Tim,

 

Ahh, thanks for the write-up.  If the fuel and air are controlled, then one must look at timing.  The setup is one mag, one Plasma ignition.  Let's assume the magneto is fixed at 25 DBTDC.  For High CRs (over 8.5:1), Klaus suggests retarding the Plasma pickup by 5 degrees.  At high power settings (measured by MAP and RPM), the Plasma may actually lag the mag firing.  At low power settings, the Plasma will precede the mag.  This does have an effect on timing (time of reaching max cylinder pressure), but it should be the same for all cylinders.  However, while the mag timing remains fixed, the Plasma is driven off MAP, RPM and data from the timing pickup device (for a 6 cylinder, it should be the crank/flywheel sensor). 

 

Does your friend have the optional Plasma Display installed? If so, it can be illuminating.  MAP and RPM should be pretty close to what the engine monitor shows and the display should not be jumping around too much.  Neither should the timing.  On my engine (9:1, Dual Plasma), the Plasma MAP and RPM read a bit lower than my engine monitor display (I trust my engine monitor - RPM checked against external sensor during prop balance) and I must assume the timing is accurate.  At take-off power (from 700 MSL), I see 22 DBTDC.  At ROP best power cruise, 3500 to 6000 MSL, I see around 25 DBTDC and at higher altitudes (less power) I will see more advance taking place.  Hmmmmm, I have not checked if there is a change in timing in level flight when switching from best power to best economy - The MAP and RPM being constant?........... Hmmmmm.....

 

I did have some excitement once when timing data from the sensors was bad (loose plug) and the engine ran rather rough.   Fixed timing from a mag/Plasma setup might cover that up somewhat.  Of course, in the case your referenced, Slick mags are capable of dropping sparks (too weak) and that would show up more in a high CR environment.

 

A note:  I have recently heard that many Lancair flyers with big 6-cyl engines have removed the Plasma system (either dual or mixed) because "it wasn't working right."  I don't know what that means exactly.  For me, the dual system works great on my IO 320.

 

Scott Krueger AKA Grayhawk

 

That is all good stuff.  I just did all that on my own plane.  We're
all talking Fuel Injected engines, that's for sure.  I just did this
write-up of my own experience doing the lean test and all:
http://www.myrv10.com/N104CD/upgrades/20090822/index.html

The problem is, there is one guy in particular who's a friend
of mine and I flew his plane, and analyzed his GAMI curves
and his are probably better than mine (and mine are < .2)
and yet he still can't get even all the way TO peak without
it stumbling.  So I'm trying to learn what other effects
to worry about.  Timing, Compression, Exhaust, and whatever
else.  My compression is 8.5:1, his is 10:1.  We have different
exhaust, but both are supposed to be good at exhaust scavenging
and good flowing.  Timing he's got to check and verify yet.
The interesting thing is, there are a few other people I'm
talking to who have plenums and circular inlets by
coincidence and they're all having the same problem...stumbling
as they get to 13gph or so...and considering I know the one
guys injectors are flowed well and peak together, I'm getting
curious as to how much of an effect things like Plenum air
pressures and the like will have on injector performance.
Looking for the things OTHER than the gph spread in the
EGT peaks that could cause things not to run at peak EGT or
leaner.

I sure hear you though....LOP can definitely be faster than
ROP when you start dropping off extra fuel stops from the
equation!!!
Tim

Sky2high@aol.com wrote:
> Listen up Grasshopper,
> 
> 1.  If the engine is carbureted, you may be out of luck as there is no
> way to control the F/A ratio in each cylinder.  However, you can try
> step 3 for informational purposes if you have the equipment listed in
> step 2.
> 
> 2. You must have an engine monitor and sensors that display (or record)
> EGT for each Cyl and the fuel flow.
> 
> 3.  Injected engines must perform the GAMI lean test before a "next
> step."  At 75% power (that means at or above 6500 MSL, 7500 if using ram
> air and KIAS is less than 190, or higher), WOT (necessary to control air
> distribution when running LOP) .
>     a. Starting sufficiently ROP, lean in small steps (less than .3 gph)
> and record the EGT for each step for Cyl 1.  Continue past peak EGT for
> that Cyl for about 50F degrees LOP or until ugly roughness.
>     b. repeat "a" for each cylinder.
>     c. Check the gph for each cyl peak EGT.  If the spread in .3 gph or
> less, you are done and you should be able to run LOP 20F, 30F, 40F
> depending on the spread).
> 
> 4.  If the spread is GT .3 gph,  you must fix the A/F ratio.  A cheap
> way to start is to provide equal air pressure to each injector - a wee
> plenum distribution to each shrouded injector) and repeat the Lean Test.
> 
> 5.  if the lean test shows the proper spread, go fly LOP (less than 75%
> power)...................
> 
> 6. If the lean test fails (GT .3 gph spread), consider different sized
> injector nozzles (either GAMI or others) for the rich/lean cyls.
> 
> Grayhawk can fly a 320 at 8500 MSL, ROP, 9.5 gph and see 195 KTAS or
> LOP, 7.5 gph and 186 KTAS (all things being equal) thus eliminating a
> fuel stop at 3.5 hours cause he could go another hour. 195 x 3.5 = 683,
> 186 x 4.5 = 837, 837/195 = 4.3 + 1 (re-fuel stop time) = 5.3  Duh!
> 
> Grayhawk
> 
> In a message dated 8/24/2009 2:40:57 P.M. Central Daylight Time,
> Tim@5000feet.com writes:
>
>     Hey guys, I am remembering a long time ago reading about
>     how the air pressure surrounding the injector can affect how
>     well the injector works.  If I'm not mistaken, turbocharged
>     engines even use some pressure manifold around the injector
>     to accommodate for the increased Manifold Pressure.
>
>     What gets me curious is that I'm having a discussion with some
>     folks right now who are all having problems running Lean of Peak
>     and their engines stumble before they get to peak EGT. Between
>     the group, there are a variety of ignitions such as 2 mags
>     or 1 lightspeed and one mag, and varieties such as 8.5:1,
>     9:1, and 10:1 compression ratios, but the one constant is that
>     they all have what is purported to be a more effective cowl...
>     the Sam James cowl with a plenum.  So I'm wondering if the
>     knowledge base some of you have would provide any insight
>     to the issue. I originally was worried that the 10:1 pistons
>     might make LOP operation harder to accomplish, but perhaps
>     there is just too much air pressure on the outside of the
>     injector?  The cowl is known to often have too little exit
>     area, so it could be that the pressure differential is
>     even too high between the top and bottom half of the
>     cowl.
>
>     Any insight you can give?
>     Thanks,
>     Tim
>
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>
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