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safe boost ????
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mustangporsche  



Joined: 08 Oct 2003
Posts: 283
Location: California Lodi

PostPosted: Mon Dec 01, 2003 4:51 pm    Post subject: safe boost ???? Reply with quote

Hi, i was wondering what safe boost is for my 81 931, i have installed a large front mount ic, how much boost can i run now safely, with no problems??? thanks
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numbbers  



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

PostPosted: Tue Dec 02, 2003 10:59 am    Post subject: Reply with quote

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.
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u000533  



Joined: 30 Sep 2003
Posts: 17
Location: Bristol, UK

PostPosted: Tue Dec 02, 2003 11:09 pm    Post subject: Reply with quote

I'm running 1 bar with no problems, any chance of some pictures of your front mounted ic?

regards

Mike..
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-nick  



Joined: 16 Nov 2002
Posts: 2699
Location: Cambridge, MA

PostPosted: Thu Dec 04, 2003 12:22 am    Post subject: Reply with quote

The factory GT was hitting 11psi on 8.5:1 pistons and an intercooler. You can certainly at least hit that very safely. I imagine 1 bar is fine with premium pump gas and the stock 8.0:1 pistons, just as Mike is doing.

I'd like to see some pics / hear about the IC install too.

-nick
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81turbo  



Joined: 03 Nov 2002
Posts: 1065
Location: Oakland, CA

PostPosted: Thu Dec 04, 2003 2:14 am    Post subject: Reply with quote

The only true safe way to raise your boost is to install a air/fuel mixture gauge.

I am currently running 10psi no intercooler and my car is running just fine. My air/fuel gauge shows that my car isn't leaning out on boost so I feel that I can run more if I want to. But without the gauge you are really just guessing.
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SprintStar  



Joined: 26 Aug 2003
Posts: 150

PostPosted: Thu Dec 04, 2003 2:38 am    Post subject: Reply with quote

-nick wrote:
The factory GT was hitting 11psi on 8.5:1 pistons and an intercooler. You can certainly at least hit that very safely. I imagine 1 bar is fine with premium pump gas and the stock 8.0:1 pistons, just as Mike is doing.

I'd like to see some pics / hear about the IC install too.

-nick


Yes, I think the CGT only had 0.75 bar of boost while the RoW 931s had 0.65 bar.

I wonder about premium gas though. What RON do you get over where you are. I have a choice of 97 or 98 but she seems to run better on 98, so does it mean I may have pinging?

Sprint.
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-nick  



Joined: 16 Nov 2002
Posts: 2699
Location: Cambridge, MA

PostPosted: Thu Dec 04, 2003 6:32 am    Post subject: Reply with quote

I think our premium (~92-94 US octane) is the same as your 97-98 RON, or 5-star, or whatever you folks call it. If you're running higher boost, then you usually need more octane than what the manual calls for.

81turbo- you can still get detonation and have an ok a/f mix. I thought the consensus is that the fuel system doesn't start to lean out until you're hitting ~250hp range? Detonation is just from too much combustion chamber heat. Careful with that boost knob!

-nick
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numbbers  



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

PostPosted: Fri Dec 05, 2003 10:49 am    Post subject: Reply with quote

OK, if we are past the "safe boost" stage, I have been running 17 PSI for several years. However, when since I installed my new turbo this fall, I have been pegging my boost guage, which only reads to 20 PSI. Still no detonation, but the cylinder pressures have to be very high. I am assuming that my wastegate is not functioning, and the car is now stored for the winter. Before I put it back on the road in the spring, I will rebuild the wastegate, and set the boost back to 17.

I am not recommending you run your boost this high, and always caveat that I am in the Denver area, so I am above 5000 feet in altitude. So, I am not sure what the effect of 17 PSI would be at lower altitudes. Anybody out there educated in fluidetics or pressures who can enlighten us on this?
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Smoothie  



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

PostPosted: Fri Dec 05, 2003 12:03 pm    Post subject: Reply with quote

Numbbers - Concerning your higher boost, I'd suspect the new turbo and a different effect it has on your boost control valve as compared with the old turbo. Just adjust the valve. I say this because I have mine set to give 12psi boost, but occasionally I'll see it go about another 5psi higher. It seems to depend on rpm's, shift points, whatever... The way I drive 99% of the time, I get 12psi with the valve set as it is, but like I said, on a rare occasion it will go higher. In your case I think the difference in the way the smaller turbo spools up is probably having an affect on the adjustment valve.

