hi, everyone. If I do a 86*86, 16v, intake port diameter 30mm, how should I set the cell length and subsurface thickness? Thank you very much.

Firstly - cell length Your minimum cell length will be decided by the scale of the smallest feature & curvature which you think is important to capture. For INtake POrt flows this will almost certainly be around the valve seat. Similarly, the minimum y+ values and hence minimum sub-surface thickness will be decided by the local velocitys around the valve seat region. You can make an estimate of this 'a priori' to running the case, using 1D theory, the lift gap area and an estimate of Cd=0.7. For your case i would expect to use minimum cell length circa 0.25mm around the valve seat and upwards of 0.25mm near wall cell thickness. Note: Wall functions fo this type of analysis are rather unreliable but should give flowrate prediction circa +/-5%. If you get any closer then it is luck not physical model accuracy! ProAm will make an admirable attempt at trimming an Inlet port mesh, but it helps if you can create a Custom mesh first with Polar regions around the valve seats.