XLR8R

Does anyone KNOW the cooling system pressure generated by the water pump's output and the engine's waste heat?

Is it too simple to assume that when the thermostat is open, exposing the radiator to the rest of the circuit - that the maximum system pressure is the radiator cap pressure rating @ *F?

Anyone ever check it with a gauge - especially with the thermostat closed?

bnold

Has to be under what 15psi for the relief on the cap right??

Raspy

The system pressure should be at the cap set pressure all the time when the engine is warmed up, or more precisely, as the temperature is increasing. In theory there is no air in the cooling system so a small temp rise will equal a large pressure rise, up to the cap set pressure. As soon as the pressure exceeds the cap pressure there is flow to the recovery bottle. The pump does not raise the system pressure it simply generates a differential pressure that causes flow. So, less pressure on one side of it and higher on the other. No net gain, but slightly higher at the radiator than in the block.

As the engine cools, like a long downgrade or being shut off, a partial vacuum will develop and coolant begins to move back to the engine from the coolant recovery bottle. But only a small amount because there is no air in the system, so a small amount makes up the difference in volume change caused by temp change. There should be no air bubbles in the system, but there is air disloved in the fluid. That's why we need the corrosion control agents in the coolant, it's not really a perfect "closed" loop

The system pressure is always changing from a partial vacuum to slightly above the cap set pressure. A total change of around 15 PSI.
Wetspirit

PEAKSTRYDE

I agree, the pump is high volume low pressure, therfore never really building pressure. The pressure it mainly made by the heating of the coolant.

dodgezilla04

outstanding post and explaination.

CamperAndy

Sorry guys but the pump though it is a centrifugal pump can easily raise the block pressure to 60 psi and above when dead heading when the thermostat is closed. Even when the thermostat is 100% open the block pressure can exceed the rad cap pressure as it creates a delta pressure across the thermostat valve.

PEAKSTRYDE

Camper,


That being said, what kind of PSI would you expect to see in other places of the system (hoses, radiator, etc.)?




Abviously you can not have pressures much over 15 PSI in the radiator or the cap would open and over fill the overflow tank, correct???????????????????

CamperAndy

Not correct. Look up Boyle's Law and Bernoulli's Principle. It explains the law that controls pressure vs volume. But basically if the volume is the same and the temperature is the same and the flow is the same. Then the addition of force will increase pressure.

So with a closed or partially closed thermostat and the addition of higher rpms (force) the pump discharge pressure and block pressure will increase. Since water is non compressible the pressure will increase until the pump begins to cavitate.

The rad cap only controls the pump suction pressure not the pump discharge pressure.

XLR8R

Just checking in...

this is off to a great start, guys - keep up the good posts!

dodgezilla04

so since i am the inspiration for all of this discussion, what do i do to not pop the plugs out of the side of the block?
i intend to block the rear plug, snip the barbells off the tstat, and when really hotrodding, put the heat control just slightly over warm. any other thoughts or ideas?

CamperAndy

Short of installing a dedicated relief system for the block you should install a cab heater core bypass valve. Put it in the engine compartment and when you really want to hot rod open this up. It will dump pressure from the top back of the head back to the pump suction. If you do not install a bypass then have the heater fully open to dump pressure. It will get hot in the cab but that is better then a blown core plug.

dodgezilla04

thanks andy, i only have to turn the heat control **** to full, the fan speed can be only on 1(slow) right? air volume will not affect the water volume, so i should be fine with this method correct.?

PEAKSTRYDE

The block pressure will increase which makes sense. Doesn't that mean that the whole systems pressure is increasing? Probably not nearly as much outside the block but you will still see and increase??????

As far a the pump cavitating, how is this affecting the system?

I'm assuming that there is no "bypass" circulation for the pump when the Tstat is closed and that is why it cavitates??????



In warmer weather (summer time) once the truck is warmed up to operating temerature does the Tstat ever fully close? I know that there are a lot of systems out there that once the system is up to operating temperature the Tstat will never close (always slightly open), and when the system gets up to the temp. that the Tstat is rated for it will fully open.

Should be safe to say that this has nothing to do with the Tstat closing "slamming shut" because the Tstat will NEVER "slam shut"!!!!


Which still leaves us with the question of when is the Plug blowing out????????

dodgezilla04

i know there is a tstat bypass port for when the stat is closed. the passage doesnt allow enough coolant to flow through back to the pump inlet to prevent excessive pressure? there has to be another reason, cummins rarely makes any engineering mistakes, and i cant see the cause. Has this ONLY happened to heavily bombed trucks? or are the few of us that are having this problem, not allowing it to FULLY warm up?

bnold

Makes sense... Jump on the fuel without the Tstat open and that pressure has to go somewhere...