boost guage and zener diode questions...

Wed Jul 12 02:50:06 EDT 2006

Here is an article I wrote for the quattro quarterly in 1995

Tech Tip No. 1
Ned Ritchie
Copyright 1995 1998

HOW DO I CONVERT BAR TO PSI?

The boost gauge on my Turbo Charged Audi reads in BAR.
What is BAR and how do I compare that to PSI?

What we're talking about here is pressure; air pressure inside the intake
manifold.

Suppose we hooked two gauges one reading BAR and the other PSI to the
manifold so that both are sensing the same pressure.  With the engine off
what do we read?  One gauge reads about 1.0 and the other reads nothing.
What does that tell you?  Some pressure must be there because the BAR gauge
reads something, but if that is so why does the PSI read 0?

Remember your physical sciences. . . the weight of the earth's atmosphere
pressing down on us constitutes atmospheric pressure and is 14.7 "pounds per
square inch" (14.7 PSI).  Generally we ignore this 15 PSI of atmospheric
pressure.  An example is when we put 30 pounds (per square inch) of air into
our tire.  We put 30 pounds more than the 15 pounds into the tire.  The
gauge ignores the 15 pounds and indicates 30 PSIG.  The PSIG refers to
"pounds per square inch gauge" and what we usually think.  Pressures less
than atmospheric are negative gauge pressures that correspond to partial
vacuums.  If our tire gauge took into account the atmospheric pressure it
would be an absolute pressure gauge and would read 45 PSIA "pounds per
square inch absolute."

Absolute pressure is the total pressure including the effect of atmospheric
pressure.  An absolute pressure of 0 corresponds to a complete vacuum.
Atmospheric pressure is 1.01 BAR.

The BAR gauge in the Audi is an absolute pressure gauge and it takes into
account the atmospheric pressure.  It is an international unit of pressure
that is used in most countries.  It is equal to 14.5 PSIA or approximately 1
atmosphere.  This is why our gauge read about 1 BAR when the engine is off
at sea level.  Those of you at a higher altitude like in Colorado will
notice your gauge is lower than that.

Some of you may have seen the aircraft gauge in my car that is hooked to the
manifold.  It uses "Inches of Mercury (inHg) to measure pressure.  This is
also an absolute method and is used in aircraft because atmospheric pressure
changes with altitude and therefore normal pressure gauges won't work.
Atmospheric pressure is 29.921 inHg.  You've heard the weather person say
something like the BARometric pressure is 29.54 and rising...  They are
referring to the pressure of the air around us.  The pressure that we
usually ignore.

Atmospheric pressure can also be ignored when it comes to BAR.  Sometimes a
European tuner will say the engine is boosted to... or is running 1.1 BAR of
boost.  In this case he is referring to what we would call 15.9 PSIG boost.
He could also say the engine runs at 2.11 BAR.

Here is a pressure conversion table to file away for future use.  Just
remember the following:

When working with PSIG you must add 14.696 PSI to get PSIA
When working with PSIA you must subtract 14.6967 PSI from PSIA to get PSIG.

=====================

The table did not come out well in text so I deleted it.
Here is a link to other of my articles for those who forgot or never knew.
http://intendedacceleration.com/technical.html

=====================
Let's try to convert something.  Conservative European engine tuners say
that 13.5 pounds is the maximum boost for the 8.5 to 1 compression ratio
early quattros running 98 RON  unleaded.  (I know you can't get 98 unleaded
at the pumps and your Audi is not 8.5 to 1)  What should the BAR gauge read?

Take the 13.5 pounds boost and add 14.7 to get 28.2 PSIA.  Then multiply
28.2 PSIA by 0.06895 to get what it should read on your BAR gauge.  The
answer is 1.9 BAR.

One more thing.  The BAR gauge on the Audi except on the early cars and the
1992 S4 usually reads low and slow.  Remember atmospheric pressure is
1.01325 BAR!
=======================

Another thing I'd like to mention.  The zener fails to protect the engine if
the engine pings, because the zener at high boost tricks the computer into
running as if the boost is low and therefore there is not the need to reduce
spark timing. 

Granted it allows for unlimited boost and if the octane of the fuel is high
enough you should have no problems.  If too low be very careful.
Intended Acceleration never used this method.

Can't believe I even looked at this list and even more surprised that I
replied, but I'm an educator.

Ned
So long until next time.

-----Original Message-----
From: quattro-bounces at audifans.com [mailto:quattro-bounces at audifans.com] On
Behalf Of Alex Kowalski
Sent: Tuesday, July 11, 2006 4:36 PM
To: quattro at audifans.com
Subject: Re: boost guage and zener diode questions...

Jason wrote:

>i ask cuz i finally got around to hooking up my
>aftermarket boost guage, and the numbers
>certainly are not jiving.

>in fact, they're not even consistently not
>jiving.  sometimes the digital guage will show
>1.2BAR, and the new guage shows 3psi, while other
>times it may show 6psi.

>weird.

Nope, it's working perfectly.  Atmospheric pressure is 14.7 PSI and the
dashboard boost gauge reads manifold pressure with the atmosphere = 1 BAR.
That's why it shows less than 1 BAR when the throttle is closed, indicating
vacuum.  Therefore 1.2BAR on the dashboard is about 3 psi of boost and you
shouldn't see more than about 8 psi before you hit the fuel pump cutoff. ~ 6
PSI sounds about right for an MC1 that is functioning relatively normally,
that would be 1.4 BAR on the dash gauge.

Zener help in Canada is going to have to come from someone else...  ;)

Cheers
Alex Kowalski
'87 5KCSTQ
'86 5KCSTQ
_______________________________________________
quattro mailing list
quattro at audifans.com
http://www.audifans.com/mailman/listinfo/quattro
---
Watch this space for ads :)