It was just an idea if you want to take care of that pressure inside the simulation. Back pressure is defined like an “extra static pressure” exerted by muffler in an engine through restriction in flow of exhaust gases. The higher the back pressure the less efficiency, more fuel consumption, overheating you might have (can also cause shutdown of an engine if the exhaust is poorly designed). We always want the lowest pressure possible in the branch of the manifold because when the exhaust valve opens it tries to push out old exhaust gas allowing more oxygen to be drawn into the cylinder on the next intake stroke. There are some good papers I found taking into account the perforations of the muffler (and the back pressure also being influenced by these perforations).
Backpressure Study in an Exhause
CFD ANALYSIS OF BACK PRESSURE OF REACTIVE MUFFLER
Those two papers should be a good start to delve into this subject. I mean if this is not relevant for your studies and the back pressure is negligible in the first place do not worry too much about it but I thought it is good to know about it.