Sorting Exhaust Drone'

[tareim]

It makes a nice sporty note with the cat gone so would be good to silence the drone with a decat

[Simon.h]

Originally Posted by Roverite

I know that many will disagree, but I had a Longlife SS exhaust fitted to my 75 CDTi ked for a quiet version, plus removal of the 'Cat'. The exhaust droned at about 1200 RPM, even though they had fitted an expansion box in place of the 'Cat'. I took it back and they replaced my old 'Cat', which restored most of the quietness and I kept the expansion box as well. Since then, Longlife have brought out a very quiet silencer for cars such as the 75, especially V6 75 Autos, which are apparently the most difficult to silence.

Removal of the 'Cat' on my car made no noticeable difference to performance, but did make it noisier. For what its worth, I would suggest re-fitting the 'Cat', which will also preclude any future MOT problems.

Roverite.

Thanks for the info but a petrol engine feels more powerful without the cat, it revs more freely and it also doesn't hold the revs between gear changes. So its not being refitted.

[johnnybbad]

Originally Posted by Simon.h

Thanks for the info but a petrol engine feels more powerful without the cat, it revs more freely and it also doesn't hold the revs between gear changes. So its not being refitted.

Well get used to the drone then

Performance exhausts are not purposely made to be noisy, its all about the flow, less boxes better flow

Putting a smaller box in the system somewhere may reduce the noise / drone , but i am sure it will have some kind of negative performance effect

Louder stereo may help lessen the noise

[jib64]

Just thought i would chip my couple of bits in mate amongst all the de-cat bashing occurring lol

For one, my local performance exhaust fabricators told me that if my straight through system turns out too loud they would fit an intermediate muffler box for £60 all-in. That's not one they have made, they buy in mid-boxes ready made and still only charge 60 beans including cutting/welding/fitting.

So i think 200 quid is extracting the urine to be honest!

You could always try the old "rolled up fibreglass matting up your tailpipe" fix, this basically breaks up a lot of the sound coming from the end of the chimney so may dampen the whole thing down a bit!

or you could always add a section of fibreglass inside the decat section somewhere, make up a baffle section within the existing pipe?

i also see that your intermediate pipe is dead straight, the original on mine had two 45 degree bends in it to swerve it around the fuel tank before the big bend up to the backbox (Google a diagram on Rimmers or similar).

Most aftermarket exhausts i scoped out didnt have these bends and just went over the fuel tank like yours, my exhaust guy is keeping them in on my straight through system as they will be critical in damping down the drone since i'm having no mid boxes!

So for me the fix would be the 3 B's: baffles, bends, or failing those boxes. Also listed in price low>high lol

As for the de-cat, if you can get away with it why the hell not not, i would if i could bud! You could always sports cat it if the day came when your mot tester failed it anyway, which would be a world better than the factory cat. The whole "put your cat back in" argument is a bit moot if a) you don't need it, b) you don't want it, and c) a sports cat would be better anyway!

[zero2]

Originally Posted by Simon.h

Hi all,
Well after fitting a full stainless exhaust to my ZT along with a de-cat, here's the exhaust-



and the de-cat-



It sounds great on the outside-
https://www.youtube.com/watch?v=bmp2xbOKxxo

But its got a bad 'drone' from about 1200rpm to 2700rpm, so I popped into a powerflow dealer and they said the drone is because it's running without the cats and it needs a small silencer added to the straight pipe but can't guarantee it will stop the drone 100% and they want £200.

So as i'm not paying £200 for a 'if' so I'm left with the option of buying a silencer from ebay and fitting it myself with clamps (have no welder and can't weld) or removing the system and replacing with std.

Then after reading this-

Drone is the magic range at which you can both hear AND feel the exhaust sound.

Factory systems do loads to prevent you from hearing this. I can't even begin to cover all of that, but most notable examples include mass dampers, valves, and specially chambered devices, which we'll get to.

Sound travels as a wave. Exhaust gas pulses travel like waves through the tubes.

It's about to get tough right here: The sound that creates drone, that pitch 94hz-125hz, these are long waves. You know how waves have wavelengths, and all that jazz? These pitches are long, basically from 9ft to 12ft long. As the pulses accumulate and travel down the exhaust (lets use a straight tube as a reference), the sound created will be a constant drone at that range. Bends and resonators help by breaking up the pathway of that wave, and absorbing the actual volume of the wave... but the low frequency tone is still there.. which is how you can have quiet exhaust that still drones. That sucks. Also, that's sometimes why louder exhaust doesn't seem to drone; you let the high frequency waves cut above it... like back at the concert reference we made before.

The best way to cut down the drone is via destructive interference. What you need to do, is reflect the trouble-causing wave back at itself, 180* out of phase.

This is like two equal people pushing an object in opposite directions- nothing happens!

However, that effectively means you need a chamber that can reflect the wave back in a straight line for 4.5 - 6ft. First up, thats difficult under our cars, to have dead tube space like that. One of the easy ways to do it would be to have a muffler or resonator that had a dead stop in a straight line down the center of the car, but the world will assume it can't flow well enough to work. This is the balance between power/flow and sound/comfort. It's always a balance.

Take APR for example, they use RSC technology. They are licensing this from Corsa, which is awesome, because Corsa has a patent on it. Because it works pretty well. What they do, is create that long length of tubing INSIDE a smaller canister- tough to manufacture as well- but it creates that destructive interference. If you've been in a car with a proper RSC system, you know what this is like- you hear the tone that should cause drone, but it doesnt. You don't feel it.

