[mreid]

To really understand the effect and both hp gains and losses, you have to tune the exhaust to the engine component selection. In fact, you should look at the entire exhaust system as a vital component in the engine build if indeed max hp is your goal. Some milder engine builds actually benefit from some back pressure as it aids scavaging of exhaust gases. Also, some sidepipes are actually more restrictive than a properly designed under car exhaust. As you see in the real world examples and experiences above, it really depends on your build and end goals.