Fuel Burn

[davevramp]

My fuel burn is averaging 10 gal/hr of 100LL. I fly at 2500 to 3500ft and once up at cruse I am at 2450 rpm and at 24 inch. My prop is a 56 in pitch on my 54b and usually see 118 mph indicated. I think that this is too high for the fuel consumption. Next move is to remove the Carb and inspect.

Any comments would be appreciated. What do you burn an hr?

thanks
dave

[jrenwick]

Dave,

Are you leaning the engine at 2500-3500'? How do you measure hours?

I have a McCauley 7653 prop and O-300. I usually run 2350-2400 RPM at 115 indicated on a cool day, 2000' MSL or so, and I expect to burn 9 to 10 GPH from takeoff to landing by the clock. On cross-country trips I'll fly higher, 4500 or 5500 usually, and expect to see about 8 GPH on average. I log all my flights with takeoff and landing times, and for the 200 or so hours I've had my 170 actually in flight, the average fuel consumption is 8.7 GPH. Most of that is at 2350 RPM.

Looking at the cruise performance chart for a '56 170B, 2450 RPM at 2500' with a 7653 prop should give 120MPH true at about 8.3GPH, leaned for maximum RPM. But they also say that due to "carburetor metering characteristics, engine and propeller condition, and turbulence of the admosphere," performance could vary by 10% or more.

By the way, the GPH numbers in the Cessna chart I referred to work out to 11.42 HP for 1 GPH, or a BSFC of 0.53 pounds/hr per horsepower. At that rate, 10GPH is 114 HP, or 79% power.

Best Regards,

John

[davevramp]

I use Tach time and have kept track of clock time off the GPS, Tach time makes the numbers better. I do lean to 100 degrees rich of peek I have the original mixture control and would like to up grade to a small vernier mixture control that would make it a little easer
thanks
dave

[blueldr]

You're burning too much fuel for the performance you're getting.
We (I) Generally fly higher out here in the west. Mountains, you know.
My overall fuel burn usually runs about 7.2 gph at around 2400 to 2450.
I lean to rough and just enrichen to smooth. You don't have to worry about being too lean if you're pulling 70% or less. Keeps the plugs clean too!

[blueldr]

As a PS to the above:
The carburetor on this eggine is so basically simple that pulling it off for anything less than a static fuel leak is almost counter productive. If your mixture control will kill the engine, you're pbobably not leaning enough.

[GAHorn]

I typically cruise up at 5500 to 7500 most of the time, running 2450 rpm on my EM7655 prop. (same as MDM7655 except for bolt-holes), leaned as described in the owner's manual. (Lean until slight drop in rpm, then enrichen to max rpm again.) I consistently burn 8 gph on any flight of more than 1-hour duration.
If you have a Brackett air filter, be certain you are installing it correctly, and replace it regularly. They're cheap. ($8 or so.) To install it, take a new one between paper-towels or newspaper and squeeze it thoroughly to remove excess wettant before installing. Never clean it or oil it for reuse....just get a new one.
Be certain your carb-heat butterfly is operating correctly, and fully to both hot/cold positions.

[mrpibb]

I have a em7653 pitched to 50" my fuel burns have been pretty consistant at 7.8 to 8.5 gph. On long trips I cruise 2550 at 110 mph at 5500 to 6500ft and always fall in the 8.0 to 8.3 gph. local flights and the such I spin 2450 103 ish mph and usually get about 7.5 to 8.0 gph.
But then again this just proves the superior ragwing airframe design

[voorheesh]

Which engine do you have? It sounds like a 0-360 with const spd prop(2450rpm/24") If that is so, 10 gph is about what you would expect.

[davevramp]

The answer to my fuel burn is a Tach that was reading 225 rpm low.
A Tach checker should be part of the annual inspection. Total time on the plane is 2085 never thought the Tach would be bad.. It worked to about a gallon for ever 100 rpm over cruse Now C-140 cruses faster that my 170 Fly and learn.
thanks
Dave