Why don't engine manufacturers advertise torque numbers

[jkust]

Hi all. Why is horsepower all we ever hear about when discussing gas marine sterndrive engines? When I buy a car/truck Torque is the important number I pay attention to yet I would never know there was such a thing as torque if marine engines were all I saw. My large car only has 240hp but 280 ft/lbs torque. 280 lbs torque translates to the car is incredibly quick and fast but you would not know that by looking at the hp number alone. Does say a volvo 4.3liter carb and a mercruiser 4.3 carb with the basic outdrive (Alpa 1 for Merc and whatever the VP standard is) and same prop have the same torque or am I way off base? I guess by that same token why are the V6 sterndrives only 225hp at most with EFI. 225HP in a v6 in a modern car would be below par for even a minivan.

 

[Euro95]

Re: Why don't engine manufacturers advertise torque numbers

Because for top speed, engine horsepower is the most important thing in a boat.

As for the amount of horsepower for a v6, in boating applications an engine has to work very hard, and has to be able to run at 3k+ rpm all the time (in some cases), a car won't pass the 3k for that long since you always stay below it for optimal gas mileage (remember, shifting gears). Merc always had that slogan, imagine trailering your boat on a 40 degree angle uphill, that is what your boat engine has to do all the time in the water. And that is why a marine engine is so down on power compared to cars.

 

[Joe_the_boatman]

Re: Why don't engine manufacturers advertise torque numbers

Horsepower is the amount of power the engine is capable of consistently producing. This power is transmitted into mechanical force by the transmission (car) or propeller (boat). Thus, torque is irrelevant, since it can be manipulated by the transmission or prop selection.

 

[Silvertip]

Re: Why don't engine manufacturers advertise torque numbers

Torque on any engine very roughly follows displacement. My truck with a GM 4.3L V6 is rated at only 195 HP but is rated at 250 lb/ft of torque and 90% of that torque is available at 2000 RPM. 4.3L = 262 cubic inches. A Honda Pilot for example was rated at 255 HP with a 3.5L motor but that HP and torque is way up on the RPM band which makes it worthless on a boat. While many V6 powered cars today are rated at higher horsepower than that, their torque figures (naturally aspirated) still follow displacement. Remember that a boat has no transmission so the engine needs to have its torque curve established for that fact. So to get out of the hole, one needs a great deal of grunt. What better engine to lug a boat on plane than a truck engine. You should not be surprised then to learn that a marine cam is very close to that found in a truck. It does no good to move torque up the rpm band because that boat would struggle to get on plane and would suck gas like crazy doing so. The cam, intake, and heads are what make horsepower and torque. The block is merely an air pump. You manipulate the other parts to move HP and torque up or down the rpm band but the fact remains, an I/O that revs to 7500 rpm is a useless engine for towing water toys or moving a large cruiser.

 

[QC]

Re: Why don't engine manufacturers advertise torque numbers

Well I don't want to completely contradict Silvertip's post as he is fairly correct about the relationship of torque to displacement. Lord knows we've had our spats, but the fact is that Peak Torque is really only valuable to a marine engine if the boat struggles to plane . . .

One way to prove this is to gradually bump your throttle up 100 RPM at a time and see if it will plane or not. Almost all boats will plane prior to having the throttle 100% mashed. If the throttle is not 100% mashed you are not at the max torque for that RPM, so basically with the test I am suggesting, with most boats, the only time you would be at max torque would be at WOT RPM. Again, the only place that you would have the throttle "wide open" and the only time you would be asking for peak torque for that RPM is at max RPM and max speed . . . If your boat needs 100% throttle to get on plane, then you do utilize peak torque and an engine that had a little more peak torque would plane your boat easier . . . But even most 3.0s if applied properly don't require 100% throttle to get on plane.

This may be easier to understand if you look at these prop load curves from a Cummins diesel spec sheet:

The lower straighter lines are the prop load or demand curves. They represent what a typical prop (any prop) would require to spin faster, faster and faster . . . The top lines are what the engine is capable of. So it is easy to see that the big jump in torque at low RPM is not needed to turn the propeller . . . BUT what the load curve does not show is any rise in demand to get the hull up on plane. Yes, that point will dip into the available "reserve" power represented by the big white area between the two lines. However, again, if the throttle is not mashed 100% you are not on the top line. Period. The available max torque/power/fuel rate for that RPM is not required, so you don't go there. Now if you like to mash the throttle 100% because it feels good in the seat of your pants (I do), then more torque will make your pants feel better . . .

