E63 Bi-Turbo Start Up

[Bryan Allman]

Having read more than a few threads on exhaust noise, both on start-up and when running, and having noted the conventional wisdom that on start-up the exhaust by-passes the cats to enable quicker warm-up, I am slightly confused on how it does that.

I can't see any means of by-passing the Cat itself (ie there is no secondary pipe) nor can I see any electronic device attached to the exaust to enable this.

As I quite like the noise it produces on start up, can anyone offer a solution as to how you could implement this state on a permanent basis - apart from removing the Cats totally ?

Thanks

 

[merc85]

Perhaps a EGR Valve bypass would do it? i dont know. just a thought.

 

[whitenemesis]

Must admit to thinking the convention (generally, not specifically AMGs) is to run in such a way so as to heat up the CATs as quickly s possible?

 

[Bellow]

Having read more than a few threads on exhaust noise, both on start-up and when running, and having noted the conventional wisdom that on start-up the exhaust by-passes the cats to enable quicker warm-up, I am slightly confused on how it does that.

I think you have confused bypassing the turbo (by opening the wastegate) with bypassing cats. Bypassing the turbo prevents the exhaust gas from cooling there so maximises heat to the cat - to achieve 'light-off' as soon as possible.

As I quite like the noise it produces on start up, can anyone offer a solution as to how you could implement this state on a permanent basis - apart from removing the Cats totally ?

Yes - but you'll have no turbo boost!

 

[st13phil]

It doesn’t bypass the cat's, it bypasses the turbo's.

The objective is to get the cat's up to temperature quickly, so by running it as (effectively) an NA engine at cold start, the exhaust gasses have higher energy, promoting rapid heat buildup in the cat's. In addition, the engine over-fuels and runs a bit retarded on the ignition to promote combustion of the unburt fuel in the cat's thus creating additional heat.

If you stand alongside the front wing at cold start, as the rev's drop you will hear the turbo's spooling up when the bypass closes.

 

[Bryan Allman]

Complete dimwit at this end !! Age is creeping up and confusing the mind so I will retire to a dark corner and have stern words with myself. Many thanks for the quick responses

 

[S2000Paul]

I take it, as it happens to mine...to bypass the bypass...when I stick it in gear from start up the noise falls away immediately...

 

[Bellow]

Another side effect of the strategy is to prevent spinning up the turbo before oil has got a chance to properly protect it.

 

[Will]

Another side effect of the strategy is to prevent spinning up the turbo before oil has got a chance to properly protect it.

^^ And presumably allowing maximum oil circulation for the engine itself at this critical stage