42 replies to this topic

[Pipo]

Pipo, is our silencer full of stuffing and if so, did you put it back?

No, it is exactly as you can see on the picture.
The only stuffing is in de two absorptionchambers you see at eather side of the backbox. As you can see on the modded picture I left that in place because I was afraid the silencer would start to resonate to much. I had that with my Renault Fuego and it was really ennoying cruising at 110km.
I wanted a better sound through the tailpipes, not more resonance.

[tlzbub]

Top Man !. I had already bought a back box for my N/A last week (£50 on ebay) with exactly this plan in mind. Thanks to you I now know what to expect when I cut it open. Armed with the photos I think I may just make 2 smaller openings rather than section the whole thing - i'm planning to use a Dremel and some H/D dremel cutting disks that make very narrow cust (about 1.0 mm). I also plan to take out the pre cat at the same time and weld in a 200 cell sports cat I just bought. Final part of the plan (stage 1) is a Piper Viper kit and then off for a remap once it's all fitted. Hopefully It'll rain all weekend to give me some encouragement to get started.

[Pipo]

Hopefully It'll rain all weekend to give me some encouragement to get started.

No bad feelings, but I hope the sun will shine the whole weekend so I can get my favorite car out

Well, looking at het weatherforcast, it looks as you will have your way

[tlzbub]

Job Done ! (Plus a few others) I've - Removed the Precat - Cut out original cat and welded in a 120 cell sportscat - Performed backbox mods as above - Fitted a Direct air system (piper) With 500 miles covered since completion (on Saturday - it's Monday !) I'd agree wholeheartedly on the comments re noise level. I managed to do the backbox mods with 2, 3 sided holes (bending up and re welding a flap of metal) rather than a wholesale sectioning of the backbox. I'd say that the engine was up around 5 - 8% on power and tourque since the mods. It also rev's more freely - but no more responsively (due to the limitations of the fly by wire throttle body I suspect). Next plan is to do some runs with a lamda meter as I suspect I may be running slightly lean post mods. If this is the case I'll get started on producing a remap. Thanks for the idea Pippo !

[Pipo]

Jippy, I just succeeded in restoring the picture of my rewelded backbox. So here it is.

[P11 COV]

Here a picture of the standard NA silencer

That costs £620

I've one for sale if anyone whats it.

[paulf-cam]

I've one for sale if anyone whats it.

So do I...

Seems theres not too much demand for std NA back boxes!

[Anarchy]

How much do you want? some have been on Ebay for as little as £30 plus postage. Id have a cheap one

[P11 COV]

How much do you want? some have been on Ebay for as little as £30 plus postage.

Id have a cheap one

£100?

[JayK]

Pipo u are god I have also done the de-cat mod, have no cats what so ever, and i am in the u.k, after speaking to Pipo and knowing i wanted to do it, i did, excellent mod!

[Scottieb]

Has anyone tried this mod on a turbo? Scott

[Pipo]

Has anyone tried this mod on a turbo?

Scott

Not that I know of. In theory it must be at easy to do as on an NA. But beware, the internals of an turbo backbox are not the same as those of an NA backbox. So you can't apply the mod exactly as I described it.

[southpaw]

A bit of a thread resurection this.... I ve just got myself a spare na exhaust from a kind forum member Whilst looking at pushing the precat removal to the next level I found this - does anyone still have the pics for this to see what is required ?

[vocky]

there is one pic of the standard backbox mod in this thread .... here

[southpaw]

cheers for the link

[2-20]

cheers for the link

And others pictures HERE

[southpaw]

And others pictures HERE

double cheers

Pre


Post


off to read about the results now.

[Pipo]

In the left and right absorption chambers you can see the sound deadening wool. The only thing I would do different if redoing this mod, is removing this wool as I did find the backbox a bit to quiet sometimes. It want make you gain more BHP though as it doesn't change the gasflow or backpressure. And it's difficult to predict if ennoying vibrations may occure at certain speeds because of removing that glasfiberwool, without actually trying it.
Replacing the 2 outlet pipes for pipes with a larger diameter would be ideal for getting the ideal sound, but I did find that to much work.

In total I gained about 11 BHP. For this result I made following mods:
- I kept the original airbox, bur removed the plastic trumpet inside. I mounted a larger and longer intake pipe which took the fresh air directly from behind the side grill.
(I did try to change the original airbox for a sportsfilter, but besides producing a lot of noise, it actually made me loose torque on lower rpm. So I went back to the original airbox)
- I changed the plastic intake pipe between the airbox and the butterfly for an aluminium 76mm pipe.
- Replaced the overpressure hose from the enginehead for a small airfilter so the oilfumes would go into the air-intake.
- Removed the precat.
- Replaced the maincat for a straigh pipe. (I kept an orignal catline in stock for the MOT just in case)
- Made the mods (see pictures) to the original backbox to improve gasflow and reduce backpressure.
- Did no reprogramming of the engine-cpu at all. (probably by injecting some more fuel, I would have gained more pouwer)

Now 11 BHP doesn't sound much for all that work, but the mods transformed the way the car drove completely. It feld a lot more alive and agile in the lower gears, and it kept on pulling till the rpm-limiter kicked in. Up to 4500 rpm accelerating in a straight line, I only slowly pulled away from a standard VX. But once above 4500 rpm the difference was amazing. I could pull right into the limiter in 5th gear, where with the completely standard VX of my friend it was impossible. You had to change back to 4th gear to reach the rpm-limiter with his car.

Here you have the exact measurements of the inside (before and after the mod). Maybe it's of some help to you. (I can send the drawings in bigger size to you if you want)
Good luck with the mod.

Edited by Pipo, 16 November 2008 - 09:57 PM.

[G-Bob]

Holy thread-resurection batman! Do you think this would be possible by putting a long drill bit in through the tailpipes and drilling some holes there or does the box have to be cut open/welded? Just a thought!

[oakmere]

Holy thread-resurection batman!

Do you think this would be possible by putting a long drill bit in through the tailpipes and drilling some holes there or does the box have to be cut open/welded? Just a thought!

It is possible but the stainless steel is very difficult to drill it just eats drill bits (if you can even get the correct drill bit that long?) so I think it would be much easier to cut the box open. I got a local fabricator to weold it back up for £10.

Cheers Oakmere