Odd banging noise finally found, but?
 

I have an '83 911SC cab late production model, as most know, no thanks to me because I did not, the late '83SC'S actually came withe the newer 3.2 case, with 3.0 internals, I have been trying to locate a bad knock comming from right rear over bad bumps (pot holes), I found out today that it is comming from the a/c compressor actually comming up and hitting the bottom of the rear lid, there is an indentaion in the metal where it hits. The compressor in these models are the large ones, and very little clearence between it and the rear lid, literally a few mm's. I think that combinded with the 3.2 case creates this problem, has anyone experienced this before? Or , probably the case, am I the last to know? Any suggestions to remedy the situation?

Are you sure the engine is mounted to the body tight? Check all of cross member bolts and the bolts that hold the engine to the mounts to see if they're tight. I've never heard of your problem but then I've only been in this while.

I agree with your suggestion, I've inspected them before, to the best of my ability, all mounts are brand new, sport type, but I do suspect that something in the crossmember is loose, it is a cab, so the crossmember is a little different, I just don't feel I'm getting to all the bolts and that center, discontinued , round bushing type thing with the 3 bolts, can't tell if thats tight and okay, it's not mentioned in my current repair manual, been experiencing a chatter, that I thought was resolved with a new clutch cable. It's much better but still present, I'll go to work on the crossmember tommorrow. But there is literally no room between the top of the compressor and the bottom of the rear lid when closed, I put a piece of rubber on top, very thin and in gets an indentation when the lid is closed and I know the engines have to move a little durring operation.

Check the condition of your engine and transaxle mounts.

It also might be possible that your engine mount dampener shock absorber is worn out, which allows the engine to shift around enough under load to get the situation you're describing. It's easy enough to remove the shock and test it (push and pull on it to deflect it and see how smooth its action is).

Another possiblity is that the rubber donut in the center of the engine mount is worn.

You're right, cab engine mounts are different.

The 3.2 engine case with a 3.0 engine is a neat combination, which is what I have on my '83 cab, too. Who needs that silly old sump plate? (Oops, guess I do on my '82 targa.)

Brian

Here's a good thread on the topic. I did a search under "cabriolet engine mount."

Brian

Brain if I understood you and your related post, the least expensive attempt is to eliminate the shock damper on the crossmember with replace it with a solid type of fabricated replacement? Made to the size of the damper in it's relaxed measurement. I'll give that a try, thanks. By the way pelican shows the damper currently in their catalog, not sure if it's in stock, but I can't wait a week to see if this solves the problem. Perhaps a temporairy fix, if I get a positive result with too much vibration, I'll order the damper.

Engine case
 

I did not know this!! What determines if it is a late model cab? VIN? Production date? How would I check if mine has this?

Check under your engine and look to see if you have the round sump plate to drain your oil, if it just has the drain plug to the left side of the case with no round sump plate, that is 3.2 case, round sump plate is std 3.0 case, it changed about april of '83 give or take a little.

I have removed the damper at the crossmenber, it is shot, all other components checked and all are tight, the center round rubber bushing is not worn and again is tight, the damper itself pulls in and out with little force, thers is no rebound to it and I am not sure it should, however once you pull or relaese the shaft, let it set a momment there is a clear 1/4 to 3/8" inch of play back and forth, I am confident that this is why the chatter persists during reverse and 1st gear take offs. Thought of fabricating a solid piece and probably will, the OEM part is out a few weeks, and I see no need to damper anything, just eliminate the movement all together as noted in a privious reply. There are no dampers on any porsche accept cabriolets and maybe turbo's. I'm kind of surprised no one has come out with prefabricated solid replacement for the damper, as an aftermarket replacement.

Maybe you have checked this... but it could be as easy as replacing the rubber stops in the engine lid... when those get old it will allow the engine lid to vibrate...

Thanks for the thought, but no, everything is in place,tight, and in correct alignment, I personally feel it has directly to do with the newer 3.2 case combined with the larger compressor as of that year, leaving next to no clearance between the top of the a/c and bottom of lid. Nobody has explained why porsche went with the different crossmember w/damper on the cab, they have the typical (normal) crossmember on the targa's, anyway there is free play in the damper and a few people here have pointed to that as a real possibility of whats happening. Will not be sure till I replace it, if available this week, the fabricating of the solid mount replacement, though not that difficult, I'll hold off on to see if the new damper solves it, I'll let you know.

