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HP or Torque for a jet???

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jet torque
7K views 73 replies 24 participants last post by  PlacecraftTunnel 
#1 ·
What is best HP or Torque? Lets say I have a motor that makes 650 HP @ 5700 RPM and also 650 HP @ 6200 slight loss in HP between 57 - 62. But at 5700 torque is 640 and at 6200 it is 610 what RPM would I want the jet to turn for best performance?:)Unsure
 
#9 ·
What is best HP or Torque? Lets say I have a motor that makes 650 HP @ 5700 RPM and also 650 HP @ 6200 slight loss in HP between 57 - 62. Unsure
You have a hp dip between 5700 and 6200 ?

What's your peak tq and what rpm ?
What's your peak hp and what rpm ?

Do you have a dyno sheet to show someone so they can help ?
Can you give more details on the boat also ?

BTW: I'm no jet boat guy, therefore I'm asking engine 'tune' questions and and other questions that may help others help you.

Oh, and 6200 will sound faster than 5700. :)hand (joke)
 
#10 · (Edited)
I love this question, it will not be easily answered. What are the observed HP #'s. I'm assuming the posted numbers are corrected ones.

Off the top of my head I would guess an slightly worked Berkley A/B cut will put you at about 6,000 rpm's. I would start with that... Now if you go for a fully worked over race pump it could be as small as a B/C cut.

You'll get about 15 different opinion's on this... Thanks for the entertainment.

There is an Impeller chart posted here somewhere. If someone else doesn't post it I'll find it. Actually there are two charts.
Edit:


Also, do you have this engine or is this hypothetical ?

Sleeper CP
Big Inch Ford Lover :D
 
#11 ·
This is one of the most debated topics I have been viewing since 2000;)

This thread should get pretty damn good...................
 
#13 ·
Yea, it is 396, and I will most likely be lurking for the rest of the fight. :D
 
#12 ·
Depends.

Depends on what RPM you want to twist the engine to.
Depends on what impeller cut you want to run.
Depends on what youy want to use the boat for.
Depends on how you want to define "performance", BY INTENDED USAGE.

"Performance" for a 1/4 mile drag jet is nowhere near the same as "performance" for a lake boat that pulls people on tubes/skiis all day and carries 5 people to fireworks shows 5 miles down-lake in the evening and then back.

Generally, larger impellers reduce RPM (thus engine wear and fuel consumption), increase acceleration (holeshot) and increase pulling ability (skiis/tubes).

Conversely, Generally, smaller impellers make slightly more top speed, at a loss of acceleration and pulling ability, and require higher RPM.

Torque is the only thing an engine generates anyhow, "twisting force by the crankshaft" is TORQUE.

All Horsepower is is a mathematical calculation of torque generated over time.
 
#33 ·
Depends.

Torque is the only thing an engine generates anyhow, "twisting force by the crankshaft" is TORQUE.

All Horsepower is is a mathematical calculation of torque generated over time.
Smokin,

Thanks for spelling that out.

I can tell 10 people that my Ford makes 765 lbs Ft at 6,200 only 1 or 2 would be able to translate that to over 900 HP and the rest would think that 765 at 6,200 is better than 702 lbs ft at 7,000. Go figure.

I think I'll just start telling people that it makes 702 lbs ft at 7,000 instead of 935 HP. :p

Sleeper CP
Big Inch Ford Lover:D
 
#18 ·
#21 ·
I'm under the thinking that continually running at max torque is not too healthy of a diet for a gas motor. Even though max hp rpms are typically 1200 or so higher than max torque and assuming the motor is built correctly for the rpms at max hp, then max hp rpm should be easier on the motor.

Again, that's my feeling. I'm open to ideas.
 
#27 · (Edited)
Torque = hp * 5252 / rpm. So torque is exactly what that chart is showing. Someone should be able to redo these charts showing torque.

At 5200 rpms with an A impeller you would be looking at around 300 hp or about 303 ft lbs of torque. If your engine puts out more than 303 ft lbs of torque at 5200 rpms, then your rpms will increase.

But suppose the following situation:
- you have an engine that puts out 1000 hp or 772 ft lbs of torque at 6800 rpms.
- and your engine can only put out 200 hp or 202 ft lbs of torque at 5200 rpms
=>Then it follows that you will never get to 6800 rpms.
 
#26 ·
Pass some of that over here. :)
 
#31 ·
a jet pump cares not if its in a heavy or light boat,still pumps water:D
if you build alot of tq hp will come with it best answer anyone has given its true.

well hell i guess it is,who am i,lmfao!!!!!!!!!!!!!!!good luck with this thread if it gets nasty ill be here.:)sphss
 
#34 ·
Exactly. Torque is a force applied to a rotating mass. Horsepower is torque applied over distance and time...in this case Revolutions Per Minute.

