Detonation in a Cont. 550

In rec.aviation.homebuilt Richard Riley wrote:
: The engine has about 60 hours since new. My friend was the pilot for
: the last 4 hours or so. As he's been flying it he's been getting much
: higher fuel burn than he expected - cruise lean was 18-20 GPH. He's
: also seen higher oil temps than expected - 220-240.

: The mags were unchanged from their original factory timing, but it
: turns out they were set at 18 and 20 degrees.

: Any ideas here? He was running it at no more than 25" and 2500 RPM.
: I'd have thought all that fuel would have kept it cool.

: Richard Riley

Was the fuel flow (i.e. pressure) gauge calibrated? If it was reading high
and adjusted for that, it would run too lean. Was he verifying fuel burn by the
amount put in the tanks after a flight? I concluded that my Cherokee was running 5%
harder than indicated via tach and MP by the fuel I put in the tanks after the flight.
I calibrated the tach with a strobe and found it reading 50 rpm low.

Also, mismatched mags off like that will increase combustion temps and
pressures. I don't know what the standard setting is for that engine, but it doesn't
sound like they were advanced too far.... if anything retarded too much. Could have
cooked the exhaust valve from being too retarded and caused preignition.

Lots of possible options.


-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering Ph.D. Graduate Student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

On Wed, 25 Aug 2004 22:17:48 -0700, Richard Riley
wrote:

The #3 cyl has the piston crown pretty much gone, along with the
rings. The other pistons show significant crown damage as well. The
local A&P believes it's detonation.

I'm no expert Richard, but that kind of damage could be pre-ignition
rather than detonation.

The mags were unchanged from their original factory timing, but it
turns out they were set at 18 and 20 degrees.

One would think, with those timing settings which are conservative,
that detonation would be unlikely because they are promoting a delayed
Peak Pressure Point, which should reduce pressure and heat. It should
also reduce power a bit, but that should also mitigate against
detonation.

Corky Scott

"Richard Riley" wrote in message
...
A friend of mine has just had a Continental 550 destroy itself. He's
asked for my advice, but all my experience is with Lycomings. If
anyone has experience with 550's and can comment, I'd be thankful.

The #3 cyl has the piston crown pretty much gone, along with the
rings. The other pistons show significant crown damage as well. The
local A&P believes it's detonation.

The engine has about 60 hours since new. My friend was the pilot for
the last 4 hours or so. As he's been flying it he's been getting much
higher fuel burn than he expected - cruise lean was 18-20 GPH. He's
also seen higher oil temps than expected - 220-240.

The mags were unchanged from their original factory timing, but it
turns out they were set at 18 and 20 degrees.

Any ideas here? He was running it at no more than 25" and 2500 RPM.
I'd have thought all that fuel would have kept it cool.

http://www.avweb.com/news/columns/182132-1.html
Pelican's Perch #43:
Detonation Myths

On Wed, 25 Aug 2004 22:17:48 -0700, Richard Riley
wrote:

I have many questions.

Was the 550 installed in a type-certificated aircraft?

Was the 550 normally aspirated?

Was the 550 equipped with the auto-lean fuel pump? If so, was your
friend trying to lean the engine manually?

When the engine was installed, was the fuel system properly set-up
following TCM SID97-3B? BTW, this is NOT optional.

Was the ignition timing accurately set/verified when the engine was
installed? A 2 degree split on an out-of-the crate engine is not
unheard of.

Did the friend have accurate CHT indication/s? What were they in
cruise?

What % power does 25 squared equal according the power chart for the
aircraft? If no power chart exists, perhaps someone in the group
flying behind a similiar 550 install can help out.

Was the cruise fuel flow an actual measured fuel flow (a la totalizer)
or was it a metered fuel pressure gage indicating fuel flow?

Detonation to the point of destruction doesn't take an engineer to
identify it. If the #3 piston was damaged to the point that the rings
seized and a slot was burned down the side of the piston (through the
rings), it was subjected to severe/extreme detonation.

I've not taken the time to read Mr. Deakin or Mr. GAMI's treatises,
but am somewhat familiar with detonation in aircraft engines . High
power setting + high CHT + advanced ignition timing + 50-100 degrees
ROP fuel flow is worst case for detonation. Reducing power or CHT,
retarding ignition timing, and either leaning or enrichening the
mixture reduces the risk.

TC

A friend of mine has just had a Continental 550 destroy itself. He's
asked for my advice, but all my experience is with Lycomings. If
anyone has experience with 550's and can comment, I'd be thankful.

The #3 cyl has the piston crown pretty much gone, along with the
rings. The other pistons show significant crown damage as well. The
local A&P believes it's detonation.

The engine has about 60 hours since new. My friend was the pilot for
the last 4 hours or so. As he's been flying it he's been getting much
higher fuel burn than he expected - cruise lean was 18-20 GPH. He's
also seen higher oil temps than expected - 220-240.

The mags were unchanged from their original factory timing, but it
turns out they were set at 18 and 20 degrees.

Any ideas here? He was running it at no more than 25" and 2500 RPM.
I'd have thought all that fuel would have kept it cool.

Richard Riley