Why is my exhaust louder for the first few mins, the suddenly getts quieter?

[sfp power]

I think the first minute is the best sound raw as a wrc !! I have full supersprint system

[J08M3]

Originally Posted by swamp2

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.

I'm not sure you have it all right here.... I'm pretty sure a lean mixture burns hotter.

I was under the impression it runs richer because it's easier to get a cold/high compression engine started with a rich mixture - think back to the days of a choke on a carburetor, it cut off air so the mixture was richer. Also a rich mixture dumps more unburned fuel into the cats which then burns there and heats them up quicker.

[BMRLVR]

On initial startup when the engine is cold the secondary air injection is turned on and the mixture is richened to heat up the cats quickly.

The combination of the extra fuel and air from the secondary air injection pump makes the exhaust note more agressive during the first ~30 seconds.

All modern OBDII cars do this it is just that with a more muffled exhaust it is harder to notice.

[swamp2]

Originally Posted by J08M3

I'm not sure you have it all right here.... I'm pretty sure a lean mixture burns hotter.

I was under the impression it runs richer because it's easier to get a cold/high compression engine started with a rich mixture - think back to the days of a choke on a carburetor, it cut off air so the mixture was richer. Also a rich mixture dumps more unburned fuel into the cats which then burns there and heats them up quicker.

Maybe I don't have it all right, I am not a combustion expert. It is a field rich with physics. There are a variety of strategies OEMs will use to optimize cold starts. A rich mixture is required for a cold engine because the fuel does not fully vaporize lowering the effective amount of fuel available. However, I think this enrichment occurs after a cat warm up procedure but before the engine is fully warmed up.

To fully understand the thermal effects of non stoichiometric burns you have to consider multiple phenomena. Flame temperature AND heat transfer to engine components. I mentioned this prior. However, I can assure you that moving from stoichiometric to EITHER rich or lean produces a cooler combustion flame temperature. Again under a rich condition the extra fuel absorbs the combustion energy (heat of vaporization). When lean the same thing applies but the cool excess oxygen is what is absorbs part of the combustion energy. See the graph and explanation below (from Combustion, Third Edition, Irvin Glassman, available on google books). This is true across most HC combustion, not just gasoline in ICEs.

Engine operating temperature depend on a many more factors other than just flame/combustion temperature. Burn rate is a key factor.

[skierman64]

Here's the way to solve this discussion.

Reset your mpg calculator. Do a cold start and drive a mile or 2. See what the mileage is.

Drive around and warm up the engine, 10 minutes or so. Go back to starting point.

Reset the mpg calculator again. restart the warm engine. drive the same route using the same rpm shifting points and use the (approximate) same amount of throttle as before.

I'll bet the cold start gets worse mileage because the engine is running rich.

[FStop7]

I'm pretty sure it's lean because when I have a wideband on a car it's hitting like a 21:1 AFR for the first 10 or 15 seconds after a cold start. Super lean. Drops down to 14.7:1 once the idle lowers to normal.

[J08M3]

Originally Posted by swamp2

Maybe I don't have it all right, I am not a combustion expert. It is a field rich with physics. There are a variety of strategies OEMs will use to optimize cold starts. A rich mixture is required for a cold engine because the fuel does not fully vaporize lowering the effective amount of fuel available. However, I think this enrichment occurs after a cat warm up procedure but before the engine is fully warmed up.

To fully understand the thermal effects of non stoichiometric burns you have to consider multiple phenomena. Flame temperature AND heat transfer to engine components. I mentioned this prior. However, I can assure you that moving from stoichiometric to EITHER rich or lean produces a cooler combustion flame temperature. Again under a rich condition the extra fuel absorbs the combustion energy (heat of vaporization). When lean the same thing applies but the cool excess oxygen is what is absorbs part of the combustion energy. See the graph and explanation below (from Combustion, Third Edition, Irvin Glassman, available on google books). This is true across most HC combustion, not just gasoline in ICEs.

Engine operating temperature depend on a many more factors other than just flame/combustion temperature. Burn rate is a key factor.

Interesting.... I always thought a lean mixture was hotter, and that's why it would burn up your engine, and that's why it's more dangerous than running too rich.

[cpiguy]

Originally Posted by swamp2

Richer initially - to heat up the cats quickly so there is less time with the cats inactive.

What cats

[swamp2]

Originally Posted by FStop7

I'm pretty sure it's lean because when I have a wideband on a car it's hitting like a 21:1 AFR for the first 10 or 15 seconds after a cold start. Super lean. Drops down to 14.7:1 once the idle lowers to normal.

Yes, lean it is, my initial suggestion was incorrect. That is what you get for simply requoting another member here on the forum. It seems it is a commom misperception.

Originally Posted by J08M3

Interesting.... I always thought a lean mixture was hotter, and that's why it would burn up your engine, and that's why it's more dangerous than running too rich.

Keep in mind: hotter flame is not always the same as hotter engine. I keep repeating that. It seems that they would have to go together but they don't. Leaner is indeed a hotter engine.

