Horsepower and Torque

Major · Post by **Major** » Apr 26th, '12, 10:38

I have been trying to get my head around horsepower and torque in referance to marine engines and could use some help. I was looking at two data sheets with curves, one on the Cummins 4BT 150 and the other the Cummina 4BTA 250.

Data Source: http://sbmar.com/Engines/Cummins_Perfor ... anical.php

The Cummins 4BT sheet didnt have a torque curve but the 4BTA sheet did. What I noticed was on the 4BTA that the torque maxed out at only 2200 RPM. Thats what got me thinking on the real relationship between torque and horsepower.

I thought torque would be the main number to look at in a marine application but I must be wrong since people running these engines do so over 2200 RPM's. From what I've read horsepower can be though of as how fast the engine can do "work" and torque is the ability to do the work. So to me it would seem that if they give the max torque at 2200 RPM's then it would be able to do the most "work" there. After that is should be overrevving so to speak in the case of the 4BTA. It just seems like if you can do the most work at a certain point below the max HP point in the curve then above that your just wasting energy in a boat that is. The only way I can see it working out is your doing less work faster so you may have less power but also less drag.

The recomended transmission ratio is about 1.4:1 or so to get to 2800 RPM's from the boatdiesel site calculator on the 150's. So if you went to a closer ratio transmission to get the shaft speed higher and tried to make 2200RPM's on a 250 that seems like it would be more efficent than going to a higher RPM with a wider ratio transmission. That is given the curve on the 4BTA 250.

In a car I may could see horsepower being important since after your rolling your not using torque as much unless you pulling something. In a boat though you really dont have that transition other than getting up on plane.

Do any of you have a good way to explain this? I know my thought process is probably wrong since I dont fully understand it so thats why I ask. Sorry if this reads like jibberish. I tend to continue thinking things out as I type.

I'm still digesting and rereading this:http://www.epi-eng.com/piston_engine_te ... torque.htm

mike ohlstein · Post by **mike ohlstein** » Apr 26th, '12, 10:47

You have a pretty good grip on it, with the exception that you should think of a boat as sort of like a car that is always going up hill.....and a deep vee boat as a car always going up hill towing a heavy trailer.

If you're thinking about 150's for a B31, stop.

In Memory Walter K · Post by **In Memory Walter K** » Apr 26th, '12, 10:57

The best way I can try to help you is to explain what happened with my boat. I had 440 Chryslers. If I went fishing with my son and went offshore to a set of numbers, fished (drifted) for several hours, returned and filled up, then did it again the next day with 5-6 big guys and all their stuff, to the same numbers and the same amount of drifting and the same sea conditions, I would find I consumed MUCH more fuel...almost half again the amount of the previous day.
I put in a set of Cummins and given the same scenario, I would use exactly the same amount of fuel whether I was with my son or a gang of very large, very heavy guys.

Bruce · Post by **Bruce** » Apr 26th, '12, 11:41

Are you trying to grasp the relationship between hp and torque for comparing the 150 and 250 in a 31?

If that's the case we would need to know the use application.
IE, planning pleasure, working trap boat, tow boat, etc.

Major · Post by **Major** » Apr 26th, '12, 11:47

The engines are not for a 31 but were originally intended for a 28 we have sitting on a trailer with no engines. After reading about some that were in use they seemed a little small. So it just happened to be one I just looked up since i had two. What i'll probably end up doing is getting them in good running order and sell them. The bad part is even if we do sell them we cant replace them even with gas engines for what we could get. I wish we could make them work in the application but logic and hope are two different things.

So in the engine swap scenario your saying the extra torque gave you more load carrying ability at a given speed? If thats the case then torque should not really make speed but gives capacity. Would a tug boat being strong yet relatively slow be an example? If you remember how did the horsepower and torque numbers compare?

Is it safe to say HP= speed but torque= load carrying ability? It makes me kind of curious when the speed calculators on boatdiesel only ask for HP and not torque though. Seems like with a heavier hull that would be of significance. I guess since the max torque and max hp are at different spots that would just complicate it too much. I thought that i've seen diesel vs gas comparisons where a diesel went faster/as fast even though it had less HP but more torque. If it can carry the load easier it should be able to use less HP though.

ETA: I posted before I saw your post Bruce. Its not so much comparing the HP and Torque between the 150 and 250 since i don't have the curve for the 150 but more so how they (HP and T) interact in the marine application. In the end Ill use the info to compare various motors in a planing hull. Specifically the 28 Bertram.

