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(leaving Bob's post for context)
I like 44gph as Bob wrote.
If you are into the APS stuff, the reason for doing this is that it slows down the wave front so that peak pressure occurs well after top dead center and reduces its magnitude. Cylinder temp is generally an indication of peak pressure because there is more thermal transfer from the combusting gasses to the cylinder wall when the pressure is higher.
High RPM also has the effect of moving peak pressure further after top dead center.
As I understand it, wide open throttle on this engine results in a richer mixture than partially closed throttle.
Putting all that together, reducing throttle and rpm actually pulls the angle at which peak pressure occurs (theta-pp) back toward top dead center and sharpens it.
It would be cool to get some data from Ada on peak pressure magnitude for these two different regimes.
My engine is actually at 45gph now and will climb indefinitely at 38"/2700 rpm/140 knots with temps all below 380 dF, at least in Vermont, and I have the normal cooling mods + exhaust tunnel extensions. There is more friction loss at higher rpm but for 10 minutes of climb I would view peak pressure as the greater evil.
So far, no one from APS has disagreed. (bcc to George and John in case they want to chime in). To John's original question, I don't know anything about PE engines. Mine is a tsio-550B with 7.5:1 pistons. Based on theory, I would not want to be leaner than 44 with a higher compression ratio.
As for the lightspeed ignition, I am not an expert on that either. My understanding (or lack of it) resulting from talking to Klaus on the phone is that the lightspeed has preprogrammed earlier timing when at lower power settings. However, since Klaus wouldn't say what the actual timings are, it's kind of hard to put this together with the TCM setup guidelines. Those guidelines are very intimately married to the ignition timing guidelines. This is one reason I have two mags and no electronic ignition. I think you can determine the timing yourself if you have the most informative Lightspeed display. There's some threads on this in the archives. If you can verify that Lightspeed timing at max power and high cruise is equivalent to TCM specs then that would be a help.
Getting all this right on the ground is difficult because there is so much power developed, it tends to move the plane despite best efforts. One approach is 4 shot bag in front of each main + 1 guy on each wingtip + pilot on the brakes + nothing in front of you. As Bob says, it takes about 10 secs for the boost to get all the way to 38.5". If you don't wait long enough, you will set it too rich. Personally I don't like having anyone standing by the engine in this regime because if the plane moves the wing will tend to push them into the propeller. ....so it's a number of restarts at best and then you want to test fly it. On test flight if the flow gets over 45 you should be ready to do something about it.
Lastly, there have been some incidents/accidents due to the bolt on the altitude compensator getting loose. It goes to the rich side when this happens. Somewhere around 48-50 gph it will make black smoke and quit (so make sure it's not loose) In some cases, backing off the throttle has helped avoid a crash. Likely CAREFULLY backing off the mixture would also help but it might take too long. In the cases I know about, this tends to happen shortly after liftoff.
Colyn
----- Original Message ----- From: "Robert Pastusek" <rpastusek@htii.com>
To: <lml@lancaironline.net>
Sent: Tuesday, July 07, 2009 8:02 AM
Subject: [LML] Re: Engine settings - TSIO-550
John Barrett wrote:
The TCM guidelines are clear and that is probably fine to go with (?) but is
there any reason to modify this with a Performance Engines engine with one
Lightspeed ignition and one mag?
John,
I originally set up my Performance Engines TSIO-550 (2 mags) to flow 43 GPH
at 2700 RPM and 38.5" MP on takeoff. My technique is to advance the throttle
all the way over about 3-5 seconds to minimize rudder/braking and prop
damage at the start of the roll, and I get to those numbers at about 10
seconds after reaching full throttle on TO.
I climb at full rich, 2500 RPM, approx 32" MP and 160 kts after the airplane
is cleaned up. This requires periodic throttle adjustment to maintain 32" MP
as I climb, and results in a fuel flow of about 30 GPH and very cool running
engine. Over the winter, I noticed that the fuel flow was slowly decreasing
during both TO and climb...to about 38 GPH and 27 GPH respectively. Also, as
the weather got warmer this spring, I saw cylinder temps (#3) reach 380
degrees for the first time ever (at about 200 hours TT engine & airframe).
During the condition inspection in May (250 hrs TT), I boosted the fuel flow
back to 43 GPH on TO with the above MP and RPM settings. This returned my
cylinder temps to below 380 until today, when I hit 380 passing 15,000' in
the climb on #3. This was over New Mexico, with surface temps above 100 and
the ISA deviation at 15,000 at +40 degrees. I leveled at cruise altitude of
16,000 and the cylinder temp quickly dropped back to its normal 340 or so.
This was an unusually hot day and the cylinders never got near the 420
recommended as max continuous, so I'm thinking the fuel flow and technique
are still OK.
The new Continental specs call for a fuel flow of 38 GPH at the above (TO)
settings. The old spec was 44 GPH. I don't know why they changed...perhaps
because the engine will quit if fed too much fuel and they wanted to
introduce a safety margin? My engine quits at about 48 GPH on TO roll on an
approximately standard day (ask me how I know). 38 GPH is not enough fuel
flow to keep my cylinders cool during less than full-power climb (32" MP @
2500 RPM). It might be OK at full throttle/2700, but I have never run the
engine that hard for the extended time it takes to climb to altitude, and
don't intend to. So for me, 43-44 GPH seems to be a good TO fuel flow
setting for my engine.
BTW, as noted in a recent post, turning the boost pump on high on TO will
run the fuel flow to about 48-50 GPH and stall the engine... It will also
stall the engine if turned on high at idle. Some run the fuel pump on low
continuously from engine start. I set my engine up to the above figures with
the boost pump off. My rationale is that the engine runs fine without it; if
the engine pump fails, the boost pump will keep it going, at least at
partial power. If the engine is set up to run optimally with both
engine-driven and electric boost pumps on, a failure of either pump will
cause a problem. At the minimum, the engine won't develop full power, will
over heat, and may stall.
This is all from reading several sources that disagree slightly, and my own
testing/experience. I'd welcome the views of others, and any better
sources/rationale!
Bob
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