In-cylinder combustion pressure
If you assume that the engine speed $N$ is constant, given in RPM (Revolution per minute), with an angular sampling of $\alpha$, you should convert it into a sampling interval:

$$ \Delta t = \frac{\alpha}{6N}\,.$$ 

The sampling frequency in Hertz is the inverse.

If the pressure data $P$ is given in bars, you should convert it to the SI unit in Pascals (about $10^{−5}$ bars) to get an absolute sound pressure. The Sound Pressure Level (SPL) is a logarithmic ratio of the absolute sound pressure and a reference level, depending on the application.

Be aware that crank angle sampling is often far for uniform, and at certain speeds the angular sampling is too high to capture phenomena without aliasing.