sschmerg

About 2 years and 5,000 miles ago I had the top end of the motor rebuilt on my '78 911SC (about 100k on the clock). At the time, the mechanic mentioned that he could do the bottom as well, but it did not appear to need it. Needless to say, we left the bottom and just did the top-end (along with the trans & clutch) to the tune of about $12k. Now, I am hearing a rattling sound at high RPM, which he diagonsed as probably being the rod bearings, and that to redo the bottom-end would be aboout $4,800.

So I have three questions: (1) was he negligent to not do the bottom-end when he was doing the top-end (when the engine was already out, thus the labor would've been far less)? (2) is the noise I'm hearing likely the rod bearings; and (3) is $4,800 about right for that job?

Thanks in advance,
Sean

Peter Zimmermann

Sean: I doubt if negligence, or rod bearings for that matter, play a part here. Let's analyze this: (1) Does the car ever get low on oil (off the dipstick)? (2) Did the car receive a follow-up valve adjustment about 1K miles after the top end job? (3) Have you over-revved the car? (4) Did the noise come on slowly and then become more apparent? Rod bearing failure is most often heard at about 2800 - 3000 rpm - not at high rpm. Please describe your "rattle". Is it someone knocking on the front door? Is it like something in the engine has come loose? Can you hear it by blipping the throttle while the car is at rest? Is it louder with a cold engine, or when it's at full temp? Some early VIN '78s had valve spring problems (they broke), so if you did not get all new springs when your top end valve job was done it might be that simple. Did a first class Porsche machine shop do your cylinder heads? Ask about the springs...
Pete

sschmerg

Peter:

Thanks for the reply. The place that did the work is considered a reputable Porsche shop. The car should've received all new springs...at least that's what they billed for. The car was over-revved, and that's why it needed the top-end done to begin with. Since then, it definitely has not been, I can tell you that! It is never low on oil either. The valves have been adjusted since the rebuild, if I recall, it probably was about 1k miles later.

I'm not sure how suddenly it came on, because I am not the only one that drives it, but I just noticed it one day. You have to be listening for it...it sounds almost like detonation...it only occurs at higher RPMS, at least 5k or above. The car feels like it's running fine until it gets up that high, where there's maybe a slight power loss. I do not think it occurs when the car is at idle, only under load, but I can't 100% confirm that since it's at the shop now. Somebody suggested the timing might be off? I am going to bring it to another shop for a second opinion.

Thanks,
Sean

Peter Zimmermann

Sean: If a car of your vintage is driven in traffic most days, maybe some short trips, etc., and it doesn't get to go to red line once a week or more it's quite possible that you have a bit of detonation. Occasionally we have to play with timing in CA, the cars run lousy on 76 & Mobil, but great on Chevron. If you use 2nd or 3rd gear and run it up to red line on three bursts in a row, do you still hear the noise?

OK, the car was over-revved, which means a piston hit a valve, yes? Were the rod bearings replaced as part of the top end (they can be replaced thru the cylinder holes)?
Pete

sschmerg

Peter:

The kind of use the car gets is occaisonal, but it usually gets to red line at least a few times when it is driven. Historically, it was not driven gingerly, which is how the over-revving happened in the first place (I was not driving it at the time!). I can give that a try when I pick up the car from these guys and see If I can still hear it. To my knowledge, the rod bearings were not replaced when the top end was done. Should they have been?

Thanks,
Sean

Peter Zimmermann

Any time that a piston contacts a valve the rods should be "checked for straight" and the bearings, which could have suffered, replaced. This is one of those "I'm there, why not do it" safety/longevity decisions. With 5K miles on the odo since engine work I would be blown away to learn that a rod bearing failed, especially on an engine that's never low on oil. If it was only 500 miles since the car was repaired I would be worried... let me know what they do with the timing.
Pete

3.2SLANTNOSE

Maybe whoever was driving it before, when they broke it, did it again.

pjc

Sorry to hear of your problem after committing a significant outlay on the engine. I guess the moral of the story is that having gone to the trouble of removing ther engine and carrying out a 'top end' the 'bottom end' is a cheap job and completes the work for 150,000 miles plus. Really, really surprised your wrench didn't try to pursuade you to go the distance, particularly after an over-rev event. Can't see you have any come back on the shop that did the work however - at the end of the day it is the owners decision on the amount of work carried out. Did you ask for a bottom end price at the time for example. Let's hope it is a minor problem that will not require another strip down

PJC

addictionms

you may want to read this thread

http://forums.pelicanparts.com/showt...hreadid=187324

over rev'd motors often suffer rod bolt failure due to stretching of the rod bolts

Jim

sschmerg

Thanks to all. I brought the car to another shop, and he says it may be a valve spring...not sure why one would've failed again so soon after the rebuild... He needs a bit more time to be sure, so I guess I'll find out soon.

-Sean

Peter Zimmermann

Sean: It's pretty easy to track down a broken valve spring, and they can be changed with the engine in the car (partial drop at the worst). Did you ever check your invoice to see if you were charged for "12" springs (they are sold as sets; 1 inner + 1 outer= 1 spring)? Also, be aware that springs can be set up wrong (to tight) - I hope that's not the problem/reason why a spring might be broken...
Pete

jet911

How high of RPMs constitutes over-revving? Also, I thought we have a rev limit built into the chip to protect against this happening. Is this something that was corrected on the 3.2s? I have an '86 Carrera and am curious - no issues, inquiring minds are looking to learn as much as possible about the limits of the car and motor.

TIA

sschmerg

Thanks Peter, I'll check the invoice.

Jim: From '84-on 911s are rev-limited by the DME. 911SC's, however, use CIS injection and do not use a computer to regulate engine management. Therefore, the '78 SC had no rev limiter (at least not that I could tell). You can easily over-rev any car though, even with a rev-limiter. Just miss a shift or downshift into too low a gear. In my car's case, the driver just exceeded the redline on acceleration and the thing just blew. He told me the smoke cloud was so large, somebody who lived in the area called the fire department!

-Sean

Bill Gregory

Not true. The 3.0L 911SC engines were rev limited by cutting off the fuel pump when the revs reached 6300-7000 rpm, depending on the model year. For 1978, the cutoff was 6700-7000 rpm.

r911

How about " Therefore, the '78 SC had no [quick acting] rev limiter"

To get more revs, you want to lighten the valves (as on the CS version), match or replace the valve springs, and replace the retainers (Ti is common here).