No, that chain was a temp fix just to drive the 1984 home, but the big sponge did get rid of the annoying vibration. I re-welded the K-frame at home. I didn't relate any scenario with these hydraulic mounts and sponges.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Unlikely there is such a "tuning" for the mount itself. The critical dynamic properties of the mount are spring constant and damping. Whether it works for a V-8 or I-5 engine app depends on the whole system. The main advantage of a hydraulic mount is the much higher damping. But, unlike the simple spring-mass model in Physics I, hydraulics is very non-linear, increasing greatly with the amplitude of the motion. I didn't notice these mounts deflecting enough to do much damping. The engine itself didn't appear to move much at idle, but some attached components did, like the AC compressor suction tube was shaking wildly and I could damp it with my hand. I tried different idle speeds, but it was pretty bad from 500 to 1500 rpm, and even on the highway. As mentioned, I think the mounts were just too stiff and transmitted everything from the engine into the car's frame.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Not quite.

Hydromounts are designed to act as a mass-spring system internally. They are designed specifically in such a way as to avoid viscous damping.

Because it is full of wonderful pictures, i'm going to cite this study: https://etd.ohiolink.edu/!etd.send_file?accession=osu1240664127&disposition=inline

Take a look at Fig 2.4. Inside the mount you have:

• The top element. This provides the static stiffness of the mount.
• The inertia track through which the glycol flows as the mount flexes
• The rubber bellow which is an extremely compliant rubber membrane that contains a reservoir of glycol
• The decoupler. I'll explain this more in a bit. For now, pretend it has 0 compliance.

As the top element moves, it pumps fluid through the inertia track. The mass of the fluid in this inertia track can be modeled as a mass-spring system with the top element above it and the bellows below. Once again, it is designed to minimize fluid losses.

The behavior of a typical non-decoupled hydromount (or in this case, a decoupled hydromount that is past its IOD but more on that later) can be roughly seen in Fig 2.12b. Dynamic stiffness (k*) is represented by the hollow squares. Phase angle (the difference between the force input and the motion, also known as damping) is represented by the solid dots.

• 6-7hz: The input frequency is before resonance of the inertia track. The presence of the fluid makes the mount softer.
• 9-20hz: The input frequency is at or around the resonance of the inertia track. This leads to the fluid's inertia fighting against the input frequency. Dynamic stiffness peaks here. Phase is at it's highest. Fig 2.15 shows this nicely (lower graph)
• Above 20hz: Phase drops off. What is actually happening here is that the inertia track is 'locking up'. When the input frequency exceeds the resonance of a mass-spring system the mass simply stops moving. The stiffness now is simply the bulge stiffness of the top element. That is to say, it is acting like a thick rubber balloon full of fluid that has nowhere to go.

It is worth noting that because of the inescapable fluid losses that do occur dynamic stiffness of a nondecoupled hydromount INCREASES as the amplitude DECREASES. This makes for lots of harshness at idle which tends to be small amplitude, medium frequency vibrations. Go back to Fig 2.4b. To solve this, a decoupler is added.

The decoupler moved up and down within the limits of it's cage to allow fluid to avoid passing through the inertia track as long as the input amplitude is not so large that the decoupler hits the limits of its travel. Unfortunately, there isnt a good diagram on that study that shows this. The effect is that at small amplitudes (0.1mm) the dynamic stiffness of the mount is very nearly the same as the static stiffness. At higher amplitudes, (~0.5mm) the decoupler is well past its limits and the peak phase angle (for the given amplitude) is able to reach its maximum (across all amplitudes). Above this, the mount behaves like a nondecoupled hydromount. The amplitude at which the peak phase angle is halfway between 0 and it's peak across all amplitudes is commonly referred to as the Initiation Of Dampening (IOD).

Almost all hydromounts are designed for maximum damping between 10 and 15hz. This is a frequency where a phenomenon called 'smooth road shake' is often observed. On an apparently smooth road, the driveline can begin bouncing on the mounts at the system's resonant frequency.

OK, enough theory. Going back to my previous post in this thread, what i said was that compared to a V8 gasoline engine, the amplitude of the vibrations that an OM617 will create easily exceed the IOD of the mount (likely around 0.3mm, peaking at no more than 0.6mm). I also said that given the first order imbalance of an I5 and an idle of 750RPM, the frequency of the largest amplitude event at idle will be 12.5hz. This is smack in the middle of the frequency range those mounts were designed to have the GREATEST dynamic stiffness at.

QED

validius,

Thanks for posting this very interesting and useful Phd thesis. The problem in comparing to my observations is that I didn't notice the hydraulic mounts deflecting at all, between 500 and 1500 rpm. The thesis explains that the hydraulic mount works in both modes (coupled or decoupled) by deflecting. For all I could tell, they are just a solid block of rubber, and much stiffer than a M-B mount. Given their source (India), one might wonder what is truly inside.

