Replacing Engine Mounts
12-31-08, 07:38 PM
#51
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I spent 10 years as an NVH & vehicle dynamics engineer---several years in suspension and several years in powertrain mount design. So let me clear this up:
First, an engine mount has three functions:
1) Locate the powertrain
2) Isolate powertrain noise & vibration
3) Control gross motion of the powertrain
The last two are of importance in this discussion:
Isolation involves selecting a proper stiffness to avoid transmission of HIGH frequency, LOW amplitude vibration. For this, you want the softest mount possible.
Gross motion control involvles selecting a proper stiffness to avoid transmission of LOW frequency, HIGH amplitude vibration. For this, you want the stiffest mount possible.
You can see how the two contradict each other. The RX-7 has a hydraulic mount which I won't get into details about, but the point is this is the best compromise as it allows you to use a softer mount for high frequency isolation and hydraulic damping to control high amplitude inputs.
Do engine mounts transmit vibration?
-Yes, doesn't matter if it's made of jelly or solid steel---it's still a transfer path for vibration. Solid mounts won't traditionally transfer high amplitude vibrations that you can feel unless you have an isolated cradle, which the RX-7 does not---it's bolted directly to the chassis. However solid mounts readily transmit high frequency vibration (read noise).
If the issue is with noise, then poly mounts could definitely be the culprit. If the issue is with vibration that you can easily feel through the steering wheel or seat track, then look somewhere else, as it's not the poly mounts that are the problem.
Just to add---I have the Banzai mounts and diff brace and they do transmit more noise, but it's not very noticable---especially if you have an aftermarket exhaust. I've also driven cars with solid nylon 6/6 mounts and didn't notice much of a difference. The trade off for the solid feel is much, much better in my opinion.
First, an engine mount has three functions:
1) Locate the powertrain
2) Isolate powertrain noise & vibration
3) Control gross motion of the powertrain
The last two are of importance in this discussion:
Isolation involves selecting a proper stiffness to avoid transmission of HIGH frequency, LOW amplitude vibration. For this, you want the softest mount possible.
Gross motion control involvles selecting a proper stiffness to avoid transmission of LOW frequency, HIGH amplitude vibration. For this, you want the stiffest mount possible.
You can see how the two contradict each other. The RX-7 has a hydraulic mount which I won't get into details about, but the point is this is the best compromise as it allows you to use a softer mount for high frequency isolation and hydraulic damping to control high amplitude inputs.
Do engine mounts transmit vibration?
-Yes, doesn't matter if it's made of jelly or solid steel---it's still a transfer path for vibration. Solid mounts won't traditionally transfer high amplitude vibrations that you can feel unless you have an isolated cradle, which the RX-7 does not---it's bolted directly to the chassis. However solid mounts readily transmit high frequency vibration (read noise).
If the issue is with noise, then poly mounts could definitely be the culprit. If the issue is with vibration that you can easily feel through the steering wheel or seat track, then look somewhere else, as it's not the poly mounts that are the problem.
Just to add---I have the Banzai mounts and diff brace and they do transmit more noise, but it's not very noticable---especially if you have an aftermarket exhaust. I've also driven cars with solid nylon 6/6 mounts and didn't notice much of a difference. The trade off for the solid feel is much, much better in my opinion.
12-31-08, 10:12 PM
#52
^^Good explanation.
I don't think it's fair to say "Your entire car will vibrate. They directly transmit every engine vibration to the frame. Trust me, you will HATE it!!!!" I've had solid urethane mounts for around 5 years now and I like them. While I notice the vibration, it's not sufficiently significant TO ME. However, when I rode in SgtBlue's car (when he still had stock mounts), it was noticeably smoother and quieter, from a NVH perspective.
Bottom line, good urethane mounts will likely transmit more NVH than stock mounts. However, the benefit is that you have a tighter driveline. I can't really explain it or justify it, but it just feels better to drive...aside from some mild vibration.
I don't think it's fair to say "Your entire car will vibrate. They directly transmit every engine vibration to the frame. Trust me, you will HATE it!!!!" I've had solid urethane mounts for around 5 years now and I like them. While I notice the vibration, it's not sufficiently significant TO ME. However, when I rode in SgtBlue's car (when he still had stock mounts), it was noticeably smoother and quieter, from a NVH perspective.
Bottom line, good urethane mounts will likely transmit more NVH than stock mounts. However, the benefit is that you have a tighter driveline. I can't really explain it or justify it, but it just feels better to drive...aside from some mild vibration.
12-31-08, 10:17 PM
#53
Someone typed ,above, to get a solid engine torque brace .Thats is a very bad idea as a solid ETB will carry vibration from the motor to the car . My design has a rubber bushing on each end .Solid or poly motor mounts should keep the motor from rocking as you shift . Several of my ETB customers are using solid or poly mounts and my ETB . Any way to change the mounts is not that hard ,just be safe and then go for a ride .
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