525SC fuel pressure
 

My father has a 2000 checkmate 259 with 525sc, chiller and higher boost pully, 1050 dominator, he just installed a Sierra 18-8860 fuel pump, seeing 7psi at idle and dropping to about 3.5 psi at W.O.T boat is running good but hes worried 3.5psi is too low? He doesnt remember the pressure being this low in past.

I read somewhere where Mercruiser states 3-7 psi @ 1,800rpm, I think hes seeing about 6psi @ 1,800rpm

Thoughts?
https://cimg7.ibsrv.net/gimg/www.off...615f34cf93.jpg

IMHO, 3.5 psi is too low at WOT.

I use one similar to this one then use an Aeromotive adjustable regulator to set press to 7.25 psi. Works great.
https://www.cpperformance.com/p-1328...ock-chevy.aspx

Thanks zz28zz, we are also thinking his sea water pump cam that runs the fuel pump may be slightly wore out to cause this issue, looking for rebuild kit or new now...

Pay no attention to me BUT... the fuel bowl is full at WOT with almost any kind of pressure. If the carb bowl was not full the pressure would be below 1 psi

Any pressure means the pump is outputting more then the carb can use

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Makes total sense, i thought same at some point, but it is not true in actuality. Under 5 psi things get a little interesting. Under 4 worse, and under 3 really not good.

I run the blue KE mechanical fuel pump on a built 525sc with no problems. Though I did increase the size of the fuel line and pickup.

Whenever a liquid flows thru a restriction (needle/seat assy) there will be a press drop. The more flow you have, the more of a press drop you will see. At some point, fuel will be taken out of bowl faster than it can be replaced due to this press vs restriction relationship. Since the restriction is basically fixed (once float drops), you'll have to increase press to maintain sufficient flow. Around 7 psi has been generally accepted as the optimum press for many many years.

IMO, I would want to see at least 5# of fuel pressure at WOT.

Understand what your saying, rural water companies have main water pressure above 100 psi to keep high flow up, and regulators on houses to bring pressure back down.

This is low pressure issue, and don't agree and appears I'm alone on this. Could prove it by setting up a fuel pump, filter, lines and carb inlet (no fuel bowl, recirculation maybe) on a test fixture of some sort. Start pumping and see if the pressure can get anything close to 3 psi with free flow
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Pressure is the product of flow VS restriction. Take a garden hose for example. Install a "T" and connect a pressure gauge at the house spigot, then connect a short hose the "T". If you don't connect anything to the other end of hose, there will be tons of flow but very little pressure on the gauge. If you add a spray nozzle to the end of the hose (a restriction) you will see an increase of press on the gauge and flow thru the hose will decrease.

The needle/seat assy provides some restriction even when it's "fully open." I agree that with free flow there will be very little pressure achieved, but it's not free flow when there's a restriction involved. To achieve more flow thru a restriction, you will need more pressure. Your logic is sound, but I think you are underestimating the effect of the restriction caused by the needle/seat assy.

Understand the needle restriction, free flow was just a term and I'm sure you understand what I was talking about without further explanation. As before, need a test to verify either of our assumptions

I'm not assuming. I assumed as you many years ago, and then the years and experiences went on.

Have ran into it on the water, at the track, and the dyno. Under 4lbs was an issue. Typically engine would go flat,no more rpm, and be lazy at that final rpm. 3psi and under, hold your nutz. lol

Once on the dyno I stood next to the engine as it had glass sight bowls. AF would shoot up, engine started to break up, and the fuel would in the sight glass would turn into a brothy mess.

Not assuming here either. It's basic physics. It takes X amount of press to force a fluid thru a restriction to achieve an X amount of flow.
I've personally seen the effects of low fuel press at WOT many times and it's never pretty.

Sounds like the OP is on the right path. If there is significant wear causing the fuel pump to not move thru it's full range of motion, that could cause low fuel press at WOT. Especially with an over-driven blower on a BB.

He ended up borrowing a used Merc pump, threw it on last night and saw 7.5lbs at idle and 5+lbs at WOT, good to go, I guess something was wrong with the brand new sierra 18-8860 or it wasn't the correct pump to begin with

I also did find on Holley website that they state the 1050 Dominator should not go below 5psi, i see what you guys are saying about low pressure high flow, but 5psi min pressure must be needed to maintain a safe carburetor operation