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From: Walter Atkinson <walter@advancedpilot.com>
Subject: Re: [LML] Ignition timing

Gary:

You have included so many variables in your question as to be 
impossible to answer without putting the engine on a computerized test 
stand and MEASURING the changes.  Based on what I've seen in testing, 
you might expect a BSFC improvement of somewhere on the order of 0.015 
or so if all you do is optimize the thetaPP by timing changes from the 
magneto setting from the manufacturer.   The one thing that you didn't 
list which plays a big part in the equation is the exact LOP mixture 
you are choosing--you would have to choose BSFC(min) which changes 
based on the power setting.  That will have a larger effect on BSFC 
than small timing tweaks.  The 25d timing may be good or horrible or 
anything in between.  Without measuring the ICPs, it's impossible to 
know.  Simply saying that all parallel valve engines should have one 
setting and all angled valves another, is a curious concept.

Basically, without some testing DATA, I'm afraid your question is 
unanswerable---at least by me.

Walter


On Dec 28, 2005, at 9:50 PM, Gary Casey wrote:

All this talk about the relative merits of EI and mags brings up a 
question that I can't seem to get answered - maybe someone out there 
knows:
Just to keep it simple, I'd like to know the BSFC of an engine (my 
engine would be even better) running at 20 inches and 2500 rpm, LOP, 
with the following configurations - dual mags, std timing; one mag std 
timing and one EI optimized timing; and finally two EI's with optimized 
timing.  To put icing on the cake it would be good to see a timing 
sweep for each of the cases.  And even better a map over more of the 
operating range, including full power. I've got a parallel-valve IO-540 
with 10:1 compression, one LSE-III and my engine builder set the base 
timing at 25 degrees.  He says that he sets all parallel-valve 
Lycomings at 25, even up to 12:1 and all angle-valve engines at 20 
degrees.  I'm uncomfortable having the setting at 25, but I have no 
data to support it.

Gary Casey

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