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Vibrations - Rubber vs Polyurethane mounts?

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Alright here is the breakdown,  I am running an SR20DET on my 72 510. Current HP is around 250 and will be tuning up to 275-300 in the near future with no plans to go higher after that. Comfortable cruising is the name of the game here and currently vibration is my biggest frustration. The vibrations do not occur at idle or while stopped and revving in neutral. No vibrations under hard acceleration or deceleration. Vibrations only occur when holding the accelerator to maintain a constant speed. After what I’ve done to try to reduce the vibrations, research is pointing me to the motor and transmission mounts.

 

-Current motor mounts are newish Datsun 620 mounts. The motor will shift almost about 3/4” under acceleration (seem almost too flexible)

-Current transmission mount bushing is a Energy Suspensison Polyurethane block

-Just replaced original mustache bar bushings with polyurethane bushings from Energy Suspension

-CV Axles installed to replace U-Joint axles

-Exhaust recently checked and does not rub on any parts of the frame or body

-All four wheels perfectly balanced and tires are new

-Drive shaft balanced

 

Looking for opinions/experiences with going poly for both motor and tranny mounts or rubber for both.  I feel rubber is the way to go for what I want however the flex/shift from the motor upon load has me concerned for transmission and drive shaft longevity. Does anyone have experience with the McKinney rubber mounts? Maybe they are stiffer than the 620s? I don’t see many other pre-made options out there and I am not opposed to making my own rubber tranny mount.

 

Thoughts?

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-Current motor mounts are newish Datsun 620 mounts. The motor will shift almost about 3/4” under acceleration (seem almost too flexible)

-Current transmission mount bushing is a Energy Suspensison Polyurethane block

-Just replaced original mustache bar bushings with polyurethane bushings from Energy Suspension

 

This should be rubber. There is nothing better for absorbing/damping vibration and movement than rubber. Sound is just a vibration. If high enough frequency you can hear it, if low enough frequency you can feel it. Poly is so dense it transmits movement directly into the body. That said, if you want your car to be a 'race car' then you put up with what a 'race car' feels and sounds like and comfort is superfluous. If you like to enjoy your ride and find vibration and noise objectionable then maybe you could get by with it being slightly less than a 'race car'. Some 'race car' innovations just don't translate well to a street car.

 

Engine movement side to side or forward and back is not going to harm the drive shaft... this is why it has a spline to collapse or expand without binding.

 

 

I would start with the transmission mount first and see. You may be able to get away with only changing it.

 

Is the exhaust pipe/muffler supported with any kind of rubber isolation to prevent vibration transmission to the body?

UNB005.jpg

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Exhaust has rubber hangers at the mid pipe/resonator and at the muffler so should be good there. Do you know of any prefabbed rubber transmission mounts for this application? I honestly don’t know what other rubber mounts can be used.  Searches seem to only lead to more polyurethane mounts, mustache bar included. If I can get away with just doing the transmission mount I would be stoked. 

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From experience, I have a 72 2 door 510 with an SR swap that's making 300+ RWHP. Up until recently, I was running McKinney poly motor mounts and Trany mount. Those damned things about rattled the car apart, and I was chaasing rattles and buzzing all around the car. I swapped the front X-member and steering box to rack & pinion, so I switched to 620 rubber and never looked back. Trust me on this one, If the name of the game is "comfortable cruising"  you don't want Poly motor mounts.

 

Do you still have the stock harmonic balancer as the crank pulley? With the SR there's a wicked harmonic vibration at about 2,700-3,000 RPM that might be rattling something that's loose, but only at that specific RPM.

 

Are your downpipe and turbo/exhaust manifold fittings V-band or bolted flange? V-band will allow a bit of rotational slip, while a bolted flange transfers everything from the motor on down the pipe.

 

Also, do you have a braided stainless flex coupler between the downpipe and exhaust? All these things serve to isolate and reduce engine vibration.

 

The 510's unibody is a metal box, so you never know where a particular wavelength of vibration in one area may have a reseinent effect in a completely different place. While driving, reproduce the sound and have someone scramble around the cabin to see if they can identify the location. 

 

BEST OF LUCK

 

 

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I do have V-bands on all exhaust pieces and the stock pulley equipped. I reached out to my friend who helped a lot with the swap and when I mentioned the vibrations and that I was thinking of swapping the transmission mount he said I should recheck the drive shaft.  He said that the guy who modded it to fit, balanced it “as best he could”...

 

Well that certainly made me rethink things. I took the drive shaft to a shop that specializes in these sort of things today and they immediately said that the original u-joints on it were likely the problem. Both ends seem to get hung up for a second while going through their range of motion. They remade the whole drive shaft with some beefier u-joints ($260). I took it out for a test drive right after reinstallation and PRESTO! No more vibrations in any RPM range or steady driving. Quieted the car down considerably.

 

Thank you Paradime and Datzenmike for your input. 

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Glad you found this as rubber probably wouldn't have totally fixed this.

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