Fuel Consumption on a 1999 Mercruiser 7.4 MPI 310 HP

[Stumpalump]

On my last carbureted 7.4 I temporarily hooked a vacuum guage to it and determined what RPM had the highest vac or best mpg. The guage does more than that because you can follow the drop in vacuum and get a range of high readings. At WOT it ran a click over 70 mph and vacuum was very low but at 50 the vacuum was just at the point of dropping fast. So my cruise was 28 mph for best milage and at 45 I was still getting good milage or high vacuum. Vacuum gauges are cheap and once you run it you can remove it and mark your tach or just memorize the point at which the vacuum falls off fast. Stay below that point at all times and you will always get good economy.

 

[Stumpalump]

Stock 7.4's need to be at the middle or lower WOT RPM for highest top end speed. They basically like to be on the lugging side like 4100-4200 to make them fly. That's fastest but it eats gas. Prop it for a higher RPM and slower top end speed and your cruise range and mpg will go up. We are only talking about one or two mph is all. Another thing you can do if your pulling skiers or slowing down constantly for no wake zones is to ventilate the prop. That makes it slip coming out of the hole and onto plane taking some load off of the engine. It also helps the out drive live a lot longer. A prop shop can add ventilation real easy and since it has no effect at speed it's a little like having a two speed prop. That's what the two stroke outboards do because they need to spin to get into the power band. It helps all engines but since slip is considered a bad word not many can wrap their heads around doing this or even talking about it.

 

[bruceb58]

I might look into installing a fuel flow meter , is it hard to install it?

It's easy if you attach it to a Sounder/GPS that you can network it to.

I have a Lowrance that I did it with.

 

[Stumpalump]

One more. Your gonna love this because it's free! If you run thru hull exhaust you need to remove the exhaust rubber tube in the lower unit. It's not really a bellows but some may call it that. When removed it frees up pressure or somthing behind the prop and is good for over a mile per hour and that means more mpg. Trim up the drive and look for it. 99% of thru hull exhaust boats still have it. It's not needed and it's costing you. It looks like this and is simple to remove:

 

[thumpar]

One more. Your gonna love this because it's free! If you run thru hull exhaust you need to remove the exhaust rubber tube in the lower unit. It's not really a bellows but some may call it that. When removed it frees up pressure or somthing behind the prop and is good for over a mile per hour and that means more mpg. Trim up the drive and look for it. 99% of thru hull exhaust boats still have it. It's not needed and it's costing you. It looks like this and is simple to remove:

That would be either the exhaust bellows or tube. It is not going to change anything.

 

[bruceb58]

That would be either the exhaust bellows or tube. It is not going to change anything.

Agreed.

 

[alldodge]

Also agree, If the exhaust is closed off and the boot is not removed there will be a slight vacuum at the rear of the prop. This vacuum is not enough to cause any change in speed but it can cause some gear lube loss around the shaft seal

 

[Stumpalump]

Whatever. It's a free mod. You can reverse it and it makes your boat faster. Have done it and seen others do it and yes it helps. You hear everthing won't help or is a waste of money all the time. The little things add up so if you can squeeze a few mph and better economy by doing all the things you are told that won't help then yes they do. File dings on prop, sand bottom remove a few pounds ect. It all adds up. A championship drag racer was repacking his front wheel bearings one night and people wanted to know why he would waste his time on somthing that only drives 1/4 mile. Reason is that everthing helps and a free 2 minute mod like this boot not performed is not performed by a fool.

 

[HT32BSX115]

It's easy if you attach it to a Sounder/GPS that you can network it to.

I have a Lowrance that I did it with.

This is the one I have

Connect it to a Lowrance NET (NMEA2000 network) it'll display all sorts of things!

Mine even displays my Bennett Tabs position

 

[bruceb58]

This is the one I have

Yep...that too. perfect thing to replace a trim gauge when the sender fails again on a Volvo. Of course you have to have a GPS module on the network in order to get MPG displayed besides just the actual fuel flow.

 

[BigDog98]

This is the one I have

Connect it to a Lowrance NET (NMEA2000 network) it'll display all sorts of things!

Mine even displays my Bennett Tabs position

I am really confused on how this whole system works , I was looking for a fuel flow the other night and came across this exact one, I don't understand what a Lowrance NET is or how it works, don't you still need to connect something to the motor in order for the gauge to show the fuel burn? Is there any other thread that goes into detail with this?

