Keeping all other parameters identical, the Compression Ratio (CR) of an air standard diesel cycle is increased from $15$ to $21$. Take ratio of specific heats $=1.3$ and cut-off ratio of the cycle $r_c=2$.

The difference between the new and the old efficiency values, in percentage, ($\eta_{\text{new}} \vert _{CR=21}) – (\eta_{\text{old}} \vert _{CR=15} ) =$ __________ $\%$ (round off to one decimal place).