Fuel Flow meters on 7.4L

[paulrfrancisco]

I just installed a pair of the Northstar F210 Fuel flow meters on my twin 7.4L motors. I have not run the motors outside of the slip as yet, only at idle and with no load on them to verify they work. Anyone wanna give me a guess of what these motors (individually) will burn in Gallons per hour? Its a 30' VHull weighing in at 9500 dry with Bravo 1/Stainless props. I hope to take it tonight for the real test...


Thanks

 

[jaxnjil]

Re: Fuel Flow meters on 7.4L

mine runs around 2.5 to 3.5 cold idle and after it warms up and has run a little it idles at around 1.8
the most it seems to run at wot is 22 gph and this is consistent. i'm guessing this is the max the q-jet can flow

 

[Mischief Managed]

Re: Fuel Flow meters on 7.4L

I just put a Lowrance LMF-200 in my boat and it drinks 13 GPH at 32-33 MPH cruise with a single 7.4 and a heavy load. Have not measured the WOT GPH yet, but I'd guess it's 27 GPH based on the HP rating, the fact that my boat tops out at about 46 MPH with a heavy load, and that drag increases with the sqaure of speed.

jaxnjill,

Somethings amiss if your 454 is only drinking 22 GPH at WOT, that's only what you'd use to make 250 Propshaft HP. Are you at high altitude or running well below 4600 RPM?

Edit:

Never mind, just looked at your profile; CO would be high altitude.

 

[paulrfrancisco]

Re: Fuel Flow meters on 7.4L

Mmmmm... Thanks for the replies... I need to double check what the settings are defaulted to. Maybe mine is set to Liters as it was idleing at 4.8-6.1. Hopefully its Liters... I'll find out tonight and let you know.

 

[BAYLINER185]

Re: Fuel Flow meters on 7.4L

Im glad I saw this post I have about the same set up as yours paul. Mine is a 1993 BAYLINER 2755 w/ 7.4 and BRAVO ONE drive.

I have no idea how much gas I consume all I know is I burn about 40 to 50 bucks worth each time I go out! and now with the price of gas I get a lot less for the money.

Id like to know your numbers when you figure it out just so I would have a place to start if I ever figure GPH on my boat.

I know all engines run and operate differantly due to use but its a good baseline I suppose.

 

[paulrfrancisco]

Re: Fuel Flow meters on 7.4L

Well, as I suspected, the meters were set to Liters... So once I reset it to US Gallons, it was idling at 1.7, 1.8 GPH. Numbers that make me feel a whole lot better than in my previous post.. I still havn't had a change tyo take it out and run it at a nice cruise speed, (3000-3300 rpms) and at WOT. I'll re-post when I get those numbers as well... (Also this is per engine)

 

[jaxnjil]

Re: Fuel Flow meters on 7.4L

I just put a Lowrance LMF-200 in my boat and it drinks 13 GPH at 32-33 MPH cruise with a single 7.4 and a heavy load. Have not measured the WOT GPH yet, but I'd guess it's 27 GPH based on the HP rating, the fact that my boat tops out at about 46 MPH with a heavy load, and that drag increases with the sqaure of speed.

jaxnjill,

Somethings amiss if your 454 is only drinking 22 GPH at WOT, that's only what you'd use to make 250 Propshaft HP. Are you at high altitude or running well below 4600 RPM?

Edit:

Never mind, just looked at your profile; CO would be high altitude.

answer you any way. yes on both. the lowest elevation is at powell and that was around 3600' and now at flaming gorge and it's around 6021'
i'm going to have to prop down. i was running 4200 at powell and on warmer afternoon with a little more weight that was droping down to 41 to 4000.
at 28 to 31mph my numbers are close to yours at 10 gph,very hard to get-only early in the morning and realy have to play with the trim and tabs, to 11.5 to 13 with a 13 gph at 28 mph on still water pretty eazy. throw in some wind, chop,and some house boat wakes and that gets to 15 to 16 gph pretty quick
best speed was 4200 rpm and 43 gpsmph at 22.something gph with full tank of fuel no water 1 cooler and my 70 lb pup a few cloths and all safty equipment
my drive is a 1.62 to 1 changed from a 1.32 to 1 stock
the boat and trailer weight 7000lb full fuel so i'm guessing boat at 5800 to 6200

 

[bruceb58]

Re: Fuel Flow meters on 7.4L

I have a 24' Wellcraft cuddy with a 7.4L Volvo Duo-Prop. The engine is carbed. I have the Lowrance flow meter which integrates with my GPS to give me MPG as well as GPH. At between 30 and 35MPH which is between 3000 and 3500 RPM, I get around 3.0 MPG in that speed range. If I go slower than that, I actually get less. A previous poster said that drag goes up with velocity. I think that depends on the hull as I think my hull drag goes down when I get going in the 30's.

