On Mon, 9 May 2005 18:44:11 -0700, "Matt Barrow"
wrote:

John Deakin has written that if you lean to the lean side of peak and
the engine runs rough, pulling on full carb heat will distribute the
fuel into the air mixture better and give you the ability to lean to
the lean side of peak.

Actually, he says "just a touch" of carb heat.

That was his initial suggestion. In his next column he posted this as
a news flash. I've posted nearly the entire portion of the column
because he has some caveats that are important to understand:

***Begin Quote***
We've always suggested trying just a touch of carb heat to improve
mixture distribution, but that was always an afterthought, secondary
to the tiny MP reduction with throttle, and we had no data.

Walter has been playing with AMPLE carburetor heat, even full
carburetor heat! On his airplane, and on a couple flat engines he's
tried, it works like a charm! It brings the EGTs and CHT into close
alignment (which isn't all that important), but more important, it
evens up the mixtures to each combustion chamber, allowing smooth and
LOP operations for the first time in some of these engines!

You heard it here first, thanks entirely to Walter. We need more data
on this to be sure it's a universal solution, or if it's peculiar to
certain engines.

There are a couple of very minor "problems" with using carb heat. One
is that on most engines, it bypasses the intake air filter and takes
combustion air from inside the warm engine compartment, or from a muff
around the exhaust tubes. Any sand, dust, or debris in the air can get
sucked into the engine, and that's not really too beneficial. On the
other hand, a lot of engines run without air filters entirely
(Walter's Twin Beech being one), apparently without much harm. Doing
this may cause a small elevation in silicone in your oil analysis, but
it's not clear how harmful this might be. Simple answer to this is
just don't use carb heat on the ground, except for testing. Or when
flying in a dust storm.

Inflight with carb heat on, some ram air effect may be lost, costing
you an inch or so of MP on some installations. But when LOP, air isn't
the determinant of power, fuel is.

Finally, using carb heat at *sea level takeoff power on a hot day* may
raise the induction air temperature so much that it will infringe on
the detonation margin if the engine is already "critical." But
full-power operation is not the real problem here, because we
recommend full power and ROP for all takeoffs. At lesser power
settings, or on cold days, even full carburetor heat is not an issue
for detonation, on most of these flat engines.

If any of you have carbureted engines with the JPI instrumentation,
and can download the data, please experiment with this, and send me
the data file. This might best be done by someone who is already a
"user" of our LOP methods, as there will be less "mystery" during the
testing. Just go up to somewhere above 5,000', and try WOT, LOP, and
play with different carb heat settings, including full heat. This is
really looking good, but we need data!

Walter thinks the warmer air is dramatically improving vaporization
and distribution, but we don't know a lot on this, yet.

You WILL run into great "resistance" at your local airport on this!
Just ask, "Do you have the data for that?" We will have, shortly.
Meanwhile, think of the IO-550B on my airplane, where we hung a turbo
on it, and we're using the same full manifold pressure as always, but
now heated dramatically by the turbo. It doesn't seem to hurt. What's
the difference between hot induction air and hot induction air?
Nothing.

This "News Flash" is the ONLY part of this column aimed at carbureted
engines! The main part of the column is about fuel-injected engines.
Sorry for the confusion, but I wanted to get this information out as
soon as possible.


February 2, 2003
Pelican's Perch #65
Where Should I Run My Engine?
(Part 3 -- Cruise)

***End Quote***

Corky Scott