W123 hydraulic motor mount trial & result
 

I got the silly idea that a hydraulic mount, as used in a few high-end cars would eliminate the cabin noise from my diesel, especially that bothersome 3000 rpm highway whine at 65 mph (great gear selection M-B geniuses!). The only one that jumped out w/ in-line studs was the 1997-2012 Corvette (& 2002-2004 GTO), which was surprisingly cheap ($12 ea), so I figured why not try. I haven't read that anyone else ever tried similar.

The biggest mod needed was to rethread one stud from M10 to M8 coarse to thread up into the M-B mount. I ASSumed the ~1" extra height wouldn't be a problem, and shouldn't a gushy hydraulic mount sink a bit. Wrong, I could barely close the hood and had to wire in the radiator shroud high to not hit the fan. I also had to jack the engine up to the max to get the mounts in, even after cutting both studs as short as I dared. It was also very hard to get the stud started since the mount rubbed the inner fender at that height, especially with one side slightly higher and tilted. I had to crow-bar the engine over to get it started, and also to drop the lower stud into the hole. I was so geared up that I found a stainless bowl that fit perfect, after cutting one side to clear the K-frame. I could fix the height issue by machining off ~3/4" on a spare set of mounts, but read below first.

How did it work? Terrible. The whole car body shook so bad I could hardly stand to even idle, and didn't get much better at highway speed. Initially, I left off the engine shocks, thinking who needs those w/ hydraulic mounts. Installing them didn't help. I also wondered if I had somehow changed the engine, like when I knocked off the fuel filter and might have got air in the hose, but cracking each injector until it bled didn't help. The problem appeared to be that the mounts are actually stiffer than the factory ones. I tested them on my hydraulic press and they need some force to compress, almost like they are solid rubber.

I finally gave up and re-installed factory mounts, w/ polyurethane fill, as before. But, this time I installed 1/4" backer plates underneath to support the poly since otherwise it will eventually put too much force on the thin sheet-metal underneath and crack it, as it did in my 1984 (earlier post). The engine now idles and runs much smoother, but no better than before the hydraulic mount fiasco.

Probably for the best since hydraulic mounts are a continual problem in Corvettes, as well as Lexus (different design). Owners regularly find them leaking brown gook and collapsed. Many change to after-market solid polyurethane mounts for hundreds of dollars and live with some buzz.

more photos.
And I had to take off that p.s. return tube while installing, which lost 1/2 qt. But it did barely clear the tube. Lots of things to interfere when you mod, as most hot rodders appreciate.

Thanks for the write up...good info.

It's confirmation for me that these diesels need soft mounts. I was shocked at how much vibration was transmitted when I did my initial urethane fill with too stiff of a urethane. I went through a long troubleshooting process finally installing new mounts that confirmed it was the mounts...which I then filled with a soft urethane that solved both the vibration and collapsing problems.

I didn't have the problem with the cracking because my installation required 1/2" spacers or I'd have discovered what you discovered with that too.

Thanks again for your work and documentation.

You probably wouldn't need the dampers with hydro mounts. I'd also look for ones from a diesel engine since they will be tuned to the different type of vibration.

I would suggest one from a mk2 or mk3 vw diesel, but those aren't set up to carry the same weight as the MB drivetrain.

-J

Great creative experiment. It was worth a try otherwise we'd never learn anything.

I worked on industrial machines in my day gig and we were always trying to kill vibration. While soft isolating pads often helped, stiffening the mounts would also help in some circumstances. The reason we found after a lot of trial and error was that peak resonant frequencies could be moved around by making a system heavier - like rigidly locking it to another heavy object, or making a flexible thing stiffer. This is because the resonant frequency of a system is the square root of the stiffness divided by the mass.

If you can move the numbers away from the frequency that they're being driven at like your 3000rpm point you can often diminish the overall vibration. It's kind of like pushing a kid in a swing at the wrong rhythm or trying to get a car to bounce by pushing it up and down at the wrong rhythm. So sometimes I've found you can get a surprise by stiffening or making something heavier which is counter intuitive. Damping is only one of the knobs we can turn down.

That being said, I killed a lot of vibes in my SD by turning in the rack damper screw (yes, not only idle but smoother highway cruising) and doing a valve adjustment. A big one was tightening my tranny mount too. You've probably done all that. The W126 has a different insulation setup anyway.

Way to go on the attempt. A negative result is still knowledge.

Static stiffness aside, any mount designed for a engine that isn't a 3cyl or a 5cyl is going to be extremely harsh at idle. The large amplitude of the vibrations are greater than what the decoupler is designed for. The mount will immediately hit IOD. Even worse, at 750RPM those vibrations are going to be at 12.5hz. This is right around where most hydro-mounts are designed to have the highest dynamic stiffness in order to combat smooth road shake.

http://i.imgur.com/LjAcrJx.jpg

Bill, good post. But I think you need to find a hydro mount that will take the weight of the 617. Your talking around a 600 lbs om617 vs a 400 lbs LS3. It would be interesting to see something that would work.


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I have been trying to improve machines for 50 years.. particularly when a part needs replacing... people make fun of me for trusting and appreciating the FSM instructions on these cars... for having some trust in the millions and millions of dollars of R and D which MB has invested.... and their general ' German ' OCD when it comes to metals and machines in general....
Sometimes their FSM instructions are not followed accidentally.... sometimes on these types of vibration items... they will specify two different firmness rates on the same item... like at the front and the back... the driveshaft has this on some models... and sometimes people will not know to order both of them since they look the same to the eye front and back... those Germans were really good at what they were trying to do... no stone left unturned.... many say that this many years later ' stuff ' has improved... sometimes yes.. sometimes not...

Don't the 60x engines have hydraulic mounts? That's what I first thought when I saw the thread title.

Yup, the same PN fit 4, 5, and 6 cyls.

IIRC the 126 applications have rubber mounts.

Sixto
83 300SD

Yes, my SDL has a similar setup to the 5 cylinders... rubber mounts and external dampers.

-J

Seemed the opposite problem. The Corvette mounts didn't deflect noticeably under the engine weight, and seemed much stiffer than M-B ones when I played with them on my shop press. I think the natural frequency comments are the best explanation, and validius post seems spot-on since it was a low frequency that seemed to shake the whole body. One can't hear 12.5 Hz, but you would hear the harmonics and sounds from parts impacting. No loss, I have other old cars I might try them on someday.

I once generated a similar vibration at idle. In securing my 1984 300D's detached-at-top K-frame, I ran a heavy chain axially back to the tranny cross-over. That chain vibrated at idle and shook the whole body at a similar low frequency that was very annoying. I stopped it by jamming some big sponges between it and the tranny.

do you still drive with sponges jammed?

thanks

As Mark noted, the V8 hydro mounts are tuned to a very different frequency than the vibrations from your I5.

VW sold I5 diesels, I know in the Audi 200 at least they used hydro mounts. Perhaps those would work better?

-J