do You know why such rule exists?
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Of course the top speed depends on the power if it's not limited by the gearing. Nearly every stock car under let's say 200 Hp is limited by power on it's top speed and You can reach higher speed driving downhill or with a wind from behind. Speaking about torque when considering top speed is very missleading and can be considered even irrelevant as torque values transform through gears while power not.
Torque is a lot harder to understand because there is no direct dependence between torque and performance. Without knowledge of the whole curve and the gearing You can't say anything about the car while the peak power says quite a lot. When driven on straight diesel car and petrol naturally aspirated car both with 200 Hp will accelerate same way (when they have same weight) while when You do the same comparison with diesel car with 300 Nm and petrol naturally aspirated car with 300 Nm the later would be a lot faster and most likely have even higher top speed. The reason is that it must have a lot higher power.
Most likely to keep reasonable durability and reliability. It's same for WRC cars.
Yes I know at some point the power will have influence on the top speed, but that seems irrelevant with the current WRC/R5 cars considering the dyno curves and limited RPM to 7500. In my understanding power overcomes the friction forces of the tyres with road, various bearings and stuff, which increase as the speed also increase, while torque is more relevant to the traction, as on low speed and cornering, thus torque is irrelevant at these high speeds, and seems this engine is well optimised about these figures. Anyway diesel car with same power probably would accelerate faster at low RPM as it would have higher torque, but that's just for a short period of time.
Hmm interesting, In f1 there are no such restrictions I believe, at least while I was still following it, and that was long time ago. AFAIR back then Ford 8-cyl Ford engines reached the cosmic 18K RPM, now they could be above 20K, I'm not aware. But of course these are N/A engines, as turbo were forbidden soon after introduced in 70's. Were the same RPM restrictions applied also for the N/A S2000 cars?
You are right with WRC/R5 as the power is always enough to accelerate up to rpm limiter. The main contributor for top speed when limited by power is drag as it is a squared function of speed. Other contributors are very minor in comparison.
The example diesel/petrol car was of course simplified. Let's say to a situation when both use continuous variable transmission with which they operate all the time on peak power ;)
Ja, natürlich, es war an A6 zwischen Tsechische Grenze und Nürnberg :p
Yes, it was on A6 between Czech border and Nürnberg.