 

[HT32BSX115]

I don't understand what a Lowrance NET is or how it works, don't you still need to connect something to the motor in order for the gauge to show the fuel burn? Is there any other thread that goes into detail with this?

Well, this would obviously be a good thread for the "Electrical, Electronics, Audio and Trolling Motors section.



Lowrance NET is a networking system for boats...It's based on the NMEA 2000 "protocol" that many other manufacturers have produced equipment for.

Essentially, you still need sensors connected for whatever you want displayed. (like oil press, temp, RPM, fuel flow etc.

The above sensors are simply connected to the network (all of them) Then the display can be the gage I indicated or ANY radar, fish-finder or other type display that is connected to the network.

I have that gage above and a Lowrance HDS-5 (the earlier non-touch model) so I can display everything that the little gage will display OR on the HDS if I want. Using the little gage, you have to "page" through each gage. The HDS will display a "page" with all or certain gages shown etc.

I bought the little gage first and used it in a stand alone configuration. Now that I have the HDS-5, I don't need the little gage anymore..... So I'll probably remove it

The nice thing about the NMEA2000 network, is you can add or remove sensors without affecting any of the other "stuff" in the network

If there's a LOT of sensors in the back of the boat for example, to add more you only need to install the sensor and connect it to the network in the back. only one cable is needed between the front and back f the boat.

Eventually, all your gages will be a tablet! or your fishfinder! (or both!)

 

[bruceb58]

For this to work as just a fuel flow meter, you would need the gauge and the fuel flow sensor on the network. If you wanted it to display in MPG you would need the gps module too on the network.

I have the Sounder/GPS, GPS Module and the Fuel Sensor.

The fuel flow sensor goes in the fuel line.

 

[Stumpalump]

Issue with a flow meter is you have to tap into the fuel line. Fine for diesel but I don't like any extra places a fuel line could leak in a boat. Adding a fuel sniffer covers you.

 

[bruceb58]

Issue with a flow meter is you have to tap into the fuel line. Fine for diesel but I don't like any extra places a fuel line could leak in a boat. Adding a fuel sniffer covers you.

It's in the line on the suction side of the pump. Not a huge worry.

 

[HT32BSX115]

It's in the line on the suction side of the pump. Not a huge worry.

Yeah. I put mine between the fuel filter and the fuel pump. (you don't want unfiltered fuel going through that sender)

For this to work as just a fuel flow meter, you would need the gauge and the fuel flow sensor on the network. If you wanted it to display in MPG you would need the gps module too on the network.

I have the Sounder/GPS, GPS Module and the Fuel Sensor.

The fuel flow sensor goes in the fuel line.

My set up is similar to the above except add fuel level and Bennett trim-tab sensors........I do not have the GPS module. (my HDS-5 has GPS built-in)

 

[bruceb58]

.I do not have the GPS module. (my HDS-5 has GPS built-in)

I bought mine very early on. I think I was required to have an external GPS module when I bought mine. Was difficult figuring everything out back then since everything was so new.

 

[HT32BSX115]

What color are your network fittings? red or blue? I think mine are blue. Which means I have the older "stuff" They changed everything requiring adapters so you can use the newer "stuff" Don't know why they did that. But it happened right after I bought my set up. Had to use the adapter to hook it up to the newer HDS-5

 

[bruceb58]

Mine are blue. I bought mine almost 10 years ago

 

[yotis]

My 24 footer with single 7.4L used an average of 6 gallons per hour for the entire season (50 hours or so and 300 gallons used at about $1,000 total cost) this includes idling, no wake speeds, water sports, cruising, etc

Hello,

I realize this is an old post, but maybe somebody still check it.

I have the same engine and I saw your statement which I have to say I'm pleasant surprised by the decent GPH consumption you post here.

If I say I know nothing about boat engines that would be too much , please bear with me.

Surprise comes because this information says something else:
MerCruiser 4.5 mpi v6 Fuel Consumption 250 HP | MPG GPH Test & Specs US Gallons per Hour (boat-fuel-economy.com) - select the 7.4 one.

According to this page if I have about 2500 RPM is already 9.9 GPH. Not sure thou if I should trust the information there.

Also maybe a dumb question, but since this is my first boat and only like a week old at the time of writing this, do I hurt the engine / fuel consumption (in a negative way) if I just take the boat out at around 1500 RPM (area is Pacific Ocean, Southern California, Marina del Rey and surroundings).
I'm not yet confident to just speed it up.

Thank you!