The biggest problem with road cars using valve springs (or indeed engines) that run at 20,000 RPM isn't technological; the issue is practical. Valve springs th

The biggest problem with road cars using valve springs (or indeed engines) that run at 20,000 RPM isn't technological; the issue is practical. Valve springs that function properly at 20k are going to be strong and produce serious resistance at low engine speeds (as a percentage of total power output). So much so that F1 cars with idle speeds of 7,000 RPM are normal. Low RPMs is where most road cars live. F1 cars don't want to be running at low engine speeds since that's not where the power is. Sacrificing power production at the low end (due to the power consumed working the valves) is an easy trade-off for a racecar. Proper valve spring design is hard: it's a balancing act between power consumption at the low end and power production at the high end. F1 engines have a (relatively) narrow operating band, whereas a road car is expected to be usable from (a low) idle to redline. (Expense is less of an issue here. Multi-million-dollar supercars with 20,000 RPM redlines would sell well.)