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.