Main bearing fears

[Patronus]

I have bought a 2.7-engined 912 a few months ago and am very happy with the car. The engine runs smooth and is very quiet, and even in the South African weather it runs relatively cool. However, I am very scared of the 2.7 engine since it has a bad reputation and our weak currency means that Porsche parts are about ten times more expensive than elsewhere in the Western World. A set of 2.7 pistons and sleeves costs more than the average yearly income in SA. For that reason, I need to know the following:
Is there an easy way to tell when the main bearings are beginning to wear? My car makes a humming noise between 2000 and 3000 rpm (sounds like 'gorrrrr') at trailing throttle. It is not the sound of over-run - it is definately a mechanical sound. It also is not the timing chains. Only I can hear it, but it bothers me. The sound disappears when the engine heats up.
Please tell me that there is an easy way to tell whether it's a main bearing.

I will frequently bother you guys with questions, if you don't mind. Thanks in advance.

Martin

[Jamie79SC]

It’s hard to comment without actually hearing your car, of course. However, you may be hearing the same thing that had me worried last week, (and needn’t have).

I had my clutch replaced in my 79SC. As long as the tranny was out of the car, I had the garage replace the fluid with fresh SWEPCO gear lube.

Once the car was warmed up, I heard the sound you described. (I’d heard it before, but not nearly so clearly.) I thought “Oh no, what’s coming apart now?” and took the car straight back to the garage. A quick ride around the block with the mechanic resulted in the comment from him, “Oh, that’s just the sound of the 915 transmission. No nothing is coming apart, nothing is wearing, and that’s just what these transmissions sound like. You just didn’t hear it with your old gunky old oil.” I was skeptical until I went for a ride in a friend’s car and heard exactly the same thing. I was just listening a little too closely after the clutch overhaul.

If this is what you are hearing, you only hear it under trailing throttle because it’s hard to hear over exhaust and other engine noise when the transmission is loaded in the other direction.

I hope I’m right and this leads to more relaxed motoring

[Superman]

I believe your main bearings are fine. Evidence as follows: First, bad main bearings do not make a gorrrrrrr noise. Many other things might (tranny, exhaust leak, tensioners, alternator bearings, clutch noises, etc), but not main bearings.

Also, main bearings do not normally make noise during a trailing throttle at 2000 to 3000 rpm. Instead, bearings can make noise at idle (due to low blood pressure) or at high rpms (due to speed and force). Th noise they make is a low, hollow thunk. Sometimes (at higher rpm) it is more like a deep rattle sound.

I suggest looking further to see if it is coming from the tranny, tensioners or alternator. Also, since you seem to be the only one who hears it, an alternative approach would be to turn the radio up a little louder.

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'83 SC

[john walker's workshop]

opening up a 2.7 is like a can of worms. you can't just do a single repair. there is a lot of machining necessary to correctly rebuild one. if you put new main bearings in one and torque up the case, the crank will bind 9+ out of 10. they all need to be align bored, head stud holes inserted, various other 8mm stud holes inserted, cylinder bases cut, heads flycut. they're just pretty worn out after 150-200k. especially one that's been in a hot climate all it's life. so if you don't have to open it up, wait until it really needs it, and you have the time, parts and machine work sources, and money.

[Patronus]

Thanks Superman, that answers my question. At first I also thought that it might be alternator bearings, but it seemed to come from deep inside the engine. And it also didn't sound chain-like. I took the car to a Porsche specialist this afternoon, and the result is as follows:

They firstly told me that the sound is indeed coming from the timing chain. The chain is resonating at a certain temperature and speed, but is not loose enough to make the 'loose chain' sound. Apart from that, the engine is perfect.

Then they told me that I must not be so paranoid about my 2.7 engine. 911 engines are strong. The 2.7 enjoyed too much bad press, they said, and I must enjoy the engine rather than scrutinize it.

And as for John Walker's advice - they said the same. Don't open up the engine before it is really necessary. Which I dont't plan to do, fortunately.

Jamie, my car still has its 901 (912) gearbox. If it was a 915 I would have blamed it, no doubt. They tend to sing along, I know.

Thanks for the advice, guys