Something is very wrong with your fuel burn.... What is your RPM numbers? Cruise RPM vs speed and WOT vs speed? What props are you running and did you change them from the 7.4's you were running? You can put a scan tool on those motors and see actual fuel burn rates, has this been done? I am asuming the easy stuff has been done like bottem and props are clean. What kind of speed/RPM were you getting before the motors were changed?

When you say you have to re-fuel, how many gallons are you actually putting in?

 

Are you saying you're burning 87gph?

 

This all comes down to WOT rpm.
If you get rated rpm for your new motors..the props are correct.
Too much rpm you need bigger or greater pitched props.
Cruise rpm should be 75% of max avail. rpm.
Of course all clean bottom and true scanned props.

 

DeltaEx,

My boat is lighter than yours, but my 8.1s', at 3400 RPM, use 15 GPH per side and get 25 MPH. That's with full fuel, full water and 4 people aboard...

As previous posters have asked, what is your WOT RPM? What is your bottom/running gear condition? That will help determine if you're propped correctly.

I'm with Turtle - I can't imagine that you are using 80+GPH at cruise.

 

Yes there is something wrong. I have a single 496 that is the 8.1 liter engine with a sterndrive. But I have done quite a bit of experimenting and recording of fuel consumption. It is the same engine with the same torque curve, so the performance should be similar.

To achieve 25 mph on our 280 rpm is at 3600 and fuel burn (when trimmed) is abt 18 gph. However go to and above 4000 rpm and the burn rate almost doubles - 30 gph.

So for a single 496 @ 25 mph/ 3600rpm 100 miles is after 4 hours, or 4 x 18 = 72 gallons. For two engines that would be 144 gallons.

To go through a tank, call it 320 gallons in 4 hours, that would require a total burn rate of 80 gph, or 40 per motor.

To do that you would have to have the engines firewalled above 4000rpm and have the prop pitch gear ratio completely out of whack.

Have you tried comparing the prop and transmission ratio data on the 7.4 370 and a newer model with the 8.1s?

Henry

 

Henry,

Yes - at WOT, my burn is like yours - 30 GPH per side...

 

WOW 40 gallons per hour! Ok I will tell I only have a single engine and I must be geared awsome after an oops, I recalculated I burn 5 gallons hour 80 gallon tank last or is that love me long time I think I should not complain with you all and your credit card melting fill-ups for just one 80 mile trip oh and on plane 2800 rpms speed is 27mph gps, WOT is 4200 36mph gps I am not as fast as alot of you but dang. maby I should be happy where I am. I was going to adjust my throtle to open more then again maby..... nawww I am going to do it.

 

WOW 40 gallons per hour! Ok I will tell I only have a single engine and I must be geared awsome after an oops, I recalculated I burn 5 gallons hour 80 gallon tank last or is that love me long time I think I should not complain with you all and your credit card melting fill-ups for just one 80 mile trip oh and on plane 2800 rpms speed is 27mph gps, WOT is 4200 36mph gps I am not as fast as alot of you but dang. maby I should be happy where I am. I was going to adjust my throtle to open more then again maby..... nawww I am going to do it.

Is it just me? :huh:

 

I have a friend with that exact same set up yr and everything docked beside me. He burns nothing near that in fuel, having said that cruise speed is 19-21 mph at 3200 rpm....mine cruises at same speed but at 2850 rpm (due to running oversized props). he burns about just a few gph less than me due to EFI. Mine is around 18-20 gph depending on conditions...I would start with gear ratio. There is a big difference in it from 7.4 to 8.1 do to the torque.

 

oh and that is per side as well....

 

We need more info. WOT RPMs, GPS top speed, cruise RPM, and cruise speed from GPS. I don't think it is even possible for a stock 8.1 to burn 40 gph.

If you have no easy way to measure fuel flow, I suggest the following: Completely fill your fuel tank, and then go for a 30 minute cruise at your normal cruise speed as per GPS. Then, come back and see how much fuel it takes to completely fill the tank. You will then know your MPGs at cruise speed.

You should be able to cruise at approx 25 mph, and burn no more than a combined 35 gph. MPG at cruise should be at least 0.7.

 

8.1 Horizon (370HP)
410 DA

This is from memory, but, reasonably close. GPH is total for both motors.

3600rpm cruise = 32 gph = 20 knot cruise
4000rpm cruise = 44 gph = 23 knot cruise
4600rpm WOT = 65+ gph = 26 knot cruise

Are you riding around at Wide Open Throttle with dirty bottom and improperly pitched props?

 

Rick,

That's total burn right?

Since that is roughly the same for my single, Carter's twins, and your twins on three different hull forms with 3 different weights that means the variables have to be gear ratio and prop size pitch. I know on the gas sterndrive line up there are basically two ratios 2.0 and 2.2. Does anyone know how varied the gearing is on the v drive transmissions? If it is fairly stable than the OP may only be looking at re-propping.


Henry

 

I just looked at the Sea Ray site for prop chart info. They don't list prop and transmission ratios for anything above the 320. For the 370 you would have to contact a dealer (or Sea Ray itself). My suggestion would be to contact them and get the prop pitch and transmission ratios for your boat as built with the 7.4s. Then get the prop and transmission info for the 370 EC (or as close a model as possible) with twin 8.1s. The 8.1 was introduced in 2001, and definitely in the SR line up by 2002. If there are differences in prop size and pitch, or transmission ratio between the two, then you know where to start.

