http://www.turnrite.com/conversions.htm gives the conversion of what i assume to be US horsepower to that of metric horsepower. The conversion ratio is from (US HP) * (1.014) = (Metric HP). So with 516Hp * 1.014 = 523.224 thus your garage discrepancy. The difference between garage hp and trackside is elevation and time of day/tempurature. Hope this helps.