Why does my boat engine specify 25w-40

[Cousineddy]

Hi Bob,

My 4.3 L mercruiser specifies 25W-40. But, the 4.3L Vortec in a GM car specifies 5W-30. I assume the Marine version has the same crank and bearings. It actually runs a lower coolant operating temperature than a car. So why the big change in the viscosity requirement?

 

[blufeb95]

Because the boat is constantly operating at high revs and beats the crap out of oil, so the oil probably stays warmer and the constant revving is more likely to shear the oil so it specifies monograde, 25W40 is generally monograde 40 oil that has a declared W rating, but should be very shear stable since it's VII free.

 

[bluesmokertdi]

because it runs under constant load at mid to high rpms and will shear down most lighter oils

a modern SAE 40 would probably qualify as a (25/15w40)

note temp range down to 10F

https://www.tractorsupply.com/tsc/product/traveller-40w-motor-oil-5-qt

or go to wally world

Robot or human?

 

[Hall]

So why the big change in the viscosity requirement?

Because the engineers that are involved in the design, etc studied what was suitable.

 

[Dogman]

Probably so you’re more likely to buy Mercury’s 25w40. Instead of buying another brands 15w40.

 

[addyguy]

Also bc boats are always operated in warmer temps, so a low W rating isn’t required.

 

[meep]

Boats are driven at 50-75-90% loading all day long. Vehicles operate at 10-20% most of the time with 40-50% to accelerate. Marine use is a significantly harder sustained use than land.

 

[Cousineddy]

Isn’t monograde horrible for engines at cold start? Wouldn’t any engine be better off with a synthetic 10w-40? Or even 5w-40? Not a 25W.

 

[Dogman]

Isn’t monograde horrible for engines at cold start? Wouldn’t any engine be better off with a synthetic 10w-40? Or even 5w-40? Not a 25W.

In a perfect world engines would run on mono 0. That’s just my peon brain way of thinking about it.

 

[bluesmokertdi]

Isn’t monograde horrible for engines at cold start? Wouldn’t any engine be better off with a synthetic 10w-40? Or even 5w-40? Not a 25W.

no, not any more, notice pour points of these mono grades, the SAE 40 is -27C

https://klondikelubricants.com/wp-content/uploads/documents/KLONDIKE-Single-Grade-Diesel-Engine-Oils-PDS.pdf

 

[blufeb95]

Isn’t monograde horrible for engines at cold start? Wouldn’t any engine be better off with a synthetic 10w-40? Or even 5w-40? Not a 25W.

Because a boat is operated at temperatures above freezing, typically and 25W is more than sufficiently pumpable above freezing, with the way a boat operates it should get up to temp pretty fast, 5W40 oils are known to shear horribly when abused, I've seen complaints from many people using Rotella T6 5W40 in shared sump motorcycles complaining it shears pretty quickly and makes the bikes shift horribly, whereas this is not a big problem with T6 15W40 or 20W40 motorcycle oil.

 

[Cousineddy]

So, the logic is that since marine engines are pushed much harder than a car engine, and operated above freezing, it needs a much thicker oil for protection. But, why wouldn’t you run 25w-40 in your car engine, which is basically the same, to give it more protection?. Wouldn’t it last longer if 25w-40 gives you more protection? Isnt it really the oil flow through the journal bearings that provide the separation force? Not necessarily viscosity and pressure? If that is true, why is it better to run a super thick oil. Just because the viscosity breaks down quickly?

 

[alarmguy]

So, the logic is that since marine engines are pushed much harder than a car engine, and operated above freezing, it needs a much thicker oil for protection. But, why wouldn’t you run 25w-40 in your car engine, which is basically the same, to give it more protection?. Wouldn’t it last longer if 25w-40 gives you more protection? Isnt it really the oil flow through the journal bearings that provide the separation force? Not necessarily viscosity and pressure? If that is true, why is it better to run a super thick oil. Just because the viscosity breaks down quickly?

40 weight oil is not "much thicker" not even close.
40 weight oil in a marine engine will ensure the oil stays at or near the 40 designation and not shear down under the incredible stress and forces that a marine engine is under pushing a vehicle (boat) through water with no wheels that make something easily roll along.
Throw in the high revolutions and incredible stress and heat on the pistons transferred to the oil when the oil is sprayed onto the bottom of the pistons to help cool them down.

