SavMan
Authorised Forum Sponsor
Exhaust & Headers
I can only speak about the exhaust on the W211 E55 so although many other models share many of the components the results may differ. My exhaust was first prepared in several areas to measure for back pressure at different stages of the exhaust and was run on a dyno for several days to gather as much data as possible. Now real data was obtained on either the primary cats
The results were as follows
Firstly the restriction
Headers – the stock headers are the log style and have 37mm inlets with a single 42mm outlet, they are twin walled and were found to have the engineer testing it have it a good to very good rating on removing heat when looked at in isolation. There is however significant back pressure added to this they join directly to the primary cats that sit only 120mm away from cylinder 4 and 8 causing further back pressure, the peak back pressure measured from header inlet to post primary cat was 0.45 bar.
The primary cats were thought to be at least 500 cell, I wish I had spent more time on this as most who viewed and did a rough measurement felt they were closer to 600 cell. I still have a set at the back of the garage so might open them up to check for sure.
Secondary cats were quite surprising as they seemed at least 300 cell if not more however as they were to be removed along with the primary’s we did not look into with as much detail as I wished now.
Centre resonator was quite a surprise the measurement from the sec cat back showed 0.3 bar and yet the measurement after the centre resonator was negligibly so the changing the rear boxes would not have had a significant effect on performance.
Exhaust pipe diameter, we could not find any improvement in flow above 70mm diameter pipe the downside is that it tends to be more expensive and harder to get. 77mm is typically favoured by most exhaust shops and the only difference that was found was that it did mean that off boost there is a slight but noticeable lag until boost builds.
You will need to have some form of balance pipe either X or H but this has been discussed many times so have a search you will find lots of pro’s and con’s for each
So in summary
Mid or Long Tube Headers are the way to go! The only real option unless you have a bottomless pocket seem to be are the Kleemann Mid style although they are a pain to fit and do require modification. Budget at least £2 - 2.2k, if you really want long tube’s you can add another 50% on top at least for a custom build as every set we could find would only fit LHD cars.
For a road car fitting mid or long tube headers that take the place of the primary cats will yield a significant improvement along similar lines as a smaller supercharger pulley or 168 crank pulley so think 550-560bhp.
Replacing the secondary cats with 200 cell cats and then removing the centre box will reduce back pressure by 0.3 bar which is significant especially when combined with Kleemann headers.
For a car that is used for track only i.e. no MOT remove all the cats and centre resonator.
If you can afford it and get hold of the right material 70mm diameter seems the best
Expect to pay anything from £1k to £2.5K depending on the material used for the exhaust.
If you went for the whole lot in one go you may be able to get it down to £2.5k including the headers.
I can only speak about the exhaust on the W211 E55 so although many other models share many of the components the results may differ. My exhaust was first prepared in several areas to measure for back pressure at different stages of the exhaust and was run on a dyno for several days to gather as much data as possible. Now real data was obtained on either the primary cats
The results were as follows
Firstly the restriction
Headers – the stock headers are the log style and have 37mm inlets with a single 42mm outlet, they are twin walled and were found to have the engineer testing it have it a good to very good rating on removing heat when looked at in isolation. There is however significant back pressure added to this they join directly to the primary cats that sit only 120mm away from cylinder 4 and 8 causing further back pressure, the peak back pressure measured from header inlet to post primary cat was 0.45 bar.
The primary cats were thought to be at least 500 cell, I wish I had spent more time on this as most who viewed and did a rough measurement felt they were closer to 600 cell. I still have a set at the back of the garage so might open them up to check for sure.
Secondary cats were quite surprising as they seemed at least 300 cell if not more however as they were to be removed along with the primary’s we did not look into with as much detail as I wished now.
Centre resonator was quite a surprise the measurement from the sec cat back showed 0.3 bar and yet the measurement after the centre resonator was negligibly so the changing the rear boxes would not have had a significant effect on performance.
Exhaust pipe diameter, we could not find any improvement in flow above 70mm diameter pipe the downside is that it tends to be more expensive and harder to get. 77mm is typically favoured by most exhaust shops and the only difference that was found was that it did mean that off boost there is a slight but noticeable lag until boost builds.
You will need to have some form of balance pipe either X or H but this has been discussed many times so have a search you will find lots of pro’s and con’s for each
So in summary
Mid or Long Tube Headers are the way to go! The only real option unless you have a bottomless pocket seem to be are the Kleemann Mid style although they are a pain to fit and do require modification. Budget at least £2 - 2.2k, if you really want long tube’s you can add another 50% on top at least for a custom build as every set we could find would only fit LHD cars.
For a road car fitting mid or long tube headers that take the place of the primary cats will yield a significant improvement along similar lines as a smaller supercharger pulley or 168 crank pulley so think 550-560bhp.
Replacing the secondary cats with 200 cell cats and then removing the centre box will reduce back pressure by 0.3 bar which is significant especially when combined with Kleemann headers.
For a car that is used for track only i.e. no MOT remove all the cats and centre resonator.
If you can afford it and get hold of the right material 70mm diameter seems the best
Expect to pay anything from £1k to £2.5K depending on the material used for the exhaust.
If you went for the whole lot in one go you may be able to get it down to £2.5k including the headers.
Last edited:
