RE: what engine to fit in my RS4
Speed is not a direct function of displacement. Torque usually is though. However, not a lot of torque is needed to propel a 1/10th nitro.
Why bother explaining this...because the displacement of racing engines is .12. The advantage of this is that all makers make at least one .12 sized engine, and they are usually the fastest engines. For example, the top racing OS .12 is much faster than the top OS .18, despite having smaller displacement, this is because it is aggressively ported and built to small tolerances for racing.
The bigger engines (.15ci and 3.0cc (.18ci)) are called 'outlaws', as they are above the size for racing. An outlaw .18 will indeed usually be a significant power upgrade to a .15, and offer more low down torque than even a racing .12, but nothing will match the top speed of a racing .12.
There is one particularly famous .18 which many use in that car, it's the OS .18tz. In case a racing .12 is outside budget, the OS .18tz is a great choice. The HPI 3.0 is not a good engine, I have experience with one in a Bullet MT.
There are a couple of good Thunder Tiger engines at modelsport.co.uk for a reasonable price, the .18bx springs to mind, but the exhaust is right side exit, which might be a problem. what side of the car is your exhaust pipe on?