I read somewhere else in here that the 2002 Sentra Spec-V (also known as the SE-R) has the same QR25DE 2.5L engine as the 2007 Altima 2.5S. If that's the case wouldn't headers for the Sentra also fit the Altima? I imagine some parts, such as the downpipe would have to be customized but considering that the 2002 Sentra's headers are below $500, would custom piping to complete the header system be a good investment? I just started learning about cars so this might be a dumb question. Any ideas would be appreciated.

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This has been discussed several times, and currently Stillen is working on headers for the 2.5 and Racingline is working on a Y-Pipe. Please look around before asking. Most questions have already been answered.