www.iFourWinns.com

In Bill Kearney's "I hate crab pots" (V348/358) posting several of us indicated our starboard engine consumes more fuel than the port engine. That certainly is the case with my boat. When I mentioned it to the dealer, they just scratched their heads and said "hmph." I am not able to figure out a reason and it seemed like the other boaters were at a loss to explain it as well. Does anybody have any ideas on this? Does it happen on all 348/358s?

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2006 348 Vista
"some days you're the bug and some days you're the windshield"

Oddly.... on my 298, the port side burns more... substantially more. By the time the port gets to 1/2, the starboard is just below 3/4.

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"ROCKHOUND"
2005 Vista 298
Georgian Bay, Ontario.

Happened to my 288.Dealer replaced some kind of valve/vent on port side fuel tank and year later on stb side.Still OK It's known problem ,even the sales guy knew about it.Hope it helps.
Rich

My port side engine uses more fuel. When the starboard is on 3/4's, the port's just over 1/2 a tank.

Maybe there is a fuel intake adjustment that could be made to even them up? I understand that the injectors should all use an even amount of fuel but just a thought...

Also, I get a few more rpm out of the starboard engine (which drinks less fuel)... like 2-300 RPM more at WOT

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Stacy & Isora in Annapolis, MD
2005 Vista 298

I've never driven a twin engined boat so please excuse my ignorance; if one engine is turning 2-300 rpm faster than the other at WOT and it's outdrive is fitted with the same props as the other one, wouldn't the boat tend to steer towards the side where the props are turning the slowest? If it doesn't, then maybe the tachometers are not accurate? Much as the fuel gauges may not be accurate as well !

Graham

Exactly the same with mine and I know its not the tach's as it is confirmed by the sync gauge.
Sounds like our 298's have identical issues.

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"ROCKHOUND"
2005 Vista 298
Georgian Bay, Ontario.

Was it a matter of the vent being blocked and causing that engine to run leaner? Or the other way 'round?

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-Bill Kearney, 2005 348 Vista

I had the same issue on my 318 and the Tech at the dealership stated that the Generator draws from one side , he also stated that the Generator required at least a 1/4 tank to run

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King Of Battle
2007 318 Vista
2003 254 Monterey
2003 Fisher F/S

I think either Bill Kearney or I first mentioned this and I thought I would repost (with some edits) my original account of the problem under this thread title. Maybe someone else (the factory maybe??) will put all of our stories together and come up with a solution.

Here goes:

This has been a persistent problem ever since I bought the boat in 2004. Four Winns had no knowledge of such a problem and the Mercury representative couldn't figure it out either. I first noticed it when the starboard tank always took more gas than the port. My generator feeds off the starboard tank so I thought that was the reason, but it was the same even when I didn't run the generator. I just lived with it until a bent strut incident, but straightening the strut (and theoretically reducing friction on the shaft) didn't fix it either.

My trouble shooting was a little more detailed than just noticing how much gas it took to refill the tanks, though. My boat has the Mercruiser Smartcraft engine monitor system installed. This gave me the real time fuel burn for each engine. By setting each engine to the same RPM's, I would see a difference in the fuel burn between engines. The starboard engine always burns more fuel than the port at equal RPM's. At 3800 RPM, the port engine burns about 16.5 gph and the starboard is 17.4 or more. The difference gets greater as the engine speed increases. As a check, I tried setting the throttles so that the fuel burn (on the Smartcraft) was the same on each engine, but the RPM's between the engines is too great for the boat to run smoothly.

The only difference I can find between the two engines is in the transmissions. Four Winns runs both engines in the same direction and gets the counter-rotation in the gearboxes. Some manufacturers use a left hand/ right hand engine arrangement to achieve counter-rotation but FW runs both engines in the same direction with one gear box running in forward and the other in reverse. These ZF transmissions have a different gear ratio in forward and reverse. It's not much different (1.48 in one direction, 1.52 in the other). I don't know if this slight difference is enough to cause up to a 20 gallon (or more) difference in fuel burn between the two tanks, but Sea Ray, with a similar arrangement actually uses a 1/2'" difference in pitch between the port and starboard props to equalize the engine speeds.

