If you start for a long race (between 12h and 24h) you may not use the boost, the boost is wearing the engine (check the engine file of the car to get the Max RPM tolerated by each car, if you keep your rpm under this limit you don't use your engine. If with boost or downshift you pass over this limit, you wear your engine and can lead to break it.
The variance is applied when you pass over the RPM limit and it's a random range for rF, so you take a risk in using the engine over his race nominal rpm range.