Originally posted by Bry5on
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Okay, let's get rid of some more NVH. Vibration this time.
I pretty much got the SS headers and section 1 to idle as well as stock by strapping the two pipes together, but after more driving I still wasn't satisfied with what I was perceiving to be some persistent exhaust natural frequency resonances. There were three frequencies that were bugging me; one at very near, but not quite, idle speed, one at about 1700rpm and one at about 3300rpm. I figured these were associated with the first, second and third natural frequency modes of the exhaust.
So let's do something about it to arrest those modes. Well, what's the best way to neutralize them? To shorten the unsuspended length of the exhaust and shift the frequency modes much higher, way up or out of the excitement of the RPM band completely. You can see using the above math that the natural frequency scales to 1/L^2, where L is the length of the unsuspended exhaust in this case. So let's use a trick from several other non-s54 BMWs used before, during and after this generation. We're going to make a bracket that ties the exhaust to the back of the transmission, using its existing but unused mounting bolts. Since the transmission and exhaust are both rigidly mounted to the engine, these parts already move together, and tying them together should not introduce any weird relative motion. And since the transmission is significantly further toward the rear of the car than the headers, this will effectively reduce the length of the exhaust left to vibrate 'freely' (note that the exhaust hangers don't do much to arrest natural frequency induced oscillations, that's what those non-m e46 cylindrical dampers do). The one thing to keep in mind is that the exhaust grows as it heats up, so the flexural orientation of the mount, and giving it enough length to bend elastically, is important.
After an iteration of carboard engineering, to borrow a term from Heinz, I designed a three bracket system, laser cut it and bolted it to the transmission. I then used one of my now favorite 2.5" exhaust clamps to attach the flexural brace directly to one of the exhaust tubes. The 'bang on the exhaust with my fist test' noted a massive reduction in vibration of the exhaust.
Here are a few shots of the brackets installed:
And with the splash guard in place, showing full clearance everywhere:
What's the verdict?
Very big improvement, I was genuinely surprised at how effective this is. A bigger and broader improvement from this change than the other two I'd made to clamp the pipes together. It idles more like an m54 now than an s54, which I think is a very good thing and it's essentially imperceptible when switching the AC on and off, which wasn't the case before. The resonance near idle and at 1700rpm are absolutely completely gone, and the resonance at 3300rpm seems a bit attenuated and spread out a little, now between 3100-3300rpm. This might be the exhaust's new first excitation mode.
Here's the thing though, it affected essentially the entire rev band under 5000rpm in a big way. The engine is so much smoother when revving, and it's especially noticeable when blipping the throttle, or when engaging the clutch and pulling away from a stop. I hadn't noticed or pinpointed this previously, but there was a 1-2 oscillation change in the exhaust when doing those things. Unnecessary sloppiness. Now the engine makes rev changes and pulls away from a stop like a 330i - smoothly and without any secondary motions. Also more like my previous porsches, responsive with the only motion being the motion that matters. And to top it off, some of the resonance and vibration above 5k stayed, so it still has a bit of that raw supersprint header feeling when you're really on it and high in the rev band. Personally I think I'd rather it be smooth, but I expect most folks would appreciate keeping the feeling of those higher frequency vibrations.The whole rev band is very noticeably smooth now under 5k, even under load, with the exception of a less prominent bit of vibration between 3100-3300. And this is going to sound crazy, but I can swear I hear the lower frequencies in the stereo better.
I've got a complete 2.5" supersprint exhaust here (-20lb), as well as a 330i ZHP muffler (-10-15lb) that I'll likely swap in to drop the mass at the end of the exhaust to help raise the natural frequency even more. I'd still like to eliminate the 3100-3300rpm subtle vibration to make it non-m-smooth cruising on the highway, as that's 75-80mph for me. An added bonus with the ZHP exhaust is that it has a vibration damper already on it that I can swap out to tune out that last vibration mode.
I really want to A/B this against a stock M3 (will do soon), as I'd bet this is smoother than the M3 came from the factory now. And this is with 75k mile, 10 year old motor and trans mounts. I'll be A/Bing against a freshly built motor with all new mounts, so this should be a conservative comparison. I also have new mounts waiting for the oil pan gasket job I'm putting off.
So the ultimate daily driver gets even better. Hope you enjoyed this and I'd recommend doing the same to your car if you have 2.5" SS headers and section 1.
