O-360 takeoff power fuel flow question

Hello

I'm looking for real-world data on full-rich, full-throttle takeoff
power fuel flow for Lycoming O-360 engines.

I have a C-172K conversion (Lyc O-360-A1A, Hartzell c/s prop and
PowerFlow tuned exhaust). The POH supplementals for the conversion are
a bit terse, but indicate that takeoff fuel flow should be in the
neighborhood of 18 GPH, with a stock Cessna exhaust. I seem to get only
about 16.0 =B1 0.5 with what I believe to be a well-calibrated fuel flow
meter (JPI EDM-700). I would expect the fuel flow to be even a bit
higher than the expected 18 GPH with the addition of a tuned exhaust.

Does 16 GPH seem too low? I seem to recall that O-320's burn about 16
GPH on takeoff.

Thanks for any info you can share.

argon39 wrote:

I seem to recall that O-320's burn about 16
GPH on takeoff.

From my aircraft performance profile data, the 160 hp O-320-B2D burns 12.25 gph
at 2700 rpm at 3,000' density altitude. From memory, it burns about 14 gph at
full takeoff power.

George Patterson
Why do men's hearts beat faster, knees get weak, throats become dry,
and they think irrationally when a woman wears leather clothing?
Because she smells like a new truck.

18 GPH is maximum allowable fuel flow for an O-360 at 2700 RPM at sea
level and standard conditions (higher fuel flows work, but cause some
loss of power).
16 GPH is maximum allowable for an O-320 under the same circumstances.

These are maximum values; minimum values are as much as 13% lower. 16
GPH would be within tolerances. 15.5 would be a bit on the low side,
but only VERY slightly, as in 1% low. It's not rare for a JPI to be
1%-2% off on fuel flow.

Some other things you should consider:

Are you at sea level? Fuel flow should decrease as air density
decreases.

Are you really making 2700 RPM? Unless you've checked with an
electronic tach, you don't know. Mechanical tachs are notoriously
inaccurate.

Michael

Michael and George

Thanks for the comments and info. Some answers:

[1] Yes, sea level (well, maybe 5 MSL at low tide).

[2] The electronic tach in the JPI EDM usually reads 2640-2650 on
takeoff.

[3] I couldn't guarantee that the JPI fuel flow sensor is accurate to
=B1 2%, but I'm pretty sure that it is not reading 10% low.

My interest in these numbers was sparked by reading the avweb.com
discussions by Deakin and others, specifically whether or not enough
excess fuel was flowing to provide cooling at full power.

The electronic tach in the JPI EDM usually reads 2640-2650 on
takeoff.

That alone is enough to account for the tiny discrepancy. At that RPM,
the minimum acceptable fuel flow would be less than 15.5 GPH. You say
it reads 16.0 +/- .5. In other words - there is really no problem,
other than the fact that your prop governor could use some adjustment.

My interest in these numbers was sparked by reading the avweb.com
discussions by Deakin and others, specifically whether or not enough
excess fuel was flowing to provide cooling at full power.

Understood. However, his discussion is primarily applicable to
big-bore engines being run close to design limits. For example, if
you're squeezing 300 hp out of 550 ci or 285 hp out of 520 ci (Which is
what the bigger Bo's do) then you have more to worry about then if
you're only making 180hp on 360ci (which is what your installation is
doing). Bigger engines are hard to cool in the first place, and
running higher compressions and greater power densities doesn't help.
When plenty of excess power is available and cooling is marginal, it
makes all kinds of sense to run the maximum allowable fuel flow on
takeoff (and maybe a might more) and trade a little power for better
cooling.

So the bottom line is that what Deakin says is correct, but not
tremendously applicable to your situation. I would watch the CHT's on
the takeoff and initial climb. If you're solidly below 400F, don't
worry. If not, maybe it would make sense to adjust the carburetor -
and tweak the prop governor while you're at it, since enriching the
mixture will cause you to lose a little bit of takeoff performance and
you're already losing some.

Michael

In fact, the presenting problem is that I have not been solidly below
400=B0F. The hottest peak CHT just after takeoff on a standard day would
be more like 430=B0F, and the coolest about 400=B0F. On really hot days I
have seen even higher values, albeit for only a short time. I think I
have solved this problem at this years's annual by installing a fairing
around the big gap between the PowerFlow exhaust pipe (much smaller
than the standard Cessna exhaust). With the fairing installed, peak
CHTs just after takeoff are now between 400=B0F and 375=B0F as a result
of better airflow inside the cowling.

Regarding design limits, I do appreciate that Deakin was referring to
bigger engines. But I still wonder what a limit for the O-360 might be
as a result of adding the tuned exhaust.

One step at a time, let's check the prop governor and maybe the
wide-open mixture.