I'm not edumicated in fluidyacallits or pressures, but your altitude would have some effect, though I couldn't say how significant. At sea level the ambient air pressure is approx. 1 Atm (14.7psi)(depending on the weather). It's the weight of 60miles of atmosphere above pressing down that causes the pressure to be 14.7psi. If you're a mile high, obviously your average ambient air pressure would be less. How much less, I don't know, but figure it's got to be something less than 0.5psi difference.
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'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox
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-nick  



Joined: 16 Nov 2002
Posts: 2699
Location: Cambridge, MA

PostPosted: Sat Dec 06, 2003 2:07 am    Post subject: Reply with quote

Actually, Denver air is pretty lean! Here on Earth the atmospheric pressure varies with height like this-

P=(14.7psi)exp(-h / 7400 meters)

5000ft = 1525 meters. Plugging it in for h, you get an air pressure of only 12psi. So numbers would need about 3psi more boost more than the rest of us who live near sea level- 17psi in Denver is about 14psi here at sea level. Still a good bit of boost!

-nick
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Smoothie  



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

PostPosted: Sat Dec 06, 2003 2:38 am    Post subject: Reply with quote

That's interesting - so a US 931 without modification in Denver air gets the equivalent of about 3psi max boost...but then again, the boost being limited by the wastegate and its' spring...even though the air is thinner, it would still have to pack the equivalent (to sea level) air charge before the wastegate began opening, right? In other words, a larger volume of the thinner air would be pumped in before the wastegate opened and the result would be the same air charge as at sea level.
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Smoothie  



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

PostPosted: Sat Dec 06, 2003 8:50 am    Post subject: Reply with quote

"The turbocharger was patented by Dr. Alfred Buchi in 1905 to improve the performance of aircraft. His original idea was based on the premise that the power of exhaust gases could be harnessed to offset the effects of diminishing air density at high altitudes."

found at: http://www3.bc.sympatico.ca/Volvo_Books/turbo4.html
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox
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wdb  



Joined: 02 Nov 2002
Posts: 2024

PostPosted: Sat Dec 06, 2003 10:31 am    Post subject: Reply with quote

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 .

and the wankle (rotory) engine was originally a supercharger .

http://www.gadgetonline.com/Super.htm
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numbbers  



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

PostPosted: Sat Dec 06, 2003 11:06 am    Post subject: Reply with quote

Yeah, you guys are going around the same way I do on the pressure issue. Obviously, atmospheric pressure is less at altitude, but 17 PSI is 17 PSI regardless of altitude. However, will air compressed to 17 PSI have the same amount of oxygen (molecules) at sea level and at 5000 feet. I beleive it will be the same, but I am not sure. I guess I will have to read up on it.

Also guys, I forgot to mention that my 931 is an '80, so I have 7.5 to 1 pistons. Therefore, my cylinder pressures are less with 17 PSI boost than an '81 or '82.
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Smoothie  



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

PostPosted: Sat Dec 06, 2003 1:28 pm    Post subject: Reply with quote

numbbers wrote:
However, will air compressed to 17 PSI have the same amount of oxygen (molecules) at sea level and at 5000 feet.


Yes - unless the ratios of the different gasses in air changes with altitute, you'll end up with the same amount of oxygen. Fill a plastic bag with a litre of air and shut it at sea level, then take it up to an altitude of 5000'... you'll have the same molecules, but the volume will increase because of the lower air pressure. -Or if you could, at 5000', transfer all the air from a cylinder that's pressurized to 17psi to a plastic bag. Bring the bag down to sea level and do the same again, transferring all the air from another 17psi cylinder at sea level into another plastic bag... Both bags will be of the same volume when compared together at the same altitude and they'll both have the same number of molecules.

I have a vague recollection of reading this somewhere, but - turbocharged engines are especially well suited to high altitude applications. They compensate for the effects of thinner air just by the physics involved in their operation.
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'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox
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