Fuel consumption port vs starbord engine

[daddy d]

1989 390 express with 7.4 340 hp gas powered, hurth trans cc port & star

I have owned the boat for 3 years, the port engine has always consumed about 20 % more fuel than the starbord engine. without operating the jenny, both engines are tuned up and run great, both props have been refurbished, carbs are original, both chokes are fully open when at operating temp, ignition timing is set the same, spark plugs have good color and look alike! :huh: Any thoughts!

 

[Ricochet]

daddy d

I have same set up in a 1990 390 express and have the same issue, not the 20%, more like 8% +/-. After 10 years I have just gotten used to it.

 

[Sr420]

1989 390 express with 7.4 340 hp gas powered, hurth trans cc port & star

I have owned the boat for 3 years, the port engine has always consumed about 20 % more fuel than the starbord engine. without operating the jenny, both engines are tuned up and run great, both props have been refurbished, carbs are original, both chokes are fully open when at operating temp, ignition timing is set the same, spark plugs have good color and look alike! :huh: Any thoughts!

Same issue on Sundancer 420, twin 8.1 gas merc's. more like 8% though.
My thought is that since both engines face same direction, both transmissions face same direction one transmission has to be running in reverse because the props rotate in different directions, right? I suspect gear ratio's may not be exactly the same in forward and reverse.... Does this float?

 

[Posh Garage]

This floats for a bit until you consider the engines are at the same RPM. Be it the Sync meter, tachs matching, or audible drone they are running the same speed before the transmissions. If off you would be able to feel the pull to one side, no?

 

[PlayDate]

On my boat the Starboard engine uses more fuel than the Port engine. The physics are the same for both engines. Both engines are within a few serial numbers of each other, same carb, ignition and plugs. Cylinder compression is also close.

So the only thing left is the torque load on the engine. When the boat is out of the water, turning the Starboard prop has always had slightly more resistance than the Port side. Even after replacing the shafts, new cutlass bearings and shaft alignment.

Sure, the Starboard throttle is a little more advanced than the Port throttle to maintain engine sync at cruise but it isn't worth losing sleep over.

 

[Posh Garage]

Just got off the phone with Alan at Simplicity. As I was going over the specs page with him regarding the Hurth transmissions I asked what A vs B was in the rpm section. Alan explained the reverse gearing is different than forward. Knowing that these transmissions operate on standard rotating engines and all you do to make counter rotating props is mount the shift cables differently it would mean one transmission is running at a different gear ratio than the other.
Problem I have is that if you sync the engines you aren't syncing the props.

 

[PlayDate]

I think it is fair to say that a .01 ratio difference is immaterial between the A and B positions on a Hurth 630 transmission.

If one transmission (engine speed 3500 rpm) is spinning at 2,333 and the other (engine speed 3500 rpm) is at 2,317 there is virtually no effect on either engine sync, fuel consumption or prop speed.

 

[sandydlc]

Following...my port engine seems to consume about 12-17% more fuel than starboard. We did recently flip our dinghy so that the engine is on the port side since when we had the engine on the starboard side we had a starboard list. Now that we have the dinghy engine on the port side I'll monitor the fuel consumption to see if there continues to be a difference. Checking back to our fuel logs from when we first purchased the boat, the consumption was within a few gallons. We ran the first 5 months with no dinghy which is why I'm pointing the finger at the dinghy as the potential cause for the lopsided fuel consumption.

Any other ideas?