Back to the OP why is lean and retarded timing loud? Once we have that I think we have the full answer.

[BMRLVR]

First, a gas engine will not start cold with a mixture that is even 14.7-1....... The mixture has to be enriched or the engine will not start and run, period. When the any engine (S65 included) is started cold I guarantee you it is running a mixture that is enriched.

Second, the secondary air injection pump pushes fresh air into the exhaust stream which causes oxidation of the unburned fuel in the exhaust stream therefore increasing the temperature of the exhaust and heating the catalysts so that they reach the temperature required for catalyzation to take place. Basically the secondary air injection pump helps takes care of the very rich cold start emissions until the catalysts are up to temperature plus it helps to heat up the cats in the process.

Third, the reason a wideband would read 21-1 AFR at initial start-up is due to two reasons. The first reason is that the wideband will not properly read until it warms up to operating temperature. The second reason is that with the air pump sending fresh air into the exhaust stream it actually causes oxidation (recombustion of the unburned fuel) within the exhaust upstream of the O2 sensors which makes the exhaust stream lean even though the mixture is rich in the combustion chamber.

Finally, an engine is running in a closed loop mode (does not use the O2 to adjust mixture) until the engine coolant temperature sensor sees operating temperature is reached. During this warm-up/closed loop period the AFR is predetermined by the ECM and not adjusted based on O2 sensor readings.

Basically these cars sound mean on a cold start till the secondary injection pump turns off, it's that simple.

[swamp2]

Good input ^.

Originally Posted by BMRLVR

Second, the secondary air injection pump pushes fresh air into the exhaust stream which causes oxidation of the unburned fuel in the exhaust stream therefore increasing the temperature of the exhaust and heating the catalysts so that they reach the temperature required for catalyzation to take place. Basically the secondary air injection pump helps takes care of the very rich cold start emissions until the catalysts are up to temperature plus it helps to heat up the cats in the process.

So is combustion happening outside of the combustion chamber or not? I don't see how just adding the oxygen would cause combustion. Of course any such combustion would be ideal for heating the cats.

Originally Posted by BMRLVR

Basically these cars sound mean on a cold start till the secondary injection pump turns off, it's that simple.

I'm not quite making the causal connection between secondary injection and the higher noise level. Don't you think that the timing affects both the rough idle and sound before the cats are hot?

[AMPowerJ]

Originally Posted by BMRLVR

First, a gas engine will not start cold with a mixture that is even 14.7-1....... The mixture has to be enriched or the engine will not start and run, period. When the any engine (S65 included) is started cold I guarantee you it is running a mixture that is enriched.

Second, the secondary air injection pump pushes fresh air into the exhaust stream which causes oxidation of the unburned fuel in the exhaust stream therefore increasing the temperature of the exhaust and heating the catalysts so that they reach the temperature required for catalyzation to take place. Basically the secondary air injection pump helps takes care of the very rich cold start emissions until the catalysts are up to temperature plus it helps to heat up the cats in the process.

Third, the reason a wideband would read 21-1 AFR at initial start-up is due to two reasons. The first reason is that the wideband will not properly read until it warms up to operating temperature. The second reason is that with the air pump sending fresh air into the exhaust stream it actually causes oxidation (recombustion of the unburned fuel) within the exhaust upstream of the O2 sensors which makes the exhaust stream lean even though the mixture is rich in the combustion chamber.

Finally, an engine is running in a closed loop mode (does not use the O2 to adjust mixture) until the engine coolant temperature sensor sees operating temperature is reached. During this warm-up/closed loop period the AFR is predetermined by the ECM and not adjusted based on O2 sensor readings.

Basically these cars sound mean on a cold start till the secondary injection pump turns off, it's that simple.

Does the car go into this mode again after being warm at idle from time to time? That was the second question in this thread. Why does the idle drop and the engine sound like it is under load?

[BMRLVR]

Originally Posted by swamp2

Good input ^.



So is combustion happening outside of the combustion chamber or not? I don't see how just adding the oxygen would cause combustion. Of course any such combustion would be ideal for heating the cats.



I'm not quite making the causal connection between secondary injection and the higher noise level. Don't you think that the timing affects both the rough idle and sound before the cats are hot?

Yes combustion is happening outside of the combustion chamber...... you see the exhaust which is already quite hot and rich only needs additional oxygen to oxidize the remaining unburned fuel in the exhaust stream just aft of the exhaust ports. This combustion is not explosive or anything like the additional combustion event and this is the reason why I prefer to refer to it as oxidization. Oxidization is technically the acceleration/intensification of combustion by adding oxygen.

This oxidization is just like when you add oxygen to an OXY-Acetylene torch....... adding the oxygen allows a hotter flame and makes for complete combustion of the fuel. If you have ever done any cutting with a torch you know when you run out of oxygen the flame from the torch is not nearly as hot and emits thick black soot/smoke which is unburned acetylene (fuel).