Thanks.

argon39 wrote:
: In fact, the presenting problem is that I have not been solidly below
: 400?F. The hottest peak CHT just after takeoff on a standard day would
: be more like 430?F, and the coolest about 40?F. On really hot days I
: have seen even higher values, albeit for only a short time. I think I
: have solved this problem at this years's annual by installing a fairing
: around the big gap between the PowerFlow exhaust pipe (much smaller
: than the standard Cessna exhaust). With the fairing installed, peak
: CHTs just after takeoff are now between 400?F and 37?F as a result
: of better airflow inside the cowling.

What type of CHT probes? If they're the spark-plug type, then you're still
pretty much fine. They tend to read 50-75 degrees hotter than the "official" locating
of the cylinder well lug type.

The other indirect reading of fuel flow will be the EGT. How much lower than
high-power cruise peak is it on full-rich takeoff? In my O-360-A3A Cherokee, I read
1600-1650 peak EGT in cruise, but 1400 on full-rich takeoff. In fact, during a long
climb I'll lean to that 1400 as I gain altitude. From what I've read, with 200-250
degrees between the two, I should be more than fine... might want to check yours

: Regarding design limits, I do appreciate that Deakin was referring to
: bigger engines. But I still wonder what a limit for the O-360 might be
: as a result of adding the tuned exhaust.

Probably not getting more than 5-10hp out of it. Although, with the
scavanging effect of a tuned exhaust, the max fuel flow could certainly require that
additional 5-10hp's worth. Making more power takes more fuel.

Of course, all that assumes the tuned exhaust actually works... ;-)

: One step at a time, let's check the prop governor and maybe the
: wide-open mixture.

... and EGT. It's the best way (short of a wideband O2 sensor, etc) to
determine the *actual* mixture of the engine on takeoff.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Thanks, Cory

(Apologies for the delay, spent last week on jury duty.)

What type of CHT probes?

Bayonet-style, not the spark plug type.


The other indirect reading of fuel flow will be the EGT. How much lower =
than
high-power cruise peak is it on full-rich takeoff?

Less than the 200=B0 you mention. Peak EGTs while leaning are perhaps
200=B0 above climb-out EGTs, but I have been advised to run 125=B0 rich
of peak by my mechanic. At 50=B0 lean of peak the exhaust pipe has a
light grey appearance that suggests a too-lean mixture.


Probably not getting more than 5-10hp out of it. Of course, all that assu=
mes
the tuned exhaust actually works... ;-)

Well, all I can say is that I noticed the difference after installing
it. But to be fair, it wasn't a very scientific experiment because I
also installed an O-360 and a c/s prop at the same time. Those three
items conspire together to make it a real hot rod, among elderly C-172s
at least. Too bad the aerodynamics resemble that of a 1978 Volvo 240DL.

: The other indirect reading of fuel flow will be the EGT. How much lower than
: high-power cruise peak is it on full-rich takeoff?

: Less than the 200?? you mention. Peak EGTs while leaning are perhaps
: 200?? above climb-out EGTs, but I have been advised to run 125?? rich
: of peak by my mechanic. At 50?? lean of peak the exhaust pipe has a
: light grey appearance that suggests a too-lean mixture.

One universal truth in aviation seems to be that there are more untruths than
can be counted. Lots of old-wives-tales, especially regarding leaning. Reading
Deakin's articles on AvWeb should be required reading. Bottom line is that the engine
cannot be harmed with the mixture knob at 65% (or 70%) power so long as CHT doesn't
get too hot. Light grey exhaust pipe to me indicates a correct mixture where there's
not an abundance of either fuel or air, and the lead is being scavanged nicely. 125
rich is burning a lot of extra fuel, depositing lots of carbon on the pistons, valves,
and valve guides.

: Probably not getting more than 5-10hp out of it. Of course, all that assumes
: the tuned exhaust actually works... ;-)

: Well, all I can say is that I noticed the difference after installing
: it. But to be fair, it wasn't a very scientific experiment because I
: also installed an O-360 and a c/s prop at the same time. Those three
: items conspire together to make it a real hot rod, among elderly C-172s
: at least. Too bad the aerodynamics resemble that of a 1978 Volvo 240DL.

That's a whole 'nother kettle of fish. The CS will let you actually *make*
rated HP at sea level. Putting a CS prop on a fixed-gear bird will not get you any
appreciable speed. It might give you a bit better economy if you can run it
over-squared. The only thing it buys you is takeoff/climb performance, and unlikely
to have increased your legal gross weight any.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Putting a CS prop on a fixed-gear bird will not get you any
appreciable speed.

The only thing it buys you is takeoff/climb performance, and unlikely
to have increased your legal gross weight any.

I never said that the prop did either of these things. What did change
things was putting a 180hp O-360 in the place of a 150hp O-320. The
extra 19hp at 65% power does in fact make it go faster. And the new
legal gross weight per the STC is now 2500.