engine torque

[aerobat]

just out of interest - do we have somewhere numbers what engine torque ( at what rpm ) our volvo penta or mercruiser engines develop ?
i did not found any information , only for horsepower .

cheers !

 

[HT32BSX115]

Re: engine torque

Howdy,

Sorry no one answered your question.

I have never seen Mercury or Volvo publish those numbers either.

You can do a search here for Torque vs HP and you'll find all sorts of discussion on the topic. It's mostly theory on which numbers are more meaningful.
And of course, there's a mathematical relationship between HP & Torque.....so you can easily calculate the torque at whatever the RPM the posted HP(s) was (were) posted.

Most engine rebuilders don't "Dyno" their engines unless you ask them to, but be prepared to pay for the service if you do! It takes more than a few hours to set up an engine and run the test. ($500-$1000)

They might do one of their "stock" engines though so they can back up their claims but don't expect it to be accurate for every engine. You only "know" when you do the engine in question.


Regards,


Rick

 

[E4ODnut]

Re: engine torque

This is one of my pet peeves.

If you are in the market for a marine diesel, most, if not all of the manufacturers will supply you with the torque, horsepower and Brake Specific fuel consumption curves as well as the propeller power demand curve. Gas engine suppliers used to do the same. No more.

It's highly unlikely that you will get this information out of Volvo, Mercruiser, or any other Company that converts any current gas engine for marine use.

I was at the Vancouver Boat Show this Spring and cornered both the Volvo and Mercruiser reps to question them on if this information was available, and if not why not. All I got was BS and double talk from Merc. To his credit, the Volvo rep sympathised, but could offer no explanation.

Could it have something to do with the fact that GM now has a virtual monopoly on supplying gas engines for marine use?

 

[Don S]

Re: engine torque

Could it have something to do with the fact that GM now has a virtual monopoly on supplying gas engines for marine use?

Neither Volvo, Mercruiser, even OMC when they existed have ever given torque specs. Not even when they used Fords.

The only reason GM is the monopoly in marine gas engines, is because the others got out of it.

 

[E4ODnut]

Re: engine torque

Don,
You could be right in the reason that GM has a monopoly now, but that still doesn't answer the question of why these very important specifications are not available.

I have a copy of the specifications which include the torque and horsepower curves for my 351 Windsors, published by Ford. I also have copies of the 300, 351 and 460 industrial versions which include the BSFC curves, also published by Ford.

My point is that in order to make an informed decision on how to choose the right engine for a particular application this information is vital, yet the marine gas engine suppliers now refuse to supply it.

Their logic escapes me.

 

[QC]

Re: engine torque

The reason most marine manufacturers do not publish torque numbers is that you can't do much with the information. If propped correctly the only time peak torque comes into play is at WOT at peak torque RPM. This occurs when you mash the throttle AND she gets stuck at that RPM. Unless she is seriously over-wheeled, or she has a really hard time planing, you just can't get there. Typically RPM rises and you continue on to some higher RPM cruise speed. You can't lug down a marine engine unless you throw out a Huge net or tow something. There is no way to force the boat down to peak torque RPM. Look at a propeller load curve, you don't get there.

This is somewhat different when it comes to some large commercial applications, but not usually what we are talking about here.

 

[aerobat]

Re: engine torque

GM by itself surpizingly provides curves for its marine blocks. lets e.g start with the 3.0 inline4

PDF

volvo or mercruiser do not this because on the one hand QC gave good arguments that this numbers do not have much value in a boat application and on the other hand surely most people do not even understand what torque is and what not.

looking again at the 3.0 GM says it peaks at 177 lb/ft at 3600 in torque. engines tend to have the best bsfc at peak torque, so if your boat/prop application permits, crusing at 3600 is a sweet spot for this engine. and heck - when volvo recommends 75% rpm for cruise and we take 4600 max rpm you land by surprize at 3500-3600 rpm, quess what ...

 

[QC]

Re: engine torque

surely most people do not even understand what torque is and what not.

Roger that!!

engines tend to have the best bsfc at peak torque

And WOT, but that can't be achieved without propping improperly. Annnnddddd, boat efficiency is way more about hull efficiency than engine efficiency. The best example I can give you is that almost all planing hulls are most efficient at wakeless speed. But Otto cycle engines deliver their worst fuel efficiency at Idle, so can't be the engine

 

[aerobat]

Re: engine torque

And WOT, but that can't be achieved without propping improperly. Annnnddddd, boat efficiency is way more about hull efficiency than engine efficiency.

fully agree with you, that even cannot be in most cases achieved at vehicles. to judge what sfc your boat engine has at a given rpm ,which is achieved out of a given power output, you would need a "muscheldiagramm" ( i do not know the english word ) and the exact cylinder pressure for this condition

http://upload.wikimedia.org/wikipedia/commons/6/60/Brake_specific_fuel_consumption.svg

thats of course far beyond interest of a boater.

in practical terms for best efficency all we can do is having a fuel flow meter and reecord the fuel flow vs boat speed. there will your find your sweet spot for most range. and correct - it far more depends on hull design than on engine SFC

 

[E4ODnut]

Re: engine torque

Aerobat,
Good link! Got any more?

