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Old 03-21-2015, 04:06 PM   #1
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Need to select right Maratron Fuel Flow

I want to install a Maratron Fuel Flow System on the boat. I have DD6-71s that are 450HP. Fuel lines that feed them are 1/2". The sensors come with either 1/4" or 3/8" ports, the smaller for lower horse power engines.
Here's where it gets tricky. I want to be able to monitor fuel flow when the boat is only running 7-9MPH (she's a planning boat cruises at 21Knt) as well as the higher cruising speed. I contacted Maratron support and they said to use the smaller 1/4" sensor. I have great reservations about this as I just can't see this restriction working.

Does anyone have a system installed rated at this horsepower that can read accurately from the low end of 1GPH up to the normal cruising consumption of 16-18GPH.
The lower end is important as I'm looking to install this in preparation for the Loop this summer.
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Old 03-21-2015, 04:24 PM   #2
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http://www.parker.com/literature/Hos..._Technical.pdf

You might have to set up your fuel system that the gauge is only online when you want to go slow.
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Old 03-21-2015, 05:04 PM   #3
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Thx for the chart, I've determined in my mind that there is no way you can get the flow required through a 1/4" line.
The M2RSP actually is rated 4-132 GPH
Which is the 3/8" unit.
What I'm hoping is that the 6-71 at 700RPM actually free flows fuel at a higher rate than the lower 4GPH. If this is the case then it should be able to calculate the difference between the incoming and return down to the level I need.
I just can't get Maratron support to commit to anything I ask.

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Old 03-21-2015, 05:38 PM   #4
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Pipe two in parallel, isolate one when in slower speed.
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Old 03-21-2015, 05:43 PM   #5
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What is your rated flow rates on the filters? A 3/8" line holds .005 gallon of water per foot, a 1/4" line holds roughly half that amount at .00235 gallon per foot. The velocity of the fuel will increase in the smaller line.
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Old 03-21-2015, 06:12 PM   #6
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Pipe two in parallel, isolate one when in slower speed.
Yes that is a possibility. I don't want this unless I absolutely have no choice.

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Old 03-21-2015, 06:14 PM   #7
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What is your rated flow rates on the filters? A 3/8" line holds .005 gallon of water per foot, a 1/4" line holds roughly half that amount at .00235 gallon per foot. The velocity of the fuel will increase in the smaller line.
dan
I'm not sure , it's the stock filter assembly that's on a J&T DD.

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Old 03-21-2015, 06:34 PM   #8
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Have you posted this to yachtforums? Lot's of knowledgeable people there, not to say there aren't any on TF, but it can't hurt to get more input.


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Old 03-21-2015, 07:12 PM   #9
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I could, but I prefer to ask here.

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Old 03-21-2015, 08:22 PM   #10
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Boatdiesel might be another source of information. I installed a Floscan system and their tech support was less than helpful.


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Old 03-21-2015, 09:09 PM   #11
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Take off the return line and time/measure into a 5 gallon container. You will no way get a DD, even at idle to flow less than 4 GPH (IMO). I have the 8V71 and I believe I return around 40 GPH at 1450 rpm.
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Old 03-21-2015, 10:08 PM   #12
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I'm not sure , it's the stock filter assembly that's on a J&T DD.

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I don't see how that matters. You know the filters will flow enough fuel at any rpm.

You either have to have two senders, one for low speed and one for high, or just the one for low speed that you can bypass at high speed. Just make an rpm/GPH/speed chart for your high end and use the meter to tune for your low end.

Have you checked what Floscan has to offer?
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Old 03-21-2015, 10:39 PM   #13
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You do not "know if the filters will flow enough fuel at any RPM" if the filters are being restricted by an undersized fuel line. Considering the flow will be on the no pressurized intake line you would not want to restrict the size of the line going to the filters. If there is presently a 1/2 or 3/8 line providing fuel to the filter you would not want to place a 1/4 in front or behind the filters. Velocity on that side of the fuel pump is much slower with a negative or suction pressure of less that 3.5psi.
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Old 03-21-2015, 10:52 PM   #14
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Quote:
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You do not "know if the filters will flow enough fuel at any RPM" if the filters are being restricted by an undersized fuel line. Considering the flow will be on the no pressurized intake line you would not want to restrict the size of the line going to the filters. If there is presently a 1/2 or 3/8 line providing fuel to the filter you would not want to place a 1/4 in front or behind the filters. Velocity on that side of the fuel pump is much slower with a negative or suction pressure of less that 3.5psi.
I was talking about the system as is. Not if you restrict it. Obviously the flow will be restricted if you introduce a restriction in the line.

