Can an after market muffler harm an engine?

I just went to an automotive store not to long ago to get an idea on a performance muffler for my 4 cylinder 2.7L and when i asked the guy there he told me that high flow mufflers will do alot more harm to a 4 cylinder engine over time as opposed to a v6 or v8. He said that the back pressure created from the stock muffler is needed for the best performance and altering the exhaust would be a bad idea. Well i went ahead and got a thrush welded muffler anyway and it sounds great and I love it, but it would be great to get another opinion if I should have anything to worry about.

 

Yeah, that guy was wrong. A muffler will not hurt your truck.

 

I dont think it would harm it , but he is right about an engine needing some back pressure for best performance.
From what i hear and see, the stock muffler is huge, no real gains on the 4cyl with an aftermarket can.
The dyno tests ive seen here or other tacoma sites show little gain in hp when using a performance muffler and a C.A.I.

 

I dont think it will harm anything. It may not give the performance people think but if you did it for the sound like I did I think you will be fine.

 

Maybe with engines made back a few decades, a free-flow muffler might cause some exhaust valve wear over years of driving but not today's engines. You will be fine.