REFLECTOR: Lycoming IO 360 Timing

[Ronnie Brown]

A while back, I asked the Reflector if anyone had any idea why Lycoming had issued a service bulletin about reducing the timing on 200 HP IO 360 engines from 25 to 22 degrees.  I didn't get an answer, but here's an interesting email re timing and CHT's and EGT's (and detonation).  I'm still running 25 degrees on my 360 and my heads have run cool from the beginning so I'm not planning on changing anything.  But those of you who have tried everything to get your CHT's down might want to consider this article I copied from the RV list.

Ronnie Brown

Time: 02:29:21 PM PST US
From: RV8ter at aol.com
Subject: Re: RV-List: engine timing vs CHTs/EGTs

--> RV-List message posted by: RV8ter at aol.com

My io-360-a1b6d lycoming manual (and info on their web site) excludes my 
engine from being allowed to be retarded to 20 BTDC from 25.  Doesn't say way.

Anybody know FOR SURE?

do not archive
In a message dated 8/22/2004 5:21:15 PM Eastern Standard Time, 
wnorth at sdccd.cc.ca.us writes:
--> RV-List message posted by: Wheeler North <wnorth at sdccd.cc.ca.us>

Well,

once again the list provided a useful jog to my rapidly degrading mental
faculties. After a recent discussion with Andre B. re his fuel injection
problems he reminded me of an old Lycoming service letter which allows most
of the IO-360s to have their timing retarded to 20 degs from the normal
factory setting of 25 degs used for most Lycoming engines. Although the SL
doesn't fully explain this, it is an option for those engine installations
where detonation and cooling are an issue. 


For some strange reason this finally gelled in my head with the previous
thirty years of experience floating around in there and answered the
question I have had in my head as to why many RVs have such a wide range of
CHTs from one aircraft to the next given they all have the same relative
engines(mostly).

Years ago I worked in a shop where we had this strange little Mercedes two
seater with a big ass engine in it. The engine was fairly high time and the
owner was always complaining about how hot the thing ran. So we eventually
went through everything including a complete teardown overhaul, reteardown
and I finally ended up calling up the old guy in Germany who said the timing
specs for that engine were wrong and to back them off two degrees. I whined
about this until as it couldn't possibly have that much effect. He promtly
reminded me who the old guy was and told me to just shut up and do it. Dang
old guy was right, two degrees of timing retard had a major effect on the
operating temperature of that engine.

Then later on I had a few customers who regularly got stuck in border
traffic so I retarded their timing a few degrees and voila, same thing,
cooler temps.

So, having had several conversations with many of you about CHTs being kinda
cold, or hot, once this re-epiphany fired from my left neuron to my right
neuron I decided to give it a try.

At 26 degs BTDC my CHTs are about 440 during a long climb out, and on a hot
day will go beyond 450 if I don't drop the nose. EGTs tend to stay about
1250 when leaned to short of peak.

After a couple of initial flights yesterday with the timing set at 22.5 degs
BTDC my CHTs were in the 325 to 350 range and the EGTs were up around 1425.

Now this jives with what I would think would happen as the heat, being
started later, is now getting more rejection through the exhaust, but I am
very suprised at how much change occured between a shift of 3.5 degrees.

Since I have the cheapo CHT/EGT gauge with the funcky switches I am not very
confident in any hard numbers I am listing here, but the relative change
does seem to be significant.

Now, with all that said one might then want to discuss the effect retarding
or advancing timing will have on power. In general retarding timing away
from that maximum point where the engine will begin to misfire or detonate
will reduce power as it shifts the maximum centroid of pressure away from
that optimum range where the pressure cycle will be fully utilized
converting to the most rotational power into the crankshaft. (energy
conversion cyclically changes from 0% at 0degsTDC to 100% at 90degsATDC)

But this would only be noticed at full power since aircraft engines don't
vary timing with engine RPM or load as cars do.  Well I didn't notice any
loss at full blast going down the runway, which is the only time I ever plow
around at that power setting. In fact it seemed to me that the engine was
running a tad bit smoother so may have actually felt a little more powerful.
(truely a subjective comment based upon not one schred of empirical
evidence)

In truth what has happened is this. There is some RPM value to which
Lycoming's setting of 25 degs causes the most efficeint conversion of
pressure into torque, (I would guess roughly 2300 to 2400 RPM) By changing
the timing one is just shifting where this occurs on the RPM scale. Since
reciprocating engines will vary power output with changes in timing at max
RPM, but if you don't fly at that RPM much, than your power really won't be
effected by a small change in timing. You might notice a slight change in
control knob position for a known MAP/RPM setting, but if you set it to that
MAP/RPM it will still produce the same power as before, or at least so close
that you wouldn't be able to tell the difference. I'm thinking the same will
be true for fuel efficiency, but will have to fly awhile to prove that.

So,

if your CHTS are very low, or too hgh, check your timing and try moving it
few degrees(retard for cooler, advanced for hotter). I would not recommend
going over 25 degs though as these aircooled engines are low RPM, large
piston, thermally critical assemblies, just looking for a detonation to
happen.

The last thing I want to say is that I played with getting the timing exact
between the mags vs being about a deg off from each other. Although I never
flew with them at very different settings, it did ground run noticeably
smoother with them set as close to each other as possible. (one degree off
was amazingly rougher at 1500 RPM)

So if some of y'all with them fancy recording engine things wanted to try
this I would love to see the hard EGT/CHT data from say 25 degs vs 22.5 degs
at max effort and cruise climb. (I know its a total pain to get the cowl off
and change this a few times, particularly if the gasket tears as it did on
my left mag, but if you're in there tinkering, it would be way cool to see
hard accurate data)

W


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