[urq] CIS airflow meter limits HP?

JShadzi at aol.com JShadzi at aol.com
Sun May 5 20:37:36 EDT 2002

[ Picked text/plain from multipart/alternative ]
Glen, as you know, the CIS metering unit delivers increased fuel via pressure
changes within the metering head, as a function of the position of the
airflow meter arm.  The arm has a range of movement of about 15 degrees or
so, the airflow plate has a diameter of about 85mm I believe, and the opening
to the air flow meter is the same diameter.  Once the airflow meter reaches
the top of its travel, it no longer can control the amount of fuel pressure
withing the metering head.

Now, the airflow meter can limit power in 2 ways:

1)  By measuring potentiometer voltage output (the center pin) you will see
that at about 5v, the airflow meter has topped out.  IME, at about 15psi
(K26) by about 5k rpm the airflow meter will have topped out, limiting fuel
indirectly - most people blame the injectors and go in search of larger ones,
when the injectors are not the problem.

2) Once the airlfow meter has topped out, it will limit airflow, no empirical
evidence needed there, its basically a throttle.  Interesting to notice, when
V8 guys upgrade their carb's, the bigger carb's are rated by CFM, but for
some reason, Audi CIS guys never think that theri CIS meter (basically a
carb) will ever become an air restriction, though by looking at it, it almost
gives me an air-flow-heartattack.  Though I've never actually done a flow
bench test on the airflow meter, IME, any car with Audi CIS is very tough to
tune over 250hp (yes, dyno #'s there), though many factors start becoming
restrictive above that number, you are trying to flow 300hp of air through an
airflow meter designed to flow 165.

It stands to reason that the CIS system is limited to certain levels of HP.
If it were an EFI car, to go from 165hp to 300hp all sorts of components
would need to be changed, why not in CIS, why do we think that it is
unlimited in its power potential?  In fact, Audi/Bosch engineers would be
poor engineers if they developed a fuel system for a 165hp car that was still
capable of fueling to 300hp.  That kind of application would cause very poor
resolution in the stock application, why overdesign a component to the point
that it would sacrifice the intended application??

The CIS system is precise and flexible, but only to a point.  It will run
well all day long with 12psi boost and about 225hp, but once you try pushing
past that point, every hp is an excersize in diminishing returns (IME tuning
Audis for over 4 years, many of them race cars).

Anyway, the point is mute, show me a CIS Audi that can actually dyno more
than 275hp, and I'll rest my case.

Also, the fact that Porsche runs CIS is completely irrelevant, its like
saying that a Geo with EFI should be able to run the same amount of power as
a Viper.  Porsches used different systems with differnent components, and
were developed for a specific application, as all CIS systems were.

Long past dreams of a better CIS,


In a message dated 05/05/2002 5:01:40 AM Pacific Daylight Time,
fundsaloracing at yahoo.com writes:

> That may well be true! Do you have any quantitative
> evidence? Dyno runs? Rumor or speculation? Any ideas
> on a quantitative test to prove at what point the
> airflow meter does start to limit HP?
> -glen

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