Exhaust back box options

[Everywhen]

On 12/26/2023 at 7:34 PM, danb said:

Has anyone got a recording of the drone? I’m wondering if you could vastly reduce it with a suitably tuned Helmholtz resonator but would need to know the primary frequency of the drone  to calculate what might work.

I know they are not uncommon on some exhausts to combat droning but I’m wondering if one could be made to sit in the cabin to tune out the noise - would make experimenting much easier than welding stuff to the exhaust….

The dB Killers from eBay work just fine and are £8, a lot less work than making a Helmholtz resonator

[02boxster968]

15 hours ago, ATM said:

What's the magic of the standard back box. 

I get the impression the versions with 2 little separate silencers just can't compete with the larger single full width silencer. Probably because the exhaust is so short on the 986.

But it is as short or shorter on the 996 - or is it. They have a cross cross arrangement at the back of the car so the left bank exits at the right and vice versa. I don't know if this length helps eliminate some resonance. The silencers on the 996 are tucked up in the bumper behind the wheel arch so right out to the side of the engine. I don't know whats going on inside but I presume not a straight through arrangement from the factory. After market options maybe are straight through but would obviously need a bend inside. I did see this one on ebay which uses 2 silencers packaged close together each side with the bend externally. It isn't cheap.

Some cars like the 997 gt3 had a single silencer similar to the 986.

The 987 and 981 manage without drone and they have separate silencers. I'm guessing they have separate chambers inside like the standard 986 box.

Or is it the thickness of the metal. Does a cheap exhaust made with thinner metal drone more?

Someone needs to buy a welding rig and experiment. 

If you look at a stock Boxster exhaust that have been cut open you can see Porsche designed the exhaust pipe from left and right banks to cross over each other internally for noise cancellation and eliminate drone 

The twin can exhausts will always drone in this setup and the cheaper exhausts have just gutted all the internals!

Edited December 28, 2023 by 02boxster968

[Norbert]

On 12/26/2023 at 4:54 PM, jonogt6 said:

Providing its as good as it looks, I think that's a good deal, even with possible import duties. Let us know when you receive it👍

This is the tax invoice eBay sent me:
Price: 108
Postage: 188
VAT (collected by eBay in line with UK legislation): 59

This does not mention import duties but I think that should only be 4%. The estimated delivery time is 2nd Jan. That sounds a bit optimistic to me, i don't think i will reciieve it in 2 weeks during holidays. I will update you and take some pictures when it arrived.

[Norbert]

11 minutes ago, 02boxster968 said:

If you look at a stock Boxster exhaust that have been cut open you can see Porsche designed the exhaust pipe from left and right banks to cross over each other internally for noise cancellation and eliminate drone 

The twin can exhausts will always drone in this setup and the cheaper exhausts have just gutted all the internals!

This looks really complicated. Now I understand why we don't have cheap oem style chinese exhausts. I wonder if this is over-engineering. 

[ATM]

4 minutes ago, Norbert said:

I wonder if this is over-engineering. 

If you've ever fitted one of these whilst laid on your back you'll know they're heavy. Real heavy. So they must have used chunky steel throughout. 

[02boxster968]

1 minute ago, ATM said:

If you've ever fitted one of these whilst laid on your back you'll know they're heavy. Real heavy. So they must have used chunky steel throughout. 

Agreed ATM significant weight in the oem factory exhaust!

[02boxster968]

This illustration is also very useful showing how the exhaust gases enter and leave the stock 986 exhaust. Blue arrow are the left side and red for right all the way to tail pipe

The setup looks like the gases from each side of the engine cross over each other via internal pipes before existing via tail pipe furthest from the entry point. German engineering at its finest 😅

Edited December 28, 2023 by 02boxster968

[edc]

28 minutes ago, 02boxster968 said:

This illustration is also very useful showing how the exhaust gases enter and leave the stock 986 exhaust. Blue arrow are the left side and red for right all the way to tail pipe

The setup looks like the gases from each side of the engine cross over each other via internal pipes before existing via tail pipe furthest from the entry point. German engineering at its finest 😅

Aside from the directional arrows you can also see the gaps and perforations. It must also contribute to back pressure. 

This topic has been discussed many times over the years with the same types of pictures, ideas and broadly conclusions. The eBay box is largely the same thing rebranded many times over. Some minor but ultimately insignificant design differences over the years. It has the fibreglass packing and is thin wall steel and weights much much lighter than the OEM exhaust. 

When the eBay exhaust was circa £99-£130 and modifications to it would have cost more than the exhaust itself for no guaranteed change. At that same time period good original exhausts were cheap. I paid £70 for one in 2013 and had it modified by Carnewal. It's not done many miles really but it shows on the old prices you could buy an exhaust get it Carnewal'd for the same price people are selling worse condition original exhausts today. 

