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Old 08-10-2009, 11:09 PM   #1
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Default max rpm and cruising rpm

does anyone know the max rpm and cruising rpm on a 2400SCR with a 2001 5.7 V8 Thunderbolt ignition (250hp)?
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Old 08-11-2009, 12:05 AM   #2
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Default Re: max rpm and cruising rpm

if your talking about WOT...Wide Open Throttle...then it should be between 4200-4800rpm...
cruising is around 3600-4k rpm depending on water/wind conditions ..load....etc....and how fast you wanna burn gas....


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Old 08-11-2009, 03:43 AM   #3
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Default Re: max rpm and cruising rpm

I was looking at my Mercruiser specs the other day for my 2001 2500 SCR with the 5.7 Bravo III and it read 4200-4600 was cruising rpm. I thought this sounded high as I usually cruise at about 3800-4000.

Not sure if this helps, maybe somebody else can chime in on these specs and correct me if I am wrong. My boat is a 2500 and not a 2400 so i couldnt imagine one foot making a significant difference.
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Old 08-11-2009, 04:09 AM   #4
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Default Re: max rpm and cruising rpm

4200-4600 rpm is usually max rpm or WOT..wide open throttle....I'd be willing to bet dollars to donuts that if your running the boat at wot..your burning the engine up....


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Old 08-11-2009, 05:40 AM   #5
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Default Re: max rpm and cruising rpm

SP,

I usually run at 3800 and when I read that I thought that 4200-4600 was high. When I am at 3800-4000 I still have plennnnttty of throttle left. Do you happen to know what normal cruise rpm should be for my 2500 with a 5.7?

Thanks (again and again and again)
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Old 08-11-2009, 08:31 AM   #6
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Default Re: max rpm and cruising rpm

Quote:
Originally Posted by seapuppy
if your talking about WOT...Wide Open Throttle...then it should be between 4200-4800rpm...
cruising is around 3600-4k rpm depending on water/wind conditions ..load....etc....and how fast you wanna burn gas....


SP irate:
Thanks SP..i should be ok then as I cruise around 3800rpm with 4 adults and a full tank of gas. My only concern lies here. When i hit the 4000rpm my engine temp is at a stable 175. Compression all good on all cyclinders, changed manifolds, risers, impeller and thermostat, no leaks in hose pipes. The thermo should keep temp at a steady 160 however i spoke to alot of mechanics here and all said that 175 is reasonable and as long as the engine alarm does not sound no damage is being done.

Can you shed some light on this?

Thanks

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Old 08-11-2009, 02:06 PM   #7
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Default Re: max rpm and cruising rpm

Rich
a 5.7ltr is the same as any other small block chevie engine....cruise should be about 3600-4k rpm....depending on load/water/winds/etc....

as for temps....175 is normal...they put a 160 deg. thermostat in but by the time it opens and the engine transfers the heat and all..the temp normally stabilizes at about 175degs.....it should stay between 175-180 degs....the alarm will go off at about 195 - 200 degs....

no damage will happen but it's not good to run it that hot.....

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Old 08-11-2009, 02:36 PM   #8
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Default Re: max rpm and cruising rpm

I think one needs to define "cruise". Is it defined as maximum engine speed for a prolonged period? Is it defined as the RPM at which both both speed and fuel economy are maximized? to me cruise is any RPM after which the boat gets on plane and below 4K RPM. More of a Cruise range if you will.

The Merc 5.0/5.7 are basically Chevrolet engines. Though the engine architecture is the same between the car engines and the marine engines, the marine engines are kitted a little differently for their intended use.

What RPM the engine should operate at during WOT really is only useful to ensure the boat is proped correctly and as a very general measure of engine health. What one really needs to know is MAX engine RPM, which is usually the RPM at which the valves will start to float, which - if it occurs, WILL eventually lead to engine damage or destruction. Stay under MAX RPM.

Engine temp. The hotter the engine runs the more efficient it will be. My '04 Chevy Silverado P/U routinely runs just below 220. All the time. My '04 2400 SC3 with 5.7 MAG and B3 drive settles in at 175. Might dip a little below that, often goes a bit above to maybe 180. Perfectly accepable. 160 is too low.

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Old 08-11-2009, 04:12 PM   #9
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Default Re: max rpm and cruising rpm

Dan
technically your correct on the max rpm for any engine...most of the new engines...ie mpi/fuel injected engines have rev limiters ..thus preventing any damage to the engine at WOT...the earlier engines do not have that feature and can exceed red line settings and thus hit Max RPM's...as stated...WOT is used for knowing the proper prop types and pitch for a given hull design.....if your WOT is faster than the recommended ideal rpm set by merc...then your under propped....if you never reach the WOT range...your over propped...meaning the prop is trying to bite too much water...

even though the merc engines are basically chevie small block engines ....most of the internals and attaching periphials are marine grade and not street grade....plus these engines are beefed up to accept the harsh environments of marine life....ie stronger rods...pistons....valves are slightly different....compression chamber and piston shape is slightly different...cam duration and overlap is way different......water jackets are marinized and pumps are different ....curve on the distr. is different.....basically the engine is designed to run with a full load all the time......kinda like driving your truck up hill in 4th gear with a full bed of junk in the back.....

temps....yes cars and trucks run at a hotter thermostat for mainly purposes of combustion effenciency and pollution laws...but remember the street engine doesn't work as hard as a boat engine does on any given day......a non freshwater cooled engine will run at 160 cuz it's getting water out of the lake or river...sound..ocean....so..it will run alot cooler...gotta remember that a boat doesn't have a giant radiator to help cool it...it gets it's cooling from the outside water....but a FWC'd engine will use antifreeze and a heat exchanger to transfer engine heat to the outside water......having an engine run at 175 gives a big margin of safety from normal running temps to hi temp alarm....this is cuz at the speeds the engine is running..it doesn't take long for the temps to rise quickly.......

just my thoughts here....got a hit of caffiene and couldn't stop typing :mrgreen:

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