It's just that last I saw, the greatest limitation of the KIVA program is that by default it can only simulate valves directly on top of the combustion chamber, and pent-roof designs are beyond the scope of the normal program and requires sub-programs and long hours to design custom meshes to simulate different valve angles and port geometry. Valve seat placement, and port angle are crucial in engine development, so I'd imagine there's a fair bit of validation work just to ensure a decent correlation of the models with experimental results. Only once this is sorted could you even begin to use your combustion CFD in ernest.
