Custom rear silencer fitted - now sounds rubbish?!

[benjy_b]

So, I've recently had a local exhaust fabricator make up a rear silencer for my MK3 Golf TDI. I asked for a similar design to standard, with larger twin outlets from the silencer to fill the bumper recess. I also didn't want there to be any additional noise. I'm happy with it from an aesthetic point of view, but the noise...!

Idle, fine. Normal driving, fine. 2800rpm, part throttle @ 80mph and it drones like hell! It really is just at that point. Really frustrating when on the motorway!

What could be causing it?

I've noticed that the rear silencer isn't a true twin exit. They've used a single exit and then a Y piece to make the twin tailpipes. Could this cause the sound?

[ChasR]

The chances are that they used a straight through silencer which for these custom peeps is oddly cheaper to make . A few places do use baffled exhausts but they also come at a cost.

Some cars require a baffled box at the rear to avoid booming at motorway speeds (my 306 GTi-6 certainly did!). Jetex tend to do something which is part of the way between the two for most cars (not quite baffled, but with two pipes running alongside each other with wadding the silencer .

[deaddave72]

you could just try blocking one outlet to see if that cures it, but if a straight through non baffled silencer it will make a odd noise. Also if stainless, that can change the exhaust note a fair bit i found.

[sowen]

It could be the silencer making the exhaust louder with he larger tailpipes like a megaphone?

On an old diesel Mondeo I used to have, the rear silencer fell off. Eventually I replaced it with a straight length of 2" diameter tube, never noticed it making a different noise at all, it was pretty well insulated from noise to start with.

[Kieran]

Sounds like it is tromboning! Line a badly tuned length exhaust!

[benjy_b]

The noise is a very deep tone. It does quieten down quick on overrun from 2800rpm and at what from 2800rpm it doesn't make the sound. It can be in any gear, part throttle (40-50%) @ 2800rpm.

I don't know if it's connected (and before anyone tells me, I know it's a diesel and they have a narrow power band) but the engine seems to be struggling at the top end of the Rev range? +3000rpm it seems sluggish and lacking torque. Could the exhaust have done this?

[ChasR]

Dec 18, 2014 19:34:41 GMT sowen said:

It could be the silencer making the exhaust louder with he larger tailpipes like a megaphone?

On an old diesel Mondeo I used to have, the rear silencer fell off. Eventually I replaced it with a straight length of 2" diameter tube, never noticed it making a different noise at all, it was pretty well insulated from noise to start with.

Dervs tend to have less silencers. In the case of the Mondeo ST TDCI they only have the Cat . The Focus TDCi doesn't even have a back box, only a mid box .

I do respect your opinion (you seem to know alot more than us mere mortals!) but I guess in this application it depends on where and what silencers it had before .

[alolympic]

Have you asked the guys that made it?

[benjy_b]

I've not approached them yet as I wanted to gather a little more information on the subject. It will be my next point of call.

[sowen]

Dec 18, 2014 23:49:27 GMT ChasR said:

Dec 18, 2014 19:34:41 GMT sowen said:

It could be the silencer making the exhaust louder with he larger tailpipes like a megaphone?

On an old diesel Mondeo I used to have, the rear silencer fell off. Eventually I replaced it with a straight length of 2" diameter tube, never noticed it making a different noise at all, it was pretty well insulated from noise to start with.

Dervs tend to have less silencers. In the case of the Mondeo ST TDCI they only have the Cat . The Focus TDCi doesn't even have a back box, only a mid box .

I do respect your opinion (you seem to know alot more than us mere mortals!) but I guess in this application it depends on where and what silencers it had before .

That's why I think it's most likely acting like a megaphone, 2800rpm at 80 isn't tickover, the engine is under load making power and if the noise is the same as original lower down in the rev/speed range then I think there lies the problem . My 2" tube was just that, a 2" tube, no fancy big bore shiny tailpipe end, in fact it slid into the original exhaust so was marginally smaller diameter

[deaddave72]

Do you have some pictures of the offending item? The "silencer" may have some markings on to help identify its manufacturer and this assist in diagnosis, also help us in getting a better idea of if it looks "right" or not.

