Engine temp question for RickB

Rick--- With all the talk that occurrs on this and other boating forums about the proper operating temperature range for a diesel I have been pondering a failry basic question for a bit now.

The short version of the question is this: is the relationship between coolant temperature and combustion chamber temperature such that if the coolant temperature is indicating properly on the gauge per the manual, does this always mean that the combustion chamber temperature is also in its correct range?

The background is this---- People often talk about their engine temperature in terms of what they see on the coolant gauge. They talk about the horrors that will occur if you run a diesel at less than it's optimal temperature range. The typical comment, which I myself have made from time to time is, "If you run your engine at too low an rpm it will run too cool and over time this could be detrimental to the engine."

When I have said this, I have meant the engine's combustion temperature being too cool.

Now I assume the engine manufacturers design the cooling systems of their engines, from coolant circulation volume and flow to thermostat characteristics to coolant temperature sensor and gauge, so that when the engine is operating in its proper temperature range (combustion) the temperature gauge on the panel will indicate the proper temperature (coolant).

So.....if on the typical diesel engine installed in the typical recreational boat the coolant temperature gauge is indicating the correct operating temperature per the manufacturer's manual, does that always mean that the combustion chamber temperature is also in the correct range?

The application of my question is this: At a cruising rpm of 1650 the coolant temperature gauges of the two FL120s in our boat indicate the proper temperature per the manual. If I pull the power back so the engines are operating at 1500 rpm (or even less) and the coolant temperatures remain at the same place on the gauges (kept there by the thermostats) can I assume that the combustion chamber temperatures are also remaining in the correct range?

So I would not have to worry about "too-cool" combustion chamber temperatures as long as the coolant temperature was at the right figure on the gauge per the operating manual.

Believe it or not, the temperature in the combustion chamber doesn't vary all that much. The heat of compression is about 500*C, the heat of combustion in the center of the "fireball" (if you don't mind me calling it that) is close to 2000*C. The temperature next to the cylinder wall is fairly close to that of the coolant and the temperature of the gas passing the exhaust valves will vary greatly with load but a good figure is around 1000*C at high load.

Now, getting back to your question. First we have to understand that there is a difference between temperature and heat. Your cooling system removes heat from the engine. The result of rejecting that heat is a lower temperature of the engine and fluids.

The amount of heat rejected to the coolant is directly related to the amount of fuel burned. At idle, the center of the "fireball" is just about what is is at max power, it is just a hell of a lot smaller and releases far less heat - fewer BTUs are available to heat the charge air, cylinder walls, pistons and all the other bits exposed to it. If you didn't control the jacket water temperature, it would go up and down proportionally to the amount of heat released by burning the fuel.

Throwing another log on the fire makes the room hotter but it doesn't change the temperature of the flame.

High power puts lots of weight of fuel in the combustion chamber, lots of weight of fuel means more heat is released. About 30 percent of that heat goes out the exhaust, another 20 percent is carried away by the coolant. When everything is right with the world, your engine can run at full power and the coolant will transfer enough heat to the ocean to maintain a constant coolant temperature that equates to a constant temperature at the cylinder wall and around the exhaust valves. The thermostat opening at full power should be enough to allow a coolant flow rate that transports just enough BTUs overboard to balance heat production with heat rejection and the coolant temperature remains constant at the thermostat setpoint.

If you reduce the heat input to the engine by reducing the fuel flow, the thermostat will reduce the coolant flow to maintain the setpoint. It will work that way until there is more heat produced than the coolant can carry away or not enough heat is produced to keep the coolant at the setpoint.

Either way, the temperature in the combustion chamber is, for all practical purposes, the same.

You may read about technologies that manage emissions by reducing combustion temperature. Some of them inject up to 50 percent by weight of water (in a fuel/water emulsion or a separate injection nozzle) to reduce the flame temperature in the fireball. But, to produce X amount of power, Y amount of fuel must be burned and the coolant doesn't even know that the flame temperature is lower, it just knows it has the same heat load to carry at the same power output as before.

You might look at it like a pot of boiling water. Water in an open pot stays at the same temperature regardless of the size of the fire under it. The only thing that changes is if you build a bigger fire under the pot, it will boil dry faster.