The fact is that the load required to spin the prop is almost linear, so you do not need the "hump" that steep torque rise (percentage of peak torque vs. torque at rated RPM) would get you. The application most common to trailer boats that likes high peak torque values is a deep-water start for a big slalom skier . . . The other biggee would be a heavy cruiser that needs all it's got to get up on plane. These are probably the only things that really require a high torque value like a typical automotive engine other than the pants thing.

Oh, and the truck cam is primarily to help prevent water ingestion aka reversion . . . and if geared and propped right the 7500 RPM engine would be great . . . it just wouldn't last as long. It would give you slower speeds at idle for trolling as the prop pitch could be significantly lower and still get good top speed with all of that available RPM . . .

 

[Philthyphil]

Re: Why don't engine manufacturers advertise torque numbers

And those 4.3s you mentioned are based off the old GM 4.3. As the 3.0 is based on the old 4cyl GM iron duke motor.

 

[Silvertip]

Re: Why don't engine manufacturers advertise torque numbers

Anyone want to try a Sprint Cup car motor? If 7500 RPM is great, then 9500 should be a hoot. They will run well over 500 miles before blowing a hole in the bottom of the boat. Lots of things can be made to work but cubic dollars prevents that for the average boater. Hence, we continue to deal with the reality of auto/truck engines in boats. Show me a normally aspirated gasoline engine of any displacement and for automotive use the torque value for that engine will generally be in the vicinity of displacement. Exceptions? Certainly! There always are.

 

[Silvertip]

Re: Why don't engine manufacturers advertise torque numbers

Here are HP and Torque figures for current GM Vortec Marine engines:

1.6L = 97 CI: HP = 100 @ 6000, Torque = 103 @ 3200
2.4L = 146 CI: HP = 173 @ 5800, Torque = 167 @ 4500
3.0L = 181 CI: HP = 143 @ 4800, Torque = 178 @ 3400
4.3L = 262 CI: HP = 226 @ 4800, Torque = 268 @ 4800
5.7L = 350 CI: HP = 292 @ 4800, Torque = 370 @ 3200
6.0L = 366 CI: HP = 380 @ 4800, Torque = 409 @ 4200 (others available)

 

[Pierutrus]

Re: Why don't engine manufacturers advertise torque numbers

Here are HP and Torque figures for current GM Vortec Marine engines:

1.6L = 97 CI: HP = 100 @ 6000, Torque = 103 @ 3200
2.4L = 146 CI: HP = 173 @ 5800, Torque = 167 @ 4500
3.0L = 181 CI: HP = 143 @ 4800, Torque = 178 @ 3400
4.3L = 262 CI: HP = 226 @ 4800, Torque = 268 @ 4800
5.7L = 350 CI: HP = 292 @ 4800, Torque = 370 @ 3200
6.0L = 366 CI: HP = 380 @ 4800, Torque = 409 @ 4200 (others available)

You wouldn't happen to have the carbureted version would you?

 

[Silvertip]

Re: Why don't engine manufacturers advertise torque numbers

I do not.

 

[Lou C]

Re: Why don't engine manufacturers advertise torque numbers

This is something I have also wondered about, since in cars we always heard that acceleration is more a function of the torque curve and gearing and horsepower is more related to top speed. However, the differences between driving a multi speed transmission on land and a single speed prop in the water may make knowledge of torque less important. I also wondered about the comparison of a 200 hp OB vs a 200 hp sterndrive, many would predict that on identical boats the OB would be faster because it could be up to 400 lbs lighter, even with the sterndrive powerplant putting out more low speed torque. I'd think that on smaller displacement V-6 OBs the torque figure would be less than the typical GM 4.3.

 

[wca_tim]

Re: Why don't engine manufacturers advertise torque numbers

"if you like to mash the throttle 100% because it feels good in the seat of your pants (I do), then more torque will make your pants feel better . . . "

excess is also kinda fun when that guy with the comb over in the new baja pulls up aside, looks over and pins the throttle... you barely have time to catch a glimpse of the frown before he passes out of sight to the rear (cause you're payin attention to the water in front of you...).

I been workin way too much lately and not playin near enough...


btw, nice primer on torque and horsepower...