There are other ways to kick that drone though. Take for example, that listed Stratmosphere exhaust- there's a mini magic trick happening in the center resonator, which works awesome on a B7 A4 because of the long straight length of tube created by the DP/center section. Where the exhaust gas splits to go to duals, its in a perforated "Y" piece.. so the waves can travel through it, hit the back of the resonator, and travel backwards (as a wave, not as exhaust gas pressure) oh, I dunno, from about 4.5-6ft right back towards the source!

So basically, there are ways to cut down drone that you can't see. There are loads of ways to deal with it.

With the mustang, using different size mufflers is just another trick. But it's not always the answer. I know older mustang guys used to claim that having two 26.5" long tubes welded at 90* off a section before the muffler would also eliminate the drone- all sorts of ways.

What further makes things interesting, is how different cylinder configurations create drone at different ranges, but all the same (to an extent).. You can calculate out where 4, 6, 8, 10, 12, 16 cylinder configs will drone. I am trying to remember without digging through documents (As this is all off the top of my head), but 4bangers create the 94-125hz pitches between 3500 and 4500rpm. Add a turbo to the mix, and its more difficult to sit at that RPM range while cruising. So often, you can have a system that is minimal, large diameter, and flows well, without having intrusive drone.
What sucks, is 6 cylinders. They create the 94-125hz tone between roughly 2450 and 3250rpm. Riiiight around the range you'll likely be cruising at on the highway. This also primarily explains why BMW has some of the fanciest tricks for exhaust stuff from the factory, what with a plethora of 6 cylinders along the lineage. It's also why the B8 S4 has light drone BONE STOCK!
8 cylinder motors are easier, as the pitches occur low, like 1850-2450rpm, which in say, the 4.2L, is barely moving along.

No, swapping various Brands mufflers will not likely be the answer. And if it was, it'd be a total fluke. A more interesting scientific test would be to make sure an exhaust system could handle full flow out of only ONE side of a dual exit exhaust.. and then cap off the other side... that'd create your long reflecting chamber..

Anywho- the point is, it is all about waves, and how to bounce them around. You might not see how it's done. And yes, true, not every aftermarket company bothers- but the ones that do, typically are worth the cash.


So after reading the above its just luck if adding a silencer will stop the drone!
The joy of fitting a stainless system to our cars!

What would you do??

Your system is similar to what I had but I've got a cat. The drone will be coming from the centre, because of the length of pipe from the back box and middle silencer is so long the gases will be bouncing around there causing the drone. I had another box similar diameter and half the length of your middle silencer and that reduced the drone and only cost £70 from Longlife Bicester.

I'd recommend a bit big than half for yours because of the lack of cat.

I've had 3 different back boxes 3 centre sections and 3 induction kits to get a good sound without loads of drone.

[mh007]

Originally Posted by Roverite

I know that many will disagree, but I had a Longlife SS exhaust fitted to my 75 CDTi and asked for a quiet version, plus removal of the 'Cat'. The exhaust droned at about 1200 RPM, even though they had fitted an expansion box in place of the 'Cat'. I took it back and they replaced my old 'Cat', which restored most of the quietness and I kept the expansion box as well. Since then, Longlife have brought out a very quiet silencer for cars such as the 75, especially V6 75 Autos, which are apparently the most difficult to silence.

Removal of the 'Cat' on my car made no noticeable difference to performance, but did make it noisier. For what its worth, I would suggest re-fitting the 'Cat', which will also preclude any future MOT problems.

Roverite.

I agree with the above & what others have said.

Unfortunately, a CAT is required for MOT & i'm sure it's only a matter of time before testers are allowed to remove undertrays for inspection !

The dreaded drone happened to me when I had a CAT-back Powerflow system fitted to my 2.0 KV6 Conn SE Auto.

They were only going to fit one intermediate box back on but I insisted on two.

The drone around 1800-2300rpm was dreadful so i took it back & they reduced the diameter of the pipe going into the back boxes (duplex system)

Although it did improve it, a drone is still noticeable & after speaking to the Powerflow tech department, this is a common issue with SS systems.

I also had a CAT-back duplex system fitted by Longlife on my ZT 190+ & asked them for their quietest system & noticed that they also fitted reduced bore rear pipes.

There is even a slight drone with that too but nothing like the R75 but in fairness, the ZT is manual so you have a bit more control of the rev range.

So, I would suggest, like others, refit the CAT & reduce the size of the pipe going to the rear silencer.

[Jim Jamieson]

I stuffed loads of soft spongy material inside the boot liners and spare wheel well. I even fitted an extra layer under the boot carpet.

Car runs super quiet.

[Skilrove]

Elaborating along the lines of reducing the flow area and creating interuptions to waves; I have an idea to fit a thick bolt (or perhaps several at various positions along the centre and/or rear runs of pipe. The bolt would be passing horizontally through across the diameter (so it doesn't weaken bending strength too much), size say about 12mm, creating a mild throttle effect. Holes each side could be sealed with a broad curved washer and some gun gum. Does that sound meaningful to you guys?

[trimani]

I had an aftermarket back box fitted which 'Boomed' like mad

A friend said 'the problem is the tail pipes are to far under the car'

I got two new 'longer tailpipes', problem solved

Not a technical answer, but for me it worked.

trimani (Alf)

[Skilrove]

googling around some more led me to some muscle car forums which spoke of "Helmholtz resonators" othrewise known as J-pipes ...take a look at these, would be interested to try it out!
http://www.camaro5.com/forums/showthread.php?t=257781
and
http://www.camaro5.com/forums/showth...69#post2252069