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dumb question about trq.

7K views 81 replies 11 participants last post by  Sleeper CP 
#1 ·
lets say you have a low reving engine that makes 800 lbs ft. at 3,500 rpm. If you put a 2:1 gear box behind it and the out-put shaft is spinning 7,000 would that be the same as having 1,066 hp ? ( less any loss to spin the gear-box)

S CP
 
#2 ·
Nope. any time you gear uup you gain RPM but lose torque, same goes the other way. Its why you car pulls like hell in low gear. Boat loads of torque traded for
RPM. My V-drive has less TORQUE at the prop than at the crank because of the overdrive. But it stills has the same HP minus frictional loss from the drive train. You don't lose HP(again except frictional losses) fro the gearing, you lose torque and gain RPM. Reverse it, and you gain torque and lose RPM. At the end, the Torque X RPM / 5252 still hold true. Make sense?
 
#4 · (Edited)
Nope. any time you gear uup you gain RPM but lose torque, same goes the other way. Its why you car pulls like hell in low gear. Boat loads of torque traded for
RPM. My V-drive has less TORQUE at the prop than at the crank because of the overdrive. But it stills has the same HP minus frictional loss from the drive train. You don't lose HP(again except frictional losses) fro the gearing, you lose torque and gain RPM. Reverse it, and you gain torque and lose RPM. At the end, the Torque X RPM / 5252 still hold true. Make sense?

No............ maybe bp can explain it to me ............ :)bulb

Power plant is turning at it's max load/power 800 lbs ft at 3,500 if the gear box is spinning at 7,000 how am I losing power ?? other than the drive-line loss ?

if I am losing power ( or trq) because of rpm how much would it be ?


Something told me it wasn't straight across but I don't get it........at least not yet.

Edit: So if the gear box takes 85 lbs ft to spin at 3,500 so the net at the out-put shaft would be 715 lbs ft that would be 953 hp vs 1,066 at 7,000?

S CP
 
#3 ·
That's a good explanation Bob.
I understand about torque multiplication and torque division but I never considered that the HP would remain the same, it just changed RPM.
 
#6 ·
Look at it this way,
Say you have an I/O which is 2:1 gear ratio underdriven.
5000 rpm and 500 ft lb input is only 2500 rpm output rpm but torque is multiplied by 2 to 1000 ft lbs.
The input gear has more leverage against the output gear.

Now consider a vdrive with 2:1 overdriven gears.
5000 rpm and 500 ft lb input gives 10000 rpm output but torque is divided by 50% to 250 ft lbs.
 
#8 ·
Gears multiply power inputed to them by their ratio.
Gears divide rpm inputed to them by their ratio.


EX:
400ftlbs from engine into a 4:00 gear ratio = output of 1600ft/lbs
5000rpm from engine into a 4:00 gear ratio = output of 1250 rpm


Hope that makes it easier.
 
#21 ·
Gears multiply power inputed to them by their ratio.
Gears divide rpm inputed to them by their ratio.

EX:
400ftlbs from engine into a 4:00 gear ratio = output of 1600ft/lbs
5000rpm from engine into a 4:00 gear ratio = output of 1250 rpm


Hope that makes it easier.
One small correction:

Gears multiply torque, not power, inputed to them by their ratio.

All your equations and thinking are 100% correct, but wanted to get that one word right as others might reference your post.
 
#13 ·
I was just thinking out loud that if someone wanted to build an impeller dyno ....would you be better off using a trq monster diesel engine and then over-driving it to get to 7,000 rpm or would you build a blown or TT gasoline engine to make 1,100 -1,200 hp at 7,000- 7,200. :)bulb

Just a "junk yard wars" thought.

S CP :)bulb
 
#14 ·
Hey sleeper! It's actually a good topic and very disregarded ! I have gotten beat up over this before for different applications. Building a impeller dyno would be tough unless you had a computer to factor in every aspect of the designed useage- Weight-drag coeficient effect and intended design. Then you need to factor in the horsepower and torgue rateings -then toss in intended usage. Which I think you have done quite well with your'e G/W. You seem to be getting the most of both worlds and maybe giving up a touch on one or the other! It seems to me you have built a boat that meets your're needs and added a sh$t pile of money to get there! JMO M
 
#20 · (Edited)
I am not certain exactly what you getting at ol guy, but I will repeat this again and again. Dyno DO NOT measure hp. HP is a math calculation of MEASURED torque at a given RPM. Scale measure weight, but they cannot determine body fat. That requires a calculation
You can have an engine that makes a peak of 500lb tq @ 3500, and you will get your ass handed to you badly by an one peaking at 500 lbs @ 5500. So tell be ol guy, what wins, torque or HP. And no, I seriously doubt you going to GEAR the 3500 engine to stay anywhere near the 5500 one. Its why formula 1 cars don't use broad torque curves and 2 speeds.