I'd also think about pulling your sales ad. The first thing a surveyor is probably going to ask you is whether you adjusted the gear ratio/propping as soon as he lays eyes on those two new bad boys. If you don't have a reasonable answer the assumption will be the worst; that it needs both new/regeared transmissions and props.

Henry

 

Mine has the 2:2 ratio BIII and 26p props. I can get approx 15GPH at 3450-3500 all day long. I have seen the GPH hit 20+ before. Scary. Rick's 3600 RPM burn matches mine. Right around 16 GPH.

 

Nope.

 

A 370 with 8.1s should scream. We moved from a 370 Express to a much heavier boat that weighs more than 19,000 pounds with full fuel tank, holding tank, water and gear. The hole shot is like a sports car compared to the 370. At 3500 RPMs we are going 31 mph and have a 200 mile range. WOT is 4600. Top speed is 41 mph.

 

I posted some numbers for our '03 360 DA in another thread. The boat is somewhat similar in size, weight, and dimensions, though I know the EC is a straight inboard vs the DA with v-drives.

As was my recollection:
4 adults, normal gear
half fuel, half water, calm sea
chain windlass, newish bottom paint
stock propellers (dimensions unknown), 4-bladed
2.5:1 gear reduction (note: on our '98 370 DA 7.4 MPI's the trasmissions were 2:1)
3230 rpm (per smartcraft)
22.1 mph (2 gps readings)
27.8 gph (both engines total)
WOT rpm is 4650 (per smartcraft)

With 250 gallons of fuel, that would translate to roughly 200 statue miles of range. Sorry to hear you're having problems...

 

I'm not sure where to start. The easiest way to sort this out is to install a fuel flow monitor such as Floscan. Then you will have the ability to match fuel consumption to rpms. My engines are different than yours but we are moving roughly the same weight. At 3200 rpm, 4 people on board and with 170 gallons of fuel and 25 gallons of water I am burning 19 gph on each engine (38 gph total). Depending on conditions and how clean the bottom is, the boat is running about 24 to 25 mph or about .63 gallons per mile. Bump up to 3400 and the secondaries on my Holley 800s kick in and fuel consumption goes to 25 gallons per hour on each engine with a 2 mph increase (.52). I presume your 8.1s are EFI but the same concept generally applies (higher rpms = high fuel consumption).

A general rule of thumb for gas engines (which is remarkably accurate) is that it takes 10 gph for each 100 horsepower applied. So using 200 horsepower per engine translates into 20 gph per engine.

I'm sorry you had to put her up for sale. However, I am curious what RPM you were running. I'm also curious where the 8.1s came from. Are they new or were they pulled from another boat?

Regardless, a fuel flow monitor will quickly sort this out. Higher fuel consumption at lower RPMS means the engines are using a lot of horsepower to turn the props. That could mean the props are not a good match for your new engines.

-John

 

Hi John,

I'm surprised your fuel burn is so high at that speed- is the carb'ed version of that motor really so much more thirsty than the MPI from a few years later? Our old 370 DA would use roughly 12 gph per engine at 3000 rpm on the floscans (the MPI 7.4's).

Sorry for the diversion from the original topic...for what it's worth, that older boat had the "twinscan" version of the Floscan meter and it was fantastic... easy to read, accurate, reliable.

 

I would have to check my Floscan (I have the same Twinscan that you used to have) at 3000 rpms....if memory serves me right I believe that is 15 gph per engine but the speed drops to 21-22 mph.

Not sure what happened to DeltaEx......he hasn't responded to any of these postings.....

-John

 

OK, the 370 EC is a great boat, and the hull/engine/prop combo is different from anything else, almost. Going from a 7.4 to a 8.1 is great...the only better combo in this hull wouls be the 3116 CAT setup. Anyway, remember, this is a straight inboard boat that is 14' wide. Changing to the 8.1 without changing anything else would first require a prop change because the 8.1 has more torque than the 7.4; and I would guess maybe 1 or 2 more inches of pitch. This would make this a much more efficient cruise setup. This is a huge boat for 2 people, and I would love to have one with diesels.

Don

 

I run 15 GPH per engine at 3500 RPM and 30 MPH with full fuel and water an two people. My boat at this setup is about 18000#.

 

My 340 Amberjack is a very similar hull design to your 370 EC (as you will see from the signature photo), and has a dry weight of 16500 lbs. Even to that, 275 gallons in the tanks, full water tank, and enough supplies for a week, she runs 25mph at 3500 rpm and will top out at 36 mph at 4500 rpm. Fuel usage at 25 mph is about .8 mpg with 8.1 horizons. I'm quite satisfied with this performance. :thumbsup:

Although you are 3' longer, 6" wider and a bit heavier, you have something wrong to generate your performance numbers. :huh:

Please respond with more information when able.

 

Did you ever resolve your issues?

 

He hasn't signed-in in over a decade. I'm starting to think he might not come back...

 

He probably sold the boat as his resolution.