Comparing oil for an automobile engine and a marine engine is like comparing apples and oranges, your assuming because the fuel is the same its ok to compare but one of them has wheels that roll on a hard surface, the other is pushing a log through water.

As far as your questions above, think of it this way. Why would you assume a 25/40 gives a car engine more protection?
An automobile engine lasts far longer then a marine engine. We are talking 5000 to 10,000 rough hours, its reall y speed dependant but lets says a truck with 152,000 miles ran 4,620 hours and still going strong, a gas marine engine may last 2000 hours.

If you use lighter weight automobile oil in it, the marine engine might only last 1000 hours. Take a marine engine out in the ocean on a good hard run with lighter automobile engine oil in it, once in safe harbor slow it down to a slow idle and listen to those valves and lifters tap from the automobile engine oil unable to handle the heat. (running thin)
SO its not a question of running a 25/40 oil in an automobile to give you more protection more then it IS if you run a lighter automobile oil in a marine engine you risk seriously shortening its already much lower engine life compared to an automobile engine.

 

[ragtoplvr]

That is why when I had the Jet boat I ran only Valvoline racing 50

 

[kschachn]

So, the logic is that since marine engines are pushed much harder than a car engine, and operated above freezing, it needs a much thicker oil for protection. But, why wouldn’t you run 25w-40 in your car engine, which is basically the same, to give it more protection?. Wouldn’t it last longer if 25w-40 gives you more protection? Isnt it really the oil flow through the journal bearings that provide the separation force? Not necessarily viscosity and pressure? If that is true, why is it better to run a super thick oil. Just because the viscosity breaks down quickly?

If you know enough to phrase the question that way then you already know the answer.

Just like you should know whether a monograde oil is "horrible" for your engine.

 

[Jetronic]

Hi Bob,

My 4.3 L mercruiser specifies 25W-40. But, the 4.3L Vortec in a GM car specifies 5W-30. I assume the Marine version has the same crank and bearings. It actually runs a lower coolant operating temperature than a car. So why the big change in the viscosity requirement?

Running WOT for longer?

 

[Jetronic]

So, the logic is that since marine engines are pushed much harder than a car engine, and operated above freezing, it needs a much thicker oil for protection. But, why wouldn’t you run 25w-40 in your car engine, which is basically the same, to give it more protection?. Wouldn’t it last longer if 25w-40 gives you more protection? Isnt it really the oil flow through the journal bearings that provide the separation force? Not necessarily viscosity and pressure? If that is true, why is it better to run a super thick oil. Just because the viscosity breaks down quickly?

It's viscosity that protects the engine, not flow through the bearings. There's a side leakage on the unloaded side but that's not really flow through the bearing. If overheating and wear materials in the oil wasn't a thing, they could seal the bearings and just run on the few drops that are between the bearing and crank. No flow or pressure required, just oil. the leakage makes the pressure necessary to ensure flow TO the bearing.

 

[K-Man]

Marine loads on engines are far more comparable to light-aircraft loads than they are to automotive loads. Boats and aircraft run at higher engine speeds and much closer to 100% of power rating all the time. Road-vehicle engines typically don't. Overhaul schedules are more similar between boat and aircraft engines than to vehicle engines. The main difference is that those in boats typically just wear out, but aircraft engines must be overhauled at assigned intervals based on hours.

This is a reason why only a limited number of purpose-built production engines are used in light aircraft, by the way, and those engines haven't changed much over the years since they were certified. It would be interesting to compare the oil specs between aircraft engines and boat engines.

 

[4WD]

Boats are driven at 50-75-90% loading all day long. Vehicles operate at 10-20% most of the time with 40-50% to accelerate. Marine use is a significantly harder sustained use than land.

Mechanic near here did a twin engine test with big Suzuki 4 strokes at WOT … “road” motor oil side all foamed up … NMMA tested oil side did just fine …

 

[Cousineddy]

Ok, if I sum up what you all are saying is that the oil type is designated based on how the engine is used, not based on the design of the crank and journal bearings or other internal parts. Im guessing the engineers felt the extra protection needed at constant WOT and 3/4 throttle justified the additional wear at cold startup. Makes sense. Thanks Bob.