[Iamlemon]

Hi guys and gals, I have been looking through different exhaust setups on forums and YouTube and have noticed something peculiar. About 1/3rd of the exhaust videos I find show people starting up their cars and in the middle of the cold start sequence, reviving them. Due to the cold start sequence, the car does not let the engine rev above 3-4k RPM and gives off no burbles, pops, or crackles. I imagine this is not great for the car. So I am convinced I am missing something. Surely the owners notice/realize that their car is not behaving normally. So my question is, what is the actual reason people rev their cars to show the exhausts while the car is in cold start mode?

PS: Please let me know if I'm posting this in the wrong Forum.

[Kohner86]

On turbo cars like the m2 the cold start sounds better than normal operation. Thus if you want to show off the sound of your car it's best to do it when the engine is cold.

If you're curious why the cold start sounds better on turbo cars it's due to the turbo being bypassed when the car is first started to warm up to catalytic converters faster.

[Iamlemon]

Originally Posted by Kohner86

On turbo cars like the m2 the cold start sounds better than normal operation. Thus if you want to show off the sound of your car it's best to do it when the engine is cold.

If you're curious why the cold start sounds better on turbo cars it's due to the turbo being bypassed when the car is first started to warm up to catalytic converters faster.

I did not know that! Thank you! However, I feel that bypassing the turbos to show off an exhaust makes no sense if it is meant to display how exactly that exhaust setup sounds as almost no one will be hearing their car in higher revs during cold start. The video I am still confused about is someone with AA midpipes revving it cold. I mean the purpose of the midpipes from AA is to reduce rasp caused by turbos. It doesn't make sense to rev it in cold start if that bypasses the turbos.

[2020m2c]

Originally Posted by Iamlemon

I did not know that! Thank you! However, I feel that bypassing the turbos to show off an exhaust makes no sense if it is meant to display how exactly that exhaust setup sounds as almost no one will be hearing their car in higher revs during cold start. The video I am still confused about is someone with AA midpipes revving it cold. I mean the purpose of the midpipes from AA is to reduce rasp caused by turbos. It doesn't make sense to rev it in cold start if that bypasses the turbos.

Apparently to make it sound better than it actually does :

[Supadice]

Never made sense to me.

A. The car sounds worse to me cold. More rasp and more metallic.

B. The normal operation of the car is when it is warm.

C. 99.9 percent of the time spent driving is when the car is fully warm so why not give sounds clips of what you will hear most of the time

[stefan]

So we can assess how mad it'll make our neighbors.

[Higgs Boson]

Originally Posted by Kohner86

If you're curious why the cold start sounds better on turbo cars it's due to the turbo being bypassed when the car is first started to warm up to catalytic converters faster.

Cold start sounding better is an opinion. Videoing it is just to show what a modification sounds like to your neighbors in the morning, in case it's a factor.

Cold start sounds different due to raised RPMs, retarded ignition timing, retarded exhaust valve close event, and retarded end of injection timing (basically delaying combustion in relation to piston location) to allow more (still combustin) fuel into the exhaust system which heats the cats more quickly.

To my knowledge there is no such thing as "bypassing the turbos."

[Kohner86]

Originally Posted by Higgs Boson

To my knowledge there is no such thing as "bypassing the turbos."

The wastgate being opened would be the more technical term.
It's the valve that lets the computer control the boost pressure.

[CalAcacian]

Originally Posted by Higgs Boson

Cold start sounding better is an opinion. Videoing it is just to show what a modification sounds like to your neighbors in the morning, in case it's a factor.

Cold start sounds different due to raised RPMs, retarded ignition timing, retarded exhaust valve close event, and retarded end of injection timing (basically delaying combustion in relation to piston location) to allow more (still combustin) fuel into the exhaust system which heats the cats more quickly.

To my knowledge there is no such thing as "bypassing the turbos."

You're incorrect there. It was an inelegant term, but they were referring to the function where the wastegate is left open at idle during cold start. On our cars, it is done to maximize the heat transfer to the catalytic converter when cold to help it reach operating temperature as quickly as possible to minimize emissions.

