I am running a big hp 581 merlin with an IMCO SC and we are pegging the pressure guage (35+ lb's) at anything above 70 mph. We have taken everything apart no restriction. Pressure tested heads and block for cracks none. We are baffled any help?

The high water pressure topic comes up whenever crossovers are used. Be easier to do a 'search' here than to retype the info. Tons of topics on that very problem.
Good luck

chip12345,

The stagnation pressure according to Bernoulli's principle at 70 mph is just over 71 psi. By the time you get to 80 mph, it is over 93 psi. If your water pickup is such that water is stagnating in front of it, there is potential for very high pressure in the system. It would be interesting to find out what others with your same water pickup and exhaust manifolds or headers are seeing. It might not be abnormal.

have you checked the cooling shower line on your drive? I run a 540 blower motor and spin it 6200 rpm and only have 17psi at wot, also run a watts valve

"IF" you are running a crossover and seeing these pressures, put on a Merc Pressure relief valve with a 1" dump over your drive and your done.
25lbs
If you are running a thermostat, make sure you have a bypass ( I'm sure you knew that) Either way use the pressure relief valve.
Dave

Just curious, what head gaskets are you running?

As an alternitve to the ultra expensive reinforced plastic Merc relief valve check out http://www.straval.com/ you will find high quality stainless and bronze relief valves cheaper.

Phil

Like USA1 said do a search on the subject and be prepared for lots of reading. Dump valves are one solution that may work for you. Modifying your pickup may be another option.
Dan

Thanks..

We have a magnaflow cam driven water pump on the motor. We also tried a relief valve (smaller brass one) and did not work very well. We just tried a modification to the impeller by cutting every other spline. Took boat out yesterday and made two 80 mph passes and pressure did not peg but still above 35 lb's. So there was some relief. Oil temp got to 140 and did not come down after idling for a while. So that did not work.

Maybe a stupid question, will the merc relief valve work with the magnaflow?

The Merc dump valve should be installed after the water pump, and before the 1st cooler. It's pretty simple, just interupt the 1 1/4" water line, and install the valve. The valve has a 1" out for the overflow, which you can either plumb to a through hull fitting, or where ever you come up with to dump it.

Don't know about Magnaflow, but as long as you are using 1 1/4 " hose , I can't see why it wouldn't.