rec.autos.simulators

I want to race with low powered cars, like if it was F2... how to
reduce the engine power of all the cars (not only mine but also the
cars of all the AI, the opponents) in GPL (the only valuable sim) ?
Thank in advance.

From http://jc.simracing.dk/

Look under "Beginners Guide to F2/FD"

"Racing F2 and FD

Offline
By default all racing in GPL is set to use the GP/F1 cars and even if
you select your F2 Driver Profile, electing to race will switch your
class to F1. To alter this you need to make a very small change to a
file in your F2 players directory.
You need to open the file player.ini in the directory e.g.
\Sierra\GPL\players\Smith_F2__John\ with a text editor (i.e. notepad).
Look for a parameter called driverRating. It's in the [ Personal
Information ] stanza. Change the value of driverRating to 2 for the
Advanced Trainer (it should be 3 by default).
Now when you choose Single Race whilst using your F2 player profile GPL
will allow you to race the F2/Advanced Trainer cars.
You Do Not need to make any changes in order to race the FDs (as these
are essentially F1 cars anyway)

Online
The only thing to bear in mind when racing online is to check that you
select the right player profile first before joining your race. If you
have only ever raced F1s with one account you won't be used to changing
this in the [Join] Dialogue box of WinVROC. If, having joined a race you
do not see the setups you thought you had, you've probably picked the
wrong profile."

GPL AI Tweaker - allows you to adjust the relative strength of each AI
driver
GPL Race Editor - allows you to change the field to all F2's or F3's
(for example), or run a split-class field.  (I don't believe that it
supports the "Engine-swapper" classes of FD, FV, etc, though)

Do a search on these names, and you hopefully should be able to find
them.  They work!