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Clarification... Posted by Larry West [Email]  (#1140)  [Profile/Gallery]
(more from Larry West) on Fri, 2 May 2008 22:40:39
In Reply to: Re: Boost is boost the purpose of a supercharger or a, Jay son, Fri, 2 May 2008 19:27:26 Members do not see ads below this line. - Help Keep This Site Online - Signup |
As Stephen uses the term (which is also the way I learned it) "boost" refers to how much pressure **over the local ambient pressure** you are adding to the motor. So, at sea level, with a pressure of (roughly...) 15 PSI, 4 PSI "boost" gives you a manifold pressure of 19 PSI. But at 10,000 feet, which is a local "standard" pressure of just 10 PSI, you're only getting a manifold pressure of 14 PSI with the same "boost"...
So the term "boost" is a relative one. In your example: "If you get 4 psi boost at sea level and the same boost at 10,000 feet you are developing the same horse power and torque..." you would actually be down about 5 PSI of manifold pressure.
Atmospheric pressure drops off at approximately 0.5 PSI per 1,000 feet of altitude. So what Stephen says is correct, that 4 PSI boost at one altitude gives him the same torque as 1 PSI at a lower altitude (though it would be about 6 thousand feet lower...), assuming the absolute pressure in the manifold is the same.
(absolute pressure is pressure relative to total vacuum - look up "pressure" in Wikipedia)
Far as the head, block, pistons, spark plugs, etc., are concerned, they do not care whether you're at sea level with zero boost and zero vacuum, or if you're at 10,000 feet with 5 PSI of "boost", if the absolute pressure in the manifold is the same, and assuming the air mass is the same, the engine should be making the same amount of torque at a given RPM (turbocharger losses disregarded at this point). So, there's no "BS" involved. You're merely giving the engine all the right ingredients to make a given amount of power.
The turbo, however does get less efficient, because it is starting with less to make the same amount of pressure. This results in a greater heating of the air from the compression, nearly negating any potential benefit from higher altitude, cooler air, etc. And at extreme ends of the turbo's performance "map", pressure starts to fall off, and it could damage the turbo.
Trionic T7 does one thing that earlier Trionics and other of Saab's APC or DI/APC systems did not. It allows the "boost" to go higher at higher altitudes, to make up for the lower ambient pressure. So, if the engine was limited to 10 PSI of "boost" at sea level (or an absolute manifold pressure of 25 PSI), at 10,000 feet, where the ambient pressure is 10 PSI, it will now allow 15 PSI of "boost", to achieve the same 25 PSI Absolute manifold pressure, and thus the same net output.
By far the best thing I have ever read on turbocharging is the venerable Hugh MacInnes book "Turbochargers" long published by HPBooks. It's got to be going on 30 years old (if not older), but I haven't read any other book that gives you the basics of turbocharging in an easy to digest form like this one. Sure, it's not new enough to cover more modern turbo technology, but the basics still have not changed. Amazon still has it. Look up "Turbochargers HP49 (HP Books)" (and disregard the $100.00 that comes up on the search results. they only want $19.95 for it).
_______________________________________ Current: 2002 9-5 Aero Kombi Cosmic Blue 2000 9-5 SE sedan, Imola Red 1990 900S Rose Quartz, Auto Past: 1999 9-5 LPT Combi 1999 9-5 LPT sedan 2002 9-5 Arc 1990 900 5-speed 1986 900 turbo Convertible 1991 9000 turbo 1980 99 GLi 1986 900 turbo 1986 900 S 1991 900 turbo 1984 900 turbo 1976 99 GL
, Fri, 2 May 2008 22:40:39 <-- Viewing This Message
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