Fuel Consumption on a 1999 Mercruiser 7.4 MPI 310 HP

[BigDog98]

Looking for advice on how to lower fuel burn on a 7.4 L merc , according to my research on average there should be 20-25g and hour burning depending on the conditions and load at WOT. Any advice on getting it lower besides dual prop? And what should be max rpm on a 7.4 ? the boat weighs 7500 dry. Does 4100 - 4400 sound about right? Thanks

 

[tpenfield]

The RPM at WOT is in the spec range (4200-4600). With 7500 lbs of boat, versus a single 7.4 (I assume), there is not much that you can do. As long as the engine is well tuned, which isn't probably is, you could try to reduce weight or move some weight forward to get an easier planing speed.

A 4 blade prop might also help.

I have twin 7.4L MPI engines on my 33 foot boat (10,000 lbs loaded) and the engines sip fuel because of the favorable length versus weight.

 

[alldodge]

For an engine to run at it's optimal level is to have an air fuel ratio of about 15:1 mix. If the mix goes above 15:1 the engine runs lean and lower it runs rich. Ted has the points mentioned on how to get better economy. To determine if your not loosing due to prop slip enter the numbers into bblades prop calculator to see what your slip is.

http://bblades.com/propeller-slip-calculator/

 

[BigDog98]

The RPM at WOT is in the spec range (4200-4600). With 7500 lbs of boat, versus a single 7.4 (I assume), there is not much that you can do. As long as the engine is well tuned, which isn't probably is, you could try to reduce weight or move some weight forward to get an easier planing speed.

A 4 blade prop might also help.

I have twin 7.4L MPI engines on my 33 foot boat (10,000 lbs loaded) and the engines sip fuel because of the favorable length versus weight.

Do you use marine gas or regular fuel from a gas station and how much is a gallon of gas where you live. also how much money on gas do you spend every time you go out?

 

[tpenfield]

Do you use marine gas or regular fuel from a gas station and how much is a gallon of gas where you live. also how much money on gas do you spend every time you go out?

I usually get about 1/2 of my fuel during the season from the gas stations and the rest from marinas.

I don't know how much a gallon of gas costs in my area, because the price keeps going down

I keep track of my fuel usage throughout the season and then can figure gallons per hour, cost per gallon, etc.

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

My 33 footer with twin 7.4L engines uses about 7.5 gallons per hour average throughout the season with similar usage, except a limited amount of tubing only, no wakeboarding. Seasonal costs are $1,200 ish

The most fuel efficient speed for the 24 footer was around 3K RPM, which is pretty typical of most boats, and I estimate it was using 12-15 gph.

I estimate that the 33 footer with twins uses about 18-20 gph at similar speed.

Have you actually measured your fuel usage with this boat? Tell us about the boat too

 

[BigDog98]

I usually get about 1/2 of my fuel during the season from the gas stations and the rest from marinas.

I don't know how much a gallon of gas costs in my area, because the price keeps going down

I keep track of my fuel usage throughout the season and then can figure gallons per hour, cost per gallon, etc.

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

My 33 footer with twin 7.4L engines uses about 7.5 gallons per hour average throughout the season with similar usage, except a limited amount of tubing only, no wakeboarding. Seasonal costs are $1,200 ish

The most fuel efficient speed for the 24 footer was around 3K RPM, which is pretty typical of most boats, and I estimate it was using 12-15 gph.

I estimate that the 33 footer with twins uses about 18-20 gph at similar speed.

Have you actually measured your fuel usage with this boat? Tell us about the boat too

Well that is the problem , you probably boat in a lake vs me i boat in the ocean with different current and condition where that matter. I dont know exactly how much this boat burn because i am taking on delivery for it on April but it is a 1999 Sea Ray Sundancer 270 with about 7500 dry weigh and i would guess close to 8500 pounds with 2 or 3 people on board with a full tank of fuel and water and gear including scuba tanks. It has a duel stainless prop which to my knowledge is suppose to burn less fuel and its a bravo 3 which makes it even better. I had a 26 footer before and its dry weight was 6200 i believe maybe less and it had a 5.7 carb motor vs this 7.4 mpi engine which is suppose to be already better because it has injectors. I know that with my 5.7 i was able to dump 5 gallons in and take a emergency 5 gallon tank and it would last me at about 2500 - 3000 rpm for a few hours and i dont mean running it non stop.

 

[tpenfield]

I boat in the ocean . . . Cape Cod . . . very choppy

 

[mr 88]

The mpi and carburetor will get the same fuel milage. The mpi starts easier and may idle a bit smoother. It takes X amount of fuel to produce y amount of hp regardless of how it gets to the cylinder.Once there it all burns the same .

 

[Scott Danforth]

Looking for advice on how to lower fuel burn on a 7.4 L merc , according to my research on average there should be 20-25g and hour burning depending on the conditions and load at WOT. Any advice on getting it lower besides dual prop? And what should be max rpm on a 7.4 ? the boat weighs 7500 dry. Does 4100 - 4400 sound about right? Thanks

Run at idle 's WOT and you burn less fuel. In other words, require less HP. Fuel consumption for a boat motor is about .35 pounds of fuel per hp hour.

 

[BigDog98]

The mpi and carburetor will get the same fuel milage. The mpi starts easier and may idle a bit smoother. It takes X amount of fuel to produce y amount of hp regardless of how it gets to the cylinder.Once there it all burns the same .

What is better MPI or Carb? I know for a fact that a MPI motor you can scan and its suppose to be more reliable.

 

[Scott Danforth]

What is better MPI or Carb? I know for a fact that a MPI motor you can scan and its suppose to be more reliable.

For fuel consumption it DOESN'T matter. The motor burns the same amount of fuel per power generated at the crank.

