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need to install fuel filters how many gph should I need to go with I have 7.4 mpi gassers

40,000,000

Or 60 if you're never going to goose it.

1962 31,
If I remember correctly adding additional Racor fuel filters will reduce the flow and possibly cause fuel starvation especially on multi-port fuel injected big blocks. I was thinking of doing the same thing to my 7.4L Merc throttle bodied fuel injected motors and I was advised against doing it. I think that Ed Murrays old express which was carbed had additional spin on/off filters in the fuel supply line. My big block Merc's came with spin on/off canister filters that are mounted just forward of the inboard exhaust manifolds.

Maybe Bruce will chime in about this subject, but if I remember correctly it's not advisable.

Pete Fallon

pete that makes sense, my motors only have the inline metal ones similar to automotive filters was thinking some water separation would be good.. Ill wait till the higher ups chime In on the subject lol

Every filter is designed for a certain capacity (through flow)… Going one size up of the recommended is only going to increase the filter change interval (more "buffer" to fill up before you HAVE to change the filter if you will)… Every filter will restrict the flow, even when brand new and perfectly clean… Therefore, adding filters in series is NOT recommended… Having said that, especially on gas engines, adding a pre-filter cum water separator to the standard filter IS recommended… Adding filters in parallel is the same thing as increasing the the capacity… Critical installations, e.g. naval craft, will have double filters separated with a T valve… One filter is being used while another is ready to be used at the turn of the T valve…

Talking about capacity, gas engines rarely if ever have a fuel return line, i.e. all the fuel flowing through the filter will be consumed by the engine… HOWEVER, all diesel engines will have a return line… In fact, with diesel engines MOST of the fuel flowing through the engine will be returned to the tank through this return line… What this basically means that if your gas engine consumes, at most, 25 gallon/hour then you'll be good with a filter that states a capacity of 30-35 gallon/hour (having a bigger filter e.g. 40-50 gallon/hour is advisable)… However, on a diesel engine which consumes an identical maximum 25 gallon/hour you'll need a fuel filter with a capacity of AT LEAST 75-100 gallon/hour…

Check the documentation of your engine, it should state the minimum capacity of any fuel filter… Take that minimum and increase it by at least 50% and you'll be good with room to spare...

mine have returns and it returns a lot learned that running it home with fuel bladder... what a mess ill look into the specs on the motor

Hopefully Bruce will jump in, but If possible, I would eliminate the inline automotive style and add the spin on fuel/water separator if you can. Suprised a spin on was not added on to the engine from the factory.

I always understood Multi Port Injection has to have a return from fuel rail due to pumps high pressure to cool and keep pump at a constant flow. If you were dead set on adding more filter media a pusher pump might be needed to overcome extra fuel flow resistance but I don't know if I would do this on a marine app. Yep, Bruce would be the yay or ney to your adding more filtration. Navatech I like that idea of your redundant system with T- valves sounds like something a aviation application would have.

Depending on the brand of mpi you could have a high flow return or a recirculating type of return.

The high flow could push the flow rate to 70 gallon per hour or higher. Like what already has been suggested, look at the marinizers specs and remember gas filters below deck or inclosed areas cannot have a see thru bowl for safety.

Many mpi already have dual filters on the engine, one on the suction side and one one the high pressure side. An extra filter before engine on the suction side as long as the micron rating does not restrict flow, never hurt anything.

Rocky wrote:Navatech I like that idea of your redundant system with T- valves sounds like something a aviation application would have.

It's not my idea at all… It's a common setup in naval and maritime applications…

Yes, modern automotive gas systems may also have a return line… Either way, the manufacturer should provide the minimum required through flow for filters…

Use a primary filter that has a large particle size (lower micron size) and a finer secondary filter (higher micron size to avoid restriction. One filter catching all of it plugs up faster.

Rawleigh wrote:Use a primary filter that has a large particle size (lower micron size) and a finer secondary filter (higher micron size to avoid restriction. One filter catching all of it plugs up faster.

With all due respect, I disagree… I look upon the engine's filter (the secondary filter as you put it) as my last ditch defense… Filter elements are cheap and it's easy to know when you need to replace them if you have a vacuum gauge on the housing… I will replace the element of the "secondary" every 3rd time I replace my Racor element…

I don't call my Racor my primary because I have a centrifugal water/crud separator upstream of my Racor… Both the razor and the "secondary" filter have the smallest particle size *2 micron IIRC)… Overhauling the high pressure fuel items (injection pump and injectors) is both a PITA and a major cost… Far more then my filter elements which I buy in bulk…