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BOOST Question

I am having difficulties reading/understanding my boost reading. First off, I have a supercharged car, but as someone pointed out boost is boost !!!

My clock is a VDO reading in inch of mercury to the left and boost in lbs to the right (common I know), and I belive to be hook up properly, was done by PO who had the whole system done by SOK I belive.

My problem,... I am seeing boost up to 5 to 5.5 lbs only in acceleration, once I am up to speed regardless of rpm my boost always level off down to 12-14 inch of mercury, therefore vaccum. Is this normal,... ? I've read somewhere that some are reading boost even at idle.
There was a thread posted not long ago about how some of the 930 guys are reading boost miles on end or until gas tank is empty.

Now I am wondering f I have a bad gauge, hook up at the wrong place, or my best guess I am leaking air somewhere.

The car pull strong while under boost, but not as much as it should as I was told by others. Can someone describe to me what should I be seeing or expecting from my boost gauge under normal operation: ie: when boost comes on, and should it stay, how about deceleration and cruising speed.

Any help would be great

thanks

Stephan
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Old 10-20-2009, 07:03 AM
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Plug it into intake, between the throttle and heads. That way, you will be able to read boost and vacuum. You should be seeing vacuum when engine-braking and at light load, and boost when on full throttle.


PO maybe T-ed boost gauge into brake booster. it has a check valve that might mess up the readings. So use separate hose and find a place to connect it to on the plenum.

I'm not familiar with inch, lbs etc. but you should be seeing anything between 10-100kPA absolute pressure at idle/coast and 100-200kPa on boost.
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Old 10-20-2009, 07:19 AM
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1 Bar = 14.7 psig

In your case 5 to 5.5 psi equals .34 to .37 Bar, well below the capability of a stock 930. Like the prior poster indicated, you might have a location issue or a bad gauge.
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Old 10-20-2009, 01:54 PM
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If you read vacuum at cruse or idle you are hooked up between the throttle plate and the intake valves. Your gage is seeing what your motor sees.

If you hook it up after the compressor and before the throttle plate and there is not a relief valve of some type to vent the excess pressure, you would see boost at idle and cruse.
Old 10-20-2009, 03:09 PM
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Quote:
Originally Posted by boxermania View Post
1 Bar = 14.7 psig

In your case 5 to 5.5 psi equals .34 to .37 Bar, well below the capability of a stock 930. Like the prior poster indicated, you might have a location issue or a bad gauge.
1 Bar = 14.5 psi
1 Atm = 14.7 psi
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Old 10-20-2009, 03:18 PM
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I thought 1 bar was suppost to be the same as so called atmospheric press. 14.7 psi.
Old 10-20-2009, 03:56 PM
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Quote:
Originally Posted by 964 T #304 View Post
I thought 1 bar was suppost to be the same as so called atmospheric press. 14.7 psi.
Nope 14.5 psi

Pressure - Wikipedia, the free encyclopedia
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Old 10-20-2009, 03:58 PM
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Nope. 1 bar = 14.5 psi

Just for fun, Google will do instant conversions. Try Googling 1 bar = ? psi
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Old 10-20-2009, 04:05 PM
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Not a Porsche, but my Procharged 5.0 (centrifigal supercharger) only sees boost during acceleration- begins around 3500 rpms, max boost at redline. At idle 14 inches and decel I am seeing 17 - 19 inches vacuum. Cruise varies 5 - 10 inches vacuum I think...
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Old 10-20-2009, 04:14 PM
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Thanks for the reply and suggestions. I will try what some have mention and see what if any changes.
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Old 10-21-2009, 11:24 AM
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You will only see boost when the engine is under load (acceleration or going up hill).
Old 10-21-2009, 11:29 AM
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So if I was to install a water meth injection (progressive system) that rely on boost and increase with boost: where I would want to hook up this gauge?

I think my system is tapped from the "T" that connects to the brake booster, where is that exactly pre or post throttle plate?

My apologies for the rookie questions, but I'm not a mechanic, but willing to learn and simply call things by what it looks to me : ex metal thingy or plastic thingy,..

Thanks for all the help
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Old 10-21-2009, 11:38 AM
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typical vacuum/boost source is after the throttlebody.
Old 10-21-2009, 01:31 PM
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Quote:
Originally Posted by drmatera View Post
You will only see boost when the engine is under load (acceleration or going up hill).
This statement seam in contradiction with what was said by the 930 crowd in the following thread:
How long for boost?

Not that I am implying that anyone is lying but if there is several ways to measure boost,... this means that boost is relative to your system and can not be compare one car to another,... am I wrong

Stephan
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Old 10-21-2009, 02:47 PM
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Boost is boost. On the other hand, mass air flow and resulting horsepower will vary enormously at any given boost level with charge temperature, humidity, port flow, exhaust back pressure, etc.. Also, all boost gauges are not born equal and boost pressure will vary with where you measure it in your intake path.
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Old 10-21-2009, 03:04 PM
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Quote:
Originally Posted by 911st View Post
If you read vacuum at cruse or idle you are hooked up between the throttle plate and the intake valves. Your gage is seeing what your motor sees.

If you hook it up after the compressor and before the throttle plate and there is not a relief valve of some type to vent the excess pressure, you would see boost at idle and cruse.
Thanks 911st, I have read your post ...probably 20 times now,... I think it starts to sink-in now. This makes more sense to me.

So again, if I was to install water/meth injection system with my boost gauge connected where it is right now,... I would only see a little squirt here and there for less than 3-5 second at each time on a progressive injection system. I wounder if these injection system can be set to inject while the gauge reads vacuum at say a preset point of 5" mercury, which would benefit my engine while coasting???

Any thoughts
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Old 10-21-2009, 04:41 PM
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You may want to read the methanol injection thread:

methanol Injection
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Quote:
Originally Posted by YZR500 View Post
This statement seam in contradiction with what was said by the 930 crowd in the following thread:
How long for boost?

Not that I am implying that anyone is lying but if there is several ways to measure boost,... this means that boost is relative to your system and can not be compare one car to another,... am I wrong

Stephan
there is no contradiction - at very high speed there is significant "load" on the engine, due to aerodynamics, that when coupled with high enough RPM will cause the turbo to generate boost. But lazily cruising down the highway at 3000rpm you most likely will NOT see boost pressure in the manifold
Old 10-22-2009, 07:28 AM
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