sabo

iTrader: (2)

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?

zz28zz

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

sabo

iTrader: (2)

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...

AllDodge

iTrader: (1)

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

SB

iTrader: (1)

h
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.

phragle

iTrader: (3)

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.

zz28zz

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.

Griff

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

AllDodge

iTrader: (1)

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
.

zz28zz

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.