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Thread: AT impeller matching?

  1. #1
    LVjetboy
    First let me say, great article Mike! And interesting results. Good to see someone stepping up and testing to try and answer the hard questions.
    Now I have a few comments.
    1) Your American Turbine Impeller cut table (p118) agrees with Seloc for A and B cuts but not AA. You had:
    AA Front: 9.25"
    AA Rear: 7.50"
    Seloc has:
    AA Front: 9.094"
    AA Rear: 7.813"
    That's a significant difference, which is it? Rpm vs Hp curves seem to support a larger diameter than Seloc, but Seloc's been wrong in the past, and not sure about these specs just wondering. The A and B cut match with power, but the AA doesn't.
    2) It's hard to draw conclusions about impeller matching without an engine power curve. If your engine power curve's similar to that published in the Nov/Dec issue (p106), there's only about 10-12 hp difference between 6200 and 6700 rpm. That engine has a very flat power curve. A flat power curve engine may benefit more from impeller efficiency than matching compared to a peaky engine. Another thing is your 6700 rpm point. 6700 rpm is on the down side of peak power if your engine's the same as the dyno results in the Nov/Dec issue. Not a good match point no matter efficiency or power.
    Then there's the rule-of-thumb that 10 hp with a great setup and light-weight jet's good for about 1 mph...OR LESS. That doesn't seem to jive with your measured speed difference of 5 mph between the AA and the B which would imply a 50 hp change. Neglecting impeller efficiency. If impeller or pump efficiency makes up the difference (4 mph) with the larger impeller, then efficiency may be more important than power matching considering your potentially flat power curve.
    I'm not saying one way or the other just commenting on your data.
    3) This is a comment for those reading the article. The impeller rpm vs size vs speed data can be deceptive. Why? Because even if you neglect pump efficiency, if you run a larger impeller at the same rpm as a smaller impeller you WILL be putting more power to that impeller. More power to the impeller means more thrust. More thrust means more speed.
    So what does that mean? More speed at a set rpm with a bigger impeller's a given no matter the payoff in performance with a correctly matched (and possibly) smaller impeller. Or...the fact that a larger impeller gives your jet a higher speed at the same rpm is no guarantee that the larger impeller will give you better acceleration or top speed.
    Someone posted, "I have always believed, the AA was the best. more mph and less rpm."
    If you think that's the best...then go with it. Maybe a bit simple-minded but no worries. Why not put an AT 9.5 in your pump? Bigger is better right? If on the other hand you want to consider the concept that impeller matching has merit and is not so simple minded...then read on.
    4) Finally, I wonder about test data and cut size. From AA to A, a gain of only 150 rpm? That doesn't seem to agree with AT's pump charts for power applied? Not to say AT's pump data is right or your data wrong. Just an observation. Your numbers from A to B match...similar power and expected rpm gain. But numbers from AA to A. Seems something missing...
    jer

