BMW M5 Forum and M6 Forums banner

1 - 8 of 8 Posts

·
Registered
Joined
·
543 Posts
Discussion Starter #1
When I first start my engine up it has the cold idle where it idles around 1000rpm, I started it up and had to throw something in my trunk one day and I happened to hear my exhaust at this point and I really like the sound. Anyone with exhaust needs to listen to it when the first fire it up in the morning sometime, it sounds really cool. I wonder if there is a way to make it always idle like this?
 

·
Registered
Joined
·
1,316 Posts
hahahahhaaa... :thumbsup:

Mine sounds like that in the AM, too. When I pull the car out in the morning, I get out to close the garage door (dang openers broken again) and it sounds really mean! It only lasts about the time it takes me to get back to the car...and then it's quiet again.

I think it's probably due to the controls throwing more fuel into the engine during warmup (I smell unburned fuel at the same time) but not really sure...

:cheers:
 

·
Registered
Joined
·
13,638 Posts
I think it's simple physics. The exhaust is designed to sound best when warm. A steel exhaust will expand as it heats up to a normal operating temparature. A cold exhaust is therefore more constricted and hence the louder exhaust note.
 

·
Registered
Joined
·
797 Posts
Re: Loud cold idle (Long)

Ashok Arora said:
I think it's simple physics. The exhaust is designed to sound best when warm. A steel exhaust will expand as it heats up to a normal operating temparature. A cold exhaust is therefore more constricted and hence the louder exhaust note.
I doubt that there is enough increase in the length of the exhaust pipes or exhaust itself to change the frequency of the exhaust note appreciably.

For example. Suppose the average temperature (along its length) of the exhaust rises from 25 degrees C to 225 degrees C (I have no idea if this is correct. Seems plausible to me though).

Then, the change in temperature of the exhaust (stainless steel lets say) is 200 degrees C. Stainless steel has a coefficient of expansion of approximately 3.6x10^-6 per degree C.

Now suppose your exhaust pipe has a length of 4 meters (forget the exhaust itself for a minute). Then the change in length of the pipe is 4x3.6x10^-6x200=0.003 m or 3 mm. The fundamental resonant frequency of an organ pipe is proportional to the inverse of its length. Hence, the resonant frequency of the exhaust pipe itself will change by 0.003m/4m =0.07%.

Now what about the exhaust system itself? Same principle. In this case, the resonant frequency of the exhaust can will be quite a bit higher because its length is smaller than the exhaust pipe. But the percent change in resonant frequency is the same - about 0.07%

These changes are not large enough for the auditory system to register as a major difference.

What about amplitude (loudness). Length change will not change the amplitude of the pressure impulses in the exhaust system.

However, the diameter of the pipes and cans also changes, but by the same small fraction. Hence there is little change in the peak pressure in the exhausts and cans.

Therefore, one must assume that the exhaust note at start up is due to something else. Perhaps cam timing?

Jeff :cheers:
 

·
Registered
Joined
·
543 Posts
Discussion Starter #5
Re: Loud cold idle (Long)

Mine does sound different accelerating when the pipes have warmed up vs. the first 5 or 10 mins when they are cold, but I know thats not the reason for the loud idle.
 

·
Registered
Joined
·
328 Posts
could be, most likely cam sensors, sounds like you just took out the
cats,then goes back to normal after awhile.
 

·
Registered
Joined
·
2,817 Posts
My guess is a combo of cam timing and ignition timing. That stupid electric air pump (just ahead of the front right tire) running for 30 seconds or so at cold start might also change the exhaust note a bit. Unless the noise we hear is the Vanos pump...
 

·
Registered
Joined
·
543 Posts
Discussion Starter #8
It is definitly exhaust since I have a really loud one. When it was stock it did the same thing but nowhere near as loud.


does it sound that much different if you take the cats off these cars?
 
1 - 8 of 8 Posts
Top