what exactly is "running rich" ?
A car can run rich, lean, or anywhere in between.
What this refers to is the air:fuel ratio of the motor.
Ideally, while not on boost, a standard gasoline engine runs a 14.7:1 air:fuel ratio.
That means that for every 14.7 parts of air in the cylinder, there is 1 part fuel. 14.7 is an ideal ratio that cars run as a good balance between power, fuel economy, and emissions.
if you run 13.7:1, you're running a little richer, and if you're running 15.7:1, you're running a little lean. Remember, the LOWER the number on the left, the MORE fuel you are running.
To understand it, take it to an extreme. If you ran a 1:1 air fuel ratio (which the engine couldn't actually run at), then your engine would burn as much fuel as it did air. On the other hand, if you ran a 100:1 air fuel ratio, you'd barely be burning any fuel at all.
Under load, most engines run a bit rich (in the neighborhood of 12:1) for two reasons.
1-a slightly richer air:fuel ratio will usually give you more power.
2-It's safer to run richer than leaner. Extra fuel in the cylinder will keep the temperatures lower (the fuel cools the combustion temps) whereas too little fuel will cause the combustion temps to rise, which is dangerous to a motor.
Most boosted engines I've tuned make the most power at 13:1 or so (which is slightly rich). However, it's dangerous to tune a car for that ratio, as it has little room for error...if it leans out a little, you could lose your motor. With that in mind, we normally go a full point richer for a safety margin (so 12:1 or so). Mind you, this is only under load (full throttle, etc). Under normal driving, you won't be boosting, so you should be running 14.7:1 or so.
Anyway, the air:fuel charts on the dyno posted earlier indicated an air:fuel ratio deep into the 9s. Mitsu obviously did this for safety...they detuned it on purpose. When a car is THAT rich, the fuel is actually hindering performance since it doesn't get to fully burn, and it tends to bog down the motor. I can almost guarantee you that if you pulled some fuel and ran it at 12:1 or so,
you could probably pick up at least an extra 12-20 horsepower (guestimate) without too much work.