Impeller Question

[Stalkaholic]

For you jet pump gurus...

Say for example I have a "C" cut impeller and my max RPM is say 5500 RPM with a max speed of 75mph.

Now let's say for example sake I switch up to an "A" impeller without any mods to the engine?

Will an "A" impeller be able to generate the same amount of thrust as the "C" impeller, just at a lower RPM?

Will I still be able to achieve a top speed of 75 mph with the A, just at a lower RPM?

[pw383426]

Depends at what RPM your engine makes horsepower.

If your engine makes the same horsepower at 5000 RPM as 5500RPM, then you should be going the same speed despite impeller

[Stalkaholic]

Depends at what RPM your engine makes horsepower.

If your engine makes the same horsepower at 5000 RPM as 5500RPM, then you should be going the same speed despite impeller

This basically means you would have to produce more torque at the lower RPM in order to make the same amount of horsepower at the lower RPM, correct?

[IMPATIENT 1]

put a bbf in frt of a a-cut and it'll do 80 :D

[bp298]

For you jet pump gurus...

Say for example I have a "C" cut impeller and my max RPM is say 5500 RPM with a max speed of 75mph.

Now let's say for example sake I switch up to an "A" impeller without any mods to the engine?

Will an "A" impeller be able to generate the same amount of thrust as the "C" impeller, just at a lower RPM??

very generally speaking, an A will generate the same thrust as the C between 400-500 less rpm. i say generally, because it depends on the impeller, how much blueprinting, who did it, etc. say you had a blueprinted, tight pump, with a C cut from one manufacturer, and you installed a box stock A from another one, you might only lose 250 rpm. reverse that (sloppy c, certain manufacturer's a that is blueprinted and tight), you could be looking at a 600-700rpm loss. it all depends on condition and brand, but for the same thrust rpm would be less by some amount. there aren't any absolutes regarding how much rpm will decrease, but a pro pump guy should be able to look at your pump as it comes apart, give you a reasonable idea of what it will do going back together.

Will I still be able to achieve a top speed of 75 mph with the A, just at a lower RPM? short answer is no, you most likely won't have the same top speed, although it may only drop off a few, but you'll improve midrange efficiency and cruise rpm for a given speed.
there are two issues here. assuming your engine hp peaks above 5500 rpm, and makes a certain hp number at 5500, the a will require that same hp number at less rpm to achieve the same, or very similar thrust. the second issue is the hp the engine makes at 5000-5100 is most likely less, by some number, so the a will require more power even further down on the curve to even get to that point, which may create a drop of another 50-100 rpm.
so, to combine both questions, you need to know your hp curve, the size/manufacturer/level of blueprint of your impeller, and condition of your pump before/after, to make an educated guess.

[Stalkaholic]

put a bbf in frt of a a-cut and it'll do 80 :D

Just when I thought the jet section was gonna offer some useful information...:D

[IMPATIENT 1]

Just when I thought the jet section was gonna offer some useful information...:D

:D seriously you'd be just as fast with a a/b cut prob, turning less rpms. all depends on the hp range of your motor. that c-cut you run is killing your holeshot, wether you have an inducer or not

[Stalkaholic]

Actually I figured out a better way to word my question.
True or False

On the same engine, an A cut impeller spun to max RPM will generate the same amount of thrust as a C cut impeller spun to max RPM...however, the C cut impeller will have a higher max RPM than the A cut impeller.

[IMPATIENT 1]

Actually I figured out a better way to word my question.
True or False

On the same engine, an A cut impeller spun to max RPM will generate the same amount of thrust as a C cut impeller spun to max RPM...however, the C cut impeller will have a higher max RPM than the A cut impeller.

pretty much sums it up

[Stalkaholic]

pretty much sums it up

OK so with that in mind...

If you were to beef the engine up to where it could spin an A impeller at the same max RPM as it spun the C impeller, you would be able to create more thrust than the C impeller could, thus making the boat go faster.

In other words, if you were to run an A impeller, then switch it with a C impeller in the same pump (with no other work done to the pump, just an impeller swap) an A impeller spun at 5500RPM will generate more thrust than a C impeller spun at 5500RPM.

Is this correct?

[bp298]

Actually I figured out a better way to word my question.
True or False

On the same engine, an A cut impeller spun to max RPM will generate the same amount of thrust as a C cut impeller spun to max RPM...however, the C cut impeller will have a higher max RPM than the A cut impeller.

false. because the same engine will be making less power at the lower rpm the A impeller will force the engine down to, plus there is more hp absorbed at a lower rpm.

and.. the general comment that holeshots are better with an A over a C is bs...

[Stalkaholic]

false. because the same engine will be making less power at the lower rpm the A impeller will force the engine down to, plus there is more hp absorbed at a lower rpm.

and.. the general comment that holeshots are better with an A over a C is bs...

OK before this thread gets misconstrewed as a 'heated debate', I'm not asking these repeated questions to try and start an argument...the point of the post is to disspell any misconceptions behind the operational theories of impeller sizing, max RPM, horsepower curves, etc etc. If it seems like I'm debating any answers that are posted here, it's because I simply do not completely understand the answer, and am in search of a complete understanding. In other words, I'm very inexperienced with jet pumps (compared to the pros) and have a big thirst for knowledge regarding jet pump theory. With that in mind I will continue with my post...

