what _IS_ a good cruise A:F ??

Tom Cloud cloud at peaches.ph.utexas.edu
Tue Mar 25 20:41:26 GMT 1997


Greg wrote (re aircraft engines) to the diy-efi list:

>In fact, with refined injector manufacturing and balanced inductions, it's
>now becoming fashionable to run the same engines LEAN of stochiometric for
>even better fuel specifics.  One of the major manufacturers actually specifies
>that a specific model of their engines be run lean of stochiometric during
>cruise.  This keeps cylinder head temperatures down while giving good fuel
>economy.
>
>I guess what I'm saying is that I find it hard to believe that you can
>damage an auto engine, running at 15-30% of its rated power in cruise, by
>running it lean of stoch. where the same operation doesn't hurt an ancient
>air-cooled aircraft engine running at 65-75% of its rated power.
>
>At least not by detonation.

I've been told by someone that my term "detonation" is incorrect,
that "pre-ignition" is the term I should be using.  Wish I knew  8^)

It was common in days of yore to run lean for cruise/economy
and rich (terms relative of stoich) for power.

Now, I am confused.  The tech books show curves that give
max economy to the lean side and power to the rich side of
stoich. It's been done for years.  I've read that open
cylinder and domed piston design increase the likelihood
of detonation, but now I don't know for sure if we're all
using the same terms ?????

 .. what is it that causes the big-end bearing to get smushed
and commonly destroys the top ring gland.  Whatever that is
called is what I had.  I did experience what I consider to
be normal clatter, on rare occasions, going up inclines at mid
to WOT that could be cured by putting higher octane fuel.

 .. I don't see how low-octane "pinging" can cause the above
problem.  To me, that is "pre-ignition", which is what I was
calling detonation.  I guess if I was to make up terms, I would
call it "self-detonation" or an "uncontrolled ignition event"
if I were a political spin doctor (an oxy-moron), whereas
"pinging" is the rapid banging of the explosion caused by
the ignition spark.

So, I'm back to the question:  if lean running has worked
for so long, what did I do wrong?  Was it something else?
I'd like to know before I finish assembling this high $$
(for me) engine and put it back in.  How do you know if
you've got "whatever it's called"?  Todd Knighton advised
me to get a "knock sensor", but it costs about $450.  Granted,
it's cheaper than an engine, but it is a large investment
for an instrument that I can't even use to tell if my
barbequed ribs are done or not.

To the knock sensor, has anyone retrofitted one to
their application -- IOW taken one from an oem vehicle
and put it on something else?  Is their a way to do
it cheaper than $450 ??

Tom Cloud <cloud at peaches.ph.utexas.edu>




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