question for jer

[Cas]

http://members.cox.net/lvjetboy/DNEBeast.jpg
In looking at your dyno (dino) sheet, a couple of questions came to mind.
What do you think would happen if your peak torque was say 500 lbs @ 5100 instead of the 735?
Do you think your top speed would be the same?
or what do you think your top speed would be if your peak TQ of 735 was at 4500?
and what about the TQ curve, what would happen if it wasn't quite as flat as it is at the top?
same thing with HB's engine, I see a definate trend with the 2.

[flat broke]

Cas,
If you start digging around through all the dyno sheets from most of the various pump gas N/A purpose built jetboat motors comming out of DNE, you'll see more of that similarity. In the quest to keep the power down in the useable RPMs, the torque curve will tend to peak and hold in the area of intended operation. Sure the motors make their peak number a little higher, and in some setups, the pump will alow them to get there. In my instance, we figgured I woudn't be spinning the impeller to more than 6k because of the size of my boat and associated impeller choices. You can see my tq curve is nice and flat, as is the power curve as well. If you were to look at other 498 build ups from different shops, you'd see some of them that might be able to get the hp a lot of us enjoy, but few have figgured out that it's much nicer to have access to that HP at lower RPMs, and how to do that as well as Dave.
http://www.***boat.com/forums/attach...achmentid=9698
Here is the sheet from my motor rigged as it went into the boat. The only difference between my motor now and during these pulls is the oil cooler, mechanical fuel pump, and a small change to the idle circuit.
Chris

[Cas]

Chris,
The dyno sheets posted is exactly what my point has been in the HP v TQ debate over the years. Without the TQ in the proper range, the engines are not going to produce the HP nor the speeds that are being obtained, IMHO.
Nice numbers, btw!
Are you spinning up that high to get to your peak hp?

[LVjetboy]

"What do you think would happen if your peak torque was say 500 lbs @ 5100 instead of the 735? Do you think your top speed would be the same?"
Way slower. For example, if I used more restrictive heads and a milder cam so my torque at 5100 was 500 ft-lbs instead of 735, my peak power would be considerably less, in fact over 200 hp less.
"...or what do you think your top speed would be if your peak TQ of 735 was at 4500?"
Assuming a matched impeller and no efficiency loss, somewhat slower. Moving peak torque from 5100 to 4500 or even 4000 while holding a peak of 735 ft-lbs, say by cranking positive cam timing, moves peak power down about the same rpm delta, but also reduces peak power by over 50 hp. Since performance depends on more power to the impeller, not more torque, your last example illustrates why focusing on the power curve instead of the torque curve is most important.
That said, impeller cut efficiency* can't be ignored. As we talked in another thread...both Mike's and Duane's tests seem to indicate efficiency loss from cutting an impeller (blade-vane spacing?) may be significant and overshadow gains from a well-matched impeller-to-engine power curve. Those tests somewhat limited so I think more needs to be done before drawing conclusions on cut efficiency. But if true, a "flat power curve" may pay off by allowing a larger and more power efficient impeller to match your engine power curve.
All that said, the bottom line is: total power applied to the impeller and how efficient that power is converted to thrust is the key to performance.
You don't need to think about torque or worry about how much torque or where it peaks to figure performance. Yes, where torque peaks and the shape of the torque curve will dictate where power peaks and the shape of the power curve. But it's the power curve we're after. Torque's only half the picture, when combined with the other half, rpm, gives the whole picture...power. And in the end, you do need to think about the shape and peak of the power curve, because that's what leads to performance.
*Efficiency = Power Out/Power In not Torque Out/Torque In.
jer

[kojac]

