HP or TQ

[Taylorman]

So do you want to cut the impeller to allow the engine to spin to peak tq or hp? Heres a dyno sheet for comparison. Im gonna be rebuilding my Olds with Edelbrock heads and im looking to achieve 500 hp at 5000 rpms. I hope to spin a AMT AA impeller to 5000.
http://www2.propichosting.com/Images/450010218/7.jpg

[lilrick]

here we go again!:argue: :boxingguy:

[centerhill condor]

does it matter with a monster like that? nice chart by the way.. :rollside:

[Heatseeker]

here we go again!:argue: :boxingguy:
Exactly what I was thinking...

[dragboat]

Where's smokinlowrider with my popcorn?

[Keyser]

Where's smokinlowrider with my popcorn?
Light on the butter please.......I'm on a diet :)

[SmokinLowriderSS]

Lordy, not again. :p
Popcorn is here, microwave is nuking more! Cold beer in the frige.
Chart them on a dyno program & spin it to the point the torque/HP curves cross.

[BOOGEYMAN]

I think that most engine torque and horsepower curves meet at 5250...

[Squirtcha?]

http://www2.propichosting.com/Images/450010218/7.jpg
Holy crap. That's quite a dyno sheet! I might have to convert.
Single 950 carb, and a flat tappet cam and only 11:1 compression and 625 hp? That must be one magic bump stick.

[Devilman]

Hopefully tryin to stay sorta on subject.
Is it correct to say that a particular cut impeller is only efficient to a given RPM? Not tryin to jack the thread or start a war. Just tryin to figure that part out...

[G-Body]

I am no expert but my gut feeling would be that if you cut it to spin at peak torque it will be the most fuel efficient, and if you cut it to spin at peak HP the boat will accelerate faster. I don`t know which would go faster but thats my gut feeling.

[Devilman]

Damn, I figured this thread would be blazing with opinions by now.... :rollside:

[Taylorman]

Holy crap. That's quite a dyno sheet! I might have to convert.
Single 950 carb, and a flat tappet cam and only 11:1 compression and 625 hp? That must be one magic bump stick.
Heres the link to the build.
http://www.realoldspower.com/phpBB2/viewtopic.php?t=14702

[YeLLowBoaT]

Well accord to JPC you will spin it alittle more then 5200 RPMs. 1.5A will spin 5500 ish ..... hell you will spin a 9.25 impler almost 5100...... thats moving some water.....

[GunninGopher]

That is a real stump puller. It looks to me like you vote for torque, Taylorman.

[inwo]

Sounds like this has been beat to death here in the past! :)
It's just a simple math test, right?
Seting up to the torque peak is no more correct than setting up to RPM peak.
Efficiency peak? maybe! Economy? maybe? If you want power translated to boat speed.........thats why they call it horsepower.
Torque and RPMs are just factors of HP. Taken to extreme, say, 1000 ft/lbs at 1 rpm. :(
RPM X Torque = Power

[LVjetboy]

So do you want to cut the impeller to allow the engine to spin to peak tq or hp? Heres a dyno sheet for comparison. Im gonna be rebuilding my Olds with Edelbrock heads and im looking to achieve 500 hp at 5000 rpms. I hope to spin a AMT AA impeller to 5000.
If your engine build matches that dyno sheet, I'd say you'd have a good shot at spining an AT AA to 5000. JPC shows 528 hp at 5000 rpm, but the dyno numbers you posted are corrected (most likely STP) and I wouldn't be surprised to see a up to a 50 hp hit at your normal boating DA.
As to your question of cutting for peak power or torque? Matching peak torque has no significance to performance. It's all about power. Yet matching peak power exactly neglects other factors influencing pump and jet performance that may be significant. For example, as an impeller's cut down, it's efficiency drops. In other words, how well it converts power in to power out. So if your engine gains 45 hp by cutting to an AT A/B, how much of that gain is offset by reduced impeller efficiency? Hard to know.
Then there's launch loading. I think a bigger impeller typically loads quicker although I don't have hard numbers to back that up. If so then quicker to plane for that bigger impeller.
Then there's cruise economy. Not to mention engines live longer at lower rpms and 900 less can mean a lot to parts life.
All things considered, impeller cuts bigger than peak power make sense and often give best top speed. I've posted this many times before but just in case some new guys haven't read my old posts on HP vs Q and assume when I say power not torque I'm suggesting the only thing important is an impeller matched exactly to peak power. Just isn't so. The pump and jet boat are a system with no perfect or simple-minded solution.
That said, matching peak torque or some magic combination of power and torque still means nothing to performance. It's not about torque, it's about power.
jer

[LVjetboy]

Is it correct to say that a particular cut impeller is only efficient to a given RPM? Not tryin to jack the thread or start a war. Just tryin to figure that part out...
I'd say pump efficiency is a function of both impeller rpm and cut size among other things. There's a sweet spot. Where that spot is rpm-wise, changes with cut size.
jer

[Jspeeddemon]

Torque is the ability to rotate that pump over one rotation, whereas power is the ability to rotate the pump over time. Since an impeller holds the motor to a certain RPM due to inability to produce any more power to rotate the pump any faster, I would cut the impeller to the HP curve. That's my opinion and I'm sticking to it. :idea:

[LVjetboy]

Torque is the ability to rotate that pump over one rotation, whereas power is the ability to rotate the pump over time...
Jspeeddemon, defining torque as the ability to rotate one revolution and power as the ability to rotate over time is a bit confusing. Instead...
Torque is defined as force times lever arm, nothing more. No movement required. In other words, by definition, the magnitude of torque applied to the impeller is independant of any rotation or amount of movement. Not that applying torque to an impeller won't in most cases cause rotation. Just that the speed of rotation depends on things other than torque applied.
By contrast, power includes rotation or movement...so also performance. Unlike torque, you can't have power without movement. And performance depends on movement.
That's why applying maximum power to the impeller, not maximum torque, is the performance driver. Yes there's pump losses that skew the engine-impeller ideal performance match point to a lower rpm. Yet those losses are also best characterised by power not torque.
jer