Fuel: electric assisting mechanical?
 

I had my TopGun out last fall for last weekend of boating when I noticed my fuel pressure drop.

boat is a 38 Gun with Merlin 588’s Mooneyham 8-71’s topped with a pair of Holley 850’s. 850hp each.

I have Mallory 110 series electric pumps feeding mechanicals... the fuel pressure dropped because the electric pump failed. The boat ran fine on just the mechanical pump and held 5psi while cruising.

Now to my questions:

1. I’m assuming the point of the electric push pump is to just assist the long delivery of fuel to the motors mechanical pump. True?

2. Is it safe to run the boat with electric pumps off? I would have thought with the pumps off they would restrict or block flow of fuel...

3. Should I only use them when starting the engines to prime the mechanicals or should they stay on at all times?

thanks for input

Electric Diaphragm pumps will flow fuel as long as there is a higher pressure on its inlet vs its outlet.
Electric gear pumps or other positive displacement pumps wont flow unless they are spinning internally. I had a Carter gear pump years ago that you could push or pull fuel thru while "off", but it caused the rotors (and motor) to spin.

if you have block mounted mechanical pumps, id swap them out for a set of Teague/baker mechanical pumps. then ditch the electric pump setups.

I didn't have much luck with the Baker pump @ 850hp, but I know that other people have. I would ditch the mechanical pumps and install a pair of Weldon 1100A pumps. No more issues.

Whatever you replace, just remember to make sure it is a marinized / Coast Guard approved pump and not Automotive replacement.

Auto pumps do not have the same internal fail-safes required in a boat.

In addition to the above I like to run a second fuel line from the electric pump to after the mechanical pump with a check valve. And only run the electric as needed. The trick is remembering to turn it on :)
Of course this requires a T after your fuel filter.

I dont think theres really a need for multiple fuel pumps, especially multiple pumps of different design, in this application.

Sometimes, all the rigging , elbows, tees, and various fittings do more harm then good. Keep it simple.

Mercury Racing ran those six valve baker style pumps on their 800sc and 900sc engines. I ran them on my engines, and several others I know ran or did run them. There are several keys to keeping an engine fed with adequate fuel.

#1 pickup in tank. Must not be less than 3/8"
#2 fuel line diameter (i recommend -10 to inlet side of pump)
#3 fuel filter . Must be of a high flow design
#4 fuel fittings. Full flow fittings. Hard 90* bends are restrictive
#5 needle and seat size. In twin carb applications, this usually isnt an issue until you get up there in HP. In single carb applications, this can be critical.

Fuel pressure is just an reference. Just because fuel psi at idle may be 7psi, and 5psi at wot, doesnt mean the engine is starving for fuel. Your spark plugs and or 02 sensor will tell you that. I believe pro stock engines ran as little as 2psi of fuel psi going down the track. You simply want enough pressure to keep the bowls full of fuel. Its like superchargers. You can make 800hp with 8psi of boost. Then put a better set of heads or exhaust on, and now make 800hp with 4psi of boost. The restriction went down, so did the gauge psi. You can have undersized needle and seats, too small of jets, incorrect float level, and so on, and still see 7psi of fuel psi, except aluminum is blowing out your tailpipes.