Now lets think about this. Torque will allow the engine to spin up to max RPM faster. A jet boat works off of pure thrust just like a jet engine does, but with water instead of a fuel/air explosion. One rule of fluid hydraulics is that a liquid cannot be compressed. Having said that, the more water you move through the pump, the more pressure is created inside the pump, which works against the impeller and makes it harder for the motor to spin, which will require more twisting force, or TORQUE, to spin it up to a higher RPM with all that pressure in the pump.

Also, keep in mind that a Chevy has absolutely no problems deliveing the kind of torque required to properly spin a fully loaded pump to max RPM :D :D :D
 
#36 ·
It dosen't make a difference FORD or CHEVY HP is HP and torque is torque!!!:D I do have some good numbers out of a Chevy I will post them up a little later to see if this battle can be settled!:)devil
 
#39 · (Edited)
Nice number's on the dyno sheet.

On the impeller I would start with an A/B cut with a detailed bowl. It should put you a tad over 6,000. Alot depends on how you load it. Most of the race pump guy's want the engine and pump loaded 100 or 200 rpm's lower than your max HP number.

On the dyno chart; that baby was running lean: A/F ratio close to 15:1 and a brake specific of .400's You might want to through a couple jet sizes in that thing before it hits the water just to be safe. And check the plugs after a few WOT run's.

Did you mention what kind of boat your putting it in ?

Edit: If you are going to run Nitrous it's a different story on the impeller. Some of the nitrous guy's look to pull an extra 300-400 rpm's on the bottle. My Ford is set up to pull another 800-1,000 rpm's. It will pull 6,200 w/o and 7,000 on when set on kill.

Sleeper CP :D
 
#41 ·
Nice number's on the dyno sheet.

On the impeller I would start with an A/B cut with a detailed bowl. It should put you a tad over 6,000. Alot depends on how you load it. Most of the race pump guy's want the engine and pump loaded 100 or 200 rpm's lower than your max HP number.

On the dyno chart; that baby was running lean: A/F ratio close to 15:1 and a brake specific of .400's You might want to through a couple jet sizes in that thing before it hits the water just to be safe. And check the plugs after a few WOT run's.

Did you mention what kind of boat your putting it in ?

Sleeper CP :D
Yes it was running lean the carb has been blueprinted since we dyno! It was also rich in low RPMs also!
 
#45 ·
It is a 19' Spectra. Had a stock 454 that pushed it good with an A impellor did not get the RPMs I was looking for we will see with the added torque!
 
#55 ·
Adding to your reading confusion! ;)

How do you convert engine torque to horsepower?

Have you ever looked at the specs of an engine in a magazine and seen something like "this engine makes 300 pound-feet of torque at 4,000 RPM," and wondered how much power that was? How much horsepower are we talking about here? You can calculate how many foot-pounds of horsepower this engine produces using a common equation:
(Torque x Engine speed) / 5,252 = Horsepower
The engine that makes 300 pound-feet of torque at 4,000 RPM produces [(300 x 4,000) / 5,252] 228 horsepower at 4,000 RPM. But where does the number 5,252 come from?

To get from pound-feet of torque to horsepower, you need to go through a few conversions. The number 5,252 is the result of lumping several different conversion factors together into one number.

First, 1 horsepower is defined as 550 foot-pounds per second (read How Horsepower Works to find out how they got that number). The units of torque are pound-feet. So to get from torque to horsepower, you need the "per second" term. You get that by multiplying the torque by the engine speed.

But engine speed is normally referred to in revolutions per minute (RPM). Since we want a "per second," we need to convert RPMs to "something per second." The seconds are easy -- we just divide by 60 to get from minutes to seconds. Now what we need is a dimensionless unit for revolutions: a radian. A radian is actually a ratio of the length of an arc divided by the length of a radius, so the units of length cancel out and you're left with a dimensionless measure.

You can think of a revolution as a measurement of an angle. One revolution is 360 degrees of a circle. Since the circumference of a circle is (2 x pi x radius), there are 2-pi radians in a revolution. To convert revolutions per minute to radians per second, you multiply RPM by (2-pi/60), which equals 0.10472 radians per second. This gives us the "per second" we need to calculate horsepower.

Let's put this all together. We need to get to horsepower, which is 550 foot-pounds per second, using torque (pound-feet) and engine speed (RPM). If we divide the 550 foot-pounds by the 0.10472 radians per second (engine speed), we get 550/0.10472, which equals 5,252.

So if you multiply torque (in pound-feet) by engine speed (in RPM) and divide the product by 5,252, RPM is converted to "radians per second" and you can get from torque to horsepower -- from "pound-feet" to "foot-pounds per second."
 
#57 ·
Look at the Fuel A and Fuel B columns. Assuming you have a mecanical secondary carb, the side of the carb that was attached to the second fuel columns flow meter is what I'd suspect first.

Unfortunately, as a result this dyno sheet doesn't give you much exact info other than showing a possible problem with your carb.

Bummer.
 
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