On 20 Jan 2005 21:57:24 -0800, wrote:

stol wrote:

My Zenith 801 is running a 347 cu in all aluminum Ford V-8. I detuned
it to about 310 hp to keep the plane from breaking in half and on
take
off I am burning 11.8-12.3 an hour.

Ben,

This equates to a BSFC of about 0.22 , essentially "impossible".

Sid, you must have missed the following, it explains a lot:

"Let me clear up some things. My plane is tied down in its hangar at
almost 7000 feet msl. So the 310 hp is down 22% right off the bat, now
it's at 240 or so. The fuel flow for that HP range is damn close. I
agree that any motor running below .38-.40 is pushing the limit on
thermal dynamics of current technology. I admit that there is some
cutting edge stuff in my motor that helps squeeze out more hp per pound of fuel.
Ben Haas N801BH Jackson Hole Wyoming"

Ben wrote:
Remember, on aircooled motors they use 30-40 % of
the fuel just to cool the heads/cylinders.

Sid asks:
Please explain the reasoning behind this (IMHO bizarre ) statement.

When properly set up aircooled engines with fixed timing (which
describes all of them except for those equipped with electronic
aftermarket timing or FADEC) are advanced to full power, the fuel
system is designed to produce a strongly overrich mixture in order to
prevent detonation and overheating.

There may be some confusion as to how an overrich mixture actually
achieves this. The answer in depth may be found at:
http://www.avweb.com/news/columns/182084-1.html

REALLY simplifying the explanation, since the timing is fixed, the
only way to vary the timing is by varying the mixture. The speed of
combustion within the combustion chamber will vary from very lean to
ideal to very rich: going from slow (relatively speaking) to fast and
back to slow again respectively.

Engineers preset the point of ignition in order for the peak pressure
point to occur at 16 degrees after top dead center while at full power
and full rich mixture. This is the point where the combustion reached
it's maximum pressure during the combustion process. Engineers have
long known that the PPP should occur at 16 degrees ATDC for maximum
power and cooling.

If the mixture is leaned while under full power the combustion will
speed up. If the combustion speeds up, the PPP may begin to occur at
close to TDC. If the PPP is occurring at TDC, all that pressure has
no where to go and pressure and temperatures skyrocket.

This is why you do NOT lean the mixture while the at full power and
taking off, unless you are at a high altitude airport, which is
another story. It's also why you should NOT pull back the throttle in
an effort to "save" the engine. Pulling back the throttle slows the
engine (fixed prop), which brings the PPP close to TDC (bad).

Anyway, the rich mixture for takeoff allows the engine to achieve it's
best PPP location at 16 degrees ATDC which allows the engine to
produce maximum power and not overheat. It does not cool the engine
by hosing down the combustion chamber with excess fuel.

Corky Scott