Can changing car exsalt system give more horse power?

Answer 1

If you had included the make, model, year and motor we could be more specific.

It depends upon how complete the replacement exhaust system is. Headers with large diameter pipes and a free flowing muffler will allow the backpressure to flow with less resistance.
Add a cold air box and a throttle body insulater spacer and depending on year make and model it's worth a 3 - 6% increase. A computer chip will add another 2 - 3%