Last year, we had a lightning strike in the area of the marina. It took out numerous battery chargers, electronics and in my case, both engine computers, the Smartcraft SC5000 display, my battery charger, and tank monitors. Because of the damage, I now have new engine comptuers and a new Mercruiser Vesselview display as well as a new battery charger and tank monitor. Mercury doesn't support the old Smartcraft SC5000 anymore so it was necessary to replace it with the Vesselview. My point here, is that the Vesselview doesn't give me individual fuel burn for each engine anymore, just a total for both engines.

Since both engine computers were replaced, that eliminated them from the mystery.

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Jerry
2004 378 Vista "Escapade", T-Mercruiser 8.1 SHO
2003 Maxum 3300SE
1987 Wellcraft San Tropez
1991 Chaparral 2550SX
1987 Bayliner 2150
!967 Caravelle 1751

I show a pretty-darn-close fuel burn with our 5.7 EVCs, but I'm always about 5 gallons more on the port tank than the starboard. Generator pulls off the starboard tank as well.

I just attribute it to the weight distribution/currents/wind, etc... As long as the difference doesn't seem to grow more than 5 gallons - I don't think there's much to worry about.

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2008 318 Vista -SOLD, but I am still around!
All Night Long, Seattle WA

I haven't noticed a difference yet. Might be reduced with the DTS?

Jerry, do you have inboards or stern drives?

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-Bill Kearney, 2005 348 Vista

I've got inboards.

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Jerry
2004 378 Vista "Escapade", T-Mercruiser 8.1 SHO
2003 Maxum 3300SE
1987 Wellcraft San Tropez
1991 Chaparral 2550SX
1987 Bayliner 2150
!967 Caravelle 1751

Same here, I'm guessing it'd be due to the transmissions as you suggest. But I'm going to give it a closer look this weekend (now that I have the boat back).

The only way I could see to check it would be to put a mark on the shafts and some sort of optical tach sensor to count the RPMs at a given engine RPM. Might not be all that difficult to do, provided you have the tach. Hmmm, here's one:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=66632

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-Bill Kearney, 2005 348 Vista

A twin engine boat provides a unique opportunity to compare the operating characteristics of two engines running ALMOST the same conditions at the same time. I can't think of another application except for a twin engine aircraft where this would occur.

It would be normal for two production engines of the same type to show a 1% or even up to 2% difference in fuel consumption while running the EXACT same conditions such as on an engine dyno. The main factor affecting fuel consumption is internal engine friction. If friction is higher, the engine has to work harder to make the same power so fuel consumption goes up. I could go into great detail about friction but the short story is, bearing clearances and piston ring to cylinder bore interface are two of the biggest contributors to friction. Tighter bearing clearances produce higher friction, normal manufacturing variation of the block and crank means that clearance varies from engine to engine. There are different bearing grades (thicker or thinner bearing shells) that are used to reduce the range of clearance but even engines within specification will vary. The piston and rings are also graded (larger and smaller sizes) to make up for variation in cylinder bore diameter. But beyond the bore diameter there are parameters for cylindricity and ovality that can vary and also affect the friction. Piston ring design is a tradeoff between sealing the combustion and blow-by versus friction. Better sealing typically means higher friction.

Engine operating temperature affects the oil viscosity. Higher viscosity (thicker) oil requires more work to pump through the engine. I haven't looked at the temperature-viscosity chart for SAE 30 wt, but at oil temps relating to coolant temps from 160 to 190 F I don't think it changes much. You have to get down to 20 deg F or lower oil temp for it to start getting thick. I'd think you would be more concerned about an engine problem with those temp differences rather than just fuel consumption. However, electronic fuel injection systems do use coolant temperature for fuel delivery. A colder engine gets more fuel, so the two factors could add up to a very slight difference if the engine coolant temps were different.

I've never been around a twin engine boat. Are there any other engine installation differences between the two engines? Does only one engine have the power steering pump?

Stern drive versus inboard could make some differences. Stern drive trim differences could put a different load on each engine and slightly change fuel consumption. For the inboards, different transmission ratio could slightly affect engine load between the two.

I can't give you any data except for the engine to engine friction variation, but just pointing out some potential differences. I don't think any of the other things would be more than a fraction of a percent fuel consumption difference but theoretically they could all add up to something measurable.

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Terry
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