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safe boost ????
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Paul  



Joined: 02 Nov 2002
Posts: 9430
Location: Southeast Wisconsin

PostPosted: Sun Dec 07, 2003 1:00 am    Post subject: Reply with quote

numbbers, what does your gauge read when the motor is not running?
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Smoothie  



Joined: 01 Jan 2003
Posts: 8030
Location: DE (the one near MD, PA, NJ)

PostPosted: Sun Dec 07, 2003 1:49 am    Post subject: Reply with quote

Hey Paul - If you're thinking what I think you're thinking, then I already thought about it, but thinking about it some more brought up another thought. (yeah I could write for Yogi Berra)

If I'm not mistakin', the boost and vacuum/boost gauges normally found in cars - their internals consist of a coiled tube that's connected to the display pointer. Boost pumps up the coil and expands it outward moving the pointer one way and vacuum pulls the coil the opposite way. The coil, being open to ambient air, would read "0" with the engine shut off at both sea level and high altitude. My initial thought on it was - I wonder if a vacuum gauge at 5000' reads a bit on the vacuum side when at rest, but if the automobile gauges are made the way I think they are, they'll read zero no matter what the altitude is. If, on the other hand you were dealing with a vacuum, boost or vacuum/boost gauge of a different design incorporating sealed internal parts...(one like that would likely be calibrated to read zero at sea level pressure), it would read negative boost or the equivalent of -3psi on a vacuum gauge. A divers' depth gauge and aircraft altimeter would be examples of gauges with sealed internals and I'm guessing there are vacuum gauges designed similarly for industry and science.
The other thought that thinking about this brought up concerns the thinner ambient air at altitude and would that thinner air surrounding the coil in the boost gauge allow the coil to expand more at a given pressure than it would at sea level. I'm thinking yes, but it would be a negligible, virtually unreadable difference.

Here's what's inside the average vacuum or boost gauge (a Bourdon tube): http://www.tpub.com/content/fc/14104/css/14104_232.htm
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'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox
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numbbers  



Joined: 05 Nov 2002
Posts: 1910
Location: Highlands Ranch, Colorado

PostPosted: Sun Dec 07, 2003 11:08 am    Post subject: Reply with quote

Well guys, My guage reads 0.0 with the engine off, and at idle. Keep in mind that my guage is tapped off of the pressure duct at the overboost pressure switch port, so my guage does not read engine vacuum.
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MunkPuppy  



Joined: 02 Jul 2003
Posts: 419
Location: New Westminster, B.C., Canada

PostPosted: Mon Dec 08, 2003 10:55 pm    Post subject: Reply with quote

wdb wrote:
and did you know that it was were they got the idea of the jet engine ,using the exhaust to power the compressor . although it took 30 + years .


A little side-note to this entry:

I was watching Junkyard Wars last week and the week's challenge was to build a jet-powered car. One team build what is called a Pulse-jet, while the other used a much more interesting method:

They salvaged a working turbocharger from a diesel transporter, made a few alterations to it, and produced a powerful, and awesome-sounding jet engine! The concept is pretty simple too:

They welded a combustion chamber between the compressor and turbine sides of the turbo. Air goes into the compressor where it is compressed, which then goes into the combustion chamber where it is mixed with fuel using a fuel injector, and ignited with a spark plug. the resulting expanded and very hot gas goes into the turbine side which drives the compressor. The gas is then thrust out the back of the turbo where it was pushed through a nozzle, resulting in thrust.

The amount of thrust this tiny jet engine produced was enough to push the golf-cart chassis beyond 70MPH, and given a tighter nozzle and afterburner, I'll bet that car could have topped 90-100MPH! Too bad the pulse-jet car won. The turbo-jet had some problems becuase the oil system stopped working, the jet nozzle wasn't constrictive enough, and the afterburner didn't work. The only other problem was that in order to get the engine started, they had to use a leaf-blower to initially get the turbine spinning.

If you get a chance to watch this episode, you SHOULD! It was awesome!
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CMXXXI  



Joined: 05 Nov 2002
Posts: 1939
Location: Vicksburg, MS

PostPosted: Tue Dec 09, 2003 4:44 am    Post subject: Reply with quote

MunkPuppy wrote:
wdb wrote:
and did you know that it was were they got the idea of the jet engine ,using the exhaust to power the compressor . although it took 30 + years .

...The amount of thrust this tiny jet engine produced was enough...

<I'm really going OT here...>
Yes it's pretty amazing isn't it. Jet engines really produce a lot of energy for their small size. Did you know that the engine in the M1 tank is actually only about 2 feet long and a foot in diameter. From that small engine, it will propel the 60+ ton vehicle over 50MPH. It's hard to fathom that something that small could have replaced an air-cooled V-12 and outperformed it as well!
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Lizard  



Joined: 03 Nov 2002
Posts: 9364
Location: Abbotsford BC. Canada

PostPosted: Tue Dec 09, 2003 11:38 am    Post subject: Reply with quote

numbbers wrote:
We have been through this many, many times. The only safe boost is the factory boost, about seven PSI. After that, you are taking your chances. However, many of us run higher boost, including me.

I will disagree with this statement

if 7 PSI is the limit why did they have 10PSI in the euro editions?

and why where they able to keep 10PSI on euro spec w/ 8.0:1 CR, so I would say w/ 7.5:1 CR that 12 PSI would be plenty safe w/o an intercooler, w/ 1 bar would be safe imo
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924 turbo  



Joined: 02 Nov 2002
Posts: 1566
Location: Simi Valley, CA, USA

PostPosted: Tue Dec 09, 2003 11:56 am    Post subject: Reply with quote

Lizard wrote:

if 7 PSI is the limit why did they have 10PSI in the euro editions?

and why where they able to keep 10PSI on euro spec w/ 8.0:1 CR, so I would say w/ 7.5:1 CR that 12 PSI would be plenty safe w/o an intercooler, w/ 1 bar would be safe imo


The Euro/ROW engines are 7.5:1 and 8.5:1, and about 10psi and 9.5psi, respectively.
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