From: svoboda@rtsg.mot.com (David Svoboda)
Subject: Engine Questions
Organization: Motorola Inc., Cellular Infrastructure Group
Date: Wed, 19 Aug 1992 20:39:51 GMT

In article <1992Aug18.195206.17141@news.eng.convex.com> mccrorey@convex.com (Martin McCrorey) writes:
|a rear exhaust OS 46 ABC.
|
|    1. Should it be broken in without the pipe, and if so, with what?
|       (There is no muffler with this engine, so I can't use that,
|       and I am going to run it in my garage, so I won't be running
|       it with an open header, either).

Even though you don't want to run an ABC too rich extensively, you are
in great danger of overheating and damaging an engine during initial
breakin.  Since a muffler tends to make an engine run hotter, you should
break in the engine without a muffler.  You really shouldn't use a tuned
pipe, as it causes the engine to heat up as much as a muffler, but
increases the RPM, which is also harder on it.  You want to be as "nice"
to your engine as possible during breakin.

Would it be possible for you to throw your breakin bench in the trunk
of your car and take it out to the boonies somewhere to do the breakin?
That's what I usually do.  Be sure to take hearing protection, too.

|    2. How do I break in the engine on the bench?  I have been advised
|       not to "four stroke" an ABC, apparently because they run too
|       tight when that cool, so I am planning to vary the speed a lot
|       while keeping it a just lean enough to hit every time.  

Short term four cycling won't hurt any engine.  The idea is to peak the
engine (quickly find the approximate peak needle setting), then find the
needle setting which will be the richest that the engine can run.  You
want to alternate peak running (for about a minute at a time) with
short forays into blubbering richness.  When the engine is that rich,
it does indeed cool the piston/sleeve, and reduce the tolerance, but
it also cools and lubricates the bearing surfaces, which are what you
will ruin if you just run it lean.

If you lean it just barely enough to two-cycle, the engine will break
in eventually, but it goes much faster and is safer for the engine if
you vary the mixture.

|    3. What is a good header length to start with?  I am more 
|       interested in a strong climb than top speed.  I looked at
|       another similar pipe, and the manufacturer recommended
|       9.25" from glow plug to the fattest part of the pipe.

The pipe resonance length, as you know, varies with engine RPM, which
is affected by prop selection, fuel nitro, temperature, humidity, and
airplane speed.  Start with the manufacturers recommendation, but err
on the long side, as it's possible to cut a header off, but difficult
to add onto it.

|    4. I plan to start with a 10x6 prop, although I will probably try
|       a variety of slightly heftier props when the plane is flying and
|       the engine is run in.  Is this a good starting point for this
|       engine?  I am also curious to know what kind of RPM to expect
|       from this engine with varous props (and how it will compare
|       with the side exhaust 46 with muffler).

Use a small prop for bench running.  A 46 might be broken in with a 10x5,
even, as long as you monitor the rpm's and mixture.  The engine will run
faster, which will make the breakin happen more quickly, but can also
stress the engine out if taken to extreme.

|    5. Is a 10% nitro, castor/synthetic mix fuel OK for break-in?

Break in an engine on the nitro content you intend to fly.  Castor is
valuable in case you screw up and run it too lean on the bench.

|    6. Can a put a tank pressure fitting on the side of the pipe
|       and have it work OK, or do I really need a pump?

Pressure fittings work best if they are back by at the widest part
of the pipe.  However, I have made them work right on the header.  (You
have to use a backing nut, becuase pipe headers are typically soft, thin
aluminum.


Good luck.

	Dave Svoboda, Palatine, IL