View Full Version : Are these RPM and fuel burn numbers reasonable?
05-07-2003, 02:35 AM
Looking for some expertise.
Got a 61 PA-18 with a 150 HP O-320 and an 8241 Borer prop. Engine has about 700 SFOH in 1997 and compressions are upper 70s. Static RPM is about 2325 and I routinely burn 10 gal/hr leaned 50 rich of peak EGT at sea level.
From what I hear and read I wonder if my RPMs aren't a little low and my fuel burn a little high. If so, what could be causing this and how can I fix it? (I know, I know, just throw $$ at it).
05-07-2003, 03:04 AM
What are you cruising at? You give static rpm but no cruise numbers.
05-07-2003, 08:18 AM
Joe........I have a 59 with the same prop. At 2450 RPM cruise I burn 8.0 - 8.4 gph average.
05-07-2003, 08:40 AM
Check the Lycoming owner's manual, which contains plots of fuel consumption (GPH) vs engine speed.
The variables that come into play when you are doing your own fuel consumption test include 1) different rates of fuel burn for taxi, takeoff, climb, descent, and landing [minimize this error by testing during long cross country flights. I have found lots of variability when I do consumption tests on short flights.], 2) variability in gas pumps [minimize this by refueling from the same pump], 3) variability in refueling technique [the line boy at my airport ALWAYS underfuels my plane by about 2 gallons / side. Eliminate this error by refueling yourself], 4) variability in fuel density [reduce this variable by refueling when temps are the same (or similar) to the temps when you last refueled], 5) tach error [the fuel consumption curve is S-shaped, meaning that there is a narrow rpm range at which fuel consumption climbs steeply. If your tach reads low, you might well be operating in the high-consumption part of the curve]. Keep in mind that there is a tolerance associated with the Lycoming data, and your engine might be operating close to the upper tolerance and still be normal.
I am not qualified to speak to the possible mechanical anomalies which you may want to investigate, but perhaps some of the A&Ps or IAs can help you there.
05-07-2003, 07:37 PM
Which carb do you have? The 10-3678-32 carb has bigger jets, a straight riser in the oil sump and burns considerably more fuel. The 10-3678-12 usually has a venturi in the oil sump riser and runs a leaner mixture.
05-07-2003, 07:58 PM
We've been going round and round on a similar topic at the Husky site (http://home.ease.lsoft.com/archives/Husky-L.html) on fuel burns, mp's, what you can and can't do with these little engines (and you can do a lot) in the April and May lists. From what I saw with my planes (SC's) that is excessive, for that engine. I don't know if you're working the plane though (towing, etc.,) and if you are then that's about right due to advanced throttle. The higher gph would then be due to having the throttle open over 75% where a carb port opens up for additional mixture enrichment (cooling). Did these readings come from a FF meter? They help manage consumption quite a bit.
05-07-2003, 08:36 PM
Try a tach checker........good chance your tach is off and you're turning a higher RPM then indicated. It's an easy and inexpensive place to start. Tach errors are very common.
With a 8244 prop at 2450-2500 im burning 6.7-7.0 gph. 160hp and a -32 carb.
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