If you're looking to make a stock car sound louder without modifying the exhaust system or adding a turbo/supercharger, there are a few options you can consider:
Remove the resonator: Some vehicles have a resonator in the exhaust system, which helps reduce noise. Removing the resonator can slightly increase the volume and alter the sound of the car. However, keep in mind that removing the resonator may also affect the backpressure and overall performance of the vehicle.
Upgrade the air intake: Installing a high-performance air intake system can improve the engine's airflow, resulting in a slightly louder engine sound. This modification doesn't directly affect the exhaust note, but it can contribute to an overall more aggressive sound.
Install an electronic sound module: There are aftermarket electronic sound modules available that can simulate the sound of a louder engine. These modules are typically installed in the engine bay and generate artificial engine sounds that are amplified through the speakers inside the car. While this option doesn't change the actual sound produced by the engine, it can create a perception of a louder exhaust note.
Adjust the engine timing: Altering the engine's timing can affect the combustion process, resulting in a slightly different sound. However, it's important to note that adjusting the engine timing without proper knowledge and expertise can lead to engine damage or poor performance. Therefore, it's recommended to consult a professional mechanic or tuning specialist if you're considering this option.
Remember that making any modifications to your vehicle may have legal implications, and it's crucial to comply with local laws and regulations regarding exhaust noise levels. Additionally, modifying your vehicle in any way may void the manufacturer's warranty, so it's essential to consider these factors before making any changes.