Yellow Rose
Suspended
- Joined
- 22 November 2001
- Messages
- 2,256
Brain teaser for the noise detectives. 95 targa five-speed, aftermarket multi-piece tri-strut bar.
Sometimes at part throttle and sometimes at full throttle, I would hear a "light rattling" noise around 40-5500 RPM with no noise on either side of this narrow RPM band. I would hear it in first and in second, but because of traffic am not able to safely continue to that RPM band in third. An interesting twist, while doing third-gear-only pulls (testing purposes only, on an isolated and private road) I would hear this noise only from 6500-7500 RPM.
Because the noise is not load dependent - different RPM and different throttle rates - I am ruling out a noise that would be otherwise attributed to electronics. (Specifically, there was a recent post here about a naturally aspirated NSX that the owner heard what he thinks may be pinging.) With throttle inducing, I could replicate my noises almost 100% of the time. This past weekend I needed to do some other work and it was easier to accomplish with the tri-strut bar removed. After finishing that task but before re-installing the tri-strut bar, I took the car out for some test runs. The noise is gone. Completely gone. Driving home scratching my noggin, I am thinking of what to look for. Back at home I find nothing, so I bolted the tri-strut bar back on. Went out for some more runs, and the noise is back. Go home, remove tri-strut bar, hit the streets again and the noise is gone.
Somewhere somehow a mechanical high-frequency, low-amplitude vibration is resonating through the tri-strut bar. I think. It is as if this occurs when a certain counter-reactive drivetrain torque is reached. Here is what I mean. When I had my coupe, at a certain engine RPM in fourth, there was a loud knock / rattle. Turned out to be a loose transmission mount that when the engine / transmission assembly "rocked" or "rotated" within the mounts, it would make that noise. Now, back to the noise with the targa. In first gear, the noise would be towards the low end of the RPM. In third gear, the noise is at the upper end of the RPM. In second gear, the noise is - you guessed it - somewhere in between. This suggests to me that at a particular engine RPM the torque is X, which translates to a drivetrain torque of Y. If there is a phenomena that excites (mechanical engineering term in the field of vibration) a resonance at a constant drivetrain torque, then it stands to reason that the noise will be heard at different RPM depending on what gear I am in.
Ok, enough of my hypothetical psychobabble. Anybody have an idea why tri-strut bar = noise, but zero without?
Sometimes at part throttle and sometimes at full throttle, I would hear a "light rattling" noise around 40-5500 RPM with no noise on either side of this narrow RPM band. I would hear it in first and in second, but because of traffic am not able to safely continue to that RPM band in third. An interesting twist, while doing third-gear-only pulls (testing purposes only, on an isolated and private road) I would hear this noise only from 6500-7500 RPM.
Because the noise is not load dependent - different RPM and different throttle rates - I am ruling out a noise that would be otherwise attributed to electronics. (Specifically, there was a recent post here about a naturally aspirated NSX that the owner heard what he thinks may be pinging.) With throttle inducing, I could replicate my noises almost 100% of the time. This past weekend I needed to do some other work and it was easier to accomplish with the tri-strut bar removed. After finishing that task but before re-installing the tri-strut bar, I took the car out for some test runs. The noise is gone. Completely gone. Driving home scratching my noggin, I am thinking of what to look for. Back at home I find nothing, so I bolted the tri-strut bar back on. Went out for some more runs, and the noise is back. Go home, remove tri-strut bar, hit the streets again and the noise is gone.
Somewhere somehow a mechanical high-frequency, low-amplitude vibration is resonating through the tri-strut bar. I think. It is as if this occurs when a certain counter-reactive drivetrain torque is reached. Here is what I mean. When I had my coupe, at a certain engine RPM in fourth, there was a loud knock / rattle. Turned out to be a loose transmission mount that when the engine / transmission assembly "rocked" or "rotated" within the mounts, it would make that noise. Now, back to the noise with the targa. In first gear, the noise would be towards the low end of the RPM. In third gear, the noise is at the upper end of the RPM. In second gear, the noise is - you guessed it - somewhere in between. This suggests to me that at a particular engine RPM the torque is X, which translates to a drivetrain torque of Y. If there is a phenomena that excites (mechanical engineering term in the field of vibration) a resonance at a constant drivetrain torque, then it stands to reason that the noise will be heard at different RPM depending on what gear I am in.
Ok, enough of my hypothetical psychobabble. Anybody have an idea why tri-strut bar = noise, but zero without?
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