Polyeurathane Engine mounts

[Rek]

Since my 5 sopeed gearbox was fitted at hard acceleration in 1st and 2nd gear for a brief period there is big noise from the prop shaft. It only lasts a second but is quite violent. The prop shaft was balanced when shortened and I can only put it down to engine movement under high torque.

I tried reinforcing the prop shaft bearing with some low strength polyeurathane and this worked at low speeds but the noise at medium to high speeds was scary. Could this indicate a problem with my prop?

Has anyone with a higher that standard engine output installed poly engine mounts? I am keen to try this but would like to get some reactions first.

 

[Blinkling]

This is fishing for clues here but try retorqueing all six bolts involved with the drive shaft and flex disk to see if the sound or vibration changes. If one has loosened even a little the vibration would appear at around 30 MPH in my experience.

 

[tochi]

Since my 5 sopeed gearbox was fitted at hard acceleration in 1st and 2nd gear for a brief period there is big noise from the prop shaft. It only lasts a second but is quite violent. The prop shaft was balanced when shortened and I can only put it down to engine movement under high torque.

I tried reinforcing the prop shaft bearing with some low strength polyurethane and this worked at low speeds but the noise at medium to high speeds was scary. Could this indicate a problem with my prop?

Has anyone with a higher that standard engine output installed poly engine mounts? I am keen to try this but would like to get some reactions first.

First, you might want to review this thread that discusses excessive engine movement. https://e9coupe.com/forum/threads/excessive-engine-movement.29378/

Unclear exactly what or how or why you reinforced the prop shaft bearing with polyurethane. Since you reference "bearing," does that mean you added polyurethane around the rubber center support bearing? Perhaps you over did it, so that the support has become so stiff as to defeat the very purpose of the compliant/flexible rubber support? Curious whether you installed the original equipment donut giubo, the later/thinner laminated frisbee version or an ultra-stiff aftermarket urethane version.

My experience with polyurethane/delrin motor mounts is limited. While they were originally chosen to limit engine movement, they transmitted too much vibration to the cabin for a street-driven vehicle. This was on standard unmodified 4speed equipment. In addition to being uncomfortable for vehicle occupants, the unwanted vibration created a concern for overstressing various body welds - potentially causing more problems/noises than avoiding them. One supposes that the results might have been different if the engine mounts were rubber and the cross member/transmission support was polyurethane or vice versa. Urethane mounts have been available in different stress-strain properties, but I do not recall which versions we tried. Further, regarding the transmission support cross member, I have observed a number of situations where the rubber transmission mount had deteriorated but vehicle occupants might not notice this except for when the vehicle is driven vigorously: "Thump" "Bang"

Other than the driveshaft alterations to accommodate the five speed's extra length, the one thing that typically differs between original four speed installations and five speed after market installations is the cross member supporting the five speed and its installation. If, for example, the new/modified cross member involves a series of home brewed cantilevered connections, it only takes one or two loose connections to make an unexpected/abnormal noise under stress. Noise might also occur if the cross member is not properly secured to the body. "Thump "Bang" Also, if the gear shifter/platform bushes are sloppy, this may result in unwanted contact with the vehicle's body, especially when the drive train is stressed under significant acceleration/deceleration. Your description of a "big noise" could not only indicate a broken weld, a loose transmission support cross member/worn transmission support bearing - but something as seemingly unrelated as a loose exhaust system contacting the vehicle undercarriage.

 

[sfdon]

If that noise is between the seats and is a rhythmic thumping- that’s the sound of a bad center bearing. Look at it closely to verify the the driveshaft is still centered in the bearing. Show us a pic?
Did you preload the center bearing?