 
#23 ·
If I had the desire to go back about a year I would bring up this whole thread again it is a set of terms (H/P Torque, Power.) Every body uses a different term for the RPM range there engine works best. To go back to sleepers question about a impeller dyno would take so much info programmed into the hard drive it would be hard to gather every usable combo for a perfect cut for every use. Thats why it goes back to trail and error. Start fat and go thin till it works best. M
 
#26 ·
OK whatever before I get yelled at again CP I dont know how torque or horsepower relate to a jet,I know that with a 4 foot wrench and a 4to1 multiplier its a bitch to get 2000 lbs ft on a nut I cannot fathom doing that with a 11 1/2 inch prop which would be about 1/2 the torque value... I'm done on this subject -no yelling I am not a getrdone exSPURT .....I will just follow and learn...
 
#30 ·
mdsheppie, lose the thought about the prop and its size.
the torque set up you are refering to is the equivilent of a 16 ft bar. Not sure why you try to equate that with any prop regardless of size. the prop has zero to do with the measured torque in the shaft that is turning the prop.

But just to get you back on the page, the multiplier you refer to is nothing more than the gear ratios we are talking about. They can be used to increase torque, but they will not increase HP.
 
#28 · (Edited)
Actually a good correction so as not to potentially mislead anyone into a incorrect direction.
I do make the mistake, as this is a good reminder to me, of generically calling tq or hp 'power.' I usually only do this when I'm speaking and/or giving specific examples of one or the other. Thinking others will be on the same page. Typing makes it harder to understand or have others understand than in person.

In good discussions like this (I feel it's good anyway, lol) it's better to clarify, and have others to clarify, exactly what's being talked about.

I'll try to remove the word power from my online diet. LOL.

May the shaft be with you,
CFM

hil:)
 
#31 · (Edited)
I got it Bob, I just dont have a clear lucid outcome on this ..and it seems to me that there should be more voices involved -meanwhile back on the ranch...I got no dog in this discussion its clear in some ways and vague in others but how do you dyno a impellor?And therefore I shut up go on with the discussion...happy hour here...
 
#39 ·
I think part of the challenge is that all the impeller charts are based on HP and none are based on torque.

Seems like it should be the other way around.
 
#41 ·
Although I understand that the pump in nothing more than the absorber on a dyno, and the cut of the impeller will effect the HP requirements for a given RPM, how would all this info on any given impeller help you select the right impeller for a particular boat. If the speed of the boat effect the inlet pressure of the pump, which in turn effect the HP requirements, how would you know which impeller to put on which type boats armed with nothing more than soem impeller dyno data. Its like trying to select the v-drive gearing and prop for a 2800lb V bottom or a 1800 lb flat with the same motor.
The same impeller on two boats as different as that but with the same power, would end up with different MAX RPMs wouldn't they. Doesn't the impeller have to also take the boat type and weight into consideration as well. I dumb as brick on these things, thats why am asking.

You and I both know, tons of data is only as good as you can decipher, and apply. Knowing what to do with the data is every bit as important as obtaining it.



 
#43 ·
You and I both know, tons of data is only as good as you can decipher, and apply. Knowing what to do with the data is every bit as important as obtaining it.

I say that same thing at least twice a day, everyday, at work.

Here's a good summary of the scientific process from Wiki. It applies to everything, including our boats:

A linearized, pragmatic scheme...offered as a guideline for proceeding:

1.Define a question
2.Gather information and resources (observe, research)
3.Form an explanatory hypothesis (including theoretical predictions based on physics laws, if possible)
4.Perform an experiment and collect data, testing the hypothesis
5.Analyze the data
6.Interpret the data, orrelate to the theoretical predictions and draw conclusions that serve as a starting point for new hypothesis
7.Publish results
8.Retest - if the results are not testable by others then the conclusions are not valid.
 
#48 ·
:D Thats the only way it could work. If there were enough sharpies out there that knew what they were looking for, and know what to do with the info. If you handed it to me, you would get a look something like this:)sphss

Now the real question is, which is harder. Analyzing the number crunching for the lottery winning ticket, or the analyzing the the results and variables from his soon to be developed impeller dyno he builds form his winnings. I banking on the dyno.

Uhhhh Jon, you have cleared the expense for this thing with the little woman haven't you;)



 
#49 ·
:D
Uhhhh Jon, you have cleared the expense for this thing with the little woman haven't you;)
Hey I'm still trying to get the boat back on the water.......visited the engine today, it "should" be together next week.