There have been some interesting points of view presented on this thread. It may well be that the technical aspects of the torque, horsepower, BSFC and propeller power demand curves are of little or no interest to the average boater, but what about the people to which they mean a great deal?

As has been pointed out, all boats are not created equal, and boats in general are a compromise in almost all respects. You have high speed planing hull boats, semi displacement cruisers, displacement hull pleasure boats, heavy duty commercial boats and everything in between. When choosing an engine for any design of boat you need to have as much information about the choices of engines available as possible to mate it with the hull design you intend to use.

If you are considering a diesel engine this information is readily available from most, if not all suppliers. Why not the gas engine suppliers? Is the industry "dumbing it down" for the masses at the expense of those who are interested and want, or even need to know?

This is the cause of my complaint.

 

[QC]

Re: engine torque

Is the industry "dumbing it down" for the masses at the expense of those who are interested and want, or even need to know?

Yes There are very few serious gasoline engine applications. Just the way it is.

 

[aerobat]

Re: engine torque

When choosing an engine for any design of boat you need to have as much information about the choices of engines available as possible to mate it with the hull design you intend to use.

in most cases we do not even choose an engine - tha boat manufacturer does and we only choose between some few engine options. in a used boat we mostly also live with that what is already installed or repower with the same block.

we could boil it down that even hp are meaningless since all we can do is to prop it correctly to reach rated rpm at WOT in a boat and it will go as fast as it goes.

but nevertheless there are boaters who like to know more about its engine and the manufacturers could indeed say a little bit more about them.

the good point - due to the curves of GM the marine setups are not bad, they incorporate a flat torque curve with a strong midrange what is good for gliders to push them quickly and poweful out of the hole.

here the 4.3 V6

PDF

5.0 V8

PDF

5.7 V8

PDF


i guess the final numbers delivered by volvo or mercruiser may vary a little due to different carbs/ fuel injection installations and even in different boats ( how many fresh air is provided to the engine compartment, what is the average engine compartment temperature when running ? ) .

further currently the prop end not engine power is provided so its also dependant on the drive efficency - does anybody know how much horses a bravo III or a volvo sx/dps eats up ? ( thats a very interesting question i have no idea about )

cheers

 

[E4ODnut]

Re: engine torque

Aerobat
Thanks for the links, that's more information than I was able to find.

Now, about the Wikipedia link. Is that a 3 dimensional BSFC map? If so, it looks like manifold pressure is plotted on the Y axis, RPM on the X axis and BSFC in the 3'd dimension. Any idea what engine this is for and what are the units of BSFC are?

I find BSFC maps fascinating, but they are very hard to find.

 

[aerobat]

Re: engine torque

Is that a 3 dimensional BSFC map? If so, it looks like manifold pressure is plotted on the Y axis, RPM on the X axis and BSFC in the 3'd dimension. Any idea what engine this is for and what are the units of BSFC are?

I find BSFC maps fascinating, but they are very hard to find.

like said ido not know the english word for it, - in german "muscheldiagramm" .

its from volkswagen TDI automotive engine.

the left side is not the manifold pressure but the break mean effective cylinder pressure expressed in metric BAR. the numbers inside the diagramm are bsfc in metric g/kwh . the top red red line is basicly the torque curve at full throttle and the according maximum achiveable cylinder pressures at full throttle and agiven rpm. the area below displays the sfc of the engine at various part loads and various rpm,s .

you see the best sfc the engine can get is 206 g/kwh and reached at about 2200 rpm and about 90 % throttle. you see also that reving the engine with only slight throttle input ( e.g driving you car in first gear at 4000 rpm but only slight throttle input ) makes the engine sfc goes as worse as 450 g/kwh .

thats a turbocharged diesel engine. in naturally aspirated petrols you usually get best sfc ( like qc said correctly ) at peak torque and full throttle. thats because at full throttle the throttle valve is fully open and so the pumping losses are the least.

maximum torque is the point where the engine achives the maximum possible cylinder pressure ( BMEP) at WOT.

