fuel difference issue

[raymond gonzalez]

I have 2 2007qsb 5.9’s in a formula 40pc with2 125 gal tanks

they both run great and have about 800 hours on them and have seen a trip tis year from nc to ny without a hiccup

issue im seeing is an up to 1gph difference between the port and starboard engine at cruise speed …starboard is the one the shows as using more. no performance issues at all … all other data is similar between the 2 and when at low speed the numbers ( gph) all match up perfectly. I would say the difference starts once the turbos kick in. and then stay that way all the way through cruise ..

I want to see if I can get any info on this and see what could be the cause of the difference .

ironically the analogue and digital fuel gauges are off also … example .. this weekend port engine was at 1/2 and starboard was between 1/2 and 1/4 . when I wen to fill up they both t reached full with the same amount of fuel ( I have 2 125 gal tanks )

any assistance would be great ..

Topic

[Tony Athens]

Engine Performance tolerances

From my own experience, I have yet to find two engines in a vessel that operate much closer that you have.. Under some conditions they something seem very even, and then they don’t under others….Look at the attached pic & always work that into the equation..

I have learned this is be true with most all of the operating parameters on the digital screen.. Besides fwd & rev gear ratios (Clutch Pos A vs. Pos B) are not always exact, you have slightly more gear loss when running thru a extra lay shaft, small imperfections in props, gauge / sending unit tolerances, etc etc.. It all adds up to where I do not believe it’s realistic to see perfect matches between Port & Starboard engines.

Tony

[Clark Leighs]

Yes, any difference between the props can account for this. There could be a tiny bit of a ding/tweak that would be tough to spot visually.

[raymond gonzalez]

what am i scanning for correct pitch ?

[Rob Schepis]

So starboard boost is higher than port, that confirms that the starboard engine is seeing more load. If you want to delve into this further, since she’ll be hauled for winter layup soon enough, pull the props for scanning…

[raymond gonzalez]

rob boost is
port : 2690 rpm 19.4
star: 2670 rpm 20.9

[raymond gonzalez]

Bill .. sorry for the delay …

TRANSMISSION
TYPE: V-drive.
MANUFACTURER: ZF, Model ZF-85-IV-2.0.
SERIAL NUMBERS:
Port: 19164
Stbd.: 19160
GEAR RATIO: A=2.01, B=2.00

[Bill Fuller]

Raymond,

Lets have a little brain fun with your situation. I will explain later. In the mean time would you mind posting a picture of the tag on one or both transmissions. Or at least both of the gear ratios shown on the tags.
We will do some calculations and see what sort of differences we come up with just do to the different ratios.

Will be fun.

Bill F.

[raymond gonzalez]

well the previous filter change was done before i bought the boat so not sure of any change sine then .. i guess will have to see when i do the filter changes … as far as the boost will take pics this weekend and post here … thanks

[Rob Schepis]

“Not a new development”….So the fuel burn delta has stayed constant through previous filter changes? Then that would eliminate a change in fuel vacuum as the culprit. Do you display Boost?

[Bill Desmarais]

As Bill F brought up.

Maybe the prop on the affected engine has a “bit more bite” than the opposite prop causing
a bit more load on the engine @ the same rpms.

[raymond gonzalez]

rob, not a new development it showed up in the numbers from the survey that we got when i was buying it … again .. its even when idle and going up t about 1400 rmp then when the turbos kick and we get up to cruise thats when you see the diff … not a great big deal but 1gal per hr is a difference …

ill try changing the last chance filter as you suggested and see what comes of it since its almost that time of year to change everything before she goes to sleep .. lol

if you guys have any other thoughts let me knwo thanks .

[Bill Desmarais]

I’m always learning Bill !

The electronic engine’s ECMs track everything !

Us (me) guys that run mechanical engines would never know the difference of fuel burn between two engines unless FloScans were installed.

Before I’m dead I think I remove the 6BTA 370 in the 25 footer and bolt on a QSB 380 so I can track all this stuff ! LOL

[Rob Schepis]

Ray, is this a new development?
Have the props “touched” anything?
Try changing the last chance filters and see if the readings get closer..

[Bill Fuller]

Bill D.

I think it would be the opposite. Starboard trans has power run through 2 shafts to get the rotation opposite the engine. Port trans has power run through 3 shafts to get the same rotation as the engine. This was also my first thought.

But, the transmission does not have the same ratio in both directions, which could contribute to the issue. Plus there is that old say that no two motors are the same. Possible reasons could go on and on. Some people adust the pitch of one pro to help make up the differences.

My suggestion is just run it.

Bill F.

[Bill Desmarais]

I’ll take a stab at the question.

Are there ZF220A gears on the back? Or other?

My “guess” is that the starboard engine is using a bit more fuel than the port because of the gear spinning clockwise (reverse) to push the boat forward. Uses a bit more fuel

The port motor gear is turning counter clockwise (forward) same as the engine. Less fuel required.

Regards, Bill D

[raymond gonzalez]

smartcraft pics attached

[Author]

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