Guide; Mid box to back box conversion

[Dum-Dum]

HDi second mid box as back box conversion

Time Taken; About 90 minutes.

Parts Needed; Spare HDi Mid Section (£15 scrap or £30 new), threaded bar (M6 or M8), nuts for bar. New exhaust clamp and rubbers if necessary.

Tools needed; See picture below



The standard back box on a HDi is a baffled affair which does a good job of keeping the car quiet but restricts power. On a standard car youd be looking at up to 2bhp from replacing, on a stage 1 car the gain is reckoned to be up to 5bhp and on a stage 2 car the gain is almost 10bhp according to my Scan Gauge Computer.

So now you know theres a power gain I bet your itching to replace it but whats the easiest way without looking and sounding like a chav or spending lots of money? Swap it for the silencer from a second standard center section as theyer a straight through design.


Heres how;

Jack the car up at the back and support on stands (see jacking guide).

Unbolt the clamp at the front end of the back box (fuel tank end) this will need either a 13mm or 15mm deep socket on a ratchet. You may need WD40.

Unhook the 2 mounting rubbers holding the box on and swear as it lands on your face.

Compare your old box and new mid section then cut the new mid section roughly to length.



Then youll need to make up some sort of mounting, you could use an extra exhaust clamp and some threaded bar but I chose to just drill the pipe and bolt the threaded bar through it then bend the bar to shape for hangers.



Give it a test mounting with a new exhaust clamp and give it a final cut to length, add a stick on tail pipe if you fancy at this point.

Clean the 2 mounting faces (if theyer not brand new) with a wire brush, a rotary one in a drill makes life a lot easier. Smear exhaust jointing paste on the mating surfaces and bolt it all up.








No go for a drive (gently at first to set the exhaust paste) and test your new found power.


NB. This is a little easier on the hatches as theyer not so long as the sedans or estates so your more likely to get it pointing straight back rather than at an angle out the side.

[Matt]

nice one dum, been thinking about doing this on the xsi as i have a spare box

[ChezHdi]

This is awesome deffinatly going to do this as searching for a box is becoming a ball ache.

But on the second picture in, where has that small tube/bit of exhaust being cut off from??

(sorry for the big bumpppp)

[Jukie88]

Would the centre box of a 1.6 petrol be good for this also or are the pipes a different size ??

[puglove]

Surly you could of just took the band with the exhaust hangers off the back box amd put it round the new one with a bit of tack weld?

[Fozzy]

Surly you could of just took the band with the exhaust hangers off the back box amd put it round the new one with a bit of tack weld?

This is exactly what i have done, although was raining when i did it so hoseclip is being used to hold it in place rather than a weld, but eventually i will.

Mid way through....

[puglove]

I knew i was talking sence thats come out in the perfect place foz! Just got to get on ebay and order a 2.5" tip to finish it off now

I.e
http://item.mobileweb.ebay.co.uk/viewite...8214948730
But you should of used a cherry bomb like me sounds porn

[Fozzy]

Ive got the original chrome back box tip to weld on its barely shows in that pic but its wedged on the end

---

I bodged up a staright through to back box out of an old gti stainless exhaust that the tip fell off and it look great and oem:



But was far too restrictive

But the double mid box will look just as good when i weld the tip on

[puglove]

Ahhh right fair do's yeah makes more sence i see what youv done with the OE tip. Got to say very tidy job you have done the especialy with the OE hanger as well as tip

[Fozzy]

Exhaust made of parts i have lying around now thats what i like.

[puglove]

Yeah iv had a weekend of that. Made up a side exit. ( pics in pics section) and it cost me £00.00 just parts left lying around

[Poodle]

This is awesome deffinatly going to do this as searching for a box is becoming a ball ache.

But on the second picture in, where has that small tube/bit of exhaust being cut off from??

(sorry for the big bumpppp)

That small bit of tube is just the remainder of the cat section, you'll find a bit like that still attached to the midbox on most scrapyard cars, it's where they just slice the cat off for the scrap metal. It's got nothing to do with the guide, just happens to be in the picture.

Hahahah tack weld would be the proper way to do it, I think Chris actually had problems with the threaded bar munching through exhaust rubbers. Or you could be a real pikey and lash it on with some strong wire like, i don't know, old handbrake cable for example...

[WiNgNuTz]

*snip*

You should have left it like that!

[ChezHdi]

Anybody got a vid of what this sounds like?? or does it not make a difference with it having 2 sliencing box's??

[cully]

heres a picture of mine
i made a short extension section to link between the centre and backbox this is so the box was raised up so i could use the cradle from the old backbox

   

[puglove]

Tidy Job there cully!

[Jukie88]

Gunna attempt this tomoz hope the 1.6 exhaust is the same size :-(

[puglove]

Gunna attempt this tomoz hope the 1.6 exhaust is the same size :-(

Not sure if you can use a petral exhaust as they are made to give back pressure and this is what you are trying to reduce with this mod, id ask in the hdi section before you go ahead

[JackLewis_Dturbo]

i dont understand why anyone bothers with this? why not a de back box...

a diesel only has to have ONE 'box' in the exhaust system to comply with MOT law! it does not state which one, therefore a mid box on its own is perfectly legal....

not to mention if you have a genuine center section its practically straight through anyway..

[cully]

but some people don't want there car sounding like a tractor!

2 baffled box's work well and arn't noisy at all

[Jonny81191]

A turbo is counted as a box, right? It silences the exhaust....

[Poodle]

not to mention if you have a genuine center section its practically straight through anyway..

Just answered your own question there haha, it's no restriction so no loss, but as Cully says it keeps the noise down.

[Jonny81191]

It does create a restriction, but it is minimal. I noticed the difference from midbox only to full blown straight through..

[aaron_one]

Will an estate midbox fit a hatch?

[Aaron _d95]

Do you end up with just a back box or do you take the back box off and use the mid box as a back box.

[cully]

my system is

turbo======empty cat box=====centrebox=====centrebox=====

[aaron_one]

Having a crack at this tomorrow!

[Jonny81191]

My system is:

Turbo==============================================empty (big) cat box==

[simonh]

I'm running a 2.25" pipe from the turbo to a single non baffled box at the back and there's virtually no noise. Certainly nothing you can hear over the xud clatter anyway.

[puglove]

my system U=========================D wut ?