That is the long version, the short version is: When the cooling system is working correctly, the temperature differential across the heat exchanger is a measure of how much heat the coolant is carrying from the combustion chamber to the sea. Combustion temperature is fairly constant and is not altered by the cooling system.

New designs created to reduce fuel consumption and emissions will give us combustion chamber temperatures that are no higher than the compression temperature and hold that temperature longer by controlling the rate of heat release. The cooling system will never know the difference.

The thermostat controls the engine temp via the coolant if all other parts in the system are working as designed. The coolant removes the excess heat in the system by circulating faster at higher loads & slows down with a lighter load. The way I understand a engine cooling system is that the removal of the thermostat will allow the coolant to move thru the engine to fast and not remove enough of the excess heat, leaving hot spots in the engine while the gauge will not indicate a problem. I believe cruising at lower RPMs will not have a significant effect on engine life or dependability if the cooling system is operating properly, the thermostat will help maintain the desired temps by not allowing heat to be removed as fast.

Rick--- Good explanation, thank you, and I think I followed it correctly.

So it would seem, then, that if there is some truth to the notion that lower combustion temperatures (heat) at the outer edges of the combustion chamber can be detrimental to an engine over time, running our engines at 1500 or even a bit less will not be detrimental because even at that lower power setting the temperatures across the chamber will still be close enough to what they are at 1650 or 1700 that there will be no detrimental effect on the engine itself. And the fact the coolant temperature on the gauge remains consistent with the factory-specified figure means the cooling system via the thermostat is keeping things on an even keel with regards to the temperature of the engine's components like valves, cylinder walls, etc.

So as long as we're on this subject, when you have time perhaps you can explain to me what ARE the detrimental effects everyone likes to talk about from running a diesel "too cool?" Everyone talks about them but I do not recall ever seeing anyone describe what they actually are. Thanks.

You had me at...................

Marin said:

Rick---

The short version of the question is this:

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Riddle me this Batman.....

I always had a bard time evacuating all of the engine oil out of the Yanmar 4LHA in my previous boat. The reason being....even though I could idle my boat at the dock with a little bit of a load on it and it would get up to a solid 180 degrees coolant temp.... The oil temp would be lukewarm at best and would still be difficult to drain due to the fact that it was a little cool. I had fantasies of running the boat hard and then stopping at anchor and changing the oil immediately but of course the reality of the "contingencies" reared their ugly head and didn't want to be at anchor with an oilless engine. Anyway, it was a solid 5 minute no wake ride to my slip and by the time I got there, the oil was damn near cold even though the coolant temp was 180!!!!!!!!!!!!

Don't know about your setup but on our engines the raw water cooling of the engine oil is not thermostat controlled. The oil just goes through the lube oil heat exchanger which has a high volume of cold raw water going through it even at idle and back to the engine. So if the engine slows down and the heat production from the movement of the crank, cam, connecting rods, etc, drops, the oil temp is probably going to drop pretty fast because it's getting less heat transferred into it plus it's getting unregulated cooling from the raw water system.

When we change oil we run each engine (separately, we do them one at a time) in gear in the slip at about 1200 rpm until the oil filter is hot, but not real hot, to the touch. This usually takes about 20 minutes. Then we shut the engine down, punch holes in the top (bottom) of the oil filter, pump out the three gallons of oil and then change the filter.

My point here is that just because the coolant temp is 180 does not mean the engine is at operating temp. I would have a tendency to believe that oil temp is a better indication of "operating temp" than coolant temp. I have discussed this with other folks and they were having a hard time believing that the coolant temp could be 180 while the oil temp is not above 100....

When I really think about it, RickB's explanation does make a lost of sense. If a 4 stroke is running at 1,800 RPMs (I picked 1,800 to make the math easier!), each cylinder fires 900 times a minute or 15 times a second. At 1,500 RPM, each cylinder fires 750 times a minute or 12.5 times a second. Even though that is a 16.7% decrease in the # of firings, it is only over a one second period. That does not seem to be enough to have any effect whatsoever on the firing chamber temperature. Perhaps that is why you don't see an actual "Engine Cylinder Temperature Gauge". In any event, that temperature seems somewhat irrelevant to me as that is indeed a constant temperature. The important trick is to maintain the metal temperatures around the firing chambers, i.e. the block and head, at temperatures that allow everything to function correctly, no (or controlled) heat expansion, etc. Thus the calculation by manufacturers that at 180 degrees, coolant is removing enough heat from the combustion that it does not adversely affect the engine operation. The thermostat increases or decreases coolant flow to maintain that temperature.