STraenkle · Post by **STraenkle** » Apr 26th, '12, 12:09

Horsepower = Torque * RPM /5252

Horsepower is just how much torque your are appling at a give rate.

Carl · Post by **Carl** » Apr 26th, '12, 12:59

My seat of the pants understanding of the big difference.

Gas motors will slow down as you climb the back of a swell...diesels won't.

Gas motors create their hp at higher RPM's. Diesels create the hp in the lower range... so with a diesel you get to use the available hp.

Bruce · Post by **Bruce** » Apr 26th, '12, 13:28

Well having repowered the 28 with both I'll tell you the absolutes.
If you want just a light use and loaded day boat, the 150 will put you up on plane barely.

Anything much more and you'll black smoke as the engines can't produce enough hp to get the boat on plane under the load.
Under stock design you can't swing a big enough prop to goto a 2:1 gear on the lower hp.
The 28 works much better with higher rpm diesels so they swing a smaller but faster prop.

JP Dalik · Post by **JP Dalik** » Apr 26th, '12, 14:17

When doing classes for Spicer Hvy Truck Mktg 20+ years ago there was a salesman in the clutch division whom had a very unique saying to differentiate between torque and horsepower.

"Horsepower" he said, "was what allowed a "thing" to rise in the morning and stay risen."

"Torque" he said;"would be the amount of pressure used in a twisting motion to no longer keep said "thing" risen".

He was a pretty colorful fellow and the explanation was always met with a glimmer of understanding and a chuckle.

Anyway I always make the distinction by horsepower moving an item on flat ground and torque as giving it the power to go up a grade at the same speed. There is more energy in diesel fuel than gasoline which in turn generally means more torques pe given horsepower rating which gives the boat more "push".

Morning Wood · Post by **Morning Wood** » Apr 26th, '12, 14:32

They sound like a good set up for my 25V Drive boat.
I might be interested in them.

Major · Post by **Major** » Apr 26th, '12, 15:22

I think i'm up to speed now. For some reason I had it in my head that HP and torque were two completely different animals but see now they are highly related. That calculation helped me on that and now i can figure it both ways.

4BT 150 hp at 281.36 ft lbs of torque @ 2800 RPM's.
4BTA 250 444 ft lbs. @ 2800 RPM's cruise 488 ft lbs @ 2200 RPM's
Ford 351 Gas : 380 ft lbs @ 3400 rpm's
Steyr 236 382 ft lbs @ ~4250 rpm's cruise 435 ft lbs @ 3300 RPM's

Just taking a quick look really shows how much lower those 150's are. It looks like if money were no issue the Steyr 236 would be a good option. If higher RPM's seem to be beneficial are many people running ratios like 1.2:1.

If we decide to sell them ill send you a PM. Im going to try to get them cranked and running soon so we know where we stand. The original owner (owner of the boatyard and Cummins Dealer) told us he turned up the pump and changed the injectors too i think. His estimation was they are right around 170 HP. How he figured that I have no idea. I'm curious if I added a 4BTA intercooler if i could bump it to closer to 200 hp by turning up the fuel.

Anyone know how to rig up a something to work like a dyno to get a rough and more real number or know what shops charge to dyno just an engine? Also what kind of places would have one other than a speed shop?

In Memory Walter K · Post by **In Memory Walter K** » Apr 26th, '12, 15:58

Another factor to consider is fuel consumption. My Chrysler 440's running at 3000 rpm had me moving at 20 kts @ about 26 gph plus. My Cummins 210's @ 23-2400 rpms gives me 21 kts @ 15 gph and I could cruise at that speed for hours without stressing them.

mike ohlstein · Post by **mike ohlstein** » Apr 26th, '12, 15:59

The thing that matters most in a boat is what you're able to do when it all turns to shit. If you're caught out in a storm in the ocean, 50 miles from shore... the gas engines make torque in a place where you'll never be able to use it. It will be all you can do to hang on while making 9 knots. 3600 rpm will simply be out of the question.

TailhookTom · Post by **TailhookTom** » Apr 26th, '12, 16:58

It gets bad when not predicted? Haha!

