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weekender

M5Di Engine Cruising RPM

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weekender

Hi All,

I been told to cruise at no more than 4-4.5k RPM on the M5Di engine, otherwise it just taxes the engine too much if your leaving it at that RPM for 5-10 minutes at a time when cruising.

Is this true? Makes sense, i think redline/max rpm on 19 23 lsv w torque prop is ~ 6 or 6.5K.

I get about 30, 35 MPH at about 5K RPM, or 25mph at 4k, which feels a bit slow for cruising.

 

makes sense to me as you wouldnt go down the highway in your car at 6k RPM, just sounds like it cant be good for the engine, and mostly, bad on gas.

 

Just curious if anything could prematurely wear if crusing at 5k+ for 10 min at a time just to move faster w the m5di engine.

 

Thanks

 

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gregtay
Posted (edited)

These boats aren't really designed or prop'ed for speed.  You could toss a speed prop on there an get the speed you want but it would take away from the things your boat is really good at.  I don't think 5k for 10 mins is going to break the motor (meaning the motor can handle it) but it is likely going to wear the motor and other parts faster than cruising at 3.5k... We don't ski but back when we did I know we turned some high RPMs for the guys who skied on the faster side of things.  Now with regards to fuel consumption... go look at some of the tests on our boats and they typically report the GPH is ~ 75-80% higher at 5k vs 4k RPM.  And 5.5k vs 4k is over 2x the GPH.  Also... I think the GM spec redline for the 5.3 is 6k but it might be lower in the Malibu marine config.  I came from the world of fast boat years ago and when I switch to a Malibu I just learned that cruising across the lake at 20-23mph and 3.5k rpm is just fine (and far more comfortable if there is any chop.)  So my opinion is... slow down and enjoy the ride:).

Below is a chart form Boating mag's 2018 23LSV tests (so this would be the Raptor... so the M5 might have lower numbers but the jump as RPMs go up will be the same.)

23LSVfuel.jpg.4db9f303c448186f2000cf531cc48cad.jpg

 

Edited by gregtay

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surfdude
Posted (edited)

I’m at 25 mph at 3500 rpm, 2019 22 lsv with the standard prop, m6di engine. Haven’t checked my top speed, but I’m thinking over 40. I Never cruise at over 3500 rpm.

Edited by surfdude

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gregtay
13 minutes ago, surfdude said:

I’m at 25 mph at 3500 rpm, 2019 22 lsv with the standard prop, m6di engine. Haven’t checked my top speed, but I’m thinking over 40. I Never cruise at over 3500 rpm.

23LSV w/ M6di and the 2277 is also 25mph @ 3500 (right on the numbers for both gauges). 20mph is 3200RPM.  Tops out just below 45mph (I only know that because it was part of my dealers pre-delivery inspection to run WOT and record the speed.  I have no plans to every go that fast.

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jjackkrash

Cruising speed is easier on the engine, for sure, but barefooters drop the hammer on their boats as part of the sport seemingly without issue.   I would bet money that running at WOT all the time would result in less service life/service hours total available for the engine, but how many less hours?  10 to 20% less maybe?  I doubt its 50%.  I bet you could run a typical new marine V8 at least the expected 1500 hours straight at WOT without killing it.  Here is little article I read while back on expected marine engine life:  

https://themarinesurveyors.com/how-long-will-my-engine-last/ 

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carguy79ta

I am thinking you can basically run the piss out of it. Do I? No. I had my first boat in the80s, an IO with a 190hp 302 Ford motor. I immediately rushed out and bought a service manual. It stated "After initial break in, the motor can be run at WOT continuously."  Wow. The difference btwn car motors and boat motors. It was a 77 Hammond Eldorado.

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pauley71
Posted (edited)

@gregtay - Interesting chart.  I have a sound pressure meter so might do some RPM comparisons although I would have no idea where they positioned the mic to do an apples to apples comparison.  I have noticed the M5di runs slightly higher RPMs than my 2018 6.2L (410) did. 

TMC - How do you get gph information?  I'm assuming some diagnostic port or is that RTD available in the engine information on the 7" screen under settings?

Edited by pauley71

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ConnollyCrew

 I cruise I keep an eye on my tach, not my speedo.I keep it around 3500-3700. Prop will determine speed. 

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Raleigh

Will the 23 LSV with M6di get better gas “mileage” than the M6di if you run at the same speeds? Seems counterintuitive but with lower RPMs for the same speed and work (500 rpms less than M5di using this thread as my research), would there be better fuel efficiency? 

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justgary

Late to the party as usual. 

That chart shows 3500 in red as the most economical cruising speed even though it clearly shows that 4000 rpm gets better mpg (and thus, total range).

Your boat requires a specific amount of power to push it a specific speed.  The drag on the hull increases with the square of speed, less a bit to account for the hull rising out of the water as it planes.  At some point the rising drag due to speed overcomes the decrease due to lifting and you have passed the hump in the economy curve. 

Don't fool yourself that one rpm is more economical than another until you test your specific setup.  It will change when you change something (prop, trim angle, extra ballast/people, etc.).  Maybe not much, but maybe more than you think. 

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