  2. #2
    Squirtin Thunder
    First let me say, great article Mike! And interesting results. Good to see someone stepping up and testing to try and answer the hard questions.
    Now I have a few comments.
    1) Your American Turbine Impeller cut table (p118) agrees with Seloc for A and B cuts but not AA. You had:
    AA Front: 9.25"
    AA Rear: 7.50"
    Seloc has:
    AA Front: 9.094"
    AA Rear: 7.813"
    That's a significant difference, which is it? Rpm vs Hp curves seem to support a larger diameter than Seloc, but Seloc's been wrong in the past, and not sure about these specs just wondering. The A and B cut match with power, but the AA doesn't.
    2) It's hard to draw conclusions about impeller matching without an engine power curve. If your engine power curve's similar to that published in the Nov/Dec issue (p106), there's only about 10-12 hp difference between 6200 and 6700 rpm. That engine has a very flat power curve. A flat power curve engine may benefit more from impeller efficiency than matching compared to a peaky engine. Another thing is your 6700 rpm point. 6700 rpm is on the down side of peak power if your engine's the same as the dyno results in the Nov/Dec issue. Not a good match point no matter efficiency or power.
    Then there's the rule-of-thumb that 10 hp with a great setup and light-weight jet's good for about 1 mph...OR LESS. That doesn't seem to jive with your measured speed difference of 5 mph between the AA and the B which would imply a 50 hp change. Neglecting impeller efficiency. If impeller or pump efficiency makes up the difference (4 mph) with the larger impeller, then efficiency may be more important than power matching considering your potentially flat power curve.
    I'm not saying one way or the other just commenting on your data.
    3) This is a comment for those reading the article. The impeller rpm vs size vs speed data can be deceptive. Why? Because even if you neglect pump efficiency, if you run a larger impeller at the same rpm as a smaller impeller you WILL be putting more power to that impeller. More power to the impeller means more thrust. More thrust means more speed.
    So what does that mean? More speed at a set rpm with a bigger impeller's a given no matter the payoff in performance with a correctly matched (and possibly) smaller impeller. Or...the fact that a larger impeller gives your jet a higher speed at the same rpm is no guarantee that the larger impeller will give you better acceleration or top speed.
    Someone posted, "I have always believed, the AA was the best. more mph and less rpm."
    If you think that's the best...then go with it. Maybe a bit simple-minded but no worries. Why not put an AT 9.5 in your pump? Bigger is better right? If on the other hand you want to consider the concept that impeller matching has merit and is not so simple minded...then read on.
    4) Finally, I wonder about test data and cut size. From AA to A, a gain of only 150 rpm? That doesn't seem to agree with AT's pump charts for power applied? Not to say AT's pump data is right or your data wrong. Just an observation. Your numbers from A to B match...similar power and expected rpm gain. But numbers from AA to A. Seems something missing...
    jer
    Was the artical like a little frog in high school science class ???
    Thank you for opening some eyes Mike !!!

  3. #3
    steelcomp
    Where does one start when trying to decide what impeller is going to be properly matched to a given pump/hull/motor combination. (for a given speed/et desired) Seems there's a lot of variables here, and a lot more info than before. Is there going to be an optimum impeller for a particular hull/pump combo which the engine will have to be built around, or will the operating parameters of the engine dictate impeller choice?

  4. #4
    blown428fe
    What bowl is that in the first pic on the ground to the left?

  5. #5
    cyclone
    Where does one start when trying to decide what impeller is going to be properly matched to a given pump/hull/motor combination. (for a given speed/et desired) Seems there's a lot of variables here, and a lot more info than before. Is there going to be an optimum impeller for a particular hull/pump combo which the engine will have to be built around, or will the operating parameters of the engine dictate impeller choice?
    It's a tough question to answer which is why I didn't attempt to answer it with the article. I think your best bet is to compare notes with your engine builder and pump builder. Knowing how much real power your engine makes and where it makes it is a good start.

  6. #6
    cyclone
    What bowl is that in the first pic on the ground to the left?
    It's an American Turbine.

  7. #7
    cyclone
    Was the artical like a little frog in high school science class ???
    Thank you for opening some eyes Mike !!!
    Yup. and the more I thought I learned, the more questions I had. fun stuff for sure.

