Sorting Exhaust Drone'

Simon.h · 4th May 2014, 08:11

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??

tareim · 4th May 2014, 08:52

I'd add a silencer or resonator as your stainless system is more missing 1 box from standard, that could be the answer

Or failing that can you make a slight bend in the straight pipe where the old cat used to be

So instead of --------- have (hopefully this will display ok)

_____
------/ \------

If that makes sense, to break up the length of straight pipe

MrDoodles · 4th May 2014, 09:00

Unless I've missed it, can you explain how this car will go through an MOT without a Cat please?

Simon.h · 4th May 2014, 09:04

Quote:

Originally Posted by MrDoodles

Unless I've missed it, can you explain how this car will go through an MOT without a Cat please?

Well the decat went through mot on my last ZT and its just down to the cat being part covered by the under tray and the tester does not spot it.

tareim · 4th May 2014, 09:04

Quote:

Originally Posted by MrDoodles

Unless I've missed it, can you explain how this car will go through an MOT without a Cat please?

lpg doesn't need a cat

Simon.h · 4th May 2014, 09:05

Quote:

Originally Posted by tareim

lpg doesn't need a cat

What's right!

MrDoodles · 4th May 2014, 09:07

Quote:

Originally Posted by Simon.h

Well the decat went through mot on my last ZT and its just down to the cat being part covered by the under tray and the tester does not spot it.

Thanks!

Quote:

Originally Posted by tareim

lpg doesn't need a cat

It may not need one to pass the gas analyser, but as the car left the factory with a Cat fitted as standard, it's a requirement that one remains on the car!

However, with the note above, let's hope that VOSA don't change the rules again to allow for removal of under-trays to check for Cats!

pab · 4th May 2014, 09:08

Quote:

Originally Posted by tareim

lpg doesn't need a cat

Are you sure..

Even LPG burns harmful gasses, the idea of a cat is to reduce this to acceptable levels.

If you remove the Cat, I'm sure you will have the MIL indicator illuminated anyway.

Simon.h · 4th May 2014, 09:31

Quote:

Originally Posted by pab

Are you sure..

Even LPG burns harmful gasses, the idea of a cat is to reduce this to acceptable levels.

If you remove the Cat, I'm sure you will have the MIL indicator illuminated anyway.

Yes I'm 100% sure lpg burns cleaner and I think its harmful gasses is half of petrol.
For the ecu light I have a MIL Emulator fitted.
Thanks.

Any way back to my reason for this thread, what would you now do?

MrDoodles · 4th May 2014, 09:38

Quote:

Originally Posted by Simon.h

Any way back to my reason for this thread, what would you now do?

Personally, I'd re-fit the Cat, to prevent any potential future issues for the MOT!

4th May 2014, 08:11	#1
Simon.h This is my second home Previously owned- MG ZT V6 160 Y plate Lpg, MG ZT 190 2003 In Rare Celestial Factory Ordered Lpg MG Join Date: Mar 2010 Location: Calne Posts: 4,531 Thanks: 8 Thanked 116 Times in 108 Posts	Sorting Exhaust Drone' 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?? __________________ Previously owned- MG ZT V6 160 Y plate Lpg, MG ZT 190 2003 In Rare Celestial Factory Ordered Lpg MG ZT 190 2004 lpg Last edited by Simon.h; 4th May 2014 at 08:27..

4th May 2014, 08:52	#2
tareim I really should get out more....... MG ZT+ 160 v6 Join Date: Jun 2013 Location: Bridgwater Posts: 2,272 Thanks: 73 Thanked 233 Times in 210 Posts	I'd add a silencer or resonator as your stainless system is more missing 1 box from standard, that could be the answer Or failing that can you make a slight bend in the straight pipe where the old cat used to be So instead of --------- have (hopefully this will display ok) _____ ------/ \------ If that makes sense, to break up the length of straight pipe __________________ https://lh5.googleusercontent.com/-x...h96-no/Sig.png My Project Thread Last edited by tareim; 4th May 2014 at 08:55..

4th May 2014, 09:00	#3
MrDoodles Banned MG ZT+ V6 190 Join Date: Nov 2011 Location: Spalding Posts: 13,245 Thanks: 369 Thanked 1,196 Times in 818 Posts	Unless I've missed it, can you explain how this car will go through an MOT without a Cat please?