SwedeSpeed - Volvo Performance Forum banner

1 - 2 of 2 Posts

·
Registered
Joined
·
4,267 Posts
Discussion Starter · #1 ·
So I guess I don't really need "advice" because I have already done what I am going to ask about. Feedback is greatly appreciated though!!!

So originally I did a full custom 3" which split at the stock locations y-pipe to 2x2.5" dual runs. In other works it looks exactly like the stock exhaust but larger piping and no muffler. The drone was unbearable. The car sounded like a beast but I could not let the drone slide. After alot of research about how dual runs can be especially tricky, I began to understand why the drone. If the dual runs aren't nearly the same length, the exiting soundwaves can be out of phase and wreak all kinds of havoc. My solution..... shove a 3" diameter sweet potato in one of the tips. Amazingly it worked, dropping the drone 10dB. On a logarithmic scale, that is half the noise!!!!

So this weekend I had my exhaust shop weld a plate in one of the dual runs to act as a permanent potato (its capped). I also had my resonator changed from a Dynomax bullet to a Vibrant performance ultra-quiet resonator to help reduce drone further. I am now at 77dB when cruising opposed to 90dB.

The advice I need is the legitimacy of what I had done. I did the math of the two systems using 16-gauge pipe for both (just a guess as to the actual gauge). The stock exhaust is a dual 1.75" run which will yield a cross-sectional area of 4.12 sq.in. when doubled. My single 2.5" pipe run yields a cross-sectional area of 4.41 sq.in. So technically it is less restrictive than stock although it is single pipe. Also consider my design is straight through and I am pretty sure the stock muffler is baffled (I confirmed the stock resonator is straight through).

What are your thoughts? If I got it tuned I would for sure mention that my exhaust is "stock" because I wouldn't want it tuned around 3".
 

·
Registered
Joined
·
11,927 Posts
Not sure exactly what you're asking, but capping one of the duals turns that capped section into a quarter-wave resonator, which if it's the correct length, will cancel out drone better than any resonator or straight-through muffler could possibly do.

The cause of the drone isn't due to slightly different pipe lengths though, the cause is simple the exhaust system has a natural frequency based on it's length, similar to an organ pipe. So when your cylinders are firing around that same frequency you get that loud hum at the resonant pitch.
 
1 - 2 of 2 Posts
Top