Re: Fire in the hole... Ross please read

[Mike Myers <mimyers@prairie.NoDak.edu>]

Ross, Ross, Ross, read this carefully, try to understand it, then read it again.

There are two major ways for computers to determine how much air is going
into the engine, they are mass-air and speed-density.

Mass-air uses a sensor in the intake to measure the volume of air going
through this. Example of this are hot wire sensors.

Speed density calculates the volume of air going through by comparing the
pressure of the air in the intake with it's temperature.  Example of this is
the HK$ VPC.

Audi uses a combination of these two.  The air flow sensor plate in the air
box is along the lines of a mass-air setup and the vacuum hoses leading to
the computer is along the lines of a speed density setup.  The computer uses
a combination of the two to get the fuel delivery right.

If you had a true speed density setup, closing the throttle would create a
vacuum in the intake manifold which along with the closed idle switch
creates a situation where the computer cuts fuel flow to the engine.

In a speed density setup, when the blow off valve opens, the mass-air sensor
is still reading X amount of air flowing into the engine, while the engine
is actually using Y amount of air.  X-Y = amount of air flowing out the
blowoff valve.  The computer doesn't know they the engine is only using Y
amount of air, it thinks the engine is using X amount and dumps the
corresponding amount of fuel, much more than it actually needs.

Audi uses a combination of these two.  It could go one way or the other, but
by Scott's posts, assuming he's correct, the computer must continue dumping
fuel into the cylinders (Continuous Intake System??).  By routing the
blowoff valve back into the intake of the turbo, you no longer need air from
an outside source, slaming the air flow sensor shut, reducing the fuel
delivery, keeping everything happy inside.

The question here is not whether the blow off valve helps or not.  It does,
look at how many people are adding one to their Audi.  The question is what
to do with the air going out the blow off valve.  I'll bet you will find
that there is very little difference in performance.

Dave Buschur and Vincent Ten:  Dave Buschur is running a VPC setup, see
above.  He is the one who I believe first tried it on the Talons.  He's the
one who has documented on the _drag strip_ between his car and others the
.1-.2 second faster quarter mile.  Vincent Ten might also be running a VPC
on his Supra, he could also have a custom computer for his engine in which
they could program it to work right.  Also, Dave runs a test pipe and race
fuel all the time in his car.  I'd bet Vincent is doing the same.

Homework has been done, now it's time to go to the lab and experiment.
Report back to us on your results.

        -Mike
        '91 Talon TSI AWD (lotsa mods)
        '86 Audi 5KCSTQ (K&N filter)