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      04-16-2010, 04:49 AM   #1
tomvcarter
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Why is my exhaust louder for the first few mins, the suddenly getts quieter?

Why is my exhaust louder for the first few mins, the suddenly getts quieter?
There is a very noticable change in tone a few mins after i start the car, as if a valve shuts in the exhaust or something...
Any ideas what causes this exactly?
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      04-16-2010, 05:15 AM   #2
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The DME has different parameters during the first minute to heat up the cats faster. I assume it runs leaner, for one thing, but specifically what it does other than that, I don't know, but it is normal behavior... It has to do with emmission controls.
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      04-16-2010, 10:52 AM   #3
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Cold start warm up and idle control valve. Revs higher for a minute than everything goes normal
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      04-16-2010, 11:15 AM   #4
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Yup.. the infamous rough idle startup
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      04-16-2010, 11:17 AM   #5
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yup rough idle start up. love it. sounds so mean esp if you have a full midpipe and muffler
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      04-16-2010, 11:21 AM   #6
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Quote:
Originally Posted by tomvcarter View Post
Any ideas what causes this exactly?
I'd like to know exactly what's taking place too. Sounds like an exhaust leak with the hood open.

And the other behavior that baffles me even more is when you let the engine idle for a couple of minutes, and all of a sudden the engine gets heavily loaded, like if you engage A/C on a little 4-banger. Rpm drop with the load and recover instantly, and the engine becomes much louder as well. What the hell is loading the engine that much??? I'd like to know about this one more than the other .
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      04-16-2010, 11:30 AM   #7
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Quote:
Originally Posted by tomvcarter View Post
Why is my exhaust louder for the first few mins, the suddenly getts quieter?
There is a very noticable change in tone a few mins after i start the car, as if a valve shuts in the exhaust or something...
Any ideas what causes this exactly?
Good thread, and good question.

My full catless set-up sounds like a machine gun is firing off uncontrollably the first minute.

I'm like, "turn it off! turn it off!"

(and then after the minute is over, I'm all relaxed, whew )
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      04-16-2010, 11:54 AM   #8
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Quote:
Originally Posted by elp_jc View Post
I'd like to know exactly what's taking place too. Sounds like an exhaust leak with the hood open.

And the other behavior that baffles me even more is when you let the engine idle for a couple of minutes, and all of a sudden the engine gets heavily loaded, like if you engage A/C on a little 4-banger. Rpm drop with the load and recover instantly, and the engine becomes much louder as well. What the hell is loading the engine that much??? I'd like to know about this one more than the other .
Great question I would love to hear this...
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      04-16-2010, 12:40 PM   #9
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Quote:
Originally Posted by MysticBlue View Post
The DME has different parameters during the first minute to heat up the cats faster. I assume it runs leaner, for one thing, but specifically what it does other than that, I don't know, but it is normal behavior... It has to do with emmission controls.
Richer initially - to heat up the cats quickly so there is less time with the cats inactive.
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      04-16-2010, 01:05 PM   #10
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Quote:
Originally Posted by tomvcarter View Post
Why is my exhaust louder for the first few mins, the suddenly getts quieter?
There is a very noticable change in tone a few mins after i start the car, as if a valve shuts in the exhaust or something...
Any ideas what causes this exactly?
Cold start enrichment.

It's to warm up the vehicle, bring the oil temps up quicker, and also warm up the catalytic converter.

It's very normal and driving REALLY hard while this step is in process can lead to engine issues and early cat failure.
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      04-16-2010, 01:16 PM   #11
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They really sound like hell when first started. Other people think there is something wrong with it!
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      04-16-2010, 02:25 PM   #12
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I know my AA custom tuned software eliminated this part of the programming. It was so nice not to wake the neighbors when I started my car.
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      04-16-2010, 04:30 PM   #13
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i love the loud rough start!
especially with my eisenmann race!
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      04-16-2010, 04:54 PM   #14
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Quote:
Originally Posted by dillon733 View Post
i love the loud rough start!
especially with my eisenmann race!
Seriously though, who doesn't love it loud and rough...
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      04-16-2010, 06:38 PM   #15
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Quote:
Originally Posted by tomvcarter View Post
Why is my exhaust louder for the first few mins, the suddenly getts quieter?
There is a very noticable change in tone a few mins after i start the car, as if a valve shuts in the exhaust or something...
Any ideas what causes this exactly?
On cold start the engine RPM is run high and the fuel mixture is run lean in order to heat the cats up as quickly as possible. This is for emissions purposes.

I was at the dealership picking my car up the other day, and I was standing next to it when the tech cold started it. It sounded so sick, I didn't realize how loud it was with my mods. It started setting off car alarms at idle.
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      04-16-2010, 07:57 PM   #16
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Quote:
Originally Posted by elp_jc View Post
And the other behavior that baffles me even more is when you let the engine idle for a couple of minutes, and all of a sudden the engine gets heavily loaded, like if you engage A/C on a little 4-banger. Rpm drop with the load and recover instantly, and the engine becomes much louder as well. What the hell is loading the engine that much??? I'd like to know about this one more than the other .
I didn't notice this until I got my Gintani exhaust. Sport exhaust or not when the engine goes into the load mode it sounds like I am deep staging at a drag strip. Scares the crap out of the little kids waiting to be picked up when I am idling in line waiting to pick up my daughter from school.
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      04-17-2010, 04:41 AM   #17
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Quote:
Originally Posted by swamp2 View Post
Richer initially - to heat up the cats quickly so there is less time with the cats inactive.
Leaner is hotter, that's why I would think it would be leaner, not richer. But I'm just guessing.
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      04-17-2010, 07:29 AM   #18
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Based on the effect of the fuel mileage after a cold start I'm guessing it's extra rich at start up. Perhaps a little unburned fuel reaching the cats helps to warm them quicker.

I reset my mpg every fill up. On the next cold start after filling up the mpg always drops significantly which is why I'm thinking extra rich.
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      04-17-2010, 08:08 AM   #19
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Quote:
Originally Posted by swamp2 View Post
Richer initially - to heat up the cats quickly so there is less time with the cats inactive.
+1 I'm pretty sure this is right.
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      04-17-2010, 09:29 AM   #20
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Quote:
Originally Posted by Green///Hell View Post
Cold start warm up and idle control valve. Revs higher for a minute than everything goes normal
+1.
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      04-17-2010, 10:24 AM   #21
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In addition to the cold-start rpm being higher, cold exhaust is actually louder than hot exhaust.
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      04-18-2010, 04:33 AM   #22
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Quote:
Originally Posted by MysticBlue View Post
Leaner is hotter, that's why I would think it would be leaner, not richer. But I'm just guessing.
Quite a few factors involved actually. But I think we are both wrong...

-Combustion temperature
-Combustion rate
-Combustion energy transferred to engine (and hence engine temperature)
-EGT, exhaust gas temperature
-Greatest power from combustion
-Engine timing

-A stoichiometric mix actually makes the hottest flame.
-A rich mixture will always make the most power.
-Both a rich and lean mixture will burn with a cooler combustion temperature than stoichiometric. Rich does so because the uncombusted fuel absorbs energy when vaporized (heat of vaporization), lean does so simply because there is less fuel energy available and hence less combustion.
-A lean mixture will also burn slower which exposes the combustion chamber to the heat for longer causing the engine to heat up more.

Both the engine and cats need to be properly warmed up and warmed up quickly to reduce emissions. Thus a lean mixture is probably employed. It probably also employs a substantially retarded timing (which also increases exhaust gas temps and hence cat temps). Probably the latter causes the rough idle more than the AFR.
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