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kellcats521's Avatar
 
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Overboost sensor

Is anyone running over 1 bar boost with an overboost sensor? Or, does anyone know if a higher value overboost sensor is available? I'm considering mapping the resistance curve of the stock sensor and adding another resistor to the circuit in series to 'recalibrate' it to cut off at say 1.2 bar, but I'll buy one if it's available....
Old 06-15-2007, 07:32 AM
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The overboost sensor you refer to is actually a overboost switch.
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Old 06-15-2007, 09:06 AM
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There are two "sensors".

One delivers variable resistance and is the one connected to boost gauge. Other one is just a switch that kills the fuel pump in case of runaway boost.
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Old 06-15-2007, 09:16 AM
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I guess it is a switch, technically, and as such could not be adjusted by adding a resistor.

So, what do people do with a 1 bar spring? Ground it and cross their fingers?

Pat K
87 930
Old 06-15-2007, 09:45 AM
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Quote:
Originally posted by kellcats521
I guess it is a switch, technically, and as such could not be adjusted by adding a resistor.
Correct. But you could always isolate it and raise the rating with a $30 manual boost controller if you had to...

Quote:

So, what do people do with a 1 bar spring? Ground it and cross their fingers?

Pat K
87 930
Are you even tripping it, or just trying to anticipate problems?

The factory test for the switch is to pressurize it with an ohmmeter across the contacts.

If it reads open (infinite resistance) at 1.5 bar, and recloses somewhere between 1.1 and 1.4 bar on the way down, that's all good according to the factory 930 manual.

If your wastegate regulates properly at the 1 bar mark, you shouldn't even trip an in-spec switch by the look of it.

And I wouldn't be depending on that guy too much to save you either (mine's disconnected).

Get a manual boost gauge and watch it, or an EBC with an programmable overboost cutout...
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Old 06-15-2007, 10:10 AM
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Mine cuts fuel at 1.0 bar - on both the OE gauge and an a VDO gauge in the clock location. My understanding is that this switch is a 1.0 bar boost cutoff, not 1.5, so I thought this was normal.

Maybe there is a 1.5 switch available as well.....
Old 06-15-2007, 10:19 AM
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Switch should short around 1.1 bar. Disconnecting is not prudent. Neither is installing 1 bar spring. Boost is much easier to raise by installing bleeder valve in series with wastegate hose, leaving everyting else the the same.

Standard paperweight turbo deadheads around 1.1 bar, while producing heaps of heat.
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Old 06-15-2007, 12:34 PM
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It is not uncommon for the stock overboost switch to trigger when using a 1.0bar spring. Any small boost spike will hit the limit. You also have the variables of spring rate and aging electrical components that may no longer be working within specs.

A controller of some sort would restore some protection from overboost and isolate the boost signal function to newer electronics that should be less prone to weakness or failure.
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Old 06-15-2007, 05:57 PM
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there has to be a pressure switch someplace that is set at 1.2 or maybe 1.5 bar. relying on a boost controller isn't safe either mine is currently grounded also but I want to replace it with something higher. with a boost controller and a k-27 hfs for instance all it would take is for the hose going to the boost controller to pop off or leak and boom no more engine.
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Old 06-15-2007, 10:45 PM
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ok Mcmaster has one that can be ordered in any set point from 5 to 300 psi in 5 pound increments. 1/8 npt threads the 20 psi switch would be 1.4 bar. Havent found anything from bosch yet.
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Old 06-16-2007, 12:47 AM
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I think you're describing a Hobbs switch. I have a picture and some info somewhere but I can't locate it right now.
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Old 06-16-2007, 07:14 AM
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ok I would think bosch probably has several different rated pressure switches that would be direct replacements but havent been able to find out for sure their online cattalogue sucks.
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88 turbo Guards red Targa slant nose, and yes I am a horsepower junkie, 3.4liter,7.5 to 1 JE pistons, Adjustable WUR, Imagine fuel head, 1 bar waste gate headers,allthe cis toys. Now apart to become the next EFI monster. fabbing my own intake, headers Individual throttle bodies, MS-3, pauter rods, Xtreme twin plugged heads, gt-2 evo cams cop's.
05 Cayenne S lapis blue
Old 06-16-2007, 07:29 PM
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Here is some info on the Hobbs device:

http://www.kipanderson.net/rs/hobbs.htm

http://content.honeywell.com/sensing/hss/hobbscorp/catswitch.asp
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Old 06-17-2007, 09:07 AM
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yup thats exactly what I was talking about. any idea what the thread pich is on the stock switch? So i can order the correct one?
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88 turbo Guards red Targa slant nose, and yes I am a horsepower junkie, 3.4liter,7.5 to 1 JE pistons, Adjustable WUR, Imagine fuel head, 1 bar waste gate headers,allthe cis toys. Now apart to become the next EFI monster. fabbing my own intake, headers Individual throttle bodies, MS-3, pauter rods, Xtreme twin plugged heads, gt-2 evo cams cop's.
05 Cayenne S lapis blue
Old 06-17-2007, 03:53 PM
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When I got my 92 turbo, the overboost sensor was grounded out. My car was insanely fast, and running super super rich. I am guessing to compensate for the high levels of boost. Once I pulled the engine, and fixed a couple of minor things (12 new plugs, plug wires, timing, mixture, etc.) I took the little ground wire off the sensor, and almost instantly starting hitting the overboost is 2nd through 5th. As my car has a Tial wastegate, we pulled it apart and found .8 and .3 bar springs in it. I removed the .3 bar spring and left the .8 bar one in and hooked everything back up. I was still hitting the overboost, but not as early in the RPM range or as often, so I went to Porsche and bought a new sensor and screwed it into the intercooler (which was somewhat of a PITA).

Long story short, no difference. I was frustrated, and did searches on here and Rennlist and did lots of reading. I read about incorrect sized header pipes, etc. Since I have B&B's on my car I was worried about that. So I went out to the garage to lay under the car, and while under there was thinking, hmmm, something doesn't look right.

Yeah, the Tial was hooked up wrong, so 5 minutes of time under the car and I was out for a test drive. I could hear the WG opeining and to this day have never had a problem with the overboost.

While that doesn't answer your question on the switch, my experience taught me to make sure everything is operating the way it should, since even with a 1.0 spring, you shouldn't be hitting the 1.2 bar cut off, unless of course it really cold where you are, smile.

Bill
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Old 06-17-2007, 08:06 PM
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