So I've been thinking about the feedback I got from my earlier post about head gasket choice and unregulated water pressure in raw-water cooled jet boats. I teach high school engineering and immediately thought of Pascal's Principle, which states that the pressure in a fluid is the same throughout. So, a "gate" or "ball valve" would have no effect on regulating the pressure in a raw water cooled engine. These would affect the rate of flow (volume) but not the pressure. A water pressure regulator (like the one in your house) that typically has a spring in it is needed to affect pressure. It was pointed out that water pressure may exceed 100 psi. My boat engine's oil pressure is frequently between 60-80 PSI. I completed the head swap on my 350 Vortec and did a compression check. Cylinder pressures are 200-206 PSI for all. Out of curiosity I Googled maximum pressure in a combustion engine. It turns out that this is roughly 100 times the compression ratio or 4 times the compression test number. That puts my Vortec firing pressure at 800-900 PSI. Other engines running turbos or diesels may be approaching 1,500-2,500 PSI. With MLS gaskets all passageways have steel around them similar to the combustion chamber. The soft 5/8" rubber heater hoses handle the pressure of the raw water cooling without blowing off or bursting. All this has me thinking that the raw water pressure is not a big deal for an engine that among other things sees 800+ PSI pulsing thousands of times a minute. Opinions?

2.

3. IF your hypothesis is correct...... and it has soe points...
Why would street cars have a 15 psi radiator cap and not 100 psi?
Wags
I used a pressure regulator on my boat.... on inlet... jet app.

4. PSI is pounds per square inch. if by closing the valve, you are limiting the area for the pressure to have effect on. not only that, but you have a free flowing system on the other side of the valve it therefore is not "holding" that much water pressure. The other big issue is that it is not just the head gaskets but also the intake gaskets that could leak.

Paul

5.

6. Originally Posted by fc-Pilot
PSI is pounds per square inch. if by closing the valve, you are limiting the area for the pressure to have effect on. not only that, but you have a free flowing system on the other side of the valve it therefore is not "holding" that much water pressure. The other big issue is that it is not just the head gaskets but also the intake gaskets that could leak.

Paul

And to add a couple cents... I believe Pascals Principal refers to a closed circuit or confined incompressible liquid, more akin to a closed cooling system or hydraulic circuit. Where a raw water cooling system is an open circuit where pressure is created by a restriction or resistance to flow. Reduce the supply via a valve, to flow less than the "restriction" after the valve and the pressure will be nearly zero on the low pressure side of the valve.

A little senseless splitting of hairs, information with little to no value... Water is compressible, or not entirely 100% incompressible. Doesn't really make a bit of difference, just an interesting point.

Cylinder pressures... I love a good cylinder pressure conversation.

7. Sounds like 2 different pressures are getting tangled up here. M

8. Pascal's principal rocks...
I am also a fan of Bernoulli's theorem.... I cant spell bern...
Wags

9. So the raw water cooling system is an open system with restrictions and reliefs throughout. Lots of variables and variations from engine to engine also that "muddy the water" making it difficult to know what the pressure is at any particular time and place in a raw-water jet engine. I'm guessing that the highest pressure in the cooling system is in the bowl take-off tube and 5/8" hose entering the system. The lowest would be where risers exit into the dual 4" exhaust tubes, but there is the pneumatic side with the engine back-feeding pressure into the hydraulic water cooling system...hmmm....

10. One thing to keep in mind is that head gaskets are designed to contain pressure inside the cylinder by the use of a steel ring around the cylinder. In engineering speak, the cross-section of this ring is slender and will buckle if enough force acts on it from the outside of the ring. Conversely, the pressure inside the cylinder adds tension stiffening that restricts the failure mode to hoop stress only.

For example, PVC pipe may be able to handle 400 psi of internal pressure, but apply the same force over the same area but from the outside and watch it crush.

11. We have a half assed jet that makes around 475-500 psi of bowl psi. We feed the engine with one-8 hose to a T and discharge trough 2 -8 hoses. On top end it shoots water about 2 feet out of the discharge ports.The block psi is around 10-12.

## Register Now

Please enter the name by which you would like to log-in and be known on this site.