03-11-2017, 11:17 AM | #1 |
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X5 50i - Cold Start? Why so loud?
While I love the cold start sound on my M3 (where it seems fitting) - why does the 50i start so loud for the first 20 seconds or so? Seems a little out of place, embarrassing almost, on a family SUV. I take it this can't be coded out unless a full tune is purchased which I don't need/want).
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03-11-2017, 11:26 AM | #2 |
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Start sequence on gasser BMWs typically utilizes the flaps in the exhaust to optimize a cold start for engine health and pollution control. Exhaust routing is modulated to increase engine temps quickly through a start sequence and to heat the catalyst quickly for emissions reduction. Gassers also tend to run a richer profile at startup (though long gone are the days of true choke). I'm not trrribly familiar with the N63 exhaust but on other models the flaps work through the duty cycle of the engine in concert with the variable vanes to optimize exhaust flow for turbine response and efficiency.
This all results in louder exhaust sounds at startup. Our 7 series is the same with the B58. Much louder than our diesel. It's the price of clean air, longer engine life, and modern technology. |
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03-11-2017, 12:02 PM | #3 |
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On 50i, there is no flap.
So the reason it is being loud is because the turbo wastegate is open to let the exhaust gas pass through while the turbo is cold and let oil to pass through the turbo bearing's first. Since when the car is cold or stop for a long time, there is no oil by the turbo's bearing. Only the oil residue from previous run.
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03-11-2017, 12:28 PM | #4 |
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I've zero knowledge such as the two incredible posts above.
But I'll add that one mans family driver is another mans sport SUV. With the big engine version a throaty seems somewhat appropriate. Personally, I was pleasantly surprised when I first heard it. Comparable vehicles I've driven recently are actually way louder. |
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03-11-2017, 01:44 PM | #5 | |
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A major portion of the sound difference is the "converter heating mode" in which the ECU purposely adjusts the engine timing to after TDC ad a means of throwing combustion heat downstream to rapidly heat the catalytic converter. That delayed ignition produces a much different noise, as it's accompanied by a second, latent injection to ensure ignition/combustion after peak cylinder pressure is reached. I've seen nothing to corroborate the idea of the wastegates opening on a cold start, though it makes a degree of sense.
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03-12-2017, 04:32 AM | #6 |
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What follows is unverified information.
I've been told that cold-started BMW V8 cats are electrically "pre-heated" to reach working temperatures fast enough for the latest emission standards. The corresponding electrical load is very high, requiring a separate high current battery and an initial 20-30 seconds higher rpm to allow the alternator to supply enough charging current. For this reason I never interrupt these 20-30 sec of higher rpm on a cold engine start. I found some information in German here: http://www.palivaventi.de/downloads/...%20Motoren.pdf I would be grateful if some better informed forum member would shed more light on this issue. |
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03-12-2017, 06:17 AM | #7 | |
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We find evidence of no electric cat pre-heat in two ways. First, the use of a rapid catalyst pre-heat engine cycle as I described above. BMW technical documents make mention of the following: "In order to bring the catalytic converters up to operating temperature as quickly as possible, the N63 engine has a catalytic converter heating mode for when the engine is started from cold. In this mode, combustion heat is intentionally introduced into the exhaust train and not used first and foremost to develop power output. The point of ignition is moved to 30° crankshaft angle after TDC." This operating mode lasts up to 60 seconds from engine starting but is terminated if the catalytic converter response temperature is reached earlier. Secondly, if you look through the BMW technical manual, the instructions for removing the catalystic converters are as follows: Remove top bracket for engine cover (pic A). Remove pre-catalyst O2 sensors (pic B) and all heat shielding from top of engine and exhaust system (after cat). Remove cable brackets (pic C) for the post-catalyst O2 sensors. Remove both catalysts (pic D) with exhaust pipes and put aside (leave post-cat O2 sensors attached to pipe). If there was any cat pre-heat via electrical means, the instructions would have to tell a technician to remove that wiring as well. The pre-heating is achieved via a delayed combustion cycle.
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03-12-2017, 08:40 AM | #8 |
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Yup, it's all about getting the catalytic converters up in temp as soon as possible (to reduce emissions) by running the exhaust a little hotter. My M3 (non-turbo V8) does it also (it's about a 45 second process), as did my previous 2 BMW's (both non-turbo six's). It's underlying purpose is not for the sake of the turbos. It very well may be true that the waste gates are held open during this process so the turbos don't spin up when they are cold and don't have lubrication yet, but that is not the reason for the higher rpm and increased sound. The waste gates being held open would be a secondary function that they would have added later in the design and development process because it was a good idea, but that's not the reason for the startup process.
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03-12-2017, 09:52 AM | #9 |
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Love cold start on my 50i!
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03-12-2017, 10:02 AM | #10 | ||
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I gratefully stand corrected. |
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03-12-2017, 10:12 AM | #11 |
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I appreciate the question! I adore digging through obscura such as that. The question I have from an engineering standpoint is why they would abandon the electric pre-heat. One assumes there must be a reliability issue or prohibitive cost. I wonder if we won't see a return to this when vehicles transition to 48-volt systems over the next decade.
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03-12-2017, 10:46 AM | #12 |
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+1
Me too. I think it sounds perfect. In fact, I wish the exhaust was this loud all the time!
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03-12-2017, 10:37 PM | #14 |
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I love cars that look like it is not supposed to be fast but can burn unsuspecting victims. My X5 50i is not a family suv but my sports car. I personally wish it had the cold start sound all the time as well.
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03-12-2017, 10:46 PM | #15 |
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@lowlevelhell , ahh, I forgot about that part.
Good mention.
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03-13-2017, 02:05 AM | #16 |
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You should hear it on my M3. Full Dinan exhaust and cat delete... When I start it up in my garage, pictures on the walls and windows literally rattle in my house. I usually leave the garage door closed until it cycles through, as a favor to my neighbors.
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03-13-2017, 07:56 AM | #17 | |
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03-13-2017, 08:42 AM | #18 | |
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03-13-2017, 09:24 AM | #19 |
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What I have learned in this thread is don't stand behind the car during this catalytic converter heating cycle because the amount of the toxic fumes in the exhaust gases is at highest.
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03-13-2017, 10:17 AM | #20 | |
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