Torque vs. Horsepower, who gets it?

[QC]

Edit: Moved this from the Luhrs restore thread. It was a hijack and needed to go, but felt like there was some good info and this subject needs to be cleared up.

For reference this was the beginning of the trend towards a hijack . . . :redface: The discussion was about gaining some speed

Was just going from memory, I thought 160 before I typed it, but was trying to be conservative with the comparison. Heck you could strap a 1970s 235 V6 JohnnyRude on the back of her and she'd be faster. Mo power = mo speed.

 

[parrisw]

Torque vs. Horsepower, who gets it?

What's the torque of the diesel compared to a 250hp 350? After all its torque that's get's chit done.

 

[QC]

Re: Torque vs. Horsepower, who gets it?

I knew this would come up :facepalm: Not true. Torque and RPM together get "chit done". 1,000 lb/ft torque at 1000 RPM is the same as 100 lb/ft at 10,000 RPM, is the same as 10 lb/ft at 100,000 RPM. All examples equal 190 bhp and would theoretically make the same boat go the same speed if all else was equal. You must have a time component to calculate speed. The M in RPM adds that to the equation and helps get us to horsepower. Horsepower is the "rate" of work, torque by itself is just work with an infinite amount of time to get it done. Sooooo, especially with marine applications, horsepower begets speed. We got no real hills to "lug down" to some low RPM peak torque value. So if this thing makes more torque than the gasser at a lower RPM, how we gonna take advantage of that?

 

[parrisw]

Re: Torque vs. Horsepower, who gets it?

So if this thing makes more torque than the gasser at a lower RPM, how we gonna take advantage of that?

More gearing.

 

[proshadetree]

Re: Torque vs. Horsepower, who gets it?

Bigger prop with more pitch.

 

[QC]

Re: Torque vs. Horsepower, who gets it?

So you guys are recommending that you prop to peak torque for WOT RPM? And in the case of this naturally aspirated diesel he will have even less horsepower? Please guys, torque x RPM/5252 = hp. Horsepower spins propellers.

Yes there are some diesels that even have higher horsepower at peak torque than they do at rated RPM, but that doesn't mean you want to run full load there 100% of the time, and those ratings are specifically designed for wheeled vehicles. I swear we need a stickie on this . . .

 

[proshadetree]

Re: Torque vs. Horsepower, who gets it?

No I am saying that a diesel has a flatter torque curve than a gasser. Maybe he could reprop to gain some speed and still have some mileage. My 27 ft formula used about 30gph wot and 20 to 25 at cruise. Guess why I sold it?

 

[sqbtr]

Re: Torque vs. Horsepower, who gets it?

I don't think bert ever gave us the WOT under load RPM of his 6.2 . Mfg's spec of 3,600 rpm. No way of knowing weather he needs to go up or most likely down in pitch to achieve that 3,300. Seeing how the 6.2 is one of the most anemic V8 diesels ever made except for the 5.7 olds, underpowered would be the word of choice.

Still gotta love the smothness of a diesel ideling out the marina. Not to mention the economy. Maybe they will bring out the 4.5 duramax and someone will marinize it. 310HP and 500 plus ft lbs. using my best Tim Allen grunt.

 

[parrisw]

Re: Torque vs. Horsepower, who gets it?

I knew this would come up :facepalm: Not true. Torque and RPM together get "chit done". 1,000 lb/ft torque at 1000 RPM is the same as 100 lb/ft at 10,000 RPM, is the same as 10 lb/ft at 100,000 RPM. All examples equal 190 bhp and would theoretically make the same boat go the same speed if all else was equal. You must have a time component to calculate speed. The M in RPM adds that to the equation and helps get us to horsepower. Horsepower is the "rate" of work, torque by itself is just work with an infinite amount of time to get it done. Sooooo, especially with marine applications, horsepower begets speed. We got no real hills to "lug down" to some low RPM peak torque value. So if this thing makes more torque than the gasser at a lower RPM, how we gonna take advantage of that?

That's all great but and engine making all its peak HP at high rpm is useless as TITS on a BULL in a boat. If you had 200hp at 100,000,000 rpm it would never get on plane, see where I'm going with this?

 

[parrisw]

Re: Torque vs. Horsepower, who gets it?

Horsepower is what sells, Torque is what wins races!!

So you guys are recommending that you prop to peak torque for WOT RPM? And in the case of this naturally aspirated diesel he will have even less horsepower? Please guys, torque x RPM/5252 = hp. Horsepower spins propellers.

Yes there are some diesels that even have higher horsepower at peak torque than they do at rated RPM, but that doesn't mean you want to run full load there 100% of the time, and those ratings are specifically designed for wheeled vehicles. I swear we need a stickie on this . . .

 

[QC]

Re: Torque vs. Horsepower, who gets it?

