This is becoming a big pain in my (^&%&*^. The Keith Eickert exhaust is the only thing that was changed from the stock HP500 system. Rebuilt engine and added the exhaust last year. Anyway the pressure is running well over the 30 psi mark. I have tried a brass 5/8 id deverter valve inline after the sea pump with two 3/4'' id hoses off the risers. This set up has helped. Now I can get to 4000 to 4500 rpms before hitting the 30 psi mark. Before I needed to stay below 4000. I just installed the Mercury Marine "rip-off" relief valve with its 1'' id hose and still using the 3/4'' hoses off the risers. 34 or more psi at wot. What the hell is going on? Are the manifolds notorious for pressure problems? Another boater has the same manifolds on a 502 EFI Lingenfelter motor. He is seeing the same psi levels. I have been chasing this problem since last summer.
The drive is a dual water pickup style. Does anyone have any helpful ideas?

Are there water restrictors in the water line going from the header to the exhaust down tube/collector?

Merc comes with restrictors at that location.
If there are, open them up.

It sounds like the restriction is in your exhaust.. since that is the only thing you have changed.

Just an idea.

Chris

I had the same problem with the EMI's but the Merc "rip-off" valve did the trick - never more than 25psi. Where are you dumping the relief valve?

P.S. you can buy pop off valves at a pool supply,plumbing supply, hardware store for next to nothing. Ran them for years. Still isnt solving your issue but thought I would let you know.

Thanks for the comments. The valve is dumping through a through hull fitting that Fountain installed to relieve pressure from the bottom of the boat. I have temporarily teed in to this line. No permanent holes until I find the right fix to the problem. I had custom tips made and I am starting to believe these are the problem. Although the problem existed when I had the short factory tips on. You would think that dumping 3/4'' id hoses before the tips would take care of any pressure problems. I am ready to go back to my Gils or try something else. Just hate to spend the money on a trial and error basis.

Thanks for the tip. I waited till the last minute and tried other valves before breaking down and paying the money for the Mercury valve which leaks water as well. It will be going back.

I forgot to mention that I did away with the 4 hoses to the exhaust, thermostat housing for the 2 hose style. Pressure problem was not affected by the change.

Thanks

Didnt you change the heads to?

Where were the 4 hoses going before you switched to the 2 hose housing???? Were they T'd together and going only to the manifold or were they run just like Gils with one to each manifold and one to each riser???

Yes heads too, AFR. I figured they would have nothing to do with the water flow.