The problem of underloading is not combustion chamber temperature.

It is combustion chamber pressure . With no load the piston does not keep the pressure up long enough most of the fuel to burn.

The only fuel that does burn is the fuel VAPOR , the excess fuel (that might have been vaporized and burned if held longer ) is blown out the exhaust , or as blowby to dilute the lube oil.

The unburned fuel in the exhaust is the cause of wet stacking (dry stack) or a fuel slick in the wet exhaust discharge.

On OTR trucks where the stack temperature is higher from more frequent high loads ,you will see black smoke (Bucky Balls) on acceleration , another over fuel condition.

AS the piston rings seal by pressure behind the rings , lo load allows the combustion gasses into the oil, and the light pressure burnishes the cylinder walls, removing the honing that controls oil on the cylinder walls.

More modern engines will use 3 or 4 shots from the injection system , in the hope of vaporizing more of the fuel, making a more efficient operating engine.

Warm , cool , not much difference , look at all the old Euro diesels that run at 140F or below.
BUT no load is a poor place to operate or even cruise.

This is why "start her up once a month" in storage is a rotten idea , and why some folks will chose to match the engine RPM and CRUISING loads to the propeller .

FF said:

The problem of underloading is not combustion chamber temperature.

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You are right - when that term is used to describe the operation of propulsion engines it is almost universally a mythical conditon.

It is combustion chamber pressure . With no load the piston does not keep the pressure up long enough most of the fuel to burn.

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Temperature, or more accurately, the amount of heat available to raise the temperature of the fuel (along with fuel quality itself) is the major factor that determines combustion quality.

Even during the phase of combustion called "afterburn" which occurs after injection is finished, and occurs only over a very small crank angle, the pressure is still well above the pressure at which injection began and the charge temperature is very much higher than at that point. The pressure is for all intents and purposes, irrelevant.

The only fuel that does burn is the fuel VAPOR , the excess fuel (that might have been vaporized and burned if held longer) is blown out the exhaust , or as blowby to dilute the lube oil.

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That is a truism that applies at all times and all loads. However, if your connection to time was accurate, an engine driving a fixed pitch propeller at low rpm or even idling at minimum rpm provides the longest period possible for all the fuel to burn. It can't be "held" much longer

The unburned fuel in the exhaust is the cause of wet stacking (dry stack) or a fuel slick in the wet exhaust discharge.

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Wet Stacking is an incredibly rare condition. How many readers have ever seen this phenomenon at all much less on their boats?

Wet stacking, when it does occur, is almost universally associated with generator engines running at a high constant speed with virtually no load, or unloaded (idling) stationary diesels in cold atmospheric conditions. Or clapped out Detroit 2-stokes. A cold stack will allow moisture in the exhaust (about a pound of water for every pound of fuel burned) to condense and wash soot and condensed hydrocarbons from the walls of the stack and eventually leak out to frighten the uninformed engine operator with visions of impending catastrophe.

AS the piston rings seal by pressure behind the rings , lo load allows the combustion gasses into the oil, and the light pressure burnishes the cylinder walls, removing the honing that controls oil on the cylinder walls.

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This is another exaggeration promoted to mythical status. Unless you idle your engine for 8 hours a day the likelihood of this condition occuring is hardly worth thinking about.

Low load is a (no pun intended) loaded concept. The published operating load range for virtually any diesel engine shows a turn-down ratio of about 5 to 1 or better. That means the manufacturer allows continuous operation (below a certain maximum power output) down to about 20 percent of maximum rated power.

If you look at the propeller curve you will see that operating outside the low end of the rating curve would mean your boat is probably moving through the water at less than walking speed. Other than approaching your dock or mooring, how often and how long do you operate that low range?

