Cummins QSB 5.9 fuel flow calibration

[Teck Foo James]

I have a pair of QSB 5.9 355 hp. 476 hours. Both engines have different consumption figures at same RPM. Both props were recently de pitched because the previous owner’s log recorded WOT at 2650 RPM 26.5 knots. Had the prop pitched reduced by 1.5 inch (could be a mistake) and now WOT 2890 RPM 26.5 knots. Props are 4 bladed 21 x 24.5. At 2600RPM 24 knots.

With the great difference in fuel consumption, it is difficult to determine which is correct. The Stbd consumption figures are much lower compared to the manufacturer’s consumption curve but the port engine consumption is way off the charts. The attached photos show the difference in figures. The LPH reading is the same as the Cummins round gauges.

How do I go about to verify the flow accuracy?

Thanks

 

Topic

[Teck Foo James]

Some updates

Installed the vacuum gauges, reading normal
Check engine rpm reading on Smartcraft with handheld tachometer – results 2 rpm difference.
Check gearbox output shaft rpm with handheld tachometer -less than 0.3% difference.
Pulled out the prop to check the pitch by the prop shop that re pitched the prop – result – both prop has same pitch reading
Not happy with the prop shop, went to another prop shop – pitch the same but the cups on the port side were missing!
Added some cupping to props but prop shop guy says he can’t measure the cups best he could do was eye balling the cups.
Result of the cupping – port engine now loading more and starboard engine running cooler.
The differential fuel consumption between both engine at WOT Port 62 Stbd 67.
Ordered a pair of new props from Nakashima..hopefully it will improve the performance.

[Teck Foo James]

Ordered a pair of vacuum gauge from Sbmar.

[Clark Leighs]

Vacuum can be measured by the use of vacuum guages installed immediately after  the off engine fuel filters using a TEE fitting.

Go to Seaboards menu above , page top >  Fuel systems and filters > Filters > vacuum guages   and look at the guages presented.   They are telltale types which will show the highest level reached on a run  which in your case would be at WOT again. The telltale can be read later back at the dock.   The most they should show is ~ 8″Hg.   Even that figure would indicate the filters are reaching the max permissable clogging.       If much over then the filters could trick the computer into a false fuel use reading.

If the filters are truly new and the vac. is high then you  could have other issues such as a clogging pick up tube, collapsing or plugging hose fuel feed hose,  plugged tank vent.

But first do the following.

You could simply change the filters, ALL of them , and do the run again to see what the fuel use reading  is. Relatively cheap, with a quick answer.

However, those guages would tell you in the future when the filters are approaching change out time BEFORE you have performance issues.

[Teck Foo James]

Tony,

Thanks for the reply. With the present prop at WOT 2870 rpm.

I prefer the actual consumption figures to be below factory rating too.

Will swap fuel lines between P n S

Have not done vacuum test on the fuel line. If I draw fuel from the line the fuel will continue to flow, how to do that test?

The fuel water separators and inline fuel filters were changed recently, about 5 running hours ago, the result are the same.

James

 

[Tony Athens]

#1———-Your engines, when REALLY propped correctly, need to reach the governor break point of 2870-2880 RPM, and at exactly 2800 RPM your load should be under 100% & your GPH should be under 18GPH/68 LPH.

Factory numbers below for an easy comparison–It’s alway good to below these numbers, but not above as to fuel consumption per RPM, which is really LOAD.

Your number are quite a bit different and would either suspect very different props and of a high fuel vacuum reading on one or both engines screwing with the computer–Ever measure fuel vacuum where you fuel line connects to the engine?

 

Tony

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