Re-powered with Mercury 8.1S Horizons

D

DeltaEx

New Member
Oct 8, 2008
3
Merritt Island, Florida
Boat Info
1998 SR 370 EC
Engines
8.1 Horizon S
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
 
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
 
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.
 
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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.
 
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?
 
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
 
mistercomputerman said:
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?
Click to expand...

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
 
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.
 
Henry Boyd said:
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
Click to expand...


Correct. Total burn for all engines.

I have put about 150 hours on the new motors this season and have a Floscan, so the numbers are pretty close.
 
eman369 said:
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.
Click to expand...


Is it just me? :huh:
 
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...
 
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