  8. #8
    cyclone
    1. I'll have to ask Tom Papp. He provided me with the measurements.
    2. Good point. I really wish there had been room to reprint the dyno sheet for the motor but there wasn't. As you can see, the story skips around in the magazine because it was quite lengthy. At the time of the test, the engine was basically in the same configuration, save for a small timing adjustment, jetting change and oil change.
    3. In my case, the larger impeller was better across the board. from holeshot to top end. But my engine also has enough power to make the big impeller work. Not the case in most applications.
    4. Here's another interesting tidbit of info. I checked my AutoMeter recall tachometer against the Qwikdata system and the data system registered higher peak rpm readings. If I zoom in closely on the data there are spots during a 10-second speed run where the engine gains 100 rpm or so. The tachometer does't reflect this though. I've noticed it happen at the races too. Guess I need to call Auto Meter.
    First let me say, great article Mike! And interesting results. Good to see someone stepping up and testing to try and answer the hard questions.
    Now I have a few comments.
    1) Your American Turbine Impeller cut table (p118) agrees with Seloc for A and B cuts but not AA. You had:
    AA Front: 9.25"
    AA Rear: 7.50"
    Seloc has:
    AA Front: 9.094"
    AA Rear: 7.813"
    That's a significant difference, which is it? Rpm vs Hp curves seem to support a larger diameter than Seloc, but Seloc's been wrong in the past, and not sure about these specs just wondering. The A and B cut match with power, but the AA doesn't.
    2) It's hard to draw conclusions about impeller matching without an engine power curve. If your engine power curve's similar to that published in the Nov/Dec issue (p106), there's only about 10-12 hp difference between 6200 and 6700 rpm. That engine has a very flat power curve. A flat power curve engine may benefit more from impeller efficiency than matching compared to a peaky engine. Another thing is your 6700 rpm point. 6700 rpm is on the down side of peak power if your engine's the same as the dyno results in the Nov/Dec issue. Not a good match point no matter efficiency or power.
    Then there's the rule-of-thumb that 10 hp with a great setup and light-weight jet's good for about 1 mph...OR LESS. That doesn't seem to jive with your measured speed difference of 5 mph between the AA and the B which would imply a 50 hp change. Neglecting impeller efficiency. If impeller or pump efficiency makes up the difference (4 mph) with the larger impeller, then efficiency may be more important than power matching considering your potentially flat power curve.
    I'm not saying one way or the other just commenting on your data.
    3) This is a comment for those reading the article. The impeller rpm vs size vs speed data can be deceptive. Why? Because even if you neglect pump efficiency, if you run a larger impeller at the same rpm as a smaller impeller you WILL be putting more power to that impeller. More power to the impeller means more thrust. More thrust means more speed.
    So what does that mean? More speed at a set rpm with a bigger impeller's a given no matter the payoff in performance with a correctly matched (and possibly) smaller impeller. Or...the fact that a larger impeller gives your jet a higher speed at the same rpm is no guarantee that the larger impeller will give you better acceleration or top speed.
    Someone posted, "I have always believed, the AA was the best. more mph and less rpm."
    If you think that's the best...then go with it. Maybe a bit simple-minded but no worries. Why not put an AT 9.5 in your pump? Bigger is better right? If on the other hand you want to consider the concept that impeller matching has merit and is not so simple minded...then read on.
    4) Finally, I wonder about test data and cut size. From AA to A, a gain of only 150 rpm? That doesn't seem to agree with AT's pump charts for power applied? Not to say AT's pump data is right or your data wrong. Just an observation. Your numbers from A to B match...similar power and expected rpm gain. But numbers from AA to A. Seems something missing...
    jer

  9. #9
    UBFJ #454
    The difference between the QwikData Info (Data) and the AutoMeter Data is a result of the digital sampling rate of the QwikData vs. the analog averaging circuit built into the AutoMeter ... The AutoMeter's Info is averaged over a longer period of time.

  10. #10
    Senior Member
    Join Date
    Feb 2009
    Posts
    303
    AA Front: 9.25"
    AA Rear: 7.50"
    Seloc has:
    AA Front: 9.094"
    AA Rear: 7.813"
    I too questioned the AA size being noted as 9.25 instead of 9".
    "Someone posted, "I have always believed, the AA was the best. more mph and less rpm."
    That was me.
    If you think that's the best...then go with it. Maybe a bit simple-minded but no worries. Why not put an AT 9.5 in your pump? Bigger is better right? If on the other hand you want to consider the concept that impeller matching has merit and is not so simple minded...then read on."
    As a matter of fact, I did run the 9.5 inch impellor for a short season. It is now broken on the front side at all four blades and three of them are broken on the back side too. The boat is a homebuilt Cheyenne splash and the motor is a 557 inch BBF. I had hoped the big impellor size would allow the aluminum to live by keeping the rpm down but I was wrong. The motor only turned 5200 w/o NOS but that rpm was good for about 95mph. The NOS bumped it up to about 5600 and the speed went somewhere over 106. I don't have a GPS but I did drive around a guy who had one and he was reading 106.
    I don't think of it as a matter of being simple minded, just having fun with a single carbed, iron headed, dual plane intake manifold, low maintenance and low tech jet boat.

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