From: svoboda@rtsg.mot.com (David Svoboda)
Subject: Re: Tuned Pipe Questions
Organization: Motorola Inc., Cellular Infrastructure Group
Date: Sat, 22 Aug 1992 00:04:52 GMT

In article <Bt8q1t.EpJ@world.std.com> jlw@world.std.com (James L Walker) writes:
|In article <1992Aug18.195206.17141@news.eng.convex.com> mccrorey@convex.com (Martin McCrorey) writes:
|>I am about to use a tuned pipe for the first time.  This will be a
|>Super Hots with a rear exhaust OS 46 ABC.  Before I start on the
|>fuselage, I would like to establish the header length as closely as
|>possible.  I have built a bench for running in and tuning the engine,
|>but I have never bench run an engine or set up a tuned pipe before, so
|>I would like any advice I can get.  In particular, I have the following
|>questions:
|>
|>    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).
|>
|There was an article in RCM this month on the construction of the q'icksilv'r
|in which the guy recommends running the SF in with the muffler.  No reason
|given.  It could be so that you don't have to mess with the pipe to keep
|the engine running at the same time as you are running it in.

The purpose of breaking in an engine w/o a muffler is to lessen the back-
pressure, thereby cooling the engine.  It also let's the engine turn up a bit
faster without stressing it out like a tuned pipe (properly tuned) does.

|>    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.  
|>
|I keep hearing this lately.  I've always run my ABC engines in on one tank
|slobberingly rich.  It seems to me that if you are trying to loosen up the
|sleeve and piston, that running them too tight is just the ticket.  

Well, you want to wear in the fit between piston and liner, true, but you
*don't* want to stress the other parts of the engine while you do so.  Running
an ABC engine overly rich (esp in the air, where the airflow tends to cool
the engine more efficiently) makes it "stick" at TDC each stroke, and overly
stresses the crankpin and wristpin.  They can easily be worn out of round
by stress long before appreciable wear takes place in the cylinder, simply
because there is that much less area to take the stress.

|After
|the slobbering rich tank, I run another tank and alternate between 5 and 
|15 clicks of the optimal needle setting every few minutes.  This is the 
|standard method at my field.

Also, it is very important to not just stake the airplane down to the
ground and run a few tanks through it.  Not many people use air cleaners,
but those who do understand very well how much crap is down near the
ground, and it is NOT good for the engine to ingest this.  At the very
least, put the airplane on a picnic table to break it in.  The best
solution is a test stand, well off the ground.

|>    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.
|>
|This construction article in RCM also says that MAC sells a pre-tuned
|pipe with header, etc for the SF version of this engine.  Perhaps they
|also sell one for the RF?  See your SIG catologue.

I used one of these MAC pre-tuned pipes on an engine once (it was the only
pipe available when I broke one once).  It worked pretty well, except
that they make some assumptions about what RPM at which you will run.
That airplane was fast, with an high-revving engine (Super Kaos, with
a OS Max 60 FSR (not the 61, the earlier 60)), with an 11x7-1/2 Zinger
prop, and it would easily fall off the top of the pipe coming out of
a split S, and not regain resonance until the airplane was half way
across the sky.  I removed 1/4" from the length, and all was beautiful.

|At my field we are pretty paranoid about 
|sound, so I would always raise the pitch before the length of the prop.

Only if you want to go faster, at the expense of acceleration and climb.
The RPM has the greatest effect on noise, so a larger diameter prop
would also quiet things down.

|Most of the fuel I buy is 18% oil
|and I add 1/3 cup of synthetic oil per gallon to make up the balance.

Or racing castor.  That will give you more high-temp protection, and will
rejuvinate a worn engine somewhat.

	Dave Svoboda, Palatine, IL