I pretty much got the SS headers and section 1 to idle as well as stock by strapping the two pipes together, but after more driving I still wasn't satisfied with what I was perceiving to be some persistent exhaust natural frequency resonances. There were three frequencies that were bugging me; one at very near, but not quite, idle speed, one at about 1700rpm and one at about 3300rpm. I figured these were associated with the first, second and third natural frequency modes of the exhaust.
So let's do something about it to arrest those modes. Well, what's the best way to neutralize them? To shorten the unsuspended length of the exhaust and shift the frequency modes much higher, way up or out of the excitement of the RPM band completely. You can see using the above math that the natural frequency scales to 1/L^2, where L is the length of the unsuspended exhaust in this case. So let's use a trick from several other non-s54 BMWs used before, during and after this generation. We're going to make a bracket that ties the exhaust to the back of the transmission, using its existing but unused mounting bolts. Since the transmission and exhaust are both rigidly mounted to the engine, these parts already move together, and tying them together should not introduce any weird relative motion. And since the transmission is significantly further toward the rear of the car than the headers, this will effectively reduce the length of the exhaust left to vibrate 'freely' (note that the exhaust hangers don't do much to arrest natural frequency induced oscillations, that's what those non-m e46 cylindrical dampers do). The one thing to keep in mind is that the exhaust grows as it heats up, so the flexural orientation of the mount, and giving it enough length to bend elastically, is important.
After an iteration of carboard engineering, to borrow a term from Heinz, I designed a three bracket system, laser cut it and bolted it to the transmission. I then used one of my now favorite 2.5" exhaust clamps to attach the flexural brace directly to one of the exhaust tubes. The 'bang on the exhaust with my fist test' noted a massive reduction in vibration of the exhaust.
Here are a few shots of the brackets installed:
And with the splash guard in place, showing full clearance everywhere:
What's the verdict?
Very big improvement, I was genuinely surprised at how effective this is. A bigger and broader improvement from this change than the other two I'd made to clamp the pipes together. It idles more like an m54 now than an s54, which I think is a very good thing and it's essentially imperceptible when switching the AC on and off, which wasn't the case before. The resonance near idle and at 1700rpm are absolutely completely gone, and the resonance at 3300rpm seems a bit attenuated and spread out a little, now between 3100-3300rpm. This might be the exhaust's new first excitation mode.
Here's the thing though, it affected essentially the entire rev band under 5000rpm in a big way. The engine is so much smoother when revving, and it's especially noticeable when blipping the throttle, or when engaging the clutch and pulling away from a stop. I hadn't noticed or pinpointed this previously, but there was a 1-2 oscillation change in the exhaust when doing those things. Unnecessary sloppiness. Now the engine makes rev changes and pulls away from a stop like a 330i - smoothly and without any secondary motions. Also more like my previous porsches, responsive with the only motion being the motion that matters. And to top it off, some of the resonance and vibration above 5k stayed, so it still has a bit of that raw supersprint header feeling when you're really on it and high in the rev band. Personally I think I'd rather it be smooth, but I expect most folks would appreciate keeping the feeling of those higher frequency vibrations.The whole rev band is very noticeably smooth now under 5k, even under load, with the exception of a less prominent bit of vibration between 3100-3300. And this is going to sound crazy, but I can swear I hear the lower frequencies in the stereo better.
I've got a complete 2.5" supersprint exhaust here (-20lb), as well as a 330i ZHP muffler (-10-15lb) that I'll likely swap in to drop the mass at the end of the exhaust to help raise the natural frequency even more. I'd still like to eliminate the 3100-3300rpm subtle vibration to make it non-m-smooth cruising on the highway, as that's 75-80mph for me. An added bonus with the ZHP exhaust is that it has a vibration damper already on it that I can swap out to tune out that last vibration mode.
I really want to A/B this against a stock M3 (will do soon), as I'd bet this is smoother than the M3 came from the factory now. And this is with 75k mile, 10 year old motor and trans mounts. I'll be A/Bing against a freshly built motor with all new mounts, so this should be a conservative comparison. I also have new mounts waiting for the oil pan gasket job I'm putting off.
So the ultimate daily driver gets even better. Hope you enjoyed this and I'd recommend doing the same to your car if you have 2.5" SS headers and section 1.
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