Craig,
Sorry for the delay in responding.
Yes.
Yes.
At high MP, Max Brake Torque on
these big bore engines happens with peak combustion pressures at around 15 to
16d ATDC.
There is a broad range of spark advance
for which the torque is within + - 1 to 2 % of MBT.
However, that range of spark advance
covers an operating spread of peak cylinder pressures that ranges from benign
to RTTSPB (ready to turn spark plugs ballistic ).
So, to pull a Willy Clinton on
you, the answer to your question all depends on the meaning of the
word “best”.
If “best” means “the
difference between 600 and 605 ft-lbs of torque, I may not want to be
part of the discussion!
If best means what is the best possible
combination of engine operating parameters for very high continuous
torque and reasonable engine durability - - then we have a lot to talk about.
The recent data that came out of the trial
in Texas
suggests that the six cylinder 300+ Hp Lycoming crankshafts are rather marginal
in design. From data we have developed at our test stand, the
crankshaft torsional issues get to be rather excited if one tries to push the peak
cylinder pressures to the 15-16d ATDC range - - even if that would make more
torque.
I put those two pieces of information
together and reach the conclusion that if there are ways to operate the engine
at high power but minimize the peak torsionals, then that method ought to
be used.
I’m not sure I answered your
question. If not, let me try again.
Regards, George
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Craig Berland
Sent: Monday, May 09, 2005 1:32 PM
To: Lancair
Mailing List
Subject: [LML] Re: Plasma III on
Hi Compression Engines - Firewall Install Notes
"I would not be comfortable with the timing set
at 25d
BTDC for a 10:1 CR experimental version of
the 8.7:1 Lycoming engine. George"
George, is 25 deg BTDC over advanced? Or is the engine not
structurally capable of carrying the added torque? What is best timing
(that is timing to produce peak torque)?