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pressure.
calculations
As the engine temperature rises, tiny gas bubbles start to form on the walls of the cooling cavities. The liquid changing to gas causes the pressure to increase. When the fluid is cold, the bubbles are condensed back into liquid right away and there is little pressure increase. As the coolant temp rises, it takes longer for them to condense, the volume of gas increases and the pressure increases.
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What the limit is of how much pressure the water pump can produce is based on the pump design and I really don't know what those values typically might be. But, the pressure in the system is controlled by the pressure cap. There's a very good description at the following link...
30 psi is putting it mildly....
Once more---A centrifugal pump--that's what is in your car--the pump discharge pressure is a function of the pump speed squared. Once the thermostat opens there is a flow path from pump to radiator back to the pump inlet. Other than a slight droop in pressure due to the flow demand, the pressure rise across the pump basically constant. Obviously, no venting occurs as long as the pressure at the radiator is less then the cap pressure setting. Pressure at the radiator is established by whatever line loses there are between the pump discharge and radiator. Venting occurs because as the coolant temperature increases it's volume will also increase. Why?? Density=Weight/Volume. Density decreases as temperature increases. Since the weight of the coolant doesn't change, it's the volume that does. As the coolant expands, the pressure in the system increases (remember, trapped volume aka closed system) and when the pressure gets to the cap setting (won't take long), the cap opens and the fluid goes into the overflow bottle. If the radiator cap didn't open--whole new scenario--things would break!! If this doesn't answer your questions, suggest you do a bit of reading on your own. MLD
The difference in pressure across the pump is equal to the difference in pressure across the radiator. Suction on one side, pressure on the other. A matter of a few psi with a good rad and the thermostat open. The pump is not capable of producing very high pressures.. The pressure on the low side can be up to 5 psi below nominal, and on the high side as much as 5psi above no
Restriction to flow affects the "head" of the pump. No restriction, no head pressure. A good water pump may produce a 5 psi head, and draw an equivalent depression on the low side of the pump for a maximum pressure differential of 10psi - but that is a "blueprinted" pump at optimal speed with an adequately restrictive radiator.
Real world numbers are generally quite significantly less. Measured
3psi on BMW 328 just this week at 3000 RPM. Above and below 3000 it dropped off. That was with diluted coolant (about 20-25% glycol) due to having just repaired a leak and having the normal "fun" bleeding all the air out of the nasty little Kraut!!
Your comment makes no sense to me. There is always water at the inlet to the pump if the system if full of water. You can see the water moving if you take the cap off, at least on some systems where it opens into the tank. Clearly the pump is pumping water, therefore it's getting some to pump and it's certainly not as vapor.
Where in the system did you take that 3 psi measurement at?
My little 1.5 hp swimming pool pump, with similar sized centrifugal impeller to a car, similar sized pipes as a radiator hose, and running at a couple thousand rpm can easily produce over 25 psi if you shut down the outlet valve. It seems likely to me that the water pump in a
150 hp car could do the same if the flow was restricted.
MLD, do you know where to find a flow rate vs RPM curve for any common stock water pumps? I'm surprised I can't seem to find anything mfr spec curves at the various mfrs and parts houses. Doesn't matter what mfr or vehicle -- just any common street car single head pump.
I was looking to see what I could find on water pumps and didn't find much. I did come across this
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pressure, or
common stock water pumps? I'm surprised I can't seem to find anything mfr spec curves at the various mfrs and parts houses. Doesn't matter what mfr or vehicle -- just any common street car single head pump.
Thanks, that's very interesting info. I'm not familiar with the particular car they are doing the testing with, but it seems odd to me that they consider coolant temps below 212 deg F as normal. They must be water with no glycol in a system open to atmosphere. However, I didn't read it that closely thought so (as most people say) I may be off.
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me...
"stock water pumps? I'm surprised I can't seem to find anything mfr spec curves at the various mfrs and parts houses. Doesn't matter what mfr or ve hicle -- just any common street car single head pump.
ar car they are doing the testing with, but it seems odd to me that they co nsider coolant temps below 212 deg F as normal. They must be water with no glycol in a system open to atmosphere. However, I didn't read it that clo sely thought so (as most people say) I may be off.
Why does normal coolant temp below 212F equate to using pure water and a system open to the atmosphere? It can be lower, depending on factors like the thermostat and where the coolant temp is being measured. It just can't be higher than 212F.
Below 212 is normal. Anything over is abnormal, but still safe untill the BP of the pressurized mixture is exceded. Normal Operating Temp is closer to 195F-215F
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l.me...
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on stock water pumps? I'm surprised I can't seem to find anything mfr spe c curves at the various mfrs and parts houses. Doesn't matter what mfr or vehicle -- just any common street car single head pump.
ular car they
A normal mix of water and antifreeze won't boil below 212F. You could run the car all day at 205F, no? It's the opposite that's the problem. You can't run a water only coolant, open air, above 212F. I would think temps below 212F would be normal, depending on the thermostat, operating conditions and again where the temp is measured.
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Thoughts?????
common stock water pumps? I'm surprised I can't seem to find anything mfr spec curves at the various mfrs and parts houses. Doesn't matter what mfr or vehicle -- just any common street car single head pump.
- Thanks, that's very interesting info. I'm not familiar with the particular car they
- are doing the testing with, but it seems odd to me that they consider coolant
- temps below 212 deg F as normal. They must be water with no glycol in a
- system open to atmosphere. However, I didn't read it that closely thought so
- (as most people say) I may be off.
Probably a good point, but I'm not sure I understand what you mean.
MLD, do you know where to find a flow rate vs RPM curve for any common stock water pumps? I'm surprised I can't seem to find anything mfr spec curves at the various mfrs and parts houses. Doesn't matter what mfr or vehicle -- just any common street car single head pump.
Found this Flow vs Delta P characteristic.
MLD
You're right, not an easy thing to find. Why not give one of the many manufacturers a call and ask for what you want. MLD
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" snipped-for-privacy@sbcglobal.net"
pressure.
Thanks. Still looking for RPM vs Floweret
Having performed thousands of automotive cooling system flow/pressure and pump performance tests, I can say you have no idea, either.
Without getting into particulars, recently....5500 pump RPM...10 psi at the pump inlet, 200+ GPM into the block at 80 PSI pump outlet pressure
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