Before the catalytic converter reaches a high enough temperature, it's ability to work and reduce NOx and CO, along with other pollutants, is severely limited.

It's a very smart setup, there is no reason to waste the heat energy partially spooling the turbo at idle on cold start when there is no demand for boost at all. The unfortunate side effect is that the car sounds extra loud and tractor-like for the first 30 seconds or so of idling when the car is cold.

Especially with high flow cats or catless downpipes, it can be extremely obnoxious at start up. That's why cold start is a very meaningful test of an exhaust. An otherwise great exhaust can be downright anti-social on cold start so it is an important data point for people to examine before purchasing an exhaust, especially if you live near neighbors or park in a communal garage.

If you want to hear it for yourself, here's how: next time you start your car in the morning, very lightly apply the throttle while still in park. You should paradoxically see the revs rise, but the exhaust decrease in volume as the wastegate(s) close. The wastegate is only open at base idle, so if you put the car in gear and apply throttle the wastegates will close, at least until the car begins idling again. After roughly 30 seconds the cat will be adequately heated and the exhaust will stay quiet.

[Higgs Boson]

Yes you’re both correct that I did not account for opening the waste gate, I guess I was thrown off by the terminology.

Regardless, the volume and tone change at startup is possibly 10% attributed to opening the waste gate and 90% atrributed to the changes I listed above.

You can detect the same tone and volume changes in naturally aspirated and supercharged cars as well due to the reasons I listed and there are no wastegstes there.

The exhaust sound flaps also play a role in the volume of the exhaust at cold start, keep in mind.

[akkando]

Originally Posted by Kohner86

On turbo cars like the m2 the cold start sounds better than normal operation. Thus if you want to show off the sound of your car it's best to do it when the engine is cold.

If you're curious why the cold start sounds better on turbo cars it's due to the turbo being bypassed when the car is first started to warm up to catalytic converters faster.

But that's not what the car sounds like, so it's useless.

[Iamlemon]

Originally Posted by Supadice

Never made sense to me.

A. The car sounds worse to me cold. More rasp and more metallic.

B. The normal operation of the car is when it is warm.

C. 99.9 percent of the time spent driving is when the car is fully warm so why not give sounds clips of what you will hear most of the time

Exactly! Especially when a company previews a product, it makes no sense.

[CalAcacian]

There's a very easy solution:

Exhaust modifications should be shown off with both a cold start and operation, and hot start and operation.

[LakiTopalovich]

Originally Posted by CalAcacian

You're incorrect there. It was an inelegant term, but they were referring to the function where the wastegate is left open at idle during cold start. On our cars, it is done to maximize the heat transfer to the catalytic converter when cold to help it reach operating temperature as quickly as possible to minimize emissions.

Before the catalytic converter reaches a high enough temperature, it's ability to work and reduce NOx and CO, along with other pollutants, is severely limited.

It's a very smart setup, there is no reason to waste the heat energy partially spooling the turbo at idle on cold start when there is no demand for boost at all. The unfortunate side effect is that the car sounds extra loud and tractor-like for the first 30 seconds or so of idling when the car is cold.

Especially with high flow cats or catless downpipes, it can be extremely obnoxious at start up. That's why cold start is a very meaningful test of an exhaust. An otherwise great exhaust can be downright anti-social on cold start so it is an important data point for people to examine before purchasing an exhaust, especially if you live near neighbors or park in a communal garage.

If you want to hear it for yourself, here's how: next time you start your car in the morning, very lightly apply the throttle while still in park. You should paradoxically see the revs rise, but the exhaust decrease in volume as the wastegate(s) close. The wastegate is only open at base idle, so if you put the car in gear and apply throttle the wastegates will close, at least until the car begins idling again. After roughly 30 seconds the cat will be adequately heated and the exhaust will stay quiet.

This is true. I live in particularly peaceful and tranquil neighborhood so I start driving the second the engine turns over in order to minimize noise. It really does make significant difference.