Right, the fact that the engine makes less power at the lower RPM is the reason why the A impeller loads the engine more, forcing it to spin at a lower RPM.

However, won't the A impeller be moving the same amount of water at the lower RPM as the C impeller moves at higher RPM?

[IMPATIENT 1]

OK so with that in mind...

If you were to beef the engine up to where it could spin an A impeller at the same max RPM as it spun the C impeller, you would be able to create more thrust than the C impeller could, thus making the boat go faster.

In other words, if you were to run an A impeller, then switch it with a C impeller in the same pump (with no other work done to the pump, just an impeller swap) an A impeller spun at 5500RPM will generate more thrust than a C impeller spun at 5500RPM.

Is this correct?

yes a a-cut should put outta more thrust at 5500 than a c-cut at 5500. but your motor has to work within the range of the impellor cut you select.
i ran a a-cut in my sj and it'd run 78-79 on motor, sprayin it would get it on up into the low 80's , but i only turned 5800 to do it :D could've went to a bcut and went faster i'm sure but that's not what i was after. we cove race and the fastest top speed boat hardly ever wins next to the quiker accelerating boat.

[Big Kahunaa]

if you are only swinging a C at 5500rpm the A would kill your motor the thing would be a slug

[IMPATIENT 1]

if you are only swinging a C at 5500rpm the A would kill your motor the thing would be a slug


yep, you'd have to step up the motor output to turn a a to 5500 if that's all you could turn the c-cut. its all about power range of the motor.

[bp298]

OK so with that in mind...

If you were to beef the engine up to where it could spin an A impeller at the same max RPM as it spun the C impeller, you would be able to create more thrust than the C impeller could, thus making the boat go faster.


In other words, if you were to run an A impeller, then switch it with a C impeller in the same pump (with no other work done to the pump, just an impeller swap) an A impeller spun at 5500RPM will generate more thrust than a C impeller spun at 5500RPM.

Is this correct?

yes, at the same rpm, an a impeller will generate more thrust than a c. because the A is absorbing a signficantly higher hp, percentage wise, than the C (i.e, it will take more than a little beefing of the engine to accomplish this).

just for comparison sake, a look at an old berkeley impeller selection chart will tell you what's necessary to do that. the numbers are taken for comparison only, they may or may not be absolutely correct, and will be different for other manufacturers. however, the concept/theory is absolutely correct for berkeley and every other manufacturer.

The chart indicates that 400hp is required to turn a berkeley C 5600. 550hp is required to turn a berkeley A 5550.

the chart also indicates that 400hp is required to turn a berkeley A 5050 rpm.

theoretically, using the chart, the A at 5050 would have the same thrust as the C at 5600, because the same amount of hp is being absorbed. also, this correlation assumes that wear ring/shoulder clearances will be identical, not going from an 0.025 clearance to an 0.040.

this does not take into account the engine hp curve. for example, if 400hp is required to turn the C 5600, and the hp curve is still on the up-slope, it is reasonable to assume that hp is less at 5050, say 25-40hp less. to turn an A 4800rpm requires a minimum 350hp. so if the hp decreases, say 30hp from 5600 to 5050, there will not be enough to achieve 5050, and rpm will land somewhere less than that, with the equivalent loss thrust for top end.

again, this is one brand and the numbers may change, but the concept remains the same.

just to add to the confusion... years ago, i ran a berk ss B, and the whole pump had been blueprinted by mpd 20 years ago. the engine would turn 6250 and run between 101-103. i had jack blueprint a legend ss B, we installed it in the same pump with the same clearances. we also made some changes to the engine that demonstrated a 50hp gain at every point, from 5500-6700, compared to a year before on the same dyno.
with the legend B, the engine turned 6050, but the boat ran 104-107 consistently, and picked up 0.25 in et with the same setup/same track/very similar conditions.
in this case, the hp change was negligable (even though rpm was lower), perhaps an increase of 1.3%. but the performance improvement was significant. another point, i did not make this change for performance reasons; the legend was constructed with a stronger alloy than the old berk, and the old berk was old - i wanted the pump to be bullet proof.
i'll also run a C impeller at certain tracks, that is very similar in design to the B. the engine runs 6250 with the c, and i've seen a best of 109.9 with it. it's just a little quicker and faster with that one.

going back to what i mentioned before, if you were to change like for like (manufacturer/level of detail in the blueprint) except for impeller cut, the result will be somewhat predictable. but if you were to change to a different cut/manufacturer/level of detail, you could very well reduce rpm and gain in performance - predictability (rpm/performance level) becomes much more difficult.

[Stalkaholic]

theoretically, using the chart, the A at 5050 would have the same thrust as the C at 5600, because the same amount of hp is being absorbed. also, this correlation assumes that wear ring/shoulder clearances will be identical, not going from an 0.025 clearance to an 0.040.



This is exactly the answer I was looking for. Thanks bp!