An engine on idle needs a rich mixture to start and run when cold however it cannot get enough oxygen into the cylinder at idle when cold to burn all of the fuel so there is quite a bit of unburned fuel in the exhaust during this warm-up period. Adding the extra fresh air via the secondary air injection pump adds the necessary oxygen to allow this unburned preheated fuel to oxidize in the exhaust before the cats. Once the engine and cats are warmed up it can sustain combustion at a stoich level the secondary air injection pump is switched off because the engine is able to burn nearly all of the fuel that is injected into the cylinder and then the cats deal with any of the unburned fuel/pollutants through catalyzation.

Originally Posted by AMPowerJ

Does the car go into this mode again after being warm at idle from time to time? That was the second question in this thread. Why does the idle drop and the engine sound like it is under load?

This loading and unloading you are talking of is not the engine going into warmup mode. What you are hearing is more than likely the AC Compressor cycling off and on. The AC compressor does not run constantly it is clutched and cycles off and on.

Please remember that the engine only enters this warm up mode only when the coolant temperature is below a certain specified value. Every time you start the car the secondary air injection pump does not run..... it only runs from a cold start which generally happens when the car is not started for more than 3-6 Hrs during summer months (depending on the outside temperature and the rate at which the engine coolant temperature drops this may be only an hour during the winter).

[FStop7]

Originally Posted by AMPowerJ

Does the car go into this mode again after being warm at idle from time to time? That was the second question in this thread. Why does the idle drop and the engine sound like it is under load?

i'm pretty sure it does. if i idle for more than a few minutes the idle drops, and the sound of the engine gets much deeper.

[BMRLVR]

Originally Posted by FStop7

i'm pretty sure it does. if i idle for more than a few minutes the idle drops, and the sound of the engine gets much deeper.

It dosen't go into a warm-up mode again..... it is more than likely the AC Compressor cycling that makes the idle slow and the the exhaust note deepen. The AC Compressor takes ~7hp to turn, that is not an insignificant load at idle.

[Donbona]

Originally Posted by darwin316

Yup.. the infamous rough idle startup

If you read your manual it actually tells you this
But remember once it is warm and you go over 4000rpm the beast awakens once again.

[AMPowerJ]

Originally Posted by BMRLVR

It dosen't go into a warm-up mode again..... it is more than likely the AC Compressor cycling that makes the idle slow and the the exhaust note deepen. The AC Compressor takes ~7hp to turn, that is not an insignificant load at idle.

It isn't the compressor. This happens with the A/C off.

[BMRLVR]

Originally Posted by AMPowerJ

It isn't the compressor. This happens with the A/C off.

Even with the switch in the off position the compressor will still run but is not used for cooling, it functions as a dehumidifier to dry damp air before it enters the cabin. Also the AC runs anytime that the windshield defrost/defog is on.

Pull the fuse for your AC compressor or unplug the clutch on the compressor itself and I guarantee it will no longer do what you are describing.

[ph_gbg]

Originally Posted by swamp2

I'm not quite making the causal connection between secondary injection and the higher noise level. Don't you think that the timing affects both the rough idle and sound before the cats are hot?

I'm thinking swamp2 has a point here. Remember that the S65 has double VANOS, so my guess is that the ECU does something with the exhaust valve timing.

Being parked in a cramped garage with Remus sport has made me quite familiar with this...

[J08M3]

Originally Posted by AMPowerJ

It isn't the compressor. This happens with the A/C off.

could be the alternator, which also has a different draw depending on how many amps it needs to feed into your battery to keep it charged.

[AMPowerJ]

That makes more sense because I don't think the A/C compressor comes on when the whole HVAC system is off.

[JCtx]

Originally Posted by BMRLVR

Finally, an engine is running in a closed loop mode (does not use the O2 to adjust mixture) until the engine coolant temperature sensor sees operating temperature is reached. During this warm-up/closed loop period the AFR is predetermined by the ECM and not adjusted based on O2 sensor readings.

You got it backwards buddy . Closed-loop is when O2 sensors are on board, since they're 'in the loop'.

Originally Posted by BMRLVR

it is more than likely the AC Compressor cycling that makes the idle slow and the the exhaust note deepen.

Originally Posted by AMPowerJ

That makes more sense because I don't think the A/C compressor comes on when the whole HVAC system is off.

It's definitely NOT the A/C compressor (I also always turn the HVAC completely off before shutting engine down). Whatever the heck happens is loading the engine A LOT more than the compressor. I engaged A/C once that happened, and as always, couldn't even feel/hear it (and it doesn't feel like it cycles either, due to using swash plates, rather than pistons). When that loading takes place, you can hear the darn engine laboring, like if we had hooked up a TIG welder or something. It has to be either that air injection pump (like a 2nd mode?), or the alternator getting a load from hell. Can't see anything else being the culprit. I'd like to know though.

Hey MBRLVR, please let your car idle until this happens so you can see what we're talking about, and then try to guess what the heck is happening . One thing I can hear is like an exhaust leak, which I know it's not. And just noticed the other day while driving with the window down (rarely happens) it's the same 'leaky' noise when I blip the throttle to rev-match. Maybe it has something to do with that air injection valve, no? It has to be connected to the exhaust, right? We might be getting somewhere .