REFLECTOR:Baffling, CHT, EGT, Ehxaust Wrap ...

Jim Sower reflector@tvbf.org
Sun, 09 Nov 2003 23:09:24 -0600


Hey guys,
I've got a puzzlment here.
My Velocity (173 FG) came at me pretty much fuel cooled.  CHT
went up really fast if I leaned off of full rich at all.
Baffling had gaps you could throw a cat through, crummy tin gap
fillers between the barrels, no attempt to seal up the exhaust
pipes at the cowl nipples, etc.

Finally a friend grabbed me by the stackin' swivel and read me
the riot act about lead deposits in the chambers, etc. and made
me do the math (I'm spending about 45 to $9 per hour extra on
fuel).  That last did it.  Wish I'd done it MUCH sooner.  ALWAYS
do the math :-)  Much as I hate baffling, I tore it all out and
did it over, improving bulkhead around alternator and starter,
etc.

Anyway, with BID-RTV around and between the barrels and tight
seals elsewhere (except the exhaust pipe wasn't sealed that
well) I tried it out.  I could lean somewhat better, but no
really dramatic improvement.  Leaning still caused CHT to rise
quickly.  I knew I had a lot better sealing of the plenum, and I
intuited that the exhaust pipe not being sealed would affect #1
& 3 more than #2 & 4 but that was not the case.  But that was
just a guess.

So ... I sealed up the exhaust-cowl gaps really well.  I figured
that would trap all the heat that the breeze out the pipe-cowl
gap had been carrying away and significantly pre-heat the
cooling air going to the cylinders, so I wrapped the exhaust
pipes (I've heard this can aggravate if not cause cracking of
the pipes and hide the cracks until something breaks, but I
thought I'd try it for a while).

Weird!  Where before I had maybe 1 or 2 "ticks" of EGT on the
Graphic Engine Monitor before the CHTs started getting away from
me, suddenly, I've got 3 tics on TAXI.  On climb out, the CHTs
still get high, but the EGTs are higher than I've ever seen.  I
figure the EGT got way high because I'm retaining all that heat
that used to escape through the walls of the pipe and it's
driving the EGTs up. Waaaay up.  But why do the CHTs go up too?
There's no more leakage, the pipes are wrapped so the cooling
air isn't preheated the way it was.  What's the problem?

The only thing I can come up with is that the high EGTs are
making the pipe really hot and the hot pipe is heating the
flange and the CHT probe is close to the exhaust flange.
Problem is, the airplane is updraft and I'm measuring the temps
on the "cool" (upstream) side and I don't know what they are on
the top of the engine.  I used to have a plug gasket CHT probe
that I could replace one of the bayonet probes with and move it
to the top of the engine to see what the delta T is across the
head, or at least across the plug positions.  Hope the wire is
long enough to reach the box where the trick wires terminate.

Any explanations for what's happening to me are most welcome ...
Jim S.

Jim Sower
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T