The newer case doesn't change anything dimensionally in the engine bay. The only difference is the older style has the big opening on the bottom for the sump plate and the Carrera-type case has a drain plug. The case design essentially went from strength to even greater strength. Notice, for example, that the engine type number didn't change even though the case design did. A U.S. 930/16 remained a 930/16.

Tyson Schmidt explained the reason for the cabriolet engine mount dampener and design like this (copied from an earlier thread):

Because the Cabriolet chassis "wiggles" over sharp one-wheel bumps. This oscillation of the chassis is undamped. The mass of the engine/trans. is used as a ballast, and the oscillations are dampened by the shock on the engine mount. The big rubber center allows the twist, and the shock dampens it, absorbing the oscillations. This gives the car the illusion of being more solid than it really is, and increases driver comfort/feel. Problem is, it doesn't help at all in the handling department. Coupes don't need it, so they don't have it.

Brian

Brian thanks for the info on both the 3.2 case and finally a note about the damper, as mentioned earlier that is exactly when the the compressor comes up and hit the bottom of the lid, it only happens over a bad pot hole like situation involving the right rear wheel. Maybe a few times a month, enough that I had to isolate and find what was happening, it took me over a month to find it, I checked everything I could imagine, from suspension, exhaust, axles, wheels etc. Thanks for that info, but my question still remains why they didn't have it on the targa? Other posts here have given the oaky to replace the crossmember back to the original or old style crossmember with no ill effects. I dont want the headache of that, I hope the new damper corrects it to the way it use to be.

The answer is that targas, with their hoop framework, didn't need that dampening system.

Brian

Brian what are thoughts of converting the damper to a fabricted solid mount exactly in it's place? Any pro or cons come to mind? Or do you fell I should just replace the old worn out damper with a new one?

I don't see any downside. The conventional wisdom is that if better handling is the priority, go to a regular firm engine mount like the targas and coupes.

Personally, I like the cab engine mount system -- it's quirky, one-off in the 911 world and it's something a buyer would be looking for if you ever sold your car. You might be able to pick up a perfectly useable used dampener shock for much less than the new-part price in the classifieds.

Or fabricate some bar stock to replace your shot dampener.

Another idea would be to mount some kind of a clamp on your existing dampener shock to limit its travel. Kind of like what 930-type tensioner collars do (except they do no limit travel, just act to prevent all-out tensioner failure).

I have to believe our particular dampener shock had way more applications in the automotive world than solely on cabriolet engine mounts. This is one example where some parts interchange information would be helpful.

Brian

I am in full agreement on all points. I will "not" go through the headache of changing the cross-bar set up back to standard. I will keep the original damper, for originality, if I fabricate a simple set up. I also have given thought on how to limit, if not eliminate, the travel of the existing damper but have kinda given up on that one. All the suppliers have said several weeks out, has to come from germany. I do agree that this is a part that has to be used in other facets of fabrication and application, I just have no way of cross referencing or cross matching this part.

Brian, after exhausting all avenue's of getting a timely replacement damper, new or used, I fabricated an "L" bracket bought at the hardware store today $7, including bolts, washers and nuts. It solved all the associated problems, the a/c no longer hits over that damn bump down the street, and all the vibration and clutch chatter is finally gone. Thanks for your patience, at least we know the answer now, that damper really does do something. P.S. when I got upstairs to send you this post my friend at "weekend rides"emailed me to say he'll have one in a couple of days. GO FIGURE! Going to install it when I get it, hope it's as good as what I made.

Glad to hear you got it sorted, Robert. Hopefully, this thread will help others who drive cabs, too.

Brian

Got another question, since we are on the subject, how does one go about replacing the center rubber (bearing) with 3 bolts in it, that mounts into the cross-bar? Looks like the would be a major PITA. I find it hard to comprehend that only the round donut assembly and the damper are all that connects the cross-bar to the engine mounting plates? Or am I missing something? There just does not seem to be anything else, no bolts etc.