That's all great but and engine making all its peak HP at high rpm is useless as TITS on a BULL in a boat. If you had 200hp at 100,000,000 rpm it would never get on plane, see where I'm going with this?

Horsepower is what sells, Torque is what wins races!!

The fact is that propeller "load" curves do not require high peak torque values. Why, if peak torque is so important, do we run our boats at high RPM? There are two reasons: one gear, and because horsepower makes speed, and in fact, wins races. High peak torque values result in better hole shot, and can help win drag races, but more importantly a steep torque curve (actually not flat and usually calculated as torque rise percentage) allows wheeled vehicles to avoid downshifting when climbing hills. How is that possible in a boat? Unless Bert is complaining about hole shot he needs more horsepower, not a higher peak torque value (low RPM). Most often when you modify an engine with things like turbochargers you get both torque and horsepower improvements. I hate playing this card, but please understand this is what I do for a living and have for 30+ years . . . There is a reason that marine manufacturers rarely display peak torque values. Ever notice how 4 strokes lose in hole shot comparisons, but the top speeds are almost identical, with maybe a slight edge to the two strokes due to lower weight? This is because two cycles have inherently higher peak torque values. Yes, better hole shot, but not top speed. If they are significantly faster, and the weight is the same, then they lied on the horsepower sticker. Horsepower makes speed. Period!

Someone brought up that because it was a diesel, the higher peak torque would somehow make up for it's low horsepower. It won't. It is that simple.

Maybe he could reprop to gain some speed and still have some mileage.

Only if he is exceeding rated RPM today. It's called Governor overrun, and if he is exceeding "rated" RPM, similar to exceeding WOT RPM on a gasser but not exactly the same, then raising pitch will indeed increase top speed. This is because fuel is pulled back above rated on a diesel. Not so on a gasser.

This is a serious hijack, and I have preached this a lot, but the best way to help those of you who want to separate torque from horsepower, and say you only need torque, need to think of torquing a head or main bolt. You can apply 300 lb/ft torque with a long breaker bar, and in fact you made some horsepower if you actually saw the bolt turn. But I absolutely guarantee you that you cannot make your boat plane with your hands. Now if you can make that 300 lb/ft at 3000+ RPM then you got something. And they call that sumpin' horsepower . . .

By going to a small block gasser Bert is going to get two things, lower weight, and more horsepower. Any bets on what will happen to top speed?

 

[scipper77]

Re: Torque vs. Horsepower, who gets it?

Along the lines of Torque vs HP, I have always viewed high torque at low rpm's to be favorable for situations where you are not running a motor anywhere near it's capacity. Why, Because higher torque at low rpm's translated to more horsepower during operation.

I'll give a real world example. My brother was shopping for a car, looking at 2002 impala's. He had an option of the 3.8 liter engine or the 3.4. Both engines made nearly the same HP but the 3.4 achieved this by having a righer red line. My brother asked me "so what does that mean?" The answer is simple. If you race the car at top rpm's you want HP plain and simple. If you want the car to feel more powerful at the rpm range you see during normal acceleration you want torque. Either engine will go plenty faster than the law will allow.

In the case of Luhrs28, speed is the issue. When he runs at top rated rpm's he is not getting enough HP, plain and simple.

 

[QC]

Re: Torque vs. Horsepower, who gets it?

Bingo ^^^^ and with a fixed propeller load curve, unless we want acceleration improvements, it is hard to take advantage of "reserve" horsepower at low speeds. That's what you get with high peak torque values below rated RPM, reserve horsepower for deep lugs (hills) and better acceleration. Not for top speed.

 

[Luhrs28]

Re: Torque vs. Horsepower, who gets it?

It sounds like I've blundered into making the right decision, having almost zero understanding of the science behind it. I love when that happens!

 

[QC]

Re: Torque vs. Horsepower, who gets it?

BTW, I don't believe that you noted 2500 Max RPM before. If so you would definitely pick up some speed if you rewheeled to get that extra 900 RPM. I'm going from memory and I think it was a 3400 rating. Noise would probably be worse though. I also saw video of a very clean wake back in the first tests. That had to be in the 11 - 12 knot range. At least the wake looked like it . . .

 

[jfalco]

Re: Torque vs. Horsepower, who gets it?

high horsepower, higher torque different prop, never mind this stuff, like i said before leave that boat alone its perfect, install a set of muffs on it and enjoy it, that boat was not designed to go fast, i do agree in the videos it is loud but a gasser is going to be loud also without muffs, 11 knots is plenty fast and that boat is special with that engine , install a gasser and its just another 25ft boat not to mention the fuel your going to burn, your cheeks are going to pucker every time you take that boat out with the fuel your burning, also this wot talk who wants to run their boat at wot anyway, not me thats for sure. lures leave that boat alone you did a fantastic job on it now enjoy it !!!!!!!

