RE: More overshooting of boost - long/nerdy

[Scott Mockry <scottmo@teleport.com>]

Boost Meisters,

Scott J. said:
>I'm still intrigued by the use of 'overboost', when in reality we are 
>speaking of an input/output loop delay resulting in a boost 'overshoot'.  A 
>faster processor and/or a better WGFV could control this better, even in the 
>early WGFV years. 

Yes, it is boost overshoot, that is done "purposely" by the use of a
feedback control system that adds pressure to the top of the waste gate
diaphragm, and results in a boost level that is slightly higher (initially)
than you would get by the use of only a mechanical Waste Gate with a
certain spring tension. 

Boost Overshoot, More Boost, Extra Boost, Extra strength Boost, Big Mama
Boost, Holy Sh*t Boost, or OverBoost, call it what you want, or whatever
makes you feel comfortable.

The whole point of my previous discussion was to point out that this
feedback control system allows a boost profile that is "better" IMHO than
would you get with only a mechanical Waste Gate with a given spring
tension. The system allows the Waste Gate to act "like" it has a stiffer
spring "initially" to prevent waste gate cracking, but then will reduce
pressure on top of the waste gate diaphragm to reduce boost after the boost
is ramped up. 

The use of some initial "overshoot" when designing a feedback control
system is often standard practice and the designers of the system can have
the system have a step response that is "overdamped", with a rolled off
front edge, or with a "underdamped" response with 5% of overshoot, 10% or
perhaps 30% of overshoot if they so desire. The designers of the system are
well aware of the intended response.

Scott M. said:
>>  Translate that boost graph into a torque output graph for the same
>>  scenario, can't you imagine that the torque is going to be higher when the
>>  boost is higher? 

Scott J. said:
>Not necessarily Scott, that's somewhat oversimplifying a turbo boost map.  If 
>you look at a turbo map of the 'standard' audi k26, increasing the PR from 
>1.8 to 2.0bar before 3000rpm decreases the turbo efficiency from 70% to 50%, 
>that surge line is really tight, and the Turbo Efficiency (TE) is right at 
>the 70% line at 1.8bar.  At 50% you are on the surge line, a really bad thing 
>in terms of turbo car performance, and ouput (air volume) from the turbo 
>doesn't increase in the above scenario (rule of thumb: increasing boost 
>increases PR, not necessarily flow).  

Very good information, and it is good that you brought up this important
piece of the puzzle. It is important to be aware of the Turbo
characteristics before raising the boost on any turbo charged car. 

I don't agree with all of your assumptions, as the dynamic response of the
engine and turbo make it difficult to assume the engine/turbo will allow
the boost to overshoot to 2.0 when the engine is under 3000 RPM for the K26
Turbo. The other assumption is whether the "transient" boost condition will
match what is found on the boost map under steady state conditions. I guess
we can agree to "dis-agree" on this one until other evidence is available.

That means the 'overshoot' boost 
>profile should yield a measured loss, not gain.  

Well, again, I don't agree with the above assumption. The acceleration
tests I did using the data acquisition system on the 1989 200TQ with the
K24 Turbo shows the recorded acceleration values (0.## g's) to max out, or
peak, the exact same moment when the boost was also at its max value during
this brief "Overboost" or "Boost Overshoot" period. This correlation was
true in 1st,  2nd, and 3rd gears when accelerating the car from 5-90MPH. 

This "data" tells me the effect of the Overboost period isn't reducing
performance, at least during this test. Other tests should be done using
the data acquisition on the K26 equipped 87 5000TQ to see what occurs
during this "Overboost", or "Boost Overshoot" period.

HTH
Scott Mockry