outriggers

What surprises will I have? Will the 7.4 fuel system support it? I think its 3/8 ID line with no return,fuel tank pick up? Wiring harness? Thanks Doug

stevesxm

if i understand you you are going from carb to EFI ? smart move and having just done all that this is what i have found.

the efi harness supplied with my 502's was plug and play.

the existing fuel system was plumbed badly w/ with 3/8 lines but for 425 hp, 3/8 is adequate. if the motors have the fuel cooler set up then you dont need a return line... it is built into the system... but if it does not then the sytem has to have a free return to the tank or it will seriously overpressure.

also... the small merc pumps as supplied w/ my motors make 200 % the volume that the motors need so no upgrade needed but bad news is that they are vane type pushers and don't " suck" very well especially if the tank is deep in the boat.. on mine i built two 2 quart surge tanks and located them higher than the merc pump. i fed each one w/ a dedicated carter pump from the primary tanks... this pump does 2 times the volume nec at WOT. that tank has a top return to the fuel tank so it sees no pressure but provides the merc pump w/ 2 quarts of gravity feed fuel regardless of what the boat is doing. i also rerouted the merc return line that normally went back into the filter housing into this tank as well... i didn't like the idea of already pumped once hot fuel being dumped back into the stream directly...

then i flicked the switches and it started up and purred like a kitten. you are going to LOVE efi after carbs...

Pat McPherson

I like that idea.
I'm putting that on my to do list for the spring...

outriggers

Will 3/8 be ok for 470HP? I haven't done so well every time I was told something bolts right in. Thanks for the help. Doug

stevesxm

yes to the fuel line.

the critical paremeter is unit measure/ unit time vs engine demand at WOT. if your motor uses 60 gallons / hour then thats all the pump/lines etc HAVE to deliver... but... what you design for is a 100 % safty factor so the primary delivery pump to the motor has to be able to make twice that at operating pressure ( not free flow) . the stad merc pump as supplied w/ the efi will do that... nice pump.

the line you run has to have a flow rate of what your engine needs at WOT at THAT operating pressure... if memory serves, a -6 line will do about 100 or 120 gals/hour at 50 psi ( my ref books are 3500 miles away)

but at any rate it is easy to know emprically... you mount the pressure gage sensor in the fuel rail and run the boat. it needs to rock steady at WOT if it ever drops at all, that tells you that demand exceeds supply.... so you give it more supply... your fuel consumption numbers will come from the WOT runs on the dyno.

as to the plug and play.... my boat is a 96 top gun... had beater blown up carb 454s in it when i got it ... all the wiring in the egine bay was , to say the least , shabby... i didn't believe for an instant what merc told me that i could take an efi harness and plug it straight in so i wrung out the pin outs according to what the factory manual wiring diagram said.... and god damn if they wern't right.... plugged em in, and absolutely everything worked except the oil temp gages ... no sending units...

i am the unltimate cynic... and this was as simple as it gets.

they key is to stick w/ factory original stuff FIRST. make it start and run perfectly on the factory stuff ... THEN start screwing around... then when it DOESN'T run anymore or runs badly, you have a usable base line to return to.

with the current state of efi, i don't know why anyone would run a carb for any reason...

Dave F

3/8 line will be fine, it will support over 525hp.