outlawinil

I was wondering if the standard 3/8" hose that fed the 496 HO was big enough for the gas line to feed the 525 EFI and I was told to go with 1/2" ID hose all the way to the tank. When I took the hose off at the tank I discovered the hose barb and the 90 degree elbow screwed into the tank are restricted down to only 5/16" ID. Do I replace them with bigger? It don't look like those fittings will come out of the tank as the are directly underneath the back wall of the rear seat framework and front engine compartment wall. They only have about 3/4" room to go up. Suggestions besides somehow sliding the gas tank forward? There is two identical fittings there with only one being used...can I draw from both to feed a 1/2" fuel line? It's a 2003 288 Sunsation and I assume the other fitting may have been for a twin engine setup?
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thirdchildhood

I'm in the process of repowering to a 525 from a 496 also. My 525 came with a 3/8 fuel fitting and the seller told me that was all it needed so I'm sticking with the original 3/8 hose.

stevesxm

without having my ref book handy, my recollection is that a 5/16 tube will flow 15 gal per MINUTE at 30 psi. thats 900 gal per hour.

you would have to have a pretty amazing boat to have a fuel consumption of 900 gal per hour....

this is a commonly misunderstood area essentially like putting on huge oil lines. flow rate in a system is defined by the output of the pump and the loss at the other end...

if the pump produces 300 gal per hour at 30 psi and your motor only uses a 100 gal per hour at WOT, then you need a line size that will deliver a minimum of 100 gph at 30 psi. but you always put in one twice that size to allow for drag and fittings etc. what you don't need is 10 times the size.

i keep saying this and keep being told that i'm crazy... but the factory guys that drew the spec on your fuel system were pretty smart and not prone to dumb mistakes like putting fuel lines on that were too small. go bigger if it makes you feel better but unless you are making a 1000 hp or so , you are probably wasting your time.

and the math always shows that they were right...

Raylar

I think your reference calculation is for pushing fuel through a line not sucking it up like a fuel inlet line to a pump will do.
Most newer boats have an anti-siphon fitting on the tank and these can restrict line size down to 5/16" or even 1/4" internal size and this can be very restrictive, so be careful to upsize this fitting if possible.
One sure way to see if the inlet line is adequate is to read the fuel pressure at the fuel rail at wide open throttle and load and if it drops more than 2-3 lbs from manufacturers desired fuel pump pressure then you could have a problem as when this pressure drops it means the fuel (sucking and pumping system) is having a problem keeping up.
Another good idea would be to call the guys at Sunsation and ask their rigging department how they rig the boat for an HP525 engine when the boat is so equipped.

Regards,
RAY @ RAYLAR

stevesxm

well... yes and no. the cross sectional area of the tube is what defines the flow rate at any given pressure... that is to say a tube of given area flows the rate whether pumping or sucking. in a lift situation, you are actually somewhat better off using the smallest pipe that will flow the volume you need. the " lift" of a pump is a function of the weight of the column of fluid that it is trying to raise. the vacuum that is created above it is a fixed number and, as such, will lift a fixed weight. the larger you make the tube, the greater that weight right up until the vacuum can't lift it. the practical example would be you sucking water up a typical straw vs you trying to suck it up thru a 2 inch pipe. the trick with all of these fuel systems is working backwards and actually designing a system that is appropriate to the demand. that starts with the gals or lbs per hour at X pressure at WOT number. That's your pump rating plus 25 % . then its just a matter of working the line sizes backwards and allowing for all the hardware in the middle and then making certain that the lift capabolity is adequate. in the race cars it was a matter of putting four submerged pumps in the fuel cell that fed a central accumulator that was above the level of the hi pressure pump. that way the hi press pump was never trying to do two jobs. on my boat, i use a typical low pressure carter pump as a lift pump to supply a 2 qt accumulator that gravity feeds the hi press pump. the carter delivers twice the volume i need at wot and circulates back to the tank. that way the hi press pump never runs dry or has to do anything other than provide 38 psi at about 75 gal per hour. no individual component works very hard. nothing gets hot. everything has a huge flow margin.

the line size is , essentially, completely irrelevant as long as it isn't something stupid small. and putting bigger ones woudn't change a thing.

the first question you ask is " how many gals per hour does this motor demand at what pressure per hour. the answer there is the answer for everything else.

outlawinil

I own a used Sunsation...no luck