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      12-25-2014, 11:54 AM   #1
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Two exhaust questions...

Hey guys,

What are your thoughts/experiences on cold air working it's way backward into the engine with a muffler delete. My dad used to do this type of stuff all the time back in the day, and the one issue he ran into was cold air working back. What are your thoughts?

Also, any recommendations for tips for a 328? I need to get some.

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      12-26-2014, 02:18 PM   #2
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      12-26-2014, 02:27 PM   #3
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How is cold air going to make its way back to the engine? When in motion the the exhaust fumes are pushed out under a good deal of pressure, so there is no way for air to get sucked back in. Tnd the exhaust system only pushes air out, while the intake pulls air in; so I don't understand how this could be a problem.
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      12-26-2014, 02:30 PM   #4
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Quote:
Originally Posted by Delta0311 View Post
How is cold air going to make its way back to the engine? When in motion the the exhaust fumes are pushed out under a good deal of pressure, so there is no way for air to get sucked back in. Tnd the exhaust system only pushes air out, while the intake pulls air in; so I don't understand how this could be a problem.
I don't fully understand it either other than the reduced back pressure can contribute to it.
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      12-26-2014, 02:39 PM   #5
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Originally Posted by WOT Bimmer View Post
I don't fully understand it either other than the reduced back pressure can contribute to it.
I'm no expert but I don't see that being a possibility given how an exhaust system works.
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      12-26-2014, 02:58 PM   #6
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      12-26-2014, 02:58 PM   #7
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Ugh, not a back pressure comment....

Anyways, I just run a single back. Lightest method for me and most economical

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      12-26-2014, 03:09 PM   #8
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Originally Posted by Kgolf31 View Post
Ugh, not a back pressure comment....

Anyways, I just run a single back. Lightest method for me and most economical
Exactly what I'm planning to have done with the upcoming race exhaust install although with a Coast Fabrication muffled 4" tip and a slight turn down. Was assured the shop could modify the Aero bumper's diffusor to make the other side's cutout disappear, cool to see it done, thanks!
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      12-26-2014, 03:23 PM   #9
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Quote:
Originally Posted by justpete View Post
Exactly what I'm planning to have done with the upcoming race exhaust install although with a Coast Fabrication muffled 4" tip and a slight turn down. Was assured the shop could modify the Aero bumper's diffusor to make the other side's cutout disappear, cool to see it done, thanks!
Keep in mind I'm in a 128i w/a M Performance Bumper so I didn't need to make any modifications to my exit. Just had a pipe bend back and out.

Once I install my AA Headers I gotta see what my dB reading is with secondary cats and a resonator and go from there. Ideally I'd like to cut the pipe after the resonator and dump there, but since I'm driving to and from events I'll probably have a exit with a turn-down as well (perhaps adjustable depending on how close I am to sound so I can turn-down away from the mic...lol

(SCCA Nationals we must maintain 95 dB at 100' IIRC)

Also, this is relevant to our interests

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      12-26-2014, 03:34 PM   #10
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Originally Posted by Kgolf31 View Post
Keep in mind I'm in a 128i w/a M Performance Bumper so I didn't need to make any modifications to my exit. Just had a pipe bend back and out.

Once I install my AA Headers I gotta see what my dB reading is with secondary cats and a resonator and go from there. Ideally I'd like to cut the pipe after the resonator and dump there, but since I'm driving to and from events I'll probably have a exit with a turn-down as well (perhaps adjustable depending on how close I am to sound so I can turn-down away from the mic...lol

(SCCA Nationals we must maintain 95 dB at 100' IIRC)
Ah, ok, had to look a lot closer. Sorry about the highjack.

328i here but changing to BMW Performance Aero bumper (not M Performance) with 335i diffusor for single pipe exhaust at the same time. Not very costly to close up the unused cutout. And I'm looking to keep the noise down to this level or lower as well. Using a 17" Coast Fab glass packed muffler without the secondaries. Built out of thinwall 304 from the headers back. Not sure if they'll fab the collector or just use one from Burns Stainless though. The muffled tip might not be necessary but it can't hurt I figure.

An adjustable turn-down to turn-out to turn-away - there's a patent in there somewhere...
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      12-26-2014, 07:12 PM   #11
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Quote:
Originally Posted by WOT Bimmer View Post
Hey guys,

What are your thoughts/experiences on cold air working it's way backward into the engine with a muffler delete. My dad used to do this type of stuff all the time back in the day, and the one issue he ran into was cold air working back. What are your thoughts?

Also, any recommendations for tips for a 328? I need to get some.
No, cold air isn't going to get back to the engine.

Why do you need tips? Damage, changing the look, etc.?
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      12-26-2014, 11:20 PM   #12
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Quote:
Originally Posted by justpete View Post
No, cold air isn't going to get back to the engine.

Why do you need tips? Damage, changing the look, etc.?
Okay, must have been an issue with older vehicles then. I thought I'd at least throw it out there.

I'm considering a muffler delete.

Thanks for the responses!
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      12-26-2014, 11:23 PM   #13
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Originally Posted by WOT Bimmer View Post
Okay, must have been an issue with older vehicles then. I thought I'd at least throw it out there.

I'm considering a muffler delete.

