40s are oval as well. They came on my car when I bought it.

 

Have to disagree with that. I had the O/R prochamber with the Mac axle backs and still had trumpeting. Only mine was around 1800 - 2200 rpm. This is caused by the lack of backpressure and can best be resolved by resonators or hi flow cats.

 

+1 When I had an o/r prochamber and magnapacks I also got trumpeting. Hi-flow cats took care of the problem.

 

I went through several setups myself before settling on one that has ZERO trumpet. That **** drove me crazy.

I was offered a ride in a 750whp turbo 3V. Kickass ride but as soon as he got on it HUGE trumpet lol. BRRRRAAAPPPPP!!! Still felt like a jet fighter so I didn't mention how bad his exhaust sounded .

 

the 40 series mufflers that you buy just as mufflers are oval, some get them installed on their cars but the 40 series axle back systems made for our cars are square. The axle back systems with the oval mufflers are 50 series and are quieter.

edit: and according to flowmaster's website both systems, the 40's and 50's are considered "American Thunder" systems

 

According to Jegs, there are three types of American thunder axlebacks for the S197.

AT 40 series

AT 50 series

AT classic 40 series