It all boils done to the fact that there is no way you are going to get a 1/4" line to flow enough fuel at all rpms.
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Old 03-21-2015, 10:59 PM   #15
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What's the rated flow range on the smaller sensor?

What's your min and max fuel draw (consumption plus return)?

What's your min and max consumption?

I'd pick the sensor that fits that range. Keep in mind that a 1/4" sensor will not pose the same restriction as changing your fuel like to 1/4". Back pressure from a pipe or hose is per-foot, and the sensor is pretty short. If it's rated for the flow you need, then I wouldn't worry too much about it. Just step down and back up immediately before and after the sensor.
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Old 03-21-2015, 11:42 PM   #16
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Page 6

http://floscan.com/html/manuals/preinstall.pdf
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Old 03-23-2015, 11:50 PM   #17
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I belive DD's like the 6-71 actually move a LOT more fuel that they burn. I cannot tell you how much but I seem to remember several times.

Talk to a mechanic or hopefully the pumped rate is in your manual or your dealer can tell you from their manuals.
The extra fuel is used to lube and COOL the injectors.

That higher figure is what you need to size for.
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Old 03-24-2015, 10:04 PM   #18
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What's the rated flow range on the smaller sensor?

What's your min and max fuel draw (consumption plus return)?

What's your min and max consumption?

I'd pick the sensor that fits that range. Keep in mind that a 1/4" sensor will not pose the same restriction as changing your fuel like to 1/4". Back pressure from a pipe or hose is per-foot, and the sensor is pretty short. If it's rated for the flow you need, then I wouldn't worry too much about it. Just step down and back up immediately before and after the sensor.
The two sensors that Maretron have available are (small) M1RSP-2R-E8 rated @ .5 - 26.5 GPH the next size M2RSP-2R-E8 rated @ 6.6 - 132 GPH.
Both sensors appear to have the same port size 1/4" . The smaller rated for 200 HP the larger 450 HP.

DD 6-71 fuel pump moves 90 GPH. Can't get a spec for low RPM flow.
The full load fuel consumption @2500RPM is 24 GPH.

Therefore based on this information you'd think the M2R would be the rated sensor. I just can't get my head around the restriction in line size of 1/2" down to 1/4". I understand that it is a short distance but I'm not convinced.
Again Maretron has stated go to the web site and based on your spec. you choose what you need. We won't get involved in the decision.
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Old 03-24-2015, 11:30 PM   #19
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What size are the hoses and pipes on the engine itself that the same fuel flows through? On most engines I've seen they are much smaller than the main supply and return hoses back to the tank. That might be a way to get comfortable with the different sizes.

90GPH is only 1.5 GPM. That's a slow water faucet.
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Old 03-25-2015, 09:36 AM   #20
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Yea, take a look at the fittings on the engine itself. Lots of places with the ID 1/4" or less.

You reall need to measure your actual flow rate before you order these. Do the five gal bucket and stopwatch test. Do it at low speed and at high.

Note that the return on the DD is a drilled orifice fitting on the head. Once fuel pressure is at pump relief valve setting, the return rate is constant regardless of load.

So you can do this with a fuel pressure gauge on secondary: Rev up to where fuel pressure plateaus, then measure return rate in bucket. Additional rpm above that will not increase return flow. And return flow will not vary with load unless load causes a drop in rail pressure.

Not a big fan of these flowmeters, although I have never used the OP's brand. You have two instruments with a certain accuracy measuring two numbers relatively close together, and subtracting them to get the flow. The inaccuracy compounds in the subtraction.
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