In the current DW12, the turbo boost is controlled by the ECU ("Computer, aka "brain"). This limit is pre-set by IndyCar officials, a team of IndyCar geeks (or "tuners" I guess) called the "Technical Group." One of their engine support engineers, Darren Crouser, asked to talk with me this morning, and he was nice enough to explain to me how the McLaren ECU works.
Darren explained that the ECU controls boost with the rev limiter — that ugly, distinctive sound that one hears during practice sessions, and will bite people during qualification this weekend. This is the so-called "hard limit." Once the car hits the "hard limit" the system then "penalizes" the driver by an amount set by — you guessed it — IndyCar. At Indy, the rev limit will effectively reduce speed by about 1 mph. IndyCar officials assured me that this penalty isn't sudden or drastic or any way unsafe (I'm sure that some drivers and teams might disagree with that bit).
In order to avoid hitting this hard limit, the teams can use a "soft limit" which keeps the driver out of the penalty box. Obviously, the closer the soft limit is to the hard limit, the faster the car will go — but the more danger of an encounter with the hard limit.
This is all important because it is definitely a factor in qualification, and perhaps on race day. Already Scott Dixon is reporting that he encountered the penalties on his 3rd and 4th laps, dropping those laps below his warm-up lap speed — about 1 mph slower than the previous lap.
BTW, the rev limiter uses a combination of fuel and ignition retard.
– Tim Wohlford, AutoRacing1.com