Changing Not-to-Exceed Speeds

There's a way to change X-Plane max speed (or other) values by altering .acf file values, but I've lost track of the specific .acf entry. (Obviously something to be done with caution lest all manner of conflicts/temperature problems result). Anyone worked with this or able to refresh my memory? Request is due low max redline speed for Aerobask's E1000 - max sustained cruise speed should be possible in the 300 kts range, but can't get above 270.