Thanks for the responses!
No worries, could've been interpreted as that bitd but I don't remember anything like that although that doesn't mean much right there.

Sure you want to do a muffler delete? Backpressure will be lower but not all that much. It is the lowest cost solution though compared to straight pipe mufflers and single run build.
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      12-27-2014, 07:32 AM   #14
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I am trying to find some tips that resemble the PE... Any suggestions? I've lucked out so far.
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      12-27-2014, 09:25 AM   #15
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Here is the deal with the OP's question. Way back in the day (and I am talking way back) there were two prevailing thoughts; one was that cold air could warp exhaust valves and two, straight pipes could burn exhaust valves. Primarily this had to do with racers running open exhausts; and with respect to this post, once an engine is shut off, cold air (by means of a very short exhaust tract) could potentially find its way back into the engine and play havoc with a hot exhaust valve. But both of these thoughts, or should I say myths, have proven to be old wives' tales.

And even if these myths were to be true, in a modern BMW that cold air would have to find its way past not one, but two very hot catalytic converters, and a very warm exhaust manifold before reaching its final destination.
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      12-27-2014, 12:50 PM   #16
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Originally Posted by Tom Droze View Post
Here is the deal with the OP's question. Way back in the day (and I am talking way back) there were two prevailing thoughts; one was that cold air could warp exhaust valves and two, straight pipes could burn exhaust valves. Primarily this had to do with racers running open exhausts; and with respect to this post, once an engine is shut off, cold air (by means of a very short exhaust tract) could potentially find its way back into the engine and play havoc with a hot exhaust valve. But both of these thoughts, or should I say myths, have proven to be old wives' tales.

And even if these myths were to be true, in a modern BMW that cold air would have to find its way past not one, but two very hot catalytic converters, and a very warm exhaust manifold before reaching its final destination.
Oh yeah! I remember the second one now, thanks. Heard of this about short straight pipes on bikes which made sense and later on cars regardless of length which never made any sense. Cold air getting back to the valves though, never heard of that one. Maybe had to do with drag engine exhausts in the sixties, dunno. I do remember being warned about running engines with an open exhaust manifold though, not a good thing. Agreed that neither applies to a passenger car with cats.
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      12-27-2014, 01:09 PM   #17
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Quote:
Originally Posted by justpete View Post
Oh yeah! I remember the second one now, thanks. Heard of this about short straight pipes on bikes which made sense and later on cars regardless of length which never made any sense. Cold air getting back to the valves though, never heard of that one. Maybe had to do with drag engine exhausts in the sixties, dunno. I do remember being warned about running engines with an open exhaust manifold though, not a good thing. Agreed that neither applies to a passenger car with cats.
So, through deductive reasoning I concluded there are two ways to potentially warp something; take something very hot and expose it to something very cold, or take something very cold and expose it to something very hot. In the dead of winter in a very cold climate with a car sitting outside, how cold do you suppose the internal components of the engine are? Pretty cold I would imagine! Yet when you start your car in these conditions the cold valves are very rapidly exposed to the heat of internal combustion, but yet they survive warpage to seal another day!

Last edited by Tom Droze; 12-27-2014 at 03:29 PM.
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      12-27-2014, 01:16 PM   #18
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So, through deductive reasoning I concluded there are two ways to potentially warp something; take some very hot and expose it to something very cold, or take something very cold and expose it to something very hot. In the dead of winter in a very cold climate with a car sitting outside, how cold do you suppose the internal components of the engine are? Pretty cold I would imagine! Yet when you start your car in these conditions the cold valves are very rapidly exposed to the heat of internal combustion, but yet they survive warpage to seal another day!
Exactly! Never seen warped valves. Burned, yes, warped, no.
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      12-28-2014, 05:53 PM   #19
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Quote:
Originally Posted by Tom Droze View Post
Here is the deal with the OP's question. Way back in the day (and I am talking way back) there were two prevailing thoughts; one was that cold air could warp exhaust valves and two, straight pipes could burn exhaust valves. Primarily this had to do with racers running open exhausts; and with respect to this post, once an engine is shut off, cold air (by means of a very short exhaust tract) could potentially find its way back into the engine and play havoc with a hot exhaust valve. But both of these thoughts, or should I say myths, have proven to be old wives' tales.

And even if these myths were to be true, in a modern BMW that cold air would have to find its way past not one, but two very hot catalytic converters, and a very warm exhaust manifold before reaching its final destination.
Yes, this makes more sense now from the wives tales I've heard. Glad to make sense of that. Thanks for the post!
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      12-28-2014, 10:46 PM   #20
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Quote:
Originally Posted by WOT Bimmer
Quote:
Originally Posted by justpete View Post
No, cold air isn't going to get back to the engine.

Why do you need tips? Damage, changing the look, etc.?
Okay, must have been an issue with older vehicles then. I thought I'd at least throw it out there.

I'm considering a muffler delete.

Thanks for the responses!
Do it! I have a 328i and i did a muffler delete yesterday. Best $225 I've spent. Drone isnt as bad as people say. Cant speak on anything if u have a 335
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      12-28-2014, 11:37 PM   #21
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Do it! I have a 328i and i did a muffler delete yesterday. Best $225 I've spent. Drone isnt as bad as people say. Cant speak on anything if u have a 335
PM coming your way... :-)
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