Money shift to 9000+
- Thread starter Shakespeares
- Start date
Shakespeares
Member
the cam seems alright. no scratch I can see there. I usually only rebuild an engine after it exploded (just got a S65 done).RedUn said:
I am as worried as anybody about the state of our engines and I treat mine very carefully when warming up (well all the time actually) but I wonder if we are over-egging the catastrophe here. If the engine over revved to 9000 for a micro second then this is likely to be within tolerance for BMW (not for extended periods granted) on the basis that redlines are always slightly lower than “burst pressure”. I had a bike that went over the line (at circa 11000 - stuck throttle) and it was fine. In this case, If the oil was hot, the extra few thousands revs would not have done the big end bearings any more harm than more than 4000 rpm when cold and we all know that we can’t say for sure that our cars weren’t treated like this when young. That’s why some of us change bearings. If it’s a spring then there will be a noise. And I can’t see cam or bore scoring happening. But if it has then the latter should produce smoke at some point. What I am saying that, while I would not like this to happen to my engine either, I would probably do no more than change the oil, get it assessed for particles, and listen to the engine carefully for anything that wasn’t there before. And I sense the chances of damage are very low. My NSX suddenly started to have a big end rattle. The forum sensed doom as it was lightly used. “Buy a high miler”….I had the oil assessed. The oil was fine. I was told I was looking at £15000 bill but it turned out to be an a/c compressor bearing. Not the same thing obviously as here but the oil assessment gave me some succour that it wasn’t all doom. And thankfully it wasn’t.
Very true Ussel but then if they did then damage would be obvious. There would be a bent valve somewhere and no compression. I don’t know about ZMs per se but don’t most cars have rev limiters that cut ignition or does this not work in this instance as we are dealing with immediate kinetic energy? I think I have just answered my point.
The damage might not be obvious - at low rpms a snapped valve spring isn't necessarily going to effect running, only potentially at high rpms when there isn't enough force to pull the valve closed.
The big thing to consider here is that the S54 is still up near the top of the list of engines with the highest piston speeds as a function of it's stroke. Putting that into perspective, the piston speeds of an S54 are higher at 8000rpm than a 991 GT3RS at 9000rpm..(!).
Arbitrarily assuming the over-rev was just to 9krpm, that equates to a 12% increase over the 8000rpm limiter; that's a potentially huge increase in piston speeds, and corresponding compression/expansion forces on the conrods and bearings.
If it were my engine, I'd be doing the following;
1. Borescope & Compression check
2. Take the cam cover off, check the valve shims are in spec and there is no evidence of spat shims, snapped valve springs, or scoring of the Followers/cam lobes,
3. Check the state of the Rod bearings and replace them whilst it's apart.
4. Visual inspection (basic) of the rods to see if any evidence of anything bent - unlikely but whilst you're there...
That's probably what £2k of work, but imo worth doing - the rest will be what it is.
The big thing to consider here is that the S54 is still up near the top of the list of engines with the highest piston speeds as a function of it's stroke. Putting that into perspective, the piston speeds of an S54 are higher at 8000rpm than a 991 GT3RS at 9000rpm..(!).
Arbitrarily assuming the over-rev was just to 9krpm, that equates to a 12% increase over the 8000rpm limiter; that's a potentially huge increase in piston speeds, and corresponding compression/expansion forces on the conrods and bearings.
If it were my engine, I'd be doing the following;
1. Borescope & Compression check
2. Take the cam cover off, check the valve shims are in spec and there is no evidence of spat shims, snapped valve springs, or scoring of the Followers/cam lobes,
3. Check the state of the Rod bearings and replace them whilst it's apart.
4. Visual inspection (basic) of the rods to see if any evidence of anything bent - unlikely but whilst you're there...
That's probably what £2k of work, but imo worth doing - the rest will be what it is.