[grubbster]

On my car, when started cold it only uses the RHS exhaust, after 30s or so the LHS kicks in. Why is this?

 

[Gazwal86]

You would not believe the amount of people at work say... "only one of your exhausts are working" when I leave to go home, I believe it's to warm the cat up quicker but don't quote me.

 

[AJRFulton]

I've noticed other cars do this too.

I don't know the technical reason for it.

 

[Toube]

If I recall reading on these forums is that it has to do something with backpressure and torque on low rpm. I have disabled the flap myself and didn't notice any changes to when it's enabled.

 

[rob180bhp]

There is no way to bypass the turbo, it's an ecu running mode to warm the cats up for emissions test purposes

That's what I thought anyway

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[rob180bhp]

Good point

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[grubbster]

That makes sense, thank you for enlightening me.

 

[colwill116]

Is it not also to create higher back pressure to increase the temp in the Cat that way too?

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[Evolution]

It's to give people something to talk about when they see you start the car

 

[DaleB90]

There is no way to bypass the turbo, it's an ecu running mode to warm the cats up for emissions test purposes

That's what I thought anyway

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It holds the wastegate fully open which has the effect of bypassing the turbo (as the turbo has no use when the wastegate is fully open)

Or that's my understanding of it

Correct

 

[pitbull666]

There is no way to bypass the turbo, it's an ecu running mode to warm the cats up for emissions test purposes

That's what I thought anyway

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It holds the wastegate fully open which has the effect of bypassing the turbo (as the turbo has no use when the wastegate is fully open)

Or that's my understanding of it

Correct

Interesting and the reason for this is?

And couldn't it be done and still have both exhaust pipes working?

 

[DaleB90]

It holds the wastegate fully open which has the effect of bypassing the turbo (as the turbo has no use when the wastegate is fully open)

Or that's my understanding of it

Correct

Interesting and the reason for this is?

And couldn't it be done and still have both exhaust pipes working?

The flap being closed is meant to create more back pressure and thus heat to help warm the cat up along with the wastegate being wide open and the exhaust gases bypassing the turbo and going straight to the cat.

It works bloody quick, you can only hold your hand on the downpipe for a few seconds before its to hot to touch on a cold start.

 

[pitbull666]

Cool thanks for the explanation

 

[br0wny]

Try a cold start with a decat if you think it's loud standarD

 

[Dajonah]

Why couldn't the valve be on the other side, thus increasing back pressure and silencing the exhaust at the same time. Embarrassing early morning.

 

[seanklein]

The exhaust gasses will pass through the cat regardless, the pipe work only separates at the back box?

The flap is there for noise control, usually vacuum operated under full throttle conditions(will open).
Manufacturers don't want loud Exhausts at cruising speeds or idle.

 

[tweeky]

By creating higher back pressures you'll slow the exhaust flow down thus increasing the temps. The waste gate will only flow so much gas and the turbo will l still spin just at a slower speed, no doubt the EGR will be doing something too probity fully closed to increase the gas temp.

 

[DaleB90]

The exhaust gasses will pass through the cat regardless, the pipe work only separates at the back box?

The flap is there for noise control, usually vacuum operated under full throttle conditions(will open).
Manufacturers don't want loud Exhausts at cruising speeds or idle.

The flap serves that purpose after cold start, but during cold start it's there to increase back pressure. The car is louder with the flap closed compared to open on cold start also.

I personally can't see it making a significant difference being that far away from the turbo and cat but I've seen the official BMW statement regarding its operation during cold start.