Fuel Flow Rate

[SavDawg8]

Hey Guys,

Looking at installing a water/fuel separator on the boat before spring and wanted to make sure I got a filter with the correct flow rate for my motor. I've google searched and can't seem to come up with anything...

I have a 1997 Johnson 112 2-stroke.
Does anyone have this motor or something similar that knows what the flow rate is?

I'm looking at a 10 micron Racor separator with a 60 gph filter.

[fitz73222]

Using the well established 10 % of rated HP rule, the most fuel your engine will burn at full throttle is about 11.2 GPH. This is a time tested calculation for older 2 stroke engines. You could run three of your engines at one time on that filter and not even come close to rated GPH of the filter.

[Levi]

What was mentioned above is partially correct . Fuel demand IS 10% of rated horsepower HOWEVER you ALWAYS want to build in a safety factor of at least 200% for your fuel filter since as your filter gets full of crud it's flow capacity will diminish and you don't want to starve the engine of fuel.
I HIGHLY RECOMMEND that you get a 2 micron Racor instead of the standard 10 micron filter. This will keep all the <10 micron trash out of your carburetors and you'll do WAY less maintenance aside from changing that filter a little more frequently.
You want your fuel to be as clean as it can possibly be BEFORE it goes to your engine because onboard fuel filters are expensive and Racors even the 2 micron ones are CHEAP by comparison.
FYI I just dropped $140 on fuel filters for my suzuki because the guy that owned it before me had a 10 micron filter ahead of the engine. HOPEFULLY I don't have the hemorrhage another $150 to have all 6 injectors cleaned.

[wingtime]

2 micron may cause too much restriction for some motors. 10 micron is plenty fine for a carburetor motor

[SavDawg8]

Thank Yall.
Went with the 10 micron 30GPH by Racor.

[fitz73222]

 Good choice, 2 micron is too small and could cause a lean condition and burn a piston if it starts to clog. Good thing about Racor filters is the pleat inside the filter is designed to swell and stop fuel flow if the filter is full of water so you know that there is an issue. I've been running 10 micron filters for 40 years in all engines and never had an issue.

[Levi]

My 2 micron for my 225 is 60 gph flow so there's no restriction until it gets REALLY gummed up. Also an important thing to note is that the flow rate on a given filter  is measured by the same amount of head pressure whether they are 10 microns or 2 micron so there's no added vacuum/ restriction between the filter and the motor with a 2 micron . Also carbed engines don't NEED the finer filtration but they do benefit from it (PARTICULARLY the idle transfer ports) but a the newer fuel injected outboards have super fine filters built into components like the injectors vst pump high pressure pumps on hpdi's, optimax's, and etecs that are EXTREMELY hard to get to or replace and expensive some of these filter CANNOT be replaced and you must replace the entire component.
So all things being equal yes a 10 micron is ok but a 2 micron catches ALL the crap in your fuel BEFORE it goes into your engine costs $3 to $5 more and saves you money in onboard filters and components.

[GoneFission]

Mercury does NOT recommend placing a filter upstream of the engine on an Optimax!  Several warnings in the rigging manual about that.  Mercury does not want any restriction in the fuel flow to the engine.  The Optimax has a built-in fuel/water separator with a warning buzzer and a notification on the Smartgauge tach.  When I got my new Optimax, I was going to keep the existing fuel filter/water separator, but the installer insisted it needed to go.  Showed me in the tech manual and in the rigging manual where Mercury says to remove any upstream filters to ensure full, consistent fuel flow. 

I don't know if mounting a filter upstream of an Optimax would void the warranty, but going against the recommended rigging could always raise a question... 

[wingtime]

DFI engines are very sensitive about fuel flow. In fact the Etecs have a maximum allowable vacuum for the fuel flow.