Re-powered with Mercury 8.1S Horizons

[PlayDate]

I have a 1997 Sea Ray 370 EC that I just repowered. It went from 7.4 to the 8.1S Horizons. My boat holds 350 Gal. of fuel. Cruise speed for me is around 25 MPH and I get 100 Miles out of the boat before I have to re-fuel :smt013 I have had the props pulled 3 seperate times and just about every mechanic here in brevard county look at this. No one kind find anything wrong. I love my boat and 2 days ago I had to list her :smt009. If anyone reading this has repowered with 8.1 horizons could you please please chime in and tell me what your getting for performance. I have been a member for quit some time here but never posted before. I NEED HELP :smt101. I would love to take her back off the market. Thank you

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

 

[terminaldegree]

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.

 

[Southpaw II]

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

 

[tbirdvet]

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#.

 

[Jimmy Buoy]

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.

 

[PlayDate]

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

 

[West32]

Did you ever resolve your issues?

 

[RollerCoastr]

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

 

[blaster]

He probably sold the boat as his resolution.