About 10 HPGetting rid of the muffler on a small engine only makes it loud. Annoys your neighbors. Draws the attention of the police. Very little power gain.Again, on a small engine, you only gain a few horsepower by changing your air filter, regardless of what the manufactures claim.When you decide to get serious about adding some power to your car without spending a lot of money, I suggest adding nitrous oxide.You can get a good dry nitrous system at most on-line auto parts websites from $300 - $500. This will get you anywhere from 50 to 75 more horsepower. Even more if you are willing to risk breaking parts.