X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [65.33.163.227] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 4.3c5) with HTTP id 941933 for lml@lancaironline.net; Thu, 12 May 2005 16:05:02 -0400 From: "Marvin Kaye" Subject: Re: [LML] Re: Plasma III on Hi Compression Engines - Firewall Install Notes To: "Lancair Mailing List" X-Mailer: CommuniGate Pro WebUser Interface v.4.3c5 Date: Thu, 12 May 2005 16:05:02 -0400 Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset="windows-1252"; format="flowed" Content-Transfer-Encoding: 8bit Posted for "George Braly" : """ "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)? """ 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