but its not maximum possible power ! the engine climbs in power output even after max torque ( and cylinder pressures falling again ) since power is the product of a given torque at a given rpm. thats because the rpm rise is more significant than the fall of torque until the engine somewhere also peaks in power output.

the formula for calculating power output from torque and rpm is TQ ( in lb-ft) X rpm / 5252 = power in hp.

take your calculator and enter any torque number from the GM block graph - you will see the result on the calculator is the power output displayed on the other curve in this graph.

you can so say that lets say 100 hp can be reached from a high reving low torque or a low reving high torque engine - and none of them is more powerful even when the torque numbers are different !

engine torque alone is virtually nothing without taking into account the rev capability of the engine and the gearing after the flywheel .

cheers !

 

[QC]

Re: engine torque

we could boil it down that even hp are meaningless since all we can do is to prop it correctly to reach rated rpm at WOT in a boat and it will go as fast as it goes.

Very good point actually. I guess the biggest reason it is important is evaluating before you buy. Where I boat there are many "custom" boat manufacturers. You can decide between many engine, and even drive, options. You can even buy without power and finish yourself, so in that regard it is important, but I get your point for sure.

I come from a world of spec'ing equipment, primarily trucks, so when I say torque values are not of much use in pleasure craft applications, that does not mean it is not an extremely valuable number. Just not so much with high speed planing hulls. Just think about OBs, They don't supply it at all. Funny to me that the two stroke guys don't as you could explain their superior hole shot this way. Oh well.

The GM site is very good, and I think Volvo actually uses the 6.0s as is. Not positive on that. I wish they would start using the blown ones!!

engine torque alone is virtually nothing without taking into account the rev capability of the engine and the gearing after the flywheel

Yep, we speak the same language. Somewhat validating as I am beat up over this assertion sometimes. Thanks!

 

[E4ODnut]

Re: engine torque

Aerobat,
Ahh. I saw the bar numbers and assumed manifold pressure from a boosted engine, never considered BMEP. I'm travelling now and don't have access to my reference books, but I seem to recall that BMEP is another way of expressing torque values.

Do you have access to any other BSFC maps?

 

[QC]

Re: engine torque

BMEP can absolutely be converted to torque, but I don't know the calcs and obviously needs to include a displacement component as well.

 

[aerobat]

Re: engine torque

but I seem to recall that BMEP is another way of expressing torque values.

Do you have access to any other BSFC maps?

you are correct gents , bmep and torque are married -and on a given engine a given bmep always results in a given torque. but you have to do the math for every engine you want to convert since calculating torque from bmep calls to know such things like piston area , piston stroke and number of cylinders .

i have some more diagramms of VW direct injected diesel engines, they can be expressed in many varieties

e.g at this here :



the maths are already done here and you have a readout in engine torque ( left) and bmep (right)

further the sfc is not expressed in g/kwh or lb/hp/h but in overall thermal efficiency. this engine tops at 0,42 or 42% and gets as worse as 26% at low loads and high rpm,s .

the great things at such graphs are that you can read the sfc at any load and rpm and have not only the full load sfc curve. and isn,t it stunning modern direct injected turbodiesels reach a bmep of 20 bar ? - thats 290 PSI !!!

it would be cool if somebody has such a graph for a marine GM block. i guess GM as well volvo and mercuiser have it but most probably they will not give it to the public.

 

[E4ODnut]

Re: engine torque

During my research I have found a few BSFC maps for naturally aspirated gas engines, but none for any relatively current North American marine gas engines. It's a pity that they are not available because it would be most helpful in estimating the correct propeller choice to run an engine in any boat at the best compromise between speed and economy. The problem with propping for maximum HP RPM is that if you want to cruise at low power settings you will almost always leave some economy on the table. If you are prepared to accept a lower full throttle RPM, which usually means a lower top speed, then a larger diameter and/or coarser pitch prop can give you better low load economy. From my experience, generally speaking, for economy, keep the revs down, the manifold pressure up, run as lean as you can without a lean stumble and adjust your timing curve to suit. Easier said than done.

 

[QC]

Re: engine torque

I understand your pursuit, but these gasoline engines can't handle a constant lug condition. If you prop below WOT RPM you truly risk issues. I have noodled on what you have here for a long time. Without a transmission, your only logical and practical solution is to prop within the recommended WOT RPM. Towards the bottom usually results in the highest top speeds anyway. Most data I have seen supports this, and theoretically should be the most efficient between the choice of bottom or top of the range. Of course hole shot will be the compromise. Personally I think the ultimate solution is a CVT, but then there is the weight and cost compromises. Can't escape it