 

[Mischief Managed]

Re: Fuel Flow meters on 7.4L

I have a 24' Wellcraft cuddy with a 7.4L Volvo Duo-Prop. The engine is carbed. I have the Lowrance flow meter which integrates with my GPS to give me MPG as well as GPH. At between 30 and 35MPH which is between 3000 and 3500 RPM, I get around 3.0 MPG in that speed range. If I go slower than that, I actually get less. A previous poster said that drag goes up with velocity. I think that depends on the hull as I think my hull drag goes down when I get going in the 30's.

There's a speed where wetted surface plays more of a role in drag than speed. When you are on plane, but slow, the boat has a lot more wetted surface than it does with higher speeds. Get over that speed and efficiency will climb, to a point. Once the wetted surface becomes fairly stable (meaning faster speeds don't really lift the hull that much further out of the water) , the drive and prop create the lion's share of the drag and efficiency starts to drop with speed. Get less of the prop and drive in the water and efficiency/speed will increase. This is why jack plates, messing with the X-dimension, and surface drives can have excellent results increasing speed.

 

[paulrfrancisco]

Re: Fuel Flow meters on 7.4L

UPDATE...

Well I finally was able to get out this weekend and figured my cruising numbers... @ 3200-3300 rpms, doing 27 mph in calm seas, I was burning 13.8 gph in one motor and 15.5 in the other motor... At WOT, it was 19.4 and 20.8 gph respectively

Question, is it normal to see that much variance between motors, and is it reasonable to assume that thay can be adjusted to consume the same rate of fuel w/o any noticable difference in performance? I also know that I will be replacing the carbs with freshly remanufactured carbs. So should I expect better numbers or the same variance in the numbers?

ps... not sure, now that I've installed these, that I want to know these numbers... especially with the price of gas. That little jaunt cost close to $80.00... Yikes!!!

 

[Mischief Managed]

Re: Fuel Flow meters on 7.4L

Assuming it was as hot where you were as it was in NH, you should be much closer to 25-26 GPH at WOT. I assume your max RPM was well below 4600. That's the first thing I would address. Your engines were only making about 220 to 240 HP. Dirty bottom perhaps?

Even with 85 degree air, and 29.79 inches Hg barometric pressure, I was able to get mine to 4500 RPM and 25.5 GPH yesterday at 500 feet above sea level. That's within .5 GPH of what I'd expect (96.5% of rated HP x .087 GPH) My gas is left over from last October so I would expect a little less than optimal performance.

 

[paulrfrancisco]

Re: Fuel Flow meters on 7.4L

Could the fact that my carbs are original 1990 QJets and I know they have the leaky plugs in the base problem, and maybe just simply operating sub par, be a reason for the lower gph? I think my rpms are approx 4300... My bottom is clean, props stainless and true... I will be replacing the carbs anyways, so I suspect It'll perform differently then...

 

[Mischief Managed]

Re: Fuel Flow meters on 7.4L

Are the carbs leaking air or fuel?

If air, be very careful; you are running very lean.

If fuel, I'd the think the GPH would increase...

4300 is in the ballpark for a loaded down boat, but if you were running with a light load, you'd be better off if the boat could get to the upper end of it's WOT RPM range.

Are the engines in a proper state of tune?

 

[paulrfrancisco]

Re: Fuel Flow meters on 7.4L

Mischeif, On the QJets, there is a jet plug in the base of the carb that corrodes over time and as a result will leak the bowl out when sitting. So when starting cold, the bowls have leaked empty and it takes a while to start... after running and hot, if you sit for a while, (1-2 hours), they start like its flooded. You need to put the throttles wide open to start it ... so, the carbs are tired, and cause of their age, I'm thinking they could also be running a tad lean robbing me of power and if the gph numbers you site are real for 330hp @ WOT, then maybe they are indeed running a little lean... BTW, the boat is pretty heavily loaded... Other than these two issues, they perform reasonably well, no hesitation, or bog down, or sputtering. I'm still inclined to replace them with reman'd carbs anyway...