myoldboat3
Member
- Joined
- Apr 6, 2023
- RO Number
- 34841
- Messages
- 26
For my 1974 Mercruiser 140, I've been trying to check my raw water pump output by holding the feed hose (disconnected from the thermostat housing) straight up and seeing how high the water column shoots at idle and 2000 rpm. But I can't find a good estimate of what those heights should be. So instead I'm going to look at the flow rate (GPM) from that disconnected hose. But I haven't found the spec for that, either (I'm guessing someone on here has it).
So anyway, I tried calculating the minimum flow rate from the heat transfer equation for water. Here are my estimates for minimum flow from raw water pump:
About 10 gpm at WOT
About 4 to 5 gpm at 2000 rpm
About 2 to 3 gpm at 1000 rpm
These are for the flow through the whole system, including restrictions from the thermostat, manifold, riser, etc. Since I will measure the flow with the raw water feed hose disconnected from thermostat housing, I would expect to see higher flow rates.
Does anyone have any flow rate numbers, GPM at various rpm?
Calculations:
1974 Mercruiser 140 would be lucky to deliver 120 hp at WOT.
Power at wide open throttle (WOT): 120 hp = 305,280 btu/hr
(1 engine hp = 2,544 btu/hr)
Assume engine is 30% efficient (30% of energy converted to rotation).
Energy loss rate = 712,320 btu/hr
Assume half of loss goes with cooling water and the rest goes with exhaust gases or convects off engine.
Loss rate into raw water = 356,160 btu/hr
Assume lake water 70 degF, engine temp 140 degF.
deltaT = 70 degF
Heat transfer equation for water:
q = (flow)(C)(deltaT)
where
q = heat flux [btu/hr]
flow = flow rate [gal/min]
C = heat transfer constant [500 (btu/hr)/(gal/min)/degF]
deltaT = cooling water rise [degF]
so
flow = q/C/deltaT
more commonly written
GPM = (btu/hr)/500/deltaT
Estimate hp linear with rpm:
4400 rpm ... 120 hp ... 10.2 gpm ... about 10 gpm at WOT
2000 rpm ... 55 hp ... 4.6 gpm ... about 4 to 5 gpm at 2000 rpm
1000 rpm ... 27 hp ... 2.3 gpm ... about 2 to 3 gpm at 1000 rpm
So anyway, I tried calculating the minimum flow rate from the heat transfer equation for water. Here are my estimates for minimum flow from raw water pump:
About 10 gpm at WOT
About 4 to 5 gpm at 2000 rpm
About 2 to 3 gpm at 1000 rpm
These are for the flow through the whole system, including restrictions from the thermostat, manifold, riser, etc. Since I will measure the flow with the raw water feed hose disconnected from thermostat housing, I would expect to see higher flow rates.
Does anyone have any flow rate numbers, GPM at various rpm?
Calculations:
1974 Mercruiser 140 would be lucky to deliver 120 hp at WOT.
Power at wide open throttle (WOT): 120 hp = 305,280 btu/hr
(1 engine hp = 2,544 btu/hr)
Assume engine is 30% efficient (30% of energy converted to rotation).
Energy loss rate = 712,320 btu/hr
Assume half of loss goes with cooling water and the rest goes with exhaust gases or convects off engine.
Loss rate into raw water = 356,160 btu/hr
Assume lake water 70 degF, engine temp 140 degF.
deltaT = 70 degF
Heat transfer equation for water:
q = (flow)(C)(deltaT)
where
q = heat flux [btu/hr]
flow = flow rate [gal/min]
C = heat transfer constant [500 (btu/hr)/(gal/min)/degF]
deltaT = cooling water rise [degF]
so
flow = q/C/deltaT
more commonly written
GPM = (btu/hr)/500/deltaT
Estimate hp linear with rpm:
4400 rpm ... 120 hp ... 10.2 gpm ... about 10 gpm at WOT
2000 rpm ... 55 hp ... 4.6 gpm ... about 4 to 5 gpm at 2000 rpm
1000 rpm ... 27 hp ... 2.3 gpm ... about 2 to 3 gpm at 1000 rpm