Return-Path: Received: from [65.33.164.252] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 4.2.5) with HTTP id 584215 for lml@lancaironline.net; Thu, 30 Dec 2004 08:56:25 -0500 From: "Marvin Kaye" Subject: Re: [LML] RAM, SLAM, BAM - MAP, WHAP, ZAP - HUH? To: lml X-Mailer: CommuniGate Pro WebUser Interface v.4.2.5 Date: Thu, 30 Dec 2004 08:56:25 -0500 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" : Skip writes: “MAP (displayed in inches of Hg) is not actually “absolute” since it is ambient pressure (AP) plus gauge sensed pressure (GP). “ Ah… Skip, actually, manifold pressure as the term has been commonly used for aircraft engine operation for the last 80 years or so, is truly an “absolute” pressure measurement, referenced to a perfect vacuum. It has nothing to do with “gage sensed pressure (GP) “ Any other notion adopted by VM, or anybody else, is inconsistent with the normal and common engineering usage of “manifold pressure” as used by engineers with respect to aircraft piston engines. It would also introduce an enormous source of error, uncertainty, and confusion in all engine test data. Ram air effects are fully accounted for by an absolute manifold pressure sensor that is properly plumbed into the induction system. Any other “adjustment” for some other source of air pressure is simply an erroneous concept. Many systems have a vent system incorporated into the pressure fuel sensor designed to pick up ambient pressure, or a designated reference pressure, but that has nothing to do with manifold pressure, even if the fuel pressure mechanism happens to be located inside the same 3” round instrument as the manifold pressure sensing system, which is the common way of doing that. In a very well designed normally aspirated engine installation, adding and installing a ram air system and bypassing the induction air filter may raise the cruise MP by up to 1.5 to 2” of MP at normal cruise speeds, compared to not having such a system. On a turbocharged engine, it can improve the critical altitude by several thousand feet, as one enjoys a multiplier on that 1.5 to 2 inches, the magnitude of the multiplier being the pressure ratio across the compressor under the applicable cruise conditions. Regards, George