Compression ratio and operating RPM mostly. Compression ratio (the 'squish' in an engine's 'suck,squish,bang,blow' cycle) is >~14:1. Dynamic compression (once

Compression ratio and operating RPM mostly. Compression ratio (the 'squish' in an engine's 'suck,squish,bang,blow' cycle) is >~14:1. Dynamic compression (once you factor in the compression of air caused by the turbo) is much higher. This is largely due to running a higher octane fuel which keeps preignition at bay. Your typical road car has around a 9:1 compression ratio. Secondly, peak operating RPM is triple the average road car. Horsepower is '(torque * RPM) / 5252'. With the engine spinning at nearly 21,000 RPM, the neglible torque from the small displacement starts to add up. Again, your average road car sees about 6,500 RPM. Lastly, for comparison, the best road cars rarely see over 120hp/liter in normally aspirated form. And 250hp/liter on 93 octane is about the limit on well tuned turbo cars. This is largely simplified, as quite a bit of engineering is needed to get even above road car values.