This is why "start her up once a month" in storage is a rotten idea.

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Idling a propulsion engine at the dock is not a good idea. Especially on those engines that use raw water cooled oil coolers. The only reason recreational boat engines use them is because they are cheaper to buy and install. A jacket water cooled oil cooler is far better for keeping the oil at the temperature is should be for viscosity and removal of contaminants.

...and why some folks will chose to match the engine RPM and CRUISING loads to the propeller .

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I have no idea of what that means. The cruising load matches the rpm of a fixed pitch propeller all by itself ... it doesn't need any help from us.

It seems then that to completely burn everything in the combustion chamber you need a larger fireball. If I understand this correctly. That means idling with a smaller fireball leaves carbon behind and running under a load which produces a larger fireball cleans out the carbon.

If that is correct, temperature isn't the issue as all combustion burns at the same temperature.

Yes, that is a pretty good way to look at it. You need enough heat (a large mass of material such as charge air) at a high enough temperature to heat the cold fuel to vaporize it and raise it to its autoignition temperature. Insufficient heat will only char some droplets to make soot and some might go out the stack as unburned fuel (white smoke) or be condensed in a wet exhasut system to form a sheen on the water.

One reason a mechanical injection engine smokes white and makes a sheen is because the engine starts at (or even beyond) maximum fuel delivery. More fuel is injected than it takes to make full power. There is no way on Earth all that ice cold fuel can be heated to ignition temperature in an equally cold engine so a lot of it goes out as white smoke and much is condensed on the cold exhaust pipe.

Since the engine doesn't come up to speed quickly, the governor keeps injecting a high volume of fuel into the cylinder to try to speed it up and that just adds more smoke and sheen to the situation. Once the engine reaches governed speed and warms up the problem goes away.

Idling with a tiny little fireball also means there is unburned fuel scattered around the edges where the mixture is too cool to vaporize or too lean to burn. A mechanical injection system doesn't atomize the fuel all that well at low rpm anyway so that means there are large droplets flying around that turn to carbon before they vaporize completely or are ignited.

Great explanations, Rick, thanks much.

Marin said:

Great explanations, Rick, thanks much.

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Ditto!! Thanks Rick

What a great forum. I have been churning over this question for a while. I have a 300 hp turbo older cummins that I run at low revs sometimes for a more economical cruise a 7.5 knots. After I do this I get a soot buildup that I clear but running her hard for 5 mins every 10 hours. From what I know that's ok but I was was still concerned about glazing/ soot in the oil. Is the fact that my oil is staying clean a indicator that all is ok?

On another note I have heard that they preheat the intake air on engines on oil rig support ships that do a lot of iding to help with the combustion. Could this reduce soot and get a better burn at lower revs?

moreton99 said:

Is the fact that my oil is staying clean a indicator that all is ok?

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That is an excellent indicator of cylinder health.

On another note I have heard that they preheat the intake air on engines on oil rig support ships that do a lot of iding to help with the combustion. Could this reduce soot and get a better burn at lower revs?

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The only application I am familiar with is using preheated jacket water in the charge air cooler so it acts as a heater during cold weather startup. Once the engine is running the turbos will heat the charge air very effectively. This is sometimes done on large medium speed engines that burn heavy oil operating in arctic conditions.

Rick, thanks for taking time to explain the above. I appreciate it!

Yes, thanks for enlightening us on that Rick, but you have sort of walked into this one, so as it's on everyone's lips, I'll ask it. Because I don't think, after all the times it has been discussed, we ever arrived at a definitive answer, even though we know even the definitive answer will still be a compromise of sorts, because clearly using the boat is the best way to keep it all good. However, accepting the reality many of us cannot get out for a proper run, what is the best thing to do when we go down to the boat to check it out, and feel that huge urge to do something because instinct says leaving the engine just sitting for maybe weeks or even months unused is not good. Do we....
1. Leave the damn thing alone...?
2. Turn it over without firing up for several revolutions to change position of parts and open and close different valves, & circulate a wee bit of oil...?
3. Run it under no load several minutes..?
4. Run it under light load for a few minutes..?
5. Run it under light load, (idle in gear, sort of thing), to operating temp..?
5a. Peter, you left out a choice. And that is to run the engines in gear under moderate load (1,200 rpm) until the coolant temperatures get to their normal reading at which point the engines continue to be run at that power and load for at least 30 minutes. This is what we do if we are unable to take the boat out for four to six weeks due to weather, my travel schedule, etc.
6. If none of the above...WTFESWD...translate that yourselves...

