Can I replace flex pipe with straight pipe ?

It is obviously not acceptable, because using a straight pipe to replace the exhaust flex pipe is not an appropriate practice. You may think that the exhaust flex pipe is only a connecting section in the exhaust system and only plays a role in keeping the pipeline connected, but actually it is not like that. During the engine operation, it takes responsibilities that a straight pipe cannot replace at all. Because when the engine is working, it will shake slightly, and the exhaust pipe connected with it will also shake in the same way, and then there will definitely be relative displacement between the components behind it. The meaning of the existence of the exhaust flex pipe is to absorb this displacement, including various vibrations, swings, torsion forces, and also the thermal expansion and contraction caused by the temperature difference of several hundred degrees. If you replace it with a straight pipe that has no buffering ability, every time you accelerate, decelerate, shift gears, or go through heat cycles, the force will be directly transmitted to the welds or flanges in the front part of the exhaust. In this case, it may seem like there is no problem for short-term use, but as time goes on, it may speed up the damage of various components of the exhaust system, and even affect the overall power performance of the vehicle.

So, if the exhaust flex pipe is already damaged, we suggest replacing it in time with a flexible section that has the same structure, length, and size as the original one, instead of using a straight pipe to replace it. And compared with waiting until it completely breaks, replacing it in advance is actually a more reliable practice, especially when the vehicle has already run to sixty or seventy thousand kilometers. If you observe that the front section starts to have slight abnormal noise, or the outer layer of the flex pipe begins to show rust spots or the metal color becomes darker, then you can consider handling it in advance.