Exhaust Chirping Sound

[jlandry@domain.elided (John A. Landry)]

Reapers,

Can someone describe to me exactly what the IH exhaust port air injection
tube "chirping" phenomenon sounds like and under what conditions you hear
it?

The reason I ask is this...

When I rebuilt the 392 I now have in my Traveler, I did some porting which
included cutting away the air injection tube "hump" that was present inside
the exhaust ports.  I made sure I contoured the opening of these air
injection ports inside the exhaust port so as to free them from any sharp
edges which might contribute to the annoying exhaust chirp I've heard about
here on the Digest.  After the porting work, I plugged the threaded air
injection ports with standard 1/2" diameter set screws.  There remained a
shallow pocket (maybe 1/4" deep) between the end of the set screw and the
surface of the exhaust port wall.

Now that the engine is installed and running, one of the first things I
noticed about it is a certain unique sound at idle I've never heard before
on my old 345.  The simplest way I can describe the overall engine sound is
sort of like the way a diesel engine sounds.  It's *very* mild mind you,
and hardly noticeable unless you had listened to another IH engine first.
I'm *not* talking about the sound of the exhaust... I'm talking about the
general mechanical sound coming from the engine itself.  I assume at this
point it's just the tight new cam, lifters, pistons, etc.  It might be
getting quieter as the miles add up, but I'm not really sure.  Anyway, it's
nothing alarming and is not the main reason for my question.

There's another sound that I hear when driving at maybe 35 mph or faster
which has me "wondering" what it might be.  I hear a very faint sound that
I initially thought was the very start of pinging.  It's the barely-audible
sound I've heard when an engine is right on the verge of pinging, and if
you were to hit the accelerator, it would ping.  I hear the sound slightly
louder... or maybe more distinctly... when accelerating moderately and
harder.  The sound seems to be less noticeable under deceleration.  The
strange thing is, depending on how I turn my head when trying to hear it,
the sound seems to almost be coming from the rear of the truck... like it's
possibly emanating and exhaust tailpipe.  The engine is not pinging though
under any operating condition that I can hear.

I have been experimenting with the ignition timing on this new engine, so I
initially assumed it *was* the start of pinging.  I had the timing at 8
degrees BTDC when I first noticed the sound, so I backed off to TDC.  The
sound still seemed to be there to me, so I went ahead and advanced the
timing to 10 degrees BTDC and *maybe* it got more noticeable, but I'm not
sure.

I next hooked up my MSD variable timing control and while acceleration up a
long moderate hill with the sound present, I retarded the timing from 10
degrees BTDC to probably 5 degrees ATDC and it made no difference.  So now
I'm convinced it's not pinging nor even ignition related.

I should mention that when Tom Mandera was over this weekend I had him ride
with me and he couldn't hear the sound even though I clearly could.  That's
how faint it is... you have to know what to listen for.

I'm starting to think this might be the faint remnants of the exhaust chirp
I've heard so much about.  If it is, it's faint enough where I can
completely ignore it.  I just hate unidentified sounds... especially on a
new engine.  Makes me wonder if I forgot something or did something wrong
during the rebuild.  I'm a natural worrier.

BTW, I've noticed something interesting about this new 392.  I'm using the
exact same distributor and ignition setup that I had on my 345.  With my
345, if I tried to advance the timing much beyond about 5 degrees BTDC, I
would get pinging under load (using 87 octane fuel at sea level).  With
this 392, I'm up to 10 degrees BTDC with no pinging on the same low octane
pump gas.  Performance and fuel economy are markedly improving the more I
advance the timing.  My last 2 tanks of gas were over 12 mpg.  My first two
tanks at TDC were less than 10 mpg.  So I can only assume the difference is
no carbon buildup in the cylinders and lower compression.

Comments on the chirping sound are appreciated.

Regards,

John L.
-------------------------------------------------------------------------- 
jlandry AT halcyon DOT com      | 
Conservative Libertarian        |  "The road to  tyranny, we must never
Life Member of the NRA          |   forget, begins with the destruction
WA Arms Collectors              |   of the truth."
Commercial Helicopter - Inst.   |                     William J. Clinton
http://www.halcyon.com/jlandry/ |    10-15-95, speech at the Univ. of CT