Hey, THD, don't distract him until he answers my questions first - get into line...and ok, Marin, I thought I had your technique covered in #5, but it's not quite the same, so I make that 5a...

Rick-this all leads me to another question-on fuel usage. Does the injection system inject a set amount of fuel on each firing cycle? Thus, the difference in fuel usage derives from the increased firing cycles at higher RPMs. Or, does the injection system, also in some manner I can't figure out, sense "load" and inject increased amounts of fuel on each firing cycle if the engine is "working" harder? If so, how does the injection system sense the engine load?

Peter, you left out a choice. And that is to run the engines in gear under moderate load (1,200 rpm) until the coolant temperatures get to their normal reading at which point the engines continue to be run at that power and load for at least 30 minutes. This is what we do if we are unable to take the boat out for four to six weeks due to weather, my travel schedule, etc.

Don't know if it's good, bad, or indifferent for the engines but it was what was suggested we do by our diesel shop and our acquaintances at Northern Lights/Lugger when we bought the boat, so we've done it for the last fifteen years. It's pretty rare we have to do this, however, as we are almost always able to take the boat out at least once every four to six weeks.

Regarding THD's question, is that not the function of the governor? The governor-- flyball as you once told me in the case of the FL120--- slows down if the load increases which therefore causes the injection pump to send more fuel to the injectors until the rpm is back where the throttle has set it to be and the governor is spinning at that speed again thus "locking" the fuel being sent to the injectors at that volume until the load changes again and the governor again adjusts and "locks" the amount of fuel being pumped. Yes, no, maybe so?

"Unless you idle your engine for 8 hours a day the likelihood of this condition occuring is hardly worth thinking about."

As the oxymoronic "Fast Trawler " with a 300hp engine putting cheap at 7K, 3gph ,is usually almost at idle.

"The cruising load matches the rpm of a fixed pitch propeller all by itself ... it doesn't need any help from us."

To load an engine a good percentage of the engines available power at the reduced RPM must be used.

Look at a HP vs prop load graph and notice the prop load at low speeds , say 1500rpm will be but a small fraction of the factory rated power available at 1500.

Using more power at the low RPM will give better efficiency , a CPP or cruise prop is the usual choice.

Read the PBB ,Calder article on electric boat propulsion , and notice his comments on Volvo overproping factory recommendation.

THD said:

... how does the injection system sense the engine load?

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Marin pretty much described it.

A diesel doesn't have a throttle that works the way a gas engine does, it doesn't vary the amount of air admitted to the cylinder as a means to control the weight of fuel in the mixture. A diesel always (though that is a highly nuanced "always" for reasons we needn't go into here) admits the same amount of air regardless of the speed or load. It is the amount of fuel that determines how fast the engine turns or how much load it can handle. The governor controls how much fuel is injected.

Moving the throttle on a mechanically injected engine does not open or close a fuel valve or throttle plate, it changes the pressure on a spring in the governor. That spring resists the force created by a spinning weight that is hinged so that increasing speed and resultant centrifual force trys to fling the weight outwards. As it moves outwards against the spring it moves a lever that, in the short version, opens or closes a valve that regulates how much fuel is above the injector pump plunger and sent to the injector itself.

The force on the flyballs is the same at a given rpm regardless of the load on the engine so we use the throttle lever (speed lever) to change the force on the spring (depending on the governor, it may increase the force or decrease it) so that the position of the valve that controls the fuel matches the amount of fuel required to oppose the spring and keep the engine turning at the rpm we command.

At no load, it requires very little fuel to reach max rpm, at high load, it takes a lot. With a properly matched fixed pitch propeller the maximum load possible matches the rated output of the engine at the "speed stop" rpm set by those screws on the side of the governor that you are not supposed to touch. The governor is set so that the range of fuel flow matches the range of power and all we have to do is move the throttle to obtain the speed we want.