 

[BigDog98]

For fuel consumption it DOESN'T matter. The motor burns the same amount of fuel per power generated at the crank.

Right i understand that but is it true that a MPI is not as loud as a carb and easier to maintain? therefore saying MPI is better?

 

[mr 88]

Do not see how it can be any quieter unless you are constantly flooring it and then you may hear a bit more noise out of the carburetor as the plates open up.Other than that all the noise is going out the exhaust and they are the same on both versions.MPI is newer and you have to deal with injectors and all related electronics-computer vs a carb rebuild that you may do once a decade,depending on how you maintain it.So it may be more costly to maintain than the carb in the long run.Not saying anything is right or wrong with MPI ,just saying it really isn't anymore efficient or better than a carb at any speed.

 

[Scott Danforth]

Right i understand that but is it true that a MPI is not as loud as a carb and easier to maintain? therefore saying MPI is better?

Mechanical noise would be identical. Intake noise is a function of the type of spark arrestor. Exhaust noise is a function of the exhaust system. An MPI system has more maintenance systems and is more complicated than a carb. So which is better.... Depends. If you change altitudes often, MPI compensates, carb does not. An MPI system can be timed for a less tempermental cold start, and easier off-idle characteristics. A carb makes same power. A carb is easy to diagnose and repair

 

[alldodge]

For WOT a carb will beat and MPI for speed, don't know about GPH. I'll take an MPI any day over a carb for cruise and mid range performance

 

[HT32BSX115]

Looking for advice on how to lower fuel burn on a 7.4 L merc , according to my research on average there should be 20-25g and hour burning depending on the conditions and load at WOT. Any advice on getting it lower besides dual prop? And what should be max rpm on a 7.4 ? the boat weighs 7500 dry. Does 4100 - 4400 sound about right? Thanks

Howdy,


You're not far off on what a 454 should burn at WOT. However, no one runs (or should run) any marine engine at WOT for any length of time if they want any sort of longevity.

I have a 454/Bravo III in my 21ft Four Winns (211 Liberator) The boat weighs about 3000lbs and the engine and drive (according to Mercruiser) around 1200lbs making the empty weight approx 4200lbs.

Add 44 gal of fuel (264lb) and 4 (200lb) people it comes in at about 5000lbs in round numbers.

Weight in any boat is critical because it can take significantly more power to maintain a particular planing speed (or achieve a plane) as weight increases, all the way up to a point where the boat will not plane with the available power. (assuming that weight is distributed for the optimum center of gravity in that hull)

Mercury (and just about all other marine manufacturers) recommend operating the engine BELOW the suggested maximum RPM range and for best performance, selecting a prop(and drive ratio) that will result in the engine turning max RPM of the range. (for my engine 4600RPM)

Can't say what yours should turn without knowing the serial number of the engine. but, the range for many of the 7.4L EFI engines is 4600-5000....... so if yours is an EFI 454, it should be propped to turn around 5000 when loaded where you will normally operate the boat. (your rev limiter is going to prevent the engine from turning much above the max RPM + approx 100)


Now having said all the above, mine will turn about 4600 at WOT. I also have a fuel flow (Lowrance) indicator......... running at 3000 RPM (@ 30mph) it indicates about 10gal per hour. It also will indicate about 15gph at 45mph.......WOT indicated around 23-25gph.

Does 4100 - 4400 sound about right? Thanks

not for an EFI 7.4L Mercruiser with a sterndrive connected. (there is an MIE carbureted 7.4 rated at 310hp that had an RPM range of 4000-4400)

Post the serial number of what you have and someone here can probably look up the ratings on your engine if you cannot find it yourself. I have several service manuals that indicate the specs on various Merc bigblock engines. I didn't see any EFI (sterndrive) bigblocks that had RPM ranges that went as low as 4100.......

Regardless of what you do with yours (assuming you're propped correctly)..........your fuel flow will still be wholly dependent on position of your right hand (on the throttle!!)

Regards,


Rick

 

[thumpar]

Just relax the right hand and you will save a bunch on fuel. I run my 5.7l EFI at about 3200rpm for cruise.

 

[tpenfield]

IIRC, the 'standard 7.4L MPI is rated for 4200-4600 top RPM and the 454 MPI 'Mag' is rated for the higher range of 4600-5000.

I believe the OP is referring to the std 7.4L MPI @ 310 HP, which is actually a bit watered down from the earlier carb'ed versions that were rated @ 330 HP.

FWIW - a friend of mine has a 1996 Sea Ray 270 with the carb'ed 7.4L and Bravo 3 It takes a bit to get it going. So, you might be running it in the 3500-4000 RPM range to keep it on plane. Twin engine versions of that boat are much better. So, plan on using a bit of fuel with the single.

The MPI is a nice feature over carb's for starting, and you might get slightly better fuel economy, but it is a negligible difference, if any. It won't turn a duck into a swan.

 

[bruceb58]

With the Wellcraft boat in my signature, I get slightly less than 3 MPG anywhere between 2800 and 3800RPM. I am using a fuel flow meter that is coupled with my GPS to calculate the mileage. Amazingly, it is very close to the same fuel usage through that entire RPM range. My engine is carb'd.

Unless you have a flow meter, it is pretty difficult to pinpoint you best fuel usage.

 

[BigDog98]

With the Wellcraft boat in my signature, I get slightly less than 3 MPG anywhere between 2800 and 3800RPM. I am using a fuel flow meter that is coupled with my GPS to calculate the mileage. Amazingly, it is very close to the same fuel usage through that entire RPM range. My engine is carb'd.

Unless you have a flow meter, it is pretty difficult to pinpoint you best fuel usage.

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