 

[Ohmess]

Also depending on which transmission mount you are using, you may want to check the location of the bolts that hold the mount to the car relative to the slots in the body that the bolts install into. I felt that using either of the 1980s BMW transmission mounts required the bolts to be located too close to the slots they mount into. If these bolts were to move into the range of the slots in the body, this could allow a significant amount of transmission movement under heavy load. I went with this rather expensive alternative to ensure I did not have this problem: : https://shop.coupeking.com/products/e9-5-speed-conversion-mount-kit

 

[Rek]

First, you might want to review this thread that discusses excessive engine movement. https://e9coupe.com/forum/threads/excessive-engine-movement.29378/

Unclear exactly what or how or why you reinforced the prop shaft bearing with polyurethane. Since you reference "bearing," does that mean you added polyurethane around the rubber center support bearing? Perhaps you over did it, so that the support has become so stiff as to defeat the very purpose of the compliant/flexible rubber support? Curious whether you installed the original equipment donut giubo, the later/thinner laminated frisbee version or an ultra-stiff aftermarket urethane version.

My experience with polyurethane/delrin motor mounts is limited. While they were originally chosen to limit engine movement, they transmitted too much vibration to the cabin for a street-driven vehicle. This was on standard unmodified 4speed equipment. In addition to being uncomfortable for vehicle occupants, the unwanted vibration created a concern for overstressing various body welds - potentially causing more problems/noises than avoiding them. One supposes that the results might have been different if the engine mounts were rubber and the cross member/transmission support was polyurethane or vice versa. Urethane mounts have been available in different stress-strain properties, but I do not recall which versions we tried. Further, regarding the transmission support cross member, I have observed a number of situations where the rubber transmission mount had deteriorated but vehicle occupants might not notice this except for when the vehicle is driven vigorously: "Thump" "Bang"

Other than the driveshaft alterations to accommodate the five speed's extra length, the one thing that typically differs between original four speed installations and five speed after market installations is the cross member supporting the five speed and its installation. If, for example, the new/modified cross member involves a series of home brewed cantilevered connections, it only takes one or two loose connections to make an unexpected/abnormal noise under stress. Noise might also occur if the cross member is not properly secured to the body. "Thump "Bang" Also, if the gear shifter/platform bushes are sloppy, this may result in unwanted contact with the vehicle's body, especially when the drive train is stressed under significant acceleration/deceleration. Your description of a "big noise" could not only indicate a broken weld, a loose transmission support cross member/worn transmission support bearing - but something as seemingly unrelated as a loose exhaust system contacting the vehicle undercarriage.

Thanks for the detailed response Tochi. I started the excessive engine movement thread, but the things noted there haven't worked hence this thread.

I reinforced the existing bearing by pouring very low level polyeurathane into the cavity around the bearing. This is the subject of another thread and this didn't work. Perhaps it was overdone, but I used the lowest level poly so no go there.

Every gearbox and engine bushes are new as is the prop shaft bearing and the guibo. Everything is tight. It might be an alignment issue where engine movement takes it out of tolerance. I also intend to look at the exhaust movement as this was an issue to the rear of the car.

I take your point on increased vibration but it might be a balnce between this and the 'tramping' as I call it.

 

[Rek]

If that noise is between the seats and is a rhythmic thumping- that’s the sound of a bad center bearing. Look at it closely to verify the the driveshaft is still centered in the bearing. Show us a pic?
Did you preload the center bearing?

Thanks Don. I will post a pic but also video the acceleration so you can hear the noise. The bearing is new and was preloaded on fitting.

 

[Rek]

Also depending on which transmission mount you are using, you may want to check the location of the bolts that hold the mount to the car relative to the slots in the body that the bolts install into. I felt that using either of the 1980s BMW transmission mounts required the bolts to be located too close to the slots they mount into. If these bolts were to move into the range of the slots in the body, this could allow a significant amount of transmission movement under heavy load. I went with this rather expensive alternative to ensure I did not have this problem: : https://shop.coupeking.com/products/e9-5-speed-conversion-mount-kit

Thats the kit I have fitted.

 

[Gransin]

If you have access to a gopro camera or similar, you could try to make a video of the driveline while accelerating. Since nothing else has worked it would be good to get a clue before continuing throwing new parts on it.

Might have to make several videos, diff, driveshaft, engine + tranny.

 

[Rek]

Great idea. That's next. The prop or bearing in the video looks a bit sad.