Do you know how to make more powerful cars with AC Car Tunner?

[Elaphe]

I'm using this tool which lets you make copies of your cars but tweaking some things, such as increasing a percentage of the original power, or swapping engines with another models. I've tried it and in most cases the cars accelerate too hard and wheels spin all the time. I've tried reducing the final drive but then the car takes forever to reach high speeds. Is there any formula for find the right balance? Also, swapping engines doesn't give me good results. If I install a powerful engine in an underpowered car and reduce the final drive, car is drivable, but the top speed is not improved. I suppose it has to do with the gears ratio.