[cobblers]

Basically, when VW made the system they spent a lot of time changing the sizes and design of boxes to make sure there wasn't any resonances at certain RPMs etc, but the custom exhaust guys just look at a gap and pick a box off the shelf to fit the hole. Sometimes it works fine, sometimes not. Usually a turbo (especially on a diesel) will mean you can get away with murder because it cuts the tops off of the individual pulses, but it seems like you've been unlucky in this case

The tailpipe design can make a difference, but the science behind it is really dependent on so many factors that it's as much of an art as anything. Twin tailpipes from a single box shouldn't really cause any issues in themselves, as people have mentioned, try blocking one of them up to see if it helps. It sounds as though you have just got unlucky with the particular combination of resonant frequency and the rest of the exhaust.

My Mk3 Golf 1.9d (no turbo!) had essentially no back box at all for over a year:


it was not at all intrusive or boomy, in fact the only thing that gave me cause to even check it was the fact that the back passenger wheel was always black with soot and I thought the brakes might be binding.

[squonk]

Dec 20, 2014 22:10:44 GMT cobblers said:

Basically, when VW made the system they spent a lot of time changing the sizes and design of boxes to make sure there wasn't any resonances at certain RPMs etc

More importantly for a turbo diesel it is critical that the exhaust is designed to geive the correct back pressure to the turbocharger, Typically around 100kPa. To little back pressure can cause the turbo to overspeed and come out of its pressure map and actually reduce performance. In the worst cases the turbo will self destruct as it is not designed to rotate at the speeds it is achieving. Too much back pressure and the turbo will never reach its correct speed or boost.

A lot of time is spent by manufacturers getting the exhaust and inlet system just right. Replacing bits of it with random pipe rarely works well in practice.

[chairchild]

Dec 21, 2014 10:56:38 GMT squonk said:

More importantly for a turbo diesel it is critical that the exhaust is designed to geive the correct back pressure to the turbocharger, Typically around 100kPa.

Any reference for this info? As this is something I've never heard of... ever. Especially as the "backpressure" (a term I despise, as it's incorrect at best) will increase with gas flow rates - so getting a fixed 1 Bar of pressure would be very difficult, plus it would increase EMP by whatever resistance the exhaust system is additionally introducing.

Turbines work via flow induced by a pressure differential. The greater the pressure difference, the greater the efficiency of the turbine - the turbine efficiency also has no effect on the compressor efficiency. You set the boost via a wastegate, so you would only encounter compressor overspeed if you were setting the boost too high anyway.

Also:

www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html



As pennance for what could turn into a debate, have some large turbo setups.. with no "backpressure"

[squonk]

Yes. I work for JCB Power Systems where we develop, from scratch, diesel engines. Our turbocharger suppliers insist that specific back pressure levels are adhered to and we have to prove this by running the engines across our certification test beds (that cost approx £1 million!!) where we measure all manner of parameters (approx 900 different parameters including ASAP data from the ECU) before they will warranty the units they supply. Specifically they are interested in turbine speed, back pressure, boost pressure, oil flow, oil priming time and oil temperature. I can tell you from experience that too low pressure can result in all kinds of anomalies in the air path including instability and surging. As I stated, both the inlet and exhaust systems require extensive design work to make them work properly. These days with the ever tightening emissions regulations the smallest of changes to either can have rather large effects on an ECU controlled engine.

You are absolutely correct regarding back pressure and flow rates. The 'standard' method of setting up the correct back pressure on a test bed is to run an engine at rated power until it is stable and then adjust a restriction in the exhaust system to give the correct pressure reading measured at the exhaust outlet of the turbocharger. Consequently the 'back pressure' is set at the highest gas flow rate (or worst case) so it will be safe at all other engine conditions. The vehicle exhaust system supplier also has to prove to the turbocharger manufacturer that their exhaust design is within the tolerances specified.