I don't follow your discussion of the internal mass inside the mount. He even says on pg 35 that the mass of the mount's steel and fluid is negligible compared to the engine and car's body. The properties of the mounts that he measured (both hydraulic and solid rubber mounts), are their spring constant and damping, both of which vary with amplitude and frequency (non-linear). Don't intend to be smug, but that is exactly what I said.

All you guys keep talking about how the large vibrations at idle were due to these mounts deflecting too much - "OM617 ... easily exceed the IOD of the mount", "OM617 is heavier than an LS engine", etc. But, I didn't notice them deflecting at all!

As I read the thesis, the hydraulic mounts worked much better than the solid mounts around 10-15 Hz (pg 38, 1st paragraph, great at 9 Hz engine resonance). The concern was that they can be a bit worse at higher frequency (>150 Hz, same page bottom). Indeed, he says the same in the intro and all thru the paper. Indeed, there is nothing in the paper to suggest any difference in design criteria for a hydraulic mount for an I4, I5 or V8 engine application. validius might need to rethink his QED.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

OK, there are a couple critical assumptions that need to be made here.

1. The mounts are built properly and are behaving as a hydromount should
2. The static stiffness of the mount is not so high that it alone could create the observed harshness

On to your reply:

With an IOD likely in the 0.3-0.6mm range peak-to-peak(p2p) you are not going to be able to perceive much deflection.

Remember, once you are at or above the resonance of the mount, the dynamic stiffness can be 2-3x+ the static stiffness, particularly at amplitudes of less than 1mm p2p (which is what it would need to be based on your observation).

The mass of the fluid inside the mount moves through tuned passageways to act as a mass damper. Outside of this scope, their mass is trivial compared to the rest of the system.

You are correct that the dynamic stiffness varies with amplitude and frequency however you originally conjectured that with increasing amplitude the dynamic stiffness would increase. This is not the case.

Once again, in the context of an IOD significantly less than 1mm and peak dynamic stiffness right around the input frequency at idle you are going to be faced with mounts operating at an extremely high dynamic stiffness, resulting in very small amplitude movements.

I applaud you for reading as far into that paper as you did. It's thick dry stuff to be sure.

The frequencies and amplitudes of the vibrations that engines create as they run varies widely not only within cylinder configurations but even more so between them. An i4 engine has a second order imbalance, resulting in a vibration at twice the frequency as the RPM of the engine. An i5 engine has a first order imbalance that causes the engine to want to rock back and fourth at the same frequency as the RPM of the engine. Mounts must be carefully tuned on an engine-by-engine basis to handle both smooth road shake, idle and many other metrics ( sometimes they even vary based on transmission because differing masses and CG have a meaningful effect on the response of the drive train ).

For more information on engine imbalance, watch the Engineering Explained series on the topic.

To visualize how a mass damper responds to differing input frequencies, check out this video: https://www.youtube.com/watch?v=Kus5nHW7Twc

Notice how the input force and the position become increasingly out of phase as the system approaches resonance. Also notice how the mass begins to stop moving as the input frequency exceeds the resonance of the system. This is what i referred to as 'locking up' in the previous post.


It is completely possible that the mounts are built so poorly that their static stiffness = their bulge stiffness at all times. Perhaps QED was a bit hasty however nothing you have observed precludes the possibility that the mounts you used were operating as designed.

I'm happy to answer questions or clarify anything you would like but you are not going to prove me wrong on hydromount theory. I used to design these things.

I found since that these Corvette mounts are made in a solid rubber version. Anchor adds "SR" to the PN to distinguish. Some Corvette guys wanted the solid rubber and were angry to find the ones in the "SR" box were hydraulic. They didn't say how they could tell. The ones I bought are Westar w/ same Anchor PN and no "SR", and were listed as "hydraulic" on rock. But, given the quality control in such India products, they might well be solid. I tried the classic "hard-boiled egg vs raw roll test", but couldn't tell if there is fluid inside (the fluid is very thick). They only cost $9 ea. Regardless, the 1" too high issue proved insurmountable. Even if I found truly hydraulic mounts, the engine brackets would have to be machined down at least 3/4" to fit. Perhaps another choice will show up someday.

In my case, the shaking was worse than I recalled after going back to factory mounts (w/ poly fill), but still much better than w/ the Corvette mounts. Turns out much was coming from a cracked Sanden bracket, so bad I removed it and found it cracked much worse than I thought (posted photos in Sanden bracket thread). Perhaps driving a few days with the Corvette brackets made it crack more. With an R4 compressor in, the idle seems OK, bothersome as always compared to most cars but apparently normal for these diesels.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Great effort Bill!

I am currently swapping a Ford Duratec engine into my BMW 2002 and I'm using hydraulic mounts from a BMW 318i. I haven't run the engine, but just shaking it by hand I can see the mounts deflect. I think this is still a viable avenue of research.