What do you think about this?

the 2.0 is v6 so you'd have to custom make front section to fit

My bad... (changed the topic)

I mean only the rear exhaust...

Would it affect performance or consumption?

Widening the bore from the cat will increase performance but not by a great deal. Backbox alone won't do anything as the restriction will still be the pipework, cat, and centre box.

Thanks for the reply Brucey76.

I already have a decat pipe.

I guess i'll be doing this change

The diameter of the exhaust tube is crucial for how the engine performs.
Taking into consider tube diameter as well as lenght and structure(grain roughness) of the tube you can calculate the flow.And specifically (what i think is the goal in the case) you can calculate Max flow.
Take into consideration the rules of thermodyinamic.
Out back the gasses are much colder than infront right after the engine.If you put larger diameter pipes the speed of the flow will drop because of the displacement itself and will cool faster than with standart exh. tubes.The flow trough the back box (if itwill be the v6 one will be small amount from that what it was design to be) so the sound will not be as the v6 one.. The flow trough it will also be slower.
If the engine needs to breath better consider a whole exhaust job, not just the back of it.
That way that you want to do exhaust upgrade you will only upgrade or downgrade the sound of the exhaust.You will still have restriction before the v6 pipes.

Thanks for the reply mazzda!

I don't have the knowledge to do such calculations...

The exhaust is original, same diameter, i've only replaced the cat with a resonator.

I see that the new mx5 ND uses the same aftermarket back box in 1.5 and 2.0.

I tough the v6 2.0 back box would lead to having less restriction (two pipes out) than the one from the 1.5...

It will be with less restriction, Yes.Due to the fact that v6 back box is originally build to flow more gases.
The effect will be incredibly ... mmmm .. wait for it ...
SMALL..
so if you change the whole pipe from before the de-cat it will be less restrictive than only a v6 back "section"..

I've done mine from the 4-2-1 to 1m of 60mm pipe.. than is the stock 1.5 mid section and Remus back box.I will have to get one v6 mid section so to get the exhaust gases to be freee at last ..
You have to do the exhaust (if not the whole) by a part from the front and then to make the rest of the tubes so to have any results from it..
If you do not free them from the front you will have no use from the rest of the exhaust.
I think you got my point.
Cheers!

I get your point mazzda.

And you make sense in what your say.

My big concern is not the gains, but the losses, because 88hp...

I didn't find any user that did it...

I want to change to dual tip exhaust, and i can get a back box for the 2.0 v6 new at about 55€...

Its almost the price of new tips, my back box is original with +330.000km and no problems...

I'll give it a try, and hope no horse runs away.

Don't worry, they won't.

http://www.ebay.co.uk/itm/Hornet-Univer ... SwuMFUdgDH that will give you a free flowing exhaust and a few decibels. i've fitted the 2.5" to my ze and the difference is amazing. as long as you dont mind getting the grinder out. The kits dont come with clamps tho so its either buy separate or weld it together.