AS far as a Pump dyno goes and paying for it, if I win the lotto the wife would probably be headed to Italy on the next flight out of town and looking for an apartment in Sorento or Florence, or maybe both. The water tanks and the electrical power needed won't be a problem netiher would the space to do it. Great Junk yard wars project.

Until then I'm just looking forward to getting the boat back on the water.

S CP
 
#52 ·
Still comments about the basics of torque, power, and such....

Folks of the Forums 'n such that might be interested;)

Gosh I would have thought that after all this time has passed nobody would even be interested in dynamometers or testing torque and calculations for horsepower (power)...:)bulb The comment "neglecting friction" is pretty common in high school or introductory physics but dealing with those issues becomes necessary in real life.

POWER by definition is WORK divided by Time.......Naw there is not any interest in that kind of stuff....then someone would have to start dissing the math or the test site or whatever.:)hand

Been really very busy for a long time doing Client work and have missed the wet forum(s). BTW a dynamometer was done long ago for water pump impeller testing. Pretty common in the oil patch and in other industries.

Hope you all have a happy 4th and hope you remember the kids deployed around the world doing our country's bidding.:cool:

Regards to All,
IG:)
 
#53 ·
Folks of the Forums 'n such that might be interested;)

BTW a dynamometer was done long ago for water pump impeller testing. Pretty common in the oil patch and in other industries.


Regards to All,
IG:)
Hi IG,

I stated "impeller dyno to measure HP", BP completed my thought process for me, I wasn't just thinking about recording hp, but having a system with 18 probes to measure whats happening from the intake, impeller face, bowl, to the exit nozzle.

with the ability to set the intake psi from 5psi-50 psi and test impellers at race rpms of 6,500-8,000 rpms. As he stated the combination's to run would be 1,000's.


Anyone know how many amps it takes to generate 1,400hp? ( @ 7,800 rpm)

S CP
 
#62 ·
So following that impeller chart,

If you had a prostock engine that makes a whopping 1432 hp and a paltry 840 torque like this,
YouTube - ‪DartTV - NHRA Pro Stock Dyno Pull‬‏

And then you had a small LS1 turbo motor that made 1200 hp and 1100 tq like Cyclones example,
Which one would turn the jet pump impeller higher rpm ?
 
#65 ·
So following that impeller chart,

If you had a prostock engine that makes a whopping 1432 hp and a paltry 840 torque like this,


And then you had a small LS1 turbo motor that made 1200 hp and 1100 tq like Cyclones example,
Which one would turn the jet pump impeller higher rpm ?
Like CP said, your looking at two totally different torque curves. Just like a v-drive with the same two engines, you would be looking at different gear ratios in the v-drive. The Pro Stock would have either a different impeller, or a gear reduction. Never seen an imeller than worked well at 10,500, so like Jon said, a gear reduction. Which, as we just discussed, will increase the torque intput to the impeller. Damn this is getting confusing. Jon quick, you need to get this thing built.



 
#63 · (Edited)
as I'm sure you are aware the Pro-Stock engine would need a gear/reduction ( underdrive) maybe to 7,800 +/-.

The trq of the turbo LS-1 would be great but limited on rpms. Compairing Power under the cuvre of both would be interesting.

Speaking of power under the curve maybe just get a new Merc Quad cam TT to spin the impellers:


That's on 91 octane, Dave at DCB told me that engine should make close to 1,800 hp at 7,000 on race gas and be able to run wide open for 30 minutes straight but probalby over an hour. Talk about power under the Curve.

S CP
 
#70 ·
If Austin Coil could build a blower dyno at Force's place from an alky burning blown BBF, you should be able to do the samething.

I was just at a bone yard the other day looking for 4.8 to install in a rider mower. The guy told $3,000 dollars. I said WTF. He let me know that ever since the test, the run on 4.8 has skyrocketed. And the artical isn't even on the stands yet. I understand Peterbuilt and Kenworth are guilty of snatching the majority of them.



 
#74 ·
The turbine gets my vote:)devil

Jon, your all over right. First you buy a Unlimited so you have something for the engine to do when it isn't spinning little firetruck water pump impellers.

Done properly, I think you could just tip the hydro up on its nose and run that puny little scroll with the prop shaft and already have it at the proper input speed.
I get first shot at sitting in the cockpit to operate the throttle. :D



 
#82 · (Edited)
FYI,

On power plant options..... if not a 2,000 hp turbine there is always the R-12:

Rotary 12 @ 1,400+ hp at 8,000 rpm on 87 octane or 2,400 hp turbo'd on 91





This thing has a hell of a service warranty: 400 hrs



and then there's always this option ;)



Hope you all had a great 4th.

S CP
 
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