Engine Pressures / head gaskets
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Engine Pressures / head gaskets

  1. #1
    Senior Member Rivernut's Avatar
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    Default Engine Pressures / head gaskets

    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?

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    Senior Member wagspe208's Avatar
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    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.

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    Village Idiot fc-Pilot's Avatar
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    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

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by fc-Pilot View Post
    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
    +1 on the intake gaskets

    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.
    Last edited by Budweiser; 08-08-2016 at 12:39 PM.

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    Senior Member ol guy's Avatar
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    Sounds like 2 different pressures are getting tangled up here. M

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    Senior Member wagspe208's Avatar
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    Pascal's principal rocks...
    I am also a fan of Bernoulli's theorem.... I cant spell bern...
    Wags

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    Senior Member Rivernut's Avatar
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    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....
    But we do know that inside the cylinders pressure is peaking at close to or above 1,000 PSI, and perhaps double that for boosted engines. The pressure in the water jackets and on the head gaskets might be a tenth of that. I'm using the Fel-Pro MLS (steel) head gaskets to counter that pressure. I'm also using the metal & rubber intake gaskets and have never had an intake leak. I use silicone heater hose and I put those thumb screw keyed hose clamps so I can check them easily and often. Lat month I had the same set up leaking water out near the rear cylinders on both banks. The engine started misfiring and I found one cylinder very low @130 while most of the others were at 175-180. New is 200-210. I inspected the gaskets and there were no breaches, but could see where water was traveling from a passage to the back lower corner and escaping. I've not seen this engine overheat. Two previous ones had tulipped valves and blowouts between cylinders 3 & 5 and damaged pistons. I have 3 spare Vortec heads and put two of those on but have not had an autopsy on old ones checking for cracks or warping. I'm assuming this recent events are from running too much timing. The MarinePower manual says to run only 8 degrees initial and 26 total. Seat of the pants / tachometer says 14 initial and 32 total. The Vortec's I've had ran between 4,250-4,450 with 34 total degrees. This one now runs 4,200 with the timing reduced. The tops of the pistons were very carbon fouled but the plugs were not. It ran great. Not sure that I want to tune it de-tuned, and get less speed and MPG, and get more carbon. Always experimenting!!!
    Last edited by Rivernut; 08-10-2016 at 07:19 AM.

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    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.

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    Ain't Right Racin piston in the wind's Avatar
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    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.
    QE 2050 W/ a Junk ass screw blower

    90% of the game is half mental... The pursuit of the unknown is priceless & gratifying . Sometimes it just takes good old fashion balls. No amount of reasoning or evaluation can deem it right or wrong. At the end of the day stick with whatever gives the person in question morning wood LOL

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