boa...marine-engines
https://mafiadoc.com/mercruiser-gm-e...ea5dd005b.html

^, The 6.2L went up to 5200 as well around that time frame.

The 350 Mag MPI MH 1996–1997, 6MH, start Gen+, Bravo, start MerCruiserMPI intake.
The 350 Mag MPI MN 2002, "low MerCruiser MPI intake" was 5000 RPM.
Then came the 2002–UP, start GM IAFM (GMEFI) intake for 5200 RPM.
The Inboard Models may not have wanted the transmission revving that high.

I mentioned some intakes in the past following link,
https://forums.iboats.com/forum/engi...9#post10157239

^, The change of intake designs again, Intake Manifold and Fuel Rails, https://www.mercruiserparts.com/350-...-0l331599-thru

Intake Manifold and Fuel Rail , https://www.mercruiserparts.com/350-...-0m300000-thru

MerCruiserMPI intake INTAKE MANIFOLD AND FUEL RAILS .... INTAKE PLENUM AND FLAME ARRESTOR

 

[alldodge]

Posted to that one about the same time you started this one. Figured the Mod's would merge, but guess not

 

[Scott Danforth]

heat exchanged cooled motors run at a more stable and consistent temperature. this is huge when it comes to engine programming and how much margin to the edge of detonation is left.

transmission its connected to is another factor.

the actual cam grind is yet another factor

 

[QBhoy]

Yeah. Good points above for sure.
Toud think that a 2003 350 mag mpi raw water cooled bravo would be the same engine as a 2003 (or similar) 350 mag mpi fresh water cooled though ? Just weird to me that they would have a different rev range