hp and torque

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divecaptchris
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hp and torque

Post by divecaptchris » Wed Jan 06, 2016 8:59 am

15' deckover, GM 435hp 383, 2.09-1 stinger, 3 blade whisper tip, my question relates to HP and torque.
the two meet at approx. 5200 rpm, then hp increases and torque starts to drop. If I set my prop to the max of 2500 prop rpms at WOT that puts me at 5250 engine rpms, would I gain anything by changing my ratio to 2.3 or is HP useless when torques starts to drop?

http://www.chevrolet.com/performance/cr ... sp383.html

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scottyroll
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Re: hp and torque

Post by scottyroll » Wed Jan 06, 2016 9:42 am

If you are spinning your prop on the 1 mark now then you can expect to spin it to the same engine RPM on the 2 mark with the ratio change IMO.

The pitch should show you quite a bit of performance gain as well as quieting down the prop because it will be spinning slower.

My experience is that I lost 1 mark on about the same setup moving down to a 2.37 from a 2.68
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1999 14ft Alumitech deckover. ZZ502. 2.37 Stinger. 4 blade WW Falcon Prop

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crowhater
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Re: hp and torque

Post by crowhater » Wed Jan 06, 2016 10:10 am

To be correct torque and HP cross at 5252rpm. The best way I have found to explain this in relationship to airboats is like this. If you prop is pitched correctly and you go full power, the rpm your engine goes to quickly is your peak torque for your engine. Now the RPM your engine muscles up to after this is HP carrying you. My rule of thumb is when running past your peak torque on the HP side I count in my head 1001, 1002, 1003, 1004, 1005, 1006, 1007, 1008, 1009, 1010. At this point I will lift out of the power and let the engine fall back to its torque number at least. From my experience running in that upper HP range for an extended period of time is asking for things to go boom. If your spraying nitrous cut that number in half and keep your butt cheeks clinched.
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dshaw94
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Re: hp and torque

Post by dshaw94 » Wed Jan 06, 2016 12:36 pm

Your torque almost certainly peaks somewhere between 3500 and 4000 rpm. There is a slow decline in torque from 3500-4000 to between 5000 and 5500 with typical small blocks. Then at 5000-5500 the torque falls out exponentially. Along this exponential decay is where maximum horsepower is achieved. Horsepower is the mechanical measure of an engines ability to do work, i.e. accelerate air with a propellor. Torque is simply one of the two factors that create horsepower. Ignoring prop efficiency, to achieve maximum thrust a prop should be pitched such that at wide open throttle the motor is operating at the rpm where max horpower is created.
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flying fish
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Re: hp and torque

Post by flying fish » Thu Jan 07, 2016 8:23 am

divecaptchris wrote:15' deckover, GM 435hp 383, 2.09-1 stinger, 3 blade whisper tip, my question relates to HP and torque.
the two meet at approx. 5200 rpm, then hp increases and torque starts to drop. If I set my prop to the max of 2500 prop rpms at WOT that puts me at 5250 engine rpms, would I gain anything by changing my ratio to 2.3 or is HP useless when torques starts to drop?

http://www.chevrolet.com/performance/cr ... sp383.html
Taking peak torque and multiplying x reduction ratio = torque @ output shaft on redrive.

After adding 2.3 ratio and governing at same engine RPM, ground running ability would be better.
Not sure about cruise RPM governing engine at same RPM. I have done this experiment but forgot what cruise RPM did governing engine equally. I was never interested in governing engine at same when adding 2.3 ratio.

If adding a 2.3 ratio and governing engine @ 5500 RPM static, boat may really come to life. Ground running ability would be better, top speed higher and boat resembling hot rod in comparison. This has been my experience changing to a 2.3 ratio as long as I had enough prop to absorb extra torque @ output shaft on redrive.

TRADE OFF
I have also found governing engine at higher RPM my cruise RPM increased. Also found when governing engine at higher RPM A/F ratio needs fine tuned when increasing WOT RPM.

FUEL CONSUMPTION
I have found my fuel consumption (GPH) burn on different engines to go up when cruise RPM goes up.
Have also found fuel consumption to have not changed when governed RPM increased.
Went from a 2.5 ratio to a 2.3 ratio governing engine 250 RPM less and fuel consumption went up by 1/2 GPH burn. I believe this due to not having enough prop with 2.5 ratio and loading engine much harder with the 2.3 ratio.

Just some thoughts. :)
15' DB - 6.2 LSA - 4 blade 82" R - 2.52 CH-4

loudmouse
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Re: hp and torque

Post by loudmouse » Thu Jan 07, 2016 7:58 pm

With that heavy of a boat and a 3 blade my choice would be a 2.55 or 2.68 for performance and the 2.37 for economy. But would also depend on diameter of ur prop.
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WhirlWind Propellers
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Re: hp and torque

Post by WhirlWind Propellers » Thu Jan 07, 2016 8:50 pm

I get to talk about this topic many times a week on the phone, and HP vs. Torque is often discussed here on SA too. I hope I can add some helpful food for thought on this topic.

HP is simply an expression of Torque and RPM, and without knowing the RPM, Torque is really meaningless.

The standard mathematical equation for the relationship between HP and Torque will always hold true:
Horsepower = (Torque x RPM)/5252
*God put order in the universe, and math is the language of the universe.

-------------------------------

I ran across this article (Debunking Horsepower and Torque Myths) that really does a good job to discuss this topic. Even though he is not talking about airboat applications, he clarifies some good facts about Torque and HP in an engine. Definitely worth a read.
Topics discussed include:
:: What is torque, and what is horsepower?
:: Does an engine with more torque “pull” better? It isn’t engine torque that decides how fast you go up the hill, it is horsepower.
:: For best acceleration (aka - performance), you should [set the max RPM] at max Torque or max Horsepower or...
:: If your engine can exceed 5252 RPM, the torque and horsepower MUST be equal at 5,252 RPM. It isn’t a matter of tuning, it is a mathematical given.

Debunking Horsepower and Torque Myths: http://www.lainefamily.com/hp.htm

Sincerely,
Patti
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glades cat
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Re: hp and torque

Post by glades cat » Fri Jan 08, 2016 7:48 am

Thanks Patti. Great explanation of torque/hp relationship.
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