Last canyon trip I went on was on the Carolina boat and even with 880hp we could only maintain 11kts. And even with a huge bow flare we still had them rollover the bow! Amen to underway and making way!

wmachovina · Post by **wmachovina** » Apr 26th, '12, 17:06

Another way to put it, when it gets sloppy, my 31 can do 16 kts at 1600rpm(250 6bs) all day, the 454sdidnt have the torque to push it up the hill without goosingit( read accelerator pumps raw gas). We just plod on comfy at 16kts. Torque s the mojo!

Carl · Post by **Carl** » Apr 27th, '12, 07:07

Major wrote: I'm curious if I added a 4BTA intercooler if i could bump it to closer to 200 hp by turning up the fuel.

Anyone know how to rig up a something to work like a dyno to get a rough and more real number or know what shops charge to dyno just an engine? Also what kind of places would have one other than a speed shop?

The more HP you squeeze out of the motors the more temperamental they become till you wind up with a high strung motor at which point I think you defeat the purpose of going diesel in the first place. My opinion anyway...

Major · Post by **Major** » Apr 27th, '12, 14:16

Yea thats what I was worried about. I thought the intercooler may help with the reliability but it probably wont be enough. If only the 6BT was a little shorter and thats what we had. I see that 250 HP is too far from the reliability margin even after some real modifications designed into the 4BTA. I was just hoping 200 may be doable but like you said at what cost. Then I have the issue of where 200 really is and matching th other engine to it.

I did find an interesting homemade Dyno that a guy made. It use a water wheel and a load cell to measure the torque. Pretty interesting little project and he said he had ~$2k in it. Thats a little hard to justify thoug if your not building more than a couple motors though. http://www.youtube.com/watch?v=D11fwgU1 ... el&list=UL

STraenkle · Post by **STraenkle** » Apr 27th, '12, 19:04

I designed Dynos and wrote the software to control them for Land & Sea for 12 years. there are only 2 things you need to make a dyno, a way to load the engine and a way to measure that load. With that said, if you want me to find a shop in your area that has a dyno, that is easy, however pushing engines to make more power than they are designed to only leads to reliability issues. We made a carb V6 1990 outboard make over 800 hp with a turbo at 16 PSI, it would make 3, maybe 4 passes before blowing up, but it was the worldâ€™s most powerful outboard at the time. I know others on this site have made even more powerful engines of various types. everything is about what the engine was designed for.

the simplest dyno is a brake rotor bolted to the flywheel and a bathroom scale 1 foot from center on the crank. apply the brake to load the engine and write down the weight recorded. It works for a short time before warping the rotor or burning the brake pads. but what is the goal? go with the engine that was design for what you want to use it for. People may not like engineers, but everything is a compromise in design and reliability is paramount in being offshore.

Let me know if you want the name of a good shop in your area.

STraenkle · Post by **STraenkle** » Apr 27th, '12, 19:12

oh and another way to think of the relationship between torque and HP. say a guy can lift 400 pounds every 4 minutes and another guy can only lift 100 pounds, but can do it every minute. they have the same horsepower, but who is stronger? one moves less stuff, but does it faster and at the end of the day they each moved the same amount in the same time. for boats, that guy that can lift more comes in handy, for a race car the opposite is true.

bob lico · Post by **bob lico** » Apr 27th, '12, 19:17

Scott we have a water Dyno in the shop for outboards . The drive connects to the output shaft and have no clue how it works been sitting for a real long time.

STraenkle · Post by **STraenkle** » Apr 27th, '12, 20:11

Bob, it most likely is one of mine if it is a water brake. We were one of the few that mounted to the prop shaft and used the cav plate to hold the brake still. does it say DYNOmite on red anodized brake? That was my first job out of college. sort of the like designing the worlds most inefficient pump, make hot water out of horsepower. if it was a hydraulic brake, then a K&O which back calculated oil pressure to a pseudo horsepower, without a strain gauge.

Major · Post by **Major** » Apr 28th, '12, 19:53

STraenkle: If you have the dyno guys info handy I would appreciate it but I'm a little ways away from starting and getting the to a point to sell or keep. My engines are on pallets too so they aren't installed in anything. The idea of making one still interests the do it yourselfer in me though. I'm located in Florence, SC 29501.

How reliable are V-drives? If i ran into some 6BT's by dumb luck and could afford them at the time along with enlarging the engine compartment that's about the only way i see not to have the shaft angled way down. I can enlarge the engine compartment myself if it came to that since I'm pretty positive they wont fit being about a foot longer than the 4BT. Do any of you see the 6BT's for sale much anymore? Especially if they want to trade for some 4BT's. :)

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Horsepower and Torque

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