Use a text editor such as Notepad to edit the "enginerestricted.cfg" file in the "\gamedata\vehicles\" folder. You will see a list of numbers similar to:

RPMTorque= (500, -30, 35)
RPMTorque= (550, -36, 42)

These numbers define how powerful the engines are. For example, at 500 RPM, the engine will generate 35 units of torque. The middle numbers (with the negative sign) control the level of torque when the throttle is off (hence a negative number). If those last numbers are made larger throughout the list, then the engine will be able to accelerate through much higher gear ratios. This means at Talladega, where top speeds are usually at 190 mph, your car can now travel over 250 mph. The AI's engines will be also improved too, but they will always use the old gear ratios, making their top speeds only slightly better. They will make about 220 mph at Talladega with full difficulty on. You should try to mimic the normal patterns for these numbers (i.e., do not go straight from 50 to 1500 units of torque in only three lines. Warning: These cars do have limits. Their brakes are not any better, so you should not try going any faster at the slower tracks. Increasing the engine power will require trial and error to get the correct balance of speed and realism. If the torque settings exceed 1800, the tires will lose grip and spin (with traction control enabled). The gear ratios can also be changed by simply making the numbers smaller. Finally, for simplicity's sake, you can also simply rename the two *.cfg files to trick the game into allowing a normal, unrestricted engine at a Superspeedway.