The Cessna 120-140 Association Forum

I had a thread on this topic on the old Forum site, thought I'd take another swing at it.

So I flew up and back to the San Juan Islands this weekend from Pearson/Vancouver WA. 5.9 hours total time. 26.53 total gallons. Moderate leaning. I come up with 4.5 gph.

This can't be right for an 0-200 running at 2450rpm in cruise.

I have only one CHT on the bottom #2 cylinder that says 350F in cruise.

Do I have some sort of super-compressed Millenium cylinders or something?

Overhaul 5 years ago, 250-ish hours now, carb oerhauled by D&G, case Divco'd, mags overhauled by Aircraft Electrical Services/Redding CA, bits and pieces yellow tagged by Premier Aircraft Engine/Troutdale OR.

Any thoughts? No complaints but I've confirmed this gph burn several times.

There are a lot of variables involved, like actual prop length and pitch, and density altitude. If you had a manifold pressure gauge and new the accuracy of the tach you could get a better idea of exactly how much power you are producing, and figure out your actual brake specific fuel consumption. The figures in the book are for full throttle operation. Hard to match without an adjustable prop.

Or, you can just sit back and smile...

John,

Yeah, I'm smiling right now...

I do have a manifold pressure gauge from a long previous owner. Don't know why he installed it on a fixed pitch prop. I was running a little under 21 inches this morning at 2450rpm, which is about usual for this time of year. The gauge shows 23 inches as being some sort of maximum in the green.

I do know that my compression tests are always 80/80. Am thinking about compression ratios, nominally supposed to be 7 to 1. How are compression ratios calculated or measured? Of course it does like ethanol free mogas 87 octane...

6597 wrote: ↑Mon Jul 27, 2026 10:22 pmHow are compression ratios calculated or measured?

It's just the ratio between the combustion chamber volume with the piston at the bottom of its stroke to the chamber with the piston at the top.

The difference between those two volumes is the displacement (per cylinder).

The nominal displacement of the O200 is 200 cubic inches, or 50 ci per cylinder. If the compression is 7:1 then the volume of the chamber at TDC is 50/6 or about 8.3 ci. So, the volume at BDC is 50+8.3 or 58.3 and at TDC it is 8.3, so the compression ratio is 58.3 : 8.3 or (58.3/8.3) : 1

Quiz on Friday...

wow that's really good fuel burn. I see and figure 6 to 6.2 always on a long distance 71 50 prop with C90 and a Marvel carb 500 smoh cruising at 2450

David,
What was your airspeed? I know you said you have a climb prop. Maybe the engine is not loaded very much at 2450rpm if your airspeeds are on the slow side? Nevertheless, those are great fuel economy numbers, sounds like your engine is happy! It would be interesting to see pics of your spark plugs to see what color they are at those low fuel burn numbers.
Josh

6898 wrote: ↑Tue Jul 28, 2026 7:24 am wow that's really good fuel burn. I see and figure 6 to 6.2 always on a long distance 71 50 prop with C90 and a Marvel carb 500 smoh cruising at 2450

Jeff...makes sense since you are only 25rpm below redline on a C90. What airspeed do you get at that rpm? My 140A at that rpm flies around 115mph but I'm running a 71x52 prop. Full throttle at 2475rpm I can get around 120mph. I think it's been said before, the airframe really seems to like 100mph...anything over just seems to take a lot more throttle for not a lot more speed.
Josh

What was your airspeed? I know you said you have a climb prop. Maybe the engine is not loaded very much at 2450rpm if your airspeeds are on the slow side?

Between 100 and 105. Been that way forever. That is a good theory on loading- Still climbs like a bat outa hell, but I really need to watch the level off to cruise otherwise she's into the red. I have the Witchita letter for approval if anyone is interested. It does have it's limitations though.

It would be interesting to see pics of your spark plugs to see what color they are at those low fuel burn numbers

Here's the typical plug pic, although this was about 18 years ago. REM40E

Seems low. I get about 5.3-5.5 gph on a stroked C-85. Do you fill the tanks to the brim each time?

Do you fill the tanks to the brim each time?

Yup, even get some dribble thru the vents when the hangar warms up. Even if I was off a half gallon it's still a pretty low gph.

Josh made an interesting point about the engine perhaps not working too hard at 2450 rpm. I do have a manifold pressure gauge, why it was installed years ago is a mystery. I will report some numbers when I fly next. IIRC they tend to be above 25 inches when I'm taking off and below 21 inches when in cruise. I do not know how that translates to anything meaningful yet with a fixed pitch prop.