If we had a controllable pitch propeller, we could "fool" the governor and overload the engine very easily. If the engine control system did not have a "combinator" or a more sophisticated governor to match pitch with rpm automatically, we could set a low rpm and a high pitch and quickly overheat and destroy the engine by overloading. Sort of like driving around in 4th gear all the time. In an overload condition more fuel is delivered than the engine can burn and all that excess fuel turns to heat and black smoke.

That is the super simple description. If you want to really get a firm grounding on governors, take a look at the Woodward governor website. they have probably built more governors than anyone else on the planet and they have some excellent training materials online. If you are the least bit of a geek, the subject is fascinating.

So do I get a more efficient "burn" at lower revs by reducing the rated hp of my engine? I am underpropped at the moment by 150 revs and can derate the motor. (mechanical fuel pump) I dont really need the 300 HP. She was a fishing trawler.

A mechanic told me we can reduce the fuel pressure to the fuel pump and "try it" (Adustable screw on the cummins fuel pump so easily undone) I have also been told to leave it all alone because it's running well but it seems logical and only provide benefits so I cant get it out of my mind. What do you think? Anyone been there?

Rick, I did have a look for info on the woodward governers but the only site I found seemed mainly about aircraft. Could you give a link?

moreton99 said:

So do I get a more efficient "burn" at lower revs by reducing the rated hp of my engine?

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the best way to do that is by pulling the throttle back. If you don't trust yourself to self limit, reset the speed lever stop on the governor so it doesn't move as far.

A mechanic told me we can reduce the fuel pressure to the fuel pump and "try it"

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You can't adjust the injection pump pressure ... the injectors "pop" at a certain pressure and anything less will create problems and anything more will create problems. The injection pump plunger is a very positive displacement device and there is no relief valve or adjustment on the outlet. Injection quantity is adjusted by changing the volume of fuel above the plunger at the start of injection, not the pressure.

I can't imagine what he was talking about. If he meant to reduce the pressure into the pump enough to starve the injector pump itself that would quickly destroy the pump by cavitation.




Woodward | Governors-W | Governors-Engine | Small

RickB-thanks for the diesel lessons! I just learned more on this thread in an hour or so than I have learned in the last 25 years of playing around with them.

Please keep in mind that all I have written about governors and injector pumps so far applies to good old fashioned mechanical injection systems only. Modern common rail systems and electronically controlled injectors operate much differently and are far more accurate..

But ... once the fuel is in the cylinder there isn't much difference.

"Please keep in mind that all I have written about governors and injector pumps so far applies to good old fashioned mechanical injection systems only."

But not for Detroit Diesel , where engine power ratings can be changed quite a bit by changing injector tips.

Big difference between power and fuel burn with 135's or 45's or anywhere in between.

Sometimes the folks in the 1930 era had great engineering!

" (CPP) we could set a low rpm and a high pitch and quickly overheat and destroy the engine by overloading. Sort of like driving around in 4th gear all the time. In an overload condition more fuel is delivered than the engine can burn and all that excess fuel turns to heat and black smoke."

It is always assumed a CPP system has an operator with the intelligence of a car operator .

No high IQ is required , few cars are operated in 4th all the time .

An EGT gauge is required for the boat driver when setting efficient pitch , about 1 /10 of a boat buck.

FF said:

But not for Detroit Diesel , where engine power ratings can be changed quite a bit by changing injector tips.

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No, it requires changing the injector which is a fuel pump in itself. Changing the tip alone would really screw things up badly.

The plunger and bushing that determine the maximum quantity of fuel that can be injected are different for different numbers and types of DD unit injectors.

No high IQ is required , few cars are operated in 4th all the time .

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Boats don't buck and jerk and rattle when severely "over-propped" either.

If there were any real benefits to putting a CP wheel on a small recreational boat there would be thousands of them in use. Just because you can tweak something doesn't mean it is worth tweaking. Reliability, low cost, and simplicity of use are the best attributes for a recreational vessel.

You could probably put an 18 speed two stick gearbox in your Toyota if you wanted but what would you really gain?