Bushings: Which Rubber and Which Poly to maximize comfort and performance

[Greeko]

The bushings on the diff are mainly to avoid or remove the "clunk" during shifting that is common from a loose diff mount.  The ES bushing has interlocking metal pieces inside so the bottom bump stop is not necessary.  The original reason for RT to develop his modified mount was because he kept breaking the stock mount, behind a powerful small block chevy engine.  It's actually "wrong" for an L6 application when used with the overhead GM mount because it lowers the diff nose compared to stock.  It fixes the clunk but probably adds a little drive shaft vibration.  When it's used with a bump stop on the top and the stock mount on the bottom it's essentially just a movement snubber, not really a "mount".

 

The urethane bushings in the mustache bar also help with the clunk removal but are known to transmit diff noise in to the cabin.  Many people go back to rubber due to the extra diff whine they hear after replacing the old worn out rubber.  Beware.

 

Many people go to polyurethane because it's cheaper than the stock rubber.  There's no huge benefit, overall, to converting from rubber to polyurethane.  You might consider evaluating each step individually before going crazy.

 

 

Thanks very much for the insight. I have been going through this in my head, this will be a summer daily driver and occasional track car. I was worried the stock rubber wouldn't keep up its form with double stock power.

 

I am not using an L6, I am using a 1JZ . I I would have to check my diff angles but I can adjust the transmission mount slightly up and down...

 

My ultimate goal is a vehicle that handles well more for street conditions.

 

Hmm I think I will have to disagree on a couple points.

 

I am not sure about the RT mount, but at least the Technoversions style of it doesn't seem to lower the nose of the diff more then stock, I didn't encounter any twisting forces putting the differential in. Drive shaft flange was close to level or within the operating angle range for a driveshaft when I measured mine to get made, although I had my diff installed wrong previously the mounting planes weren't changed with the correction. With the mustache poly bushings it is pretty sensitive to deflections, I had to line up everything and jack it up slowly or it would bind. I suppose if you mounted the mustache the bar itself would be capable of deflecting quite a bit due to its construction.

 

I would say there is a fairly noticeable difference with polyurethane bushings in the mustache bar, I'll look for it, but there is a video of an EVO out there with a camera under the car and you can see the diff move drastically on power and off power, especially on shifts. In the scheme of things probably not a thing most people will notice when just normally driving, but I find that pre-movement a bit nauseating as the force comes a bit after the input. 

 

I did the GM mount up top as the nose wants to come up when you are on the throttle the GM mount is supposed to be mounted to the bottom of a cross member so it is working upside down, while the casting of it kind of supports the tabs being pushed down when it is mounted correctly, mounted upside down the tabs kind of do bend away polyurethane with just gravity. I added the snubbing piece (good name for it) to give support while it is not under load. 

 

I will agree though it would be a waste to poly mount things if NVH is your concern. Newzed is right, the poly kits are popular as replacing with stock bushings can be a real chore requiring quite a bit of press work if you can even find all the rubber bits. I have a mostly stripped interior, 3 inch exhaust with a bullet muffler, and a whiny fuel pump, so I have no reference for hearing diff whine really. I've added some sound deadening since then and I find it acceptable, but I have a daily driver so I don't have to live with the car.

 

I am rolling with a 3" exhaust with a muffler and resonance chamber. I am not HUGELY worried about noise..I just don't want the thing to Shake like hell... I don't want to go through the effort of pressing in new bushings and having a sore back every time I drive...

[Greeko]

I also don't want my rubber flexing so hard and wearing out after 5000 miles... Maybe im being completely unrealistic lol

Edited January 5, 2017 by Greeko

[seattlejester]

L6 usually refers to inline 6 just for reference. So a JZ motor is indeed an L6 or I6 depending on what you are more familiar with.

 

Honestly I don't mind it. I had Poly engine mounts on an L4/I4 in my FWD car and that literally shook the whole car and I absolutely hated them and I did change that back to regular mounts.

 

On my 240z it isn't bad in my opinion, granted I have maybe 2k miles in the last 6 years on that car so not the best person to take livability advice from. 

[grannyknot]

When I installed my Technoversion R/T mount I had to cut off about 3/4"-7/8" off of the stepped poly snubber for the front of the R200 diff to come back up enough for the drive shaft to line up properly but after that there was no movement and more to the point, NO clunk, it was glorious!

Edited January 6, 2017 by grannyknot

[NewZed]

 I was worried the stock rubber wouldn't keep up its form with double stock power.

 

I am not using an L6, I am using a 1JZ . I I would have to check my diff angles but I can adjust the transmission mount slightly up and down...

 

 I am not HUGELY worried about noise..I just don't want the thing to Shake like hell... I don't want to go through the effort of pressing in new bushings and having a sore back every time I drive...

Well, that was the impetus for the RT design so you're on the right track.  Your angles might be the determining factor in how you proceed.  If you need a higher nose, maybe a shimmed stock mount and a snubber on top.  If you need a lower nose, maybe the GM mount on top with or without a snubber on the bottom.  The basic RT design has a lot of flexibility in how it's used.

 

The mustache bar ends are pretty loosely held so urethane makes it less floppy in general.  I solved my diff noise transmission problem by using washers on the ends of the sleeves to create a small air gap.  Basically created a longer inner sleeve).  The way that they're designed with their inner sleeve. the urethane gets compressed when installed and transmits more noise.  My theory.

