Are all RPM's the same?

[Scarab Jet]

OK guys, this might sound like an obvious question but here it goes:
Let's assume we have the same boat with the same jet pump and the same impeller. Now, if we were to compare a 350 vs. a 502 motor in this boat and run each of them at let's say 4500 RPM, would both of them go just as fast at that RPM? ( or for that matter at any given RPM?
If the answer to the above is yes, then can it be assumed that the only [noticeable] difference between the two engines is that the bigger engine has the ability to get to the given RPM quicker than the smaller engine?
Thanx in advance...

[BK]

Good ? but in this case we will need to compare motors where the weight is the same since a 350 would be much lighter than the 502.

[Scarab Jet]

Good point ... OK then, let's assume that both weigh the same too. For the sake of discussion, the 502 is all aluminum and 350 is all cast iron.

[Stupid Fast]

Scarab Jet:
Good point ... OK then, let's assume that both weigh the same too. For the sake of discussion, the 502 is all aluminum and 350 is all cast iron.OK I will play: My thoughts
The 350 would be wide open. The 502 would have alot left over. But at the same rpm the boats should be going the same speed.
.
.
.....I think
Eric

[malcolm]

That sounds pretty good to me. Depending on the impeller cut, the small block will power out at a certain rpm where the big block will go higher. Check out one of the impeller hp curve charts.

[LVjetboy]

Hmmmm...sounds like hp vs Q vs Wt to me. Where were you guys in the other thread??? Did I intimidate? Was that "Hp vs Q" thread shut down by the good ol' racer boys?
A good question Scarab...
jer
[ October 01, 2002, 07:18 PM: Message edited by: LVjetboy ]

[77charger]

put it this way the impeller is still spinning 4500 rpm it doesnt know what motor is powering it.
You will still be pushing the same amount of water thru the pump.

[bigerich]

All of this would be assuming. If you have a motor that could get the boat to the particular RPM, yes, the impeller would spin at the same speed, but would the small block be able to get that water moving?????

[Blown 472]

I have had 429's and 460's in my boat, they ran the same rpm, the 60 got there quicker than the 29 due to the longer stroke but ran the same speed and rpm.

[pops1]

Scarab Jet:
OK guys, this might sound like an obvious question but here it goes:
Let's assume we have the same boat with the same jet pump and the same impeller. Now, if we were to compare a 350 vs. a 502 motor in this boat and run each of them at let's say 4500 RPM, would both of them go just as fast at that RPM? ( or for that matter at any given RPM?
Thanx in advance...You must make a assumption based on your write up that both impellers are the same cut.
That is whare the two motors will fall apart- ON Torque! The small block is a lighter weight but that is all. The big block may even make the same HP but it make Torque at a lower RPM. To off set this, you are be forced to cut the impeller-Which shortens the duration of water ride (a critical producer). Also on the smaller impeller you cut the vane away (smaller) which causes the bowl pressure to become the offset for the lower thrust comming off the impeller. You now have a larger exit port opening on the smaller impeller(flushing) with a lower thrust entering into the bowl. You also now have reduced your frontal draft by flushing the water out the back (and so on). Torque is the answer or gear reduction to develop comp torque to run the biggest impeller possable at the highest RPM.
What you have developed to offset the torque to HP is design in slippage which is non-functional loss -Dave wink