451 Exhaust Types

The smart fortwo 451 ran from 2007 to 2014 and in those 7 years we saw a tightening of the emissions laws in Europe which meant that 451 exhausts changed. Due to the confusion, it seems necessary to create a visual list so people know what they need should they need to fit a replacement exhaust.

Any car created after September 2009 had to conform to Euro 5 emissions laws, however, smart may have fitted Euro 5 exhausts before this time.

Euro 4 Non Turbo and Euro 4 Turbo

It was the same for both engines.

Euro 4 Non Turbo Brabus and Euro 4 Turbo Brabus

It was the same for both engines.

Euro 5 MHD

The non turbo model has a large silencer and a short pipe from the flexi-joint.

Euro 5 MHD Brabus

The non turbo model has a large silencer and a short pipe from the flexi-joint.

Euro 5 Turbo

The turbo model has a narrow silencer and a long pipe from the flexi-joint.

Euro 5 Turbo Brabus

The turbo model has a narrow silencer and a long pipe from the flexi-joint.

Euro 4 Diesel (45bhp)

I don’t have a photo yet but it looks like the Euro 5 diesel exhaust but without the pressure and temperature pipe connections coming out of the silencer can. If you have a photo, please send it over.

Euro 5 Diesel (54bhp) With DPF

This version has 3 extra connections. 2 on the top and 1 on the left. These are for temperature and pressure sensors.

American Non Turbo

Thanks to Phil for the photo of the US exhaust.

It looks exactly the same as the MHD exhaust.

This would make sense as America have strict emissions laws (believe it or not. Ask VW). So for our non turbo MHD to adopt their exhaust type seems to add up.

Flexi Failure

Pretty much all of the 451 exhausts have a flexible section of pipe between the catalytic converter and the silencer (muffler). They take up the vibration from the engine so it doesn’t pass through to the silencer. The problem is that the smart was built cheap so the flexible section doesn’t generally last more than 10 years but they often fail after about 7.

Inside is a convoluted metal hose and the whole thing is wrapped in a stainless steel braid to add protection and keep the hose inside in position and straight.

There are 3 issues that cause problems.

1 – The stainless steel braid become detached from 1 end which allows a bit more movement than designed.
2 – Dirt, debris, mud and water get through the braid and get trapped between the brain and the convoluted hose. This causes wear of both eventually leading to a hole.
3 – Either issue 1 or 2 causes a hole or poor combustion can cause the water vapour in the emissions to become acidic, eventually rusting through the convoluted hose.

The first you normally know about it is when your car sudden sounds really loud.

The Fix

Smart would obviously like you to buy a new exhaust. Ebay sellers would like you to buy their 2nd hand exhausts.

However, the best option is to measure the outside diameter of the hard pipe either side of the flexi section, then do a search for a replacement flexi pipe section that can be welded in once the old one has been cut out. If you can’t find a flexi pipe section long enough, you can just get some more hard pipe section and extend what is already there. If you find an exhaust shop that does custom work, they should easily be able to do all of that work for you and it really shouldn’t cost much at all.