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Old 10-14-2010, 06:06 AM
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I have a 454 with a Mercury pressure relif valve.
I run between 7-15 max depending on trim.
I also have a shorty.

I thought this was way too low as the pressure releif is "set" for 30 or so pounds so I am told. Changed out the water pressure gauge with another new one (Autometer) and they both read the same.

I have no over heating issues, boat runs 160-170.

Ken
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Old 10-14-2010, 12:42 PM
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Originally Posted by minxguy
I have a 454 with a Mercury pressure relif valve.
I run between 7-15 max depending on trim.
I also have a shorty.

I thought this was way too low as the pressure releif is "set" for 30 or so pounds so I am told. Changed out the water pressure gauge with another new one (Autometer) and they both read the same.

I have no over heating issues, boat runs 160-170.

Ken
No problem with those pressures as long as the engine maintains temp. The merc relief valves are set at 25psi. You really don't need one with your pressures and it should not be dumping any water. Where are you getting your pressure reading from????
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Old 10-14-2010, 07:14 PM
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well i had the plastic pump crack when the impeller came apart so i put the stainless pump on. lot of water comes in when the pump cracks !! have about 5 hours on the pump then blew the hose going from the pump to the oil cooler. sooo i put good hose back on there instead of the molded hose junk and an autometer pressure gauge in. just a stock 502 mag mpi in a 96 272 but i was expecting the pressure to be much higher with a new pump and cold with the thermostat closed. hardin directions say the side of the pump is tapped for a pressure gauge but those can't be the right readings. but then again you guys seem to have low readings too. twin spins says he gets about 20-30 running, that was more what i was expecting. thanks

Last edited by perky2196; 10-14-2010 at 07:19 PM.
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Old 10-14-2010, 10:31 PM
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Originally Posted by perky2196
well i had the plastic pump crack when the impeller came apart so i put the stainless pump on. lot of water comes in when the pump cracks !! have about 5 hours on the pump then blew the hose going from the pump to the oil cooler. sooo i put good hose back on there instead of the molded hose junk and an autometer pressure gauge in. just a stock 502 mag mpi in a 96 272 but i was expecting the pressure to be much higher with a new pump and cold with the thermostat closed. hardin directions say the side of the pump is tapped for a pressure gauge but those can't be the right readings. but then again you guys seem to have low readings too. twin spins says he gets about 20-30 running, that was more what i was expecting. thanks
The way I think about it, is most vehicles have a 15psi radiator cap. Trucks, cars whatever. I don't see any reason why a boat motor would need over that either. As long as the water temp holds at your thermostat setting.
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Old 10-15-2010, 06:09 AM
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Originally Posted by Griff
No problem with those pressures as long as the engine maintains temp. The merc relief valves are set at 25psi. You really don't need one with your pressures and it should not be dumping any water. Where are you getting your pressure reading from????
Griff, I have my pressure line plumbed into the T-stat housing on the starboard side
There was a plug there and i used the hole.

Dumps water at 1500 RPM.

Ken
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Old 10-15-2010, 09:38 AM
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i have 468s., at idel ive never had pressure acording to my guage, but at running speeds i get anywhare 5 to 20 psi, boat never runs hot idel after running temp is about 150- running about 120-140, my boat has always run cool and very good, and i have a crossover pipe!
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Old 10-15-2010, 09:57 AM
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Stock 454, 5 psi high idling (with a new impeller), about 10 psi when running.
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Old 10-15-2010, 03:37 PM
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Originally Posted by cloudmaster_321
The way I think about it, is most vehicles have a 15psi radiator cap. Trucks, cars whatever. I don't see any reason why a boat motor would need over that either. As long as the water temp holds at your thermostat setting.
excellent point. i don't know why i didn't think of that. you are correct. for some reason i thought it would be higher. thanks
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Old 10-18-2010, 06:51 AM
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I was just told this past weekend that I should plumb the water pressure line somewere BELOW the thermostat. Right now I am above it.

Should this make a difference?

Ken
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Old 10-18-2010, 10:43 AM
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Mine is plumbed into the intake, next to the thermostat.
1/8 tube to the autometer gauge.

barely reads anything at idle. 10-15psi at speeds below 50 20-25 at 50-60mph. 25-35 at 60-80mph. I also have a low water pickup, which I think adds to the higher psi.

I plan to instal a relief valve pretty soon.
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