This was a well known issue with Chevrolet V8s a long time back.
Along with my 1940 Chevy street rod, I have a 69 chevy pickup. The pickup has a cable on the driver's side of the engine - it bolts to the exhaust manifold, runs through a hook in the frame and back up to the mainfold. It's only needed on the driver's side because the engine torque lifts the driver's side and compresses the passenger side. The angle of the mounts limits the 'spin' factor and act as pivot points.
Aftermarket motor mounts have interlocking steel plates that eliminate this problem.
In most cases, (as in my parent's 1964 Oldsmobile) I believe the engine mounts usually fail due to petroleum products (usually engine oil, or power steering fluid leaks) deteriorating/weakening the bonded rubber vibration isolation engine mounts. "Hot rodding" can be the straw-that-broke-the-camels-back. Alternatively of course, with a very stout engine being raced hard, even a new mount can be broken (also a frame can be bent by a racing engine without sub-frame connectors and a robust roll-cage.).
In the case of the Oldsmobile, I believe (based upon examination of the worn failure surface) the "hot rodding" just identified a problem that was already there for some time, but undetected. I do not feel I broke their mounts, but the identification of the problem also pointed-out that I was "hot-rodding". Even though this action was somewhat mild, it was contrary to their boundaries.
As a point of interest, to address the root cause on my parent's car, I also replaced the leaking power steering hoses, and valve cover gaskets, to get rid of all the fluid leaks. That particular car was still going strong at 199,000 miles when I sold it to a friend. He still had no problems with it by 230,000 miles when I last saw him in 1984!
Thanks for your story, David. The Northern California mechanic who told me about the 2 destroyed engine mount gaskets looked at my like I was crazy for driving it like that for so long. This was the first time I'd taken my car to him, and it was for a once over checkup. When he explained the situation, I must have turned pretty pale, and assured him I was not in the habit of driving my car in such a dangerous condition. Maybe he also thought I'd been hot-rodding it, although that was pretty unlikely by that time in my life. I can say I've since never taken any unusual car sounds for granted, especially those that occur at 65 mph.
The National Highway Traffic Safety Administration (NHTSA) Case IR 162 correspondance with GM between December 1970 and January 1971 indicated 172 reports of failed motor mounts, with 63 accidents and 18 injuries. There were NO reported fatalities.
I apologize for my misinformation from my inaccurate memory of the report.
In the mid-1970's I built-up a V8 Chevy muscle car and Chevrolet had recently come out with "safety mounts" that had interlocking steel T and slot features to limit travel even when the rubber failed. This was popular with performance oriented applications; although, serious racers also went to either a back-up movement limiter (chain or steel cables), or else solid steel mounts (which transmitted unacceptable engine vibration for all but the most hard-core race-only cars, even rattling your vision).
What I only learned recently was that this "Safety Motor Mount" was precipitated by an investigation in 1971 of 127 Chevrolet motor mount failures resulting in 63 accidents and a few fatalities. The result of this National Highway Safety Transportation Board investigation was the motor mount redesign, and a recall where a steel limiting cable was installed (presumably cheaper than retrofitting new motor mounts). What made this type of failure a greater problem leading to the investigation was the cascading problems from the motor mount failures. The lifting engine pulled the throttle linkage "on" further lifting the engine even more. The engine movement also locked-up the transmission linkage preventing a shift into neutral, and the engine movement sometimes ripped loose hoses to the power brakes and power steering making steering and braking MUCH more difficult. Hmmmm, sounds like an accident waiting to happen similar to the more recent drive by wire Toyota throttle issues.
I guess Chrysler anticipated this problem better, or experienced it sooner with their monster 429 Hemis.
I know the GM and Ford sub-frame module constructions, such as in the Ford Taurus and GM X body cars, did allow for better road vibration isolation in a unibody car construction as well as provided a convenient module for efficient assembly. I don't think the concept is flawed, but it does need to executed properly, and reliably, or things can become very unglued in an ugly way.
The 1970s were rough years for the automotive industry. The workers on the line were an angry bunch. There was plenty of worker sabotage going on. Not a happy time in Detroit.
At the time, I worked for Celanese Coatings. They produced much of the paint for Chrysler. In the lab, we tested paint against multiple environments. The salt spray was the worst. At the time there was no way to protect paint against salt. Cars in Detroit started to rot through the floorboards after five or six years.
It was in the seventies, probably around 1974, although it may have been in development for a while by then. I think that it may have made the engine drop in a bit easier as well, but I can't vouche for that.
Back when I worked at Chrysler they solved the motor mount failure problem by creating a motor mount that put the rubber in a hole in a support frame, so that if the rubber failed the engine was still trapped, since the two mount sections were interlocked. I think that they did it to protect the radiator in minor collisions, but it seemed like a very good idea at the time. And it would certainly aviod a disaster if the rubber failed completely.
As for the engine and trans, along with the whole front suspension being on a subframe, GM has done that on a few vehicles, probably others as well, because it does allow better isolation from the road noise, and it makes building the car much simpler, and therefore much cheaper. Of course, when the engine assembly does come unglued it is a very big deal, as you discovered in the driveway.
Of course the salt damage from road salt here in Michigan is usually far worse than the military salt spray test, mostly because our roads are far saltier than any ocean could ever be. And nobody will admit that a few tons of salt per mile every year constitutes pollution.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.