 

[kdh]

10 hours ago, Everywhen said:

The dB Killers from eBay work just fine and are £8, a lot less work than making a Helmholtz resonator

Do you have a link to these please?

 

[½cwt]

6 hours ago, ATM said:

If you've ever fitted one of these whilst laid on your back you'll know they're heavy. Real heavy. So they must have used chunky steel throughout. 

And they last 20+ years too.  Also bear in mind Porsche set out to design this as part of the vehicle development to fit these of thousands of them and to comply with all sorts of international rules for noise and emissions and to meet their own engineering standards.  To do it properly costs money...

[iborguk]

Interesting comments below this video...

 

 

 

Edited December 28, 2023 by iborguk

[Norbert]

9 hours ago, ATM said:

If you've ever fitted one of these whilst laid on your back you'll know they're heavy. Real heavy. So they must have used chunky steel throughout. 

I did and i will do it again really soon.  I just wonder if the goal is to eliminate the drone (around 100 Hz) why not just use a simple J-pipe with the proper lenght. That could work with the cheap eBay exhaust too, never seen anybody try it.

[Norbert]

2 hours ago, iborguk said:

Interesting comments below this video...

 

 

 

Did anybody check what happens if you move those caps? I am guessing the position of the caps change the frequency you want to eliminate. I would still prefer to play with J-pipes to be honest. Apperantly V6 engines suffer similar issues. Here is a good disuccusion: https://www.chargerforums.com/threads/j-pipe-calculations-to-kill-drone.393462/

[iborguk]

So to summarise is it fair to say that unless you want to get into fiddling the best options to guarantee a non droner are a decent 2nd hand OE exhaust or a Dansk OE TUV equivalent ? 

 

 

[Norbert]

15 minutes ago, iborguk said:

So to summarise is it fair to say that unless you want to get into fiddling the best options to guarantee a non droner are a decent 2nd hand OE exhaust or a Dansk OE TUV equivalent ? 

 

 

* decent 2nd hand OE from abroad. It wil be really hard to find a decent one in the UK. 

[kdh]

Agreed. Stick with OE for no drone and it you want it sportier higher up the revs, do a by-pass.

I've not heard the Dansk so can't comment.

[½cwt]

1 hour ago, Norbert said:

* decent 2nd hand OE from abroad. It wil be really hard to find a decent one in the UK. 

I can see two good S boxes (no seam deterioration) from Lithuania at about £500+600 landed in the UK on eBay currently.  There's also a half decent UK one that with a bit of attention from a skilled welder should work out as sound for under £400 (the entry pipes and seams near them show some cracking/corrosion) as it can be done off the car before fitting.

[Everywhen]

8 hours ago, kdh said:

Do you have a link to these please?

 

There are literally countless Sellers of these on eBay, all the same, just search for “dB killer”

[kdh]

Thanks, I've seen lots of these around, but all for motorbikes. I thought you may have found one specifically to fit a car with an exhaust the same size as the 986 - around 75mm - 3`".

[iborguk]

Fixed it

 

[iborguk]

32 minutes ago, kdh said:

Thanks, I've seen lots of these around, but all for motorbikes. I thought you may have found one specifically to fit a car with an exhaust the same size as the 986 - around 75mm - 3`".

This kind of thing but a bit smaller....https://www.ebay.co.uk/itm/195894292038?_trkparms=amclksrc%3DITM%26aid%3D777008%26algo%3DPERSONAL.TOPIC%26ao%3D1%26asc%3D20230811123856%26meid%3D8c37db6f6fb642228a0374c275634e40%26pid%3D101770%26rk%3D1%26rkt%3D1%26itm%3D195894292038%26pmt%3D1%26noa%3D1%26pg%3D4375194%26algv%3DRecentlyViewedItemsV2&_trksid=p4375194.c101770.m146925&_trkparms=parentrq%3Ab1712e4018c0a8d1e737c9d8fffecc06|pageci%3A68acdcdd-a5a5-11ee-9a97-3a59f20ff616|iid%3A1|vlpname%3Avlp_homepage

 

[02boxster968]

4 hours ago, ½cwt said:

I can see two good S boxes (no seam deterioration) from Lithuania at about £500+600 landed in the UK on eBay currently.  There's also a half decent UK one that with a bit of attention from a skilled welder should work out as sound for under £400 (the entry pipes and seams near them show some cracking/corrosion) as it can be done off the car before fitting.

I think that would be money well spent!

[Fat Rat]

Reading all this has given me the kick up the ar*e to finish putting my box back together that I chopped open months ago. New Years Resolution. Get it finished next year. 😂🤙🏻

[brillomaster]

https://www.ebay.co.uk/itm/315030418875

48mm is the size you need. Dont get painted ones as the paint gets hot and melts!

[kdh]

My tailpipes are 80mm wide.

48mm wont fit.

 

Edited December 28, 2023 by kdh