Mark Mallory wrote:

MikeM wrote:
It sure shows what a ****TY job Cessna did when they "designed" the
cowling/baffling in my L model Skylane. I have improved the cht
balance greatly with some added baffling. Front two chts are still
40 deg F colder than the back four.

Interesting, Mike. Where did you add the baffling?

Front two, 5&6 CHTs are too cold due to being blasted directly
with cold air via the intake scoops, (in addition to 5&6 having
the lowest EGTs, as discussed below). Cessna realized this, and
has made the scoop openings progressively smaller over the years.
(Also done for drag reduction).
I installed a "cold weather kit", which partially shields the
front two cyls from the direct air blast. It reduces the CHT
differenial between 5-3 and 6-4. When I started, the front
cyl ran ~300degF, now they are about 345.

The middle two, 3&4, were always the hottest, while
the back two, 1&2 ran cooler. I played with the baffles below the
cylinders, sealed all the holes in the air dam (surrounding
the magnetos), and stiffened the baffle silicone seals (used
stiffer 3/32 silicone instead of 1/16) and that reduced the CHT
differentials between 3-1 and 4-2.

It also shows how POOR the mass-flow balance between the front and
rear cyclinders is on the Continental O470R. Due to the brain-dead
induction plumbing, the front two cyl get much less fuel/air than the
other four.

How do you tell the difference between fuel/air QUANTITY and fuel/air
RATIO?

I have never subscribed to the notion that the carburettor knows if
the air moving through it is destined for the front cylinders or the
rear ones. In other words, the mixture is established as the air
flows through the carb. Once mixed, there is a pretty big slug
of fuel/air sitting in the induction tubes waiting for the next intake
valve to open.

When an intake valve opens, the fuel/air mix is sucked into the
cylinder. The EGT is effected by the total volume
sucked in, (as well as the mix ratio). My EGTs have consistently been
hottest on the back two, 1&2, and lowest on the front two, 5&6; inversely
proportional to the length of the induction tubes between the carb
and the intake port. I believe that it is the flow resistance along
the induction tube that prevents cyl 5&6 from getting as much fuel/air
as 3&4 as much as 1&2. I dont see this distribution on Lycoming engines.
On Lycs, I attribute this to the carb being under the center of the
engine, and the induction tube lengths from carb to each cyl are more
equal...

I'd like someone to explain how the ratio of fuel/air can change
depending on which cyl is sucking? In other words, how could the front
cylinders be getting a different mixure ratio?

MikeM