"What do you think would happen if your peak torque was say 500 lbs @ 5100 instead of the 735? Do you think your top speed would be the same?"
Way slower. For example, if I used more restrictive heads and a milder cam so my torque at 5100 was 500 ft-lbs instead of 735, my peak power would be considerably less, in fact over 200 hp less.
"...or what do you think your top speed would be if your peak TQ of 735 was at 4500?"
Assuming a matched impeller and no efficiency loss, somewhat slower. Moving peak torque from 5100 to 4500 or even 4000 while holding a peak of 735 ft-lbs, say by cranking positive cam timing, moves peak power down about the same rpm delta, but also reduces peak power by over 50 hp. Since performance depends on more power to the impeller, not more torque, your last example illustrates why focusing on the power curve instead of the torque curve is most important.
That said, impeller cut efficiency* can't be ignored. As we talked in another thread...both Mike's and Duane's tests seem to indicate efficiency loss from cutting an impeller (blade-vane spacing?) may be significant and overshadow gains from a well-matched impeller-to-engine power curve. Those tests somewhat limited so I think more needs to be done before drawing conclusions on cut efficiency. But if true, a "flat power curve" may pay off by allowing a larger and more power efficient impeller to match your engine power curve.
All that said, the bottom line is: total power applied to the impeller and how efficient that power is converted to thrust is the key to performance.
You don't need to think about torque or worry about how much torque or where it peaks to figure performance. Yes, where torque peaks and the shape of the torque curve will dictate where power peaks and the shape of the power curve. But it's the power curve we're after. Torque's only half the picture, when combined with the other half, rpm, gives the whole picture...power. And in the end, you do need to think about the shape and peak of the power curve, because that's what leads to performance.
*Efficiency = Power Out/Power In not Torque Out/Torque In.
jer
Jer,
In trying to establish a pattern in dyno horsepower/torque and Impellar curves I would estimate by your dyno readings that your impellar would peak out at about 5650 rpm's with an "A+" cut impellar.
For my own edification could you substantiate this hypothesis. Or correct me if I am wrong.
Thanks Kojac.

[LVjetboy]

"I would estimate by your dyno readings that your impellar would peak out at about 5650 rpm's with an "A+" cut impellar. For my own edification could you substantiate this hypothesis. Or correct me if I am wrong.
Thanks Kojac."
I believe you are correct. In fact, as I posted before, for my estimated lake power (dyno corrected or measured hp adjusted to my lake density altitude) I'd guess a Berkeley A2 a better match to my engine. After 5600 rpm, my engine power falls flat. Even neglecting impeller efficiency with cut size, there's not much power to gain by going higher on my engine.
Not saying all engines have the same flat power curve, or won't benefit from a smaller cut. Just talking about mine.
For example, my engine gains about 10-15 hp from 5600 to 6200, not significant in performance terms. Another engine may gain 100 hp in the same range. Also, Duane and Mike's tests (though limited and not necessarily conclusive), seem to imply impeller cut efficiency's a significant factor...in some situations equal to or exceeding engine power output curve gains from matching.
That's the trade-off in question.
If we know how cut size efficiency affects power out and combine that with an accurate engine power curve...then we'd have a better clue.
jer

[kojac]

Thanks Jer,
Just needed confirmation. It helps when trying to figure out motor/impellar combinations.
Kojac

[Cas]

"What do you think would happen if your peak torque was say 500 lbs @ 5100 instead of the 735? Do you think your top speed would be the same?"
Way slower. For example, if I used more restrictive heads and a milder cam so my torque at 5100 was 500 ft-lbs instead of 735, my peak power would be considerably less, in fact over 200 hp less.
"...or what do you think your top speed would be if your peak TQ of 735 was at 4500?"
Assuming a matched impeller and no efficiency loss, somewhat slower. Moving peak torque from 5100 to 4500 or even 4000 while holding a peak of 735 ft-lbs, say by cranking positive cam timing, moves peak power down about the same rpm delta, but also reduces peak power by over 50 hp.
jer
you said everything that needed to be said right there....can't have the power without the torque and you can't get the power with a jet boat without the torque in the right place.
Now that you've finally admitted it, case closed.

[PC Rat]

Now that you've finally admitted it, case closed.
Nice trick - I like it!