Interesting pictures, not one is of a vehicle that would be expected to last for 100k+ miles without a major rebuild!!

[chairchild]

This is one of those time, where I have enjoyed being wrong - as you've given decent information for a reply, instead of a "nu-uh" that I've generally received in debates

Could I ask which particular brands of turbo JCB use? I'd guess at the obvious Holset/CTT and Garrett, but I'm not so familiar with the big stuff I can imagine surge and choke of turbos to be far more pronounced when playing with the big boys too!

[benjy_b]

There's a lot of substantial information coming out of this thread. Thank you!

I suppose it was a little nieve of me to think I could replace the rear silencer with a custom item and not change the dynamics of the engine.

I'm going to look into getting a replacement VW silencer as I don't want to start causing damage to my engine.

As a matter of interest, companies like Milltek sell performance exhaust systems for the newer VAG TDI engines.

Would these companies put in the R&D to maintain engine/turbo reliability?

Also, when tuning TDI engines the EGT seems to be the main measurement that's looked at to reduce damage. I've never read about people looking into exhaust system pressure. Is this because exhaust pressure is difficult to measure?

[squonk]

Surprisingly the turbo's are not that big. The most powerful four cylinder engine we make is 129kW (about 173 bhp) and are a relatively large capacity (4.4 or 4.8 litre) for a four cylinder. Hence there is a hell of a lot of usable torque even without the turbo. Also bear in mind that rated power is produced at a lowly 2200 rpm so a smaller turbo that spools up quickly is far more important than a large lazy one. Yes, we have dabbled with twin turbos (one small and one large) but to be honest they didn't give much of an improvement over a well matched single turbo. Certainly from a production point of view the cost of the installation far outweighed the performance gains!!

However, in answer to you question, they are primarily Garrett or Borg Warner turbos depending on rating and application, although we have dabbled with others.

[chairchild]

sounds like my dream job tbh...... incredibly envious


Any chance you could sneak out any training manuals/etc? Always eager to learn

[squonk]

Dec 21, 2014 19:01:46 GMT benjy_b said:

As a matter of interest, companies like Milltek sell performance exhaust systems for the newer VAG TDI engines.

Would these companies put in the R&D to maintain engine/turbo reliability?

That's a very good question, and the same question can be asked of tuning chip manufacturers. In most cases I suspect that their development test beds are customers cars!!

The investment required to develop products properly on a test bed is massive. For a start, an emissions analyser capable of operating to certification standards (not MoT standards as they are much lower in comparison) costs in excess of £120k. A 300kW transient dynamometer will cost around £50k, the control system another £100k and on top of that you need the infrastructure to provide cooled water, temperature controlled fuel, temperature and humidity controlled combustion air etc etc etc. Before you know it you have blown between £500k and £1 million on the test facility. It would take a hell of a lot of sales to recoup that level of investment!!

Some tuners use a rolling road to develop their products but these are a bit rough and ready compared to a proper development test bed and can be installed for less than £100k all in. However, they are really only designed to measure and plot power and very little else.

Also bear in mind that when a manufacturer is developing an engine, part of the process is testing for deterioration factor (DF). 10,000 hours continuous running with regular inspections and emissions tests. I cannot see any tuner doing that!!

It won't be too far in the future when cars will be pulled up on the road at random and tested to comply with production emissions regulations - and I think we can guess which cars will be picked on can't we!!. This is when we will find out just how well developed the various tuning mods are - I suspect there will be an awful lot of people forced into de-chipping their vehicles just so that they can use them. There is also talk of the introduction of roadside vehicle checks via the OBD socket to identify non-standard states of tune. It's possible that any vehicles found to be non-compliant will be confiscated or forced to undergo a production certification test at the owners cost, and that could run into tens of thousands of pounds (hiring a development test bed costs in excess of £10k per day). Big brother has got his claws into everything and likes to have total control!!