[Greeko]

Well, that was the impetus for the RT design so you're on the right track.  Your angles might be the determining factor in how you proceed.  If you need a higher nose, maybe a shimmed stock mount and a snubber on top.  If you need a lower nose, maybe the GM mount on top with or without a snubber on the bottom.  The basic RT design has a lot of flexibility in how it's used.

 

The mustache bar ends are pretty loosely held so urethane makes it less floppy in general.  I solved my diff noise transmission problem by using washers on the ends of the sleeves to create a small air gap.  Basically created a longer inner sleeve).  The way that they're designed with their inner sleeve. the urethane gets compressed when installed and transmits more noise.  My theory.

 

 

I ordered the RT Mount with a new lower diff isolation mount to start. We will see if i need to go for the GM mount if my alignment is off.

 

So you feel the rubber doesn't hold the diff firmly enough?   I am trying to envision the washers on the end of sleeves....

 

Thanks so much for the great feedback everyone!...

[seattlejester]

Given what we found out in this thread, I would make sure you get the taller mount so you can shim down. Granted I don't know too many people that make the RT style mount other then RT himself or Technoversions, so I imagine you got a Technoversions kit.

 

The rubber piece on the the front diff mount is acting in the wrong direction which is the main fault. The diff pulls up as you go forward, eventually it will pull the mount apart, it should have a mount on top to fight the diff from moving up and another mount to support the diff when it is not under load. That is what the snubber does is it fights that motion of the diff pulling up and prevents the diff from pulling the bottom mount apart. This was done with a strap early on, but those deteriorate and fall apart very quickly on the underside of the car.

 

What newzed means if I read it correctly is that he added very small washers that are the same size as the inner sleeve of the poly bushing to space the actual poly bushing away from the subframe that way the contact area for the bushing is much less. I imagine it had to be a series or at least two different size washers to keep the top bushing from being pulled towards the top. That would isolate noise with the trade off being that the mustache bar bolt is now encountering an axial load through the bolt more directly. Honestly with the size of the bolt that would be a very minor concern.

 

I want to say there was a better example, but 

was one of the reasons I wanted to run all poly instead of rubber. Solid would be better to prevent movement, but would most likely propagate cracking somewhere down the line especially in our cars. Edited January 10, 2017 by seattlejester

[Greeko]

Given what we found out in this thread, I would make sure you get the taller mount so you can shim down. Granted I don't know too many people that make the RT style mount other then RT himself or Technoversions, so I imagine you got a Technoversions kit.

 

The rubber piece on the the front diff mount is acting in the wrong direction which is the main fault. The diff pulls up as you go forward, eventually it will pull the mount apart, it should have a mount on top to fight the diff from moving up and another mount to support the diff when it is not under load. That is what the snubber does is it fights that motion of the diff pulling up and prevents the diff from pulling the bottom mount apart. This was done with a strap early on, but those deteriorate and fall apart very quickly on the underside of the car.

 

What newzed means if I read it correctly is that he added very small washers that are the same size as the inner sleeve of the poly bushing to space the actual poly bushing away from the subframe that way the contact area for the bushing is much less. I imagine it had to be a series or at least two different size washers to keep the top bushing from being pulled towards the top. That would isolate noise with the trade off being that the mustache bar bolt is now encountering an axial load through the bolt more directly. Honestly with the size of the bolt that would be a very minor concern.

 

I want to say there was a better example, but 

was one of the reasons I wanted to run all poly instead of rubber. Solid would be better to prevent movement, but would most likely propagate cracking somewhere down the line especially in our cars.

Very cool man,

I totally get it. I am going to play around a bit.

 

I ordered the technoversion RT mount.

[seattlejester]

Very solid piece, just something you may run into, you may have to shave the little lip in the tunnel near the bolts.

[Greeko]

Very solid piece, just something you may run into, you may have to shave the little lip in the tunnel near the bolts.

Thanks for the heads up

[ZHoob2004]

Since we're talking about the RT mount here anyway, what is the recommended mount configuration for a nearly stock l28? I'm experiencing some clunking and if I find it's the mount, would it be a mistake to simply add a RT snubber on top of it?

 

Essentially, keeping the stretched factory isolator and adding poly above.

[seattlejester]

If the mount has failed then it would be best to replace the lower mount before adding on the RT mount if you plan on running a factory style rubber mount on the bottom.

 

The snubber setup will be fine as it basically replaces the limiter strap.

 

Keep in mind clunking can come from other sources, so inspecting your mustache bar, axles etc would be a good idea. One source of a clunk that is missed fairly often are the control arms in the rear. If the bolts become loose that retain the bushing on power on or braking the entire control arm can shift forward or rear ward.

[ZHoob2004]

Yeah I'm going to investigate further tomorrow (need to pull the driveshaft for an output shaft seal). Everything else should be be good since I just replaced all bushings. It's either the mount or a part of the driveline.

 

Any comments on vibration with your setup?

[Greeko]

If the mount has failed then it would be best to replace the lower mount before adding on the RT mount if you plan on running a factory style rubber mount on the bottom.

 

The snubber setup will be fine as it basically replaces the limiter strap.

 

Keep in mind clunking can come from other sources, so inspecting your mustache bar, axles etc would be a good idea. One source of a clunk that is missed fairly often are the control arms in the rear. If the bolts become loose that retain the bushing on power on or braking the entire control arm can shift forward or rear ward.

 

Great suggestions!