small block vs big block

[LVjetboy]

"PowerIn/PowerOut,
dont trust me"
Ok, I won't.
Pump efficiency is a number less than 100% By definition, Output divided by Input. That efficiency definition applies to more than just pumps of course, but in pump guy terms...
Pump efficiency = WHP/BHP
Where,
WHP = pump output or water horsepower (WHP) the liquid horsepower delivered by the pump.
BHP = Pump input or brake horsepower (BHP) the actual horsepower delivered to the pump shaft.
In other words, Efficiency = PowerOut/PowerIn as I posted.
Brake horsepower or input to a pump is greater than the hydraulic horsepower or output due to the mechanical and hydraulic losses incurred in the pump. That's why the ratio is always less than one. Mechanical losses are things like bearing friction. Hydraulic losses are things like wear ring clearance and impeller cut size, but also depend on flow path design. How fast you spin it (rpm) can contribute to both mechanical and hydraulic losses. Without pump performance curves, we have no real map of how these complex losses affect impeller selection. Only trial and error.
Back to the essence of the thread. Assume a 100% efficient pump...no losses. Then PowerOut = PowerIn. In that situation, it may be easier to understand how applying maximum power to the impeller (not torque) gives maximum thrust: Best acceleration and top speed performance. In that situation, a small block delivering the same power to the impeller as a big block will win because that small block will be lighter...less hull drag. All that regardless of torque applied to the impeller.
Sure real world pump losses and engine build costs may influence your decision. But I believe if you know the difference between torque and power and how those terms affect performance, you'll be better able to choose the best engine for your goals.
It's not about torque.
jer

[miketsouth]

dont trust me
Ok, I won't.
good thing. you right. sorry jer. when i looked at it yesterday it looked backward and would result in a figure greater than unity.
and i was sober. Dam. the torque/hp dissertation that followed was never in doubt.
Its about SBvsBB, not Q or Hp. how about $/Hp or even Lbs./Hp.
The small block can be made to work. If you got to go fast looks like the BB is the ticket. Big boat, Big Block.
Light boat, maybe. Light wallet, definitely.
mikeT