[200q20v] bar: from .1 to 1.8...is this normal?

Phil Rose pjrose at frontiernet.net
Mon Apr 23 17:06:03 EDT 2001

A speculation:

Both the OEM and the 2.5-bar sensors may have good linearity and 
produce exactly the same voltage _change_ over their rated pressure 
ranges. But I wouldn't be surprised if replacing the OEM device with 
a 2.5-bar sensor also introduces a voltage-intercept change (I'll 
call it "offset") along with the expected slope change in the 
calibration curve (line). AFAIK, the ECU chip modification 
procedure(s) do not include a correction for that.

When there's no offset,  then of course the simple correction factor 
(1.25) should work as Calvin states. But the presence of an offset 
can make the needed correction factor different from 1.25 and causes 
the factor to change along with the manifold  pressure. The fact that 
some people with 2.5 bar sensors (including Calvin) claim  to see 
extra-low readings (beyond a 1.25X factor) at the vacuum-end seems to 
indicate that a negative offset (of 2.5-bar sensor-output relative to 
the OEM gauge) may be common.

Suppose the calibration offset of a 2.5 bar sensor were -5% relative 
to the OEM sensor. Then the correction factor will be higher than 
1.25, and--it has an _inverse_ dependence on the dash gauge reading 
(and manifold pressure) as shown in the following table.

	bar (dash LCD)	correction factor
	2.0		1.31
	1.8		1.32
	1.5		1.33
	1.0		1.38
	0.5		1.50
	0.3		1.67
	0.2		1.88
	0.1		2.5

That--at least--might account for those really low vacuum readings. 
As I mentioned at the outset, this is just a speculation. I've not 
seen the specs on either the OEM or the higher-rated sensors.


At 10:59 PM -0600 4/22/01, Calvin & Diana Craig wrote:
>I see this comment about the boost gauge being useless after a chip change
>quite frequently.  Given the inherent inaccuracies of the digital gauge in
>the first place, the "usefulness" of the gauge doesn't change with a change
>in the pressure sensor.  The pressure sensor (motorola variety) just sends
>out a 0 to Vcc signal in relation to MAP.  The stock sensor is 0 to 2.0 bar,
>usually the ones that are insterted with a chip are 2.5 Bar. If you multiply
>the reading on your digital display by 1.25 (2.5/2.0), it should be right
>on, hence 1.8 * 1.25 would be approximately 2.25 bar. This is exactly what
>Scott Mockry measured on the graph of the 1991 Wagon's boost vs. RPM with
>the modified ECU.  On the low end, the 0.1 bar is probably a little off.  I
>don't know what the reason is behind this but both 200's that I have develop
>this problem sometimes where the boost gauge likes to read too low for a
>little while.  I do see .2 occasionally when it's working properly, but I am
>at 5k to 6k feet (stock ECU).
>Assuming that stage 3 means that the pressure sensor has been
>upgraded, your dash boost gauge is now almost meaningless. Surely you
>expect that your maximum boost is actually above 1.8 bar, right? Best
>advice: install an analogue boost gauge. Quick, cheap and easy to
>do--except deciding on a spot to install the gauge. Mine's in the
>glove box.
>200q20v mailing list
>200q20v at audifans.com


Phil Rose
Rochester, NY
mailto:pjrose at frontiernet.net

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