 

[parrisw]

Re: Torque vs. Horsepower, who gets it?

high horsepower, higher torque different prop, never mind this stuff, like i said before leave that boat alone its perfect, install a set of muffs on it and enjoy it, that boat was not designed to go fast, i do agree in the videos it is loud but a gasser is going to be loud also without muffs, 11 knots is plenty fast and that boat is special with that engine , install a gasser and its just another 25ft boat not to mention the fuel your going to burn, your cheeks are going to pucker every time you take that boat out with the fuel your burning, also this wot talk who wants to run their boat at wot anyway, not me thats for sure. lures leave that boat alone you did a fantastic job on it now enjoy it !!!!!!!

I agree. It would be a cold day in hell before I took a Diesel out of a boat and put in a gas engine.

 

[Willyclay]

Re: Torque vs. Horsepower, who gets it?

QC, thanks for educating me on yet another aspect of boating!

Bert, I don't have a dog in this diesel vs gasser fight but as an interested lurker who has his eyes on a Bertram 20 with a tired Mercruiser 165, it sure would be great to see the full development of your diesel project with the "perfect" prop, mufflers and additional sound-proofing. Regardless of what decision you make at this point, it has been an outstanding thread to follow. Good luck!

 

[g1sammons]

Re: Torque vs. Horsepower, who gets it?

Have seen some mis leading info on th hp vs torque
Im a long time engine builder mostly race engines
All you really have is torque
Hp is nothing more than a expression for torque at a given rpm!
When you factor in a transmission With multiple gear that changes
Because if you take a engine that makes 500 lbs of torque at 10,000 rpm
And gear reduce it to 5,000 rpm suddenly we are making 1,000 lbs of torque
But boats don't have mutiple gears and are almost alway fixed prop pitch
So we are back to flat out torque being the issue at hand
Ther are some other issues at play with the gasser vs diesel
Gassers usually have a wider operating rpm range than a diesel which leads to a wider torque band or curve
Meaning they might not make as much torque as diesel but the make torque over a larger rpm range
Which is more useful in a fixed gear ratio
Peak torque is not as important as a big torque rpm range
Meaning you would be better off with a engine that makes 400 lbs of torque from 2000 to 8000 rpm
Than a engine that makes 800 lbs of torque from 1000 to 2000 rpm
This is where the diesel get hurts because it only make 3500 rpm at best
Where your gasser can easily make good power thru 6000 rpm

Now ther are several reason most boat engines are built with low rpm torque values
1 it takes more torque to get you out of the hole than it does to push you at top speed
2 have to do with cooling if you take a engine like a race engine that makes big torque numbers
Let's say 800 lbs at 8000 rpm and you put it in a big boat like this one it generates so much heat
That over heats not talking about it boiling water but rather it starts knocking detonating the fuel instead of burning it if you get a better fuel than we melt pistons ect
3 the higher the torque is made in the rpm range the more fuel it uses !

Now if we are looking for speed ideally we should have max torque at top speed
If we have enough to get out of the hole any ways
If your engine makes 400 lbs at 3500 than we want top speed rpm range to be very close to that
How ever engines very rarely work that way because your torque range is determined but your cam shaft timing duration ect
And like I said it takes more energy to get you moving than it does to top speed


Now something els that was said was about hp selling more than torque in a engine
We that come from back in the day when we use to measure engine power from brake horse power
That when hp really mattered now it's nothing more than a selling tool

Back to my reason for my response
Torque is the measurement of twisting force which is what a engine does
Horse power is is a measurement of work done in a given time
1 horse power is 550 ft-lb of torque for the period of 1 second
Ther are several others way to measure it thru watts ect
But that the it of it

Other things to consider for the general argument with this boat
Is diesel is more costly than gas At the end of the day it balances out
You cold ad a supper charger to make better torque with the 6.2 but for not much more
You can update that 360 rebuild it make a lot better torque than it's original design
Fuel inj is a lot cheaper than it use to be and the would help with gas savings

 

[QC]

Re: Torque vs. Horsepower, who gets it?

Hp is nothing more than a expression for torque at a given rpm!

Nothing more?

Horse power is is a measurement of work done in a given time

Which translates to speed. Speed is distance over time, you show me where time is included in the force calc and I'll shut up. How do we differ here? If you make "better torque" you make better horsepower. If you make more horsepower, you go faster. I am dumbfounded as why people seem to think they have to separate these two to make some sort of logical plan or point. You can't!!! Torque is how hard you twists things, how fast you twist them results in horsepower. Horsepower makes your boat go. Period.

My only goal in these discussions is to counter the "all you need is torque" mantra. You say that, and then contradict yourself. Yes, higher torque at ANY RPM get's you more horsepower, but how do you use 400 hp at 2000 RPM if propeller demand is only 125? How? Lug it? How do you lug it with a single gear? How? Except for acceleration this point is 100% valid with regards to marine. Period.