HP or TORQUE

[LVjetboy]

On the which is better, Q or HP, 4 qts or a gallon subject.
Lots of good thoughts here. A thread worth more thoughts.
Consider when you mash the throttle on a jet, rpms climb quickly to max power. Like within a second. So issues of gearing, torque to rear wheels, and traction may not apply?
For example, an engine with gobs of low end Q can use higher gears to get things rolling and still maintain traction. But if your jet boat engine revs to max power in 0.5 seconds, why worry about low rpm Q??? Or traction for that matter...what is that all about? Not that Q is independent of HP for any engine that is...both being joined at the hip so to speak. One is derived from the other. And why worry about gearing for that matter?
Your jet accelerates by the difference between water jet thrust (a direct result of bowl pressure and nozzle size), water drag and boat mass. More along the lines of say...a rocket than a car. Or maybe a car with infinitely variable gearing? And no wheels. But with different forces motivating and opposing? Just some thoughts.
So what does this all mean? I'm open for discussion...

jer

[Infomaniac]

LVjetboy:
[QB}
So what does this all mean? I'm open for discussion...
)
jer[/QB]The guy asking the question runs his engine at 4,800 RPM. As he mentioned 5,200 RPM max. There is not an engine in the world that makes more HP than torque at this RPM. It is impossible.
He should build his engine to make as much torque as possible with the flattest torque curve. If he were to build an engine that makes big HP, his engine would never see the RPM that makes the HP. His torque would suffer also to make the HP that he cannot use.
This does not apply to all jet boats obviously.

[77charger]

look at it this way 4500 and under build torque motor above go for hp.Another ex a diesel makes gobs of torque but is slow in the long run a race car makes lots of hp and accelerates fast!!

[MikeC]

"This does not apply to all jet boats obviously."
InfoMan, What does this mean? Do you mean all Jet Pumps?
MikeC

[Avenger 1975]

As Oldsquirt said:
"More correctly, HP = Torque x RPM /5252"
Put some thought into the formula and the discussion begins to make sense.
Another aspect I didn't see discussed here is how you make torque. Torque comes from internal momentum, for example longer stroke and/or more massive internal moving parts. But that long stroke doesn't like the high rpms that build big HP. So the guys saying you have to decide your max rpm target first are right, if you plan to be over 5000 rpm you don't want all that internal mass to accelerate and decelerate each stroke, you want lighter parts and shorter stroke so the engine will stay together at high rpms.

[Infomaniac]

MikeC:
"This does not apply to all jet boats obviously."
InfoMan, What does this mean? Do you mean all Jet Pumps?
MikeCThat means not all jet boats run RPM's as low as this discussion.

[Infomaniac]

Avenger 1975
[QB]
Another aspect I didn't see discussed here is how you make torque.[QB]Almost too many ways to mention.
Stroke is the best due to the leverage, actually cubic inches any way you can get them.
The longest rod possible will flatten out the torque curve. It will cause the piston to dwell at TDC longer. This allows the pressure during combustion to remain in the smallest possible area, while the crank continues to turn.
Compression, boost, NOS. (combustion pressure)
Short duration cams keep the power range down for best torque.
An interesting fact: An engine makes it's peak torque when the cylinder fills the best.
Big HP engines generally make the big numbers at high RPM's. At high RPM's there is not enough time during the intake stroke to fill the cylinders. The increased number of power strokes make up for less cylinder filling.
wink

[LVjetboy]

Not done yet. Here's more questions to stir the pot. Anyone know the answers?
So in Germany where the dyno reads Power = Watts and Torque = Newton-meters, would that magic crossing point of 5250 engine rpm (so often refered to on the net) still be significant from the engine design perspective? Or would some other rpm be the design of choice, driven by unit conversion with no engine design significance implied?
Can an engine's torque at any given rpm be increased without also increasing the hp at that rpm? 4 qts or a gallon.
Can the shape of the torque curve be changed without affecting the shape or location of the hp curve? Joined at the hip?
On a large or heavy boat, will the pump impeller accelerate measurably slower to maximum power with full throttle?
If a jet pump can accelerate within a second to maximum power (unlike a car) even for a large boat for example, is peak impeller torque more important than peak power? (No traction or gearing issues)
Should we be matching peak Q to impeller Q required or peak Hp to impeller Hp required...or are these also the same?
Are larger impellers somehow more efficient than smaller, from the pump friction and flow loss point of view?
Just getting warmed up!
jer

[Oldsquirt]

LVjetboy:
Not done yet. Here's more questions to stir the pot. Anyone know the answers?
So in Germany where the dyno reads Power = Watts and Torque = Newton-meters, would that magic crossing point of 5250 engine rpm (so often refered to on the net) still be significant from the engine design perspective? Or would some other rpm be the design of choice, driven by unit conversion with no engine design significance implied?Jer, I dug up this info earlier this year. The original thread is no longer in the archives, but this is a link to a page which explains how the formula: HP=Torque x RPM / 5252 is derived. Torque to Horsepower It's not magic, just some simple math, physics, geometry and unit conversion. The 5252 RPM crossing point is not by design, but rather by definition and the unit conversion math.
Can an engine's torque at any given rpm be increased without also increasing the hp at that rpm? 4 qts or a gallon.Nope. Look at the formula. If you increase Torque at any given RPM, the only variable left to change is HP. The converse is true as well. If HP changes, so must Torque.
Can the shape of the torque curve be changed without affecting the shape or location of the hp curve? Joined at the hip?Essentially the same question as above. If you change each point on one curve you automatically change each corresponding point on the other.
[ September 26, 2002, 10:31 AM: Message edited by: OLDSQUIRT ]

[Oldsquirt]

oops
[ September 26, 2002, 10:30 AM: Message edited by: OLDSQUIRT ]