Exhaust drone after decat (not "raspiness")

[msm]

I'm experiencing (another) strange problem after my decat. Before I spend some money on what I think is the solution, I thought I'd post the symptoms in case anyone experienced similar.

Firstly, some background: Had the stock header replaced with 328i catless headers and had the centre cats replaced with a normal "muffler" box. Noticeable performance increase above 3K RPM. Car pulls much more cleanly to the red line.

Anyway, here's the issue: At 120K/Hr to 130K/Hr ONLY, I'm getting a "droning" noise with some vibration coming through the entire car. The closest I can describe it to, is like when you drive on the freeway at high speed and open one of your back windows - it almost feels like that same buffeting noise/feeling. It's also most noticeable in 5th gear. If I drop to sports mode and change to 4th or 3rd it goes away. There is a slight raspiness in all gears at around 2.5K RPM, but it's not that at 120K/Hr - it's something very different. Alistair (Powerflow) suggested that it could be the back OEM box failing, due to the additional flow coming out after the decat. He mentioned something about the stock box having fibreglass bits that could be causing this.

Before I replace the back exhaust boxes, any other suggestions as to what it could be, as it is extremely annoying :banghead: At speeds below 120, it is fine - slight raspiness only, but I can live with that. At 120, it is driving me crazy :cursin:

 

[andrewbuch]

I would say box is finished..

had similar on the E36 when I put on a 2nd hand free flow.. the one box was gone & it droned so much

 

[moranor@axis]

what rpm do you sit at when at 120? all droning should go away over 3000 rpm

 

[msm]

what rpm do you sit at when at 120? all droning should go away over 3000 rpm

Can't recall exactly, but it is slightly under 3000rpm at 120. Yes, it does go away around 140 KM/Hr, but the problem is 120/130 is my normal cruising speed, hence the annoyance.

 

[George Smooth]

I got the same problem on my 335 auto after fitting a free flow. Mine drones at 1900-2000 making even the coins in the ashtray rattle. It is much worse on auto cars.
What happens is the free flowing exhaust creates a type of noise frequency that the droning happens at, the only away around it is fitting resonators at a certain point in the exhaust system but there is no rule of thumb and most times its trial and error to try get it right.

 

[moranor@axis]

what rpm do you sit at when at 120? all droning should go away over 3000 rpm

Can't recall exactly, but it is slightly under 3000rpm at 120. Yes, it does go away around 140 KM/Hr, but the problem is 120/130 is my normal cruising speed, hence the annoyance.

if you have done the golf T mod then you should plug the line back in... it helps alot to block sound below 3000 rpm it will probably solve your problem... your other option is to run a shorter ratio

 

[msm]

if you have done the golf T mod then you should plug the line back in... it helps alot to block sound below 3000 rpm it will probably solve your problem... your other option is to run a shorter ratio

Did not think of that. My flap is actually stuck open since I got the car. Maybe I should look into getting it fixed.

 

[moranor@axis]

if its an easy fix it will be worth it i would not spend alot on fixing it thou because it might not solve your problem and then you will probably end up changing your box anyway...

 

[tcal69]

Or drive under 120 haha jk kidding man... hope you get the problem fix