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      07-07-2017, 03:55 PM   #1
The Wind Breezes
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Rod bearings in N54 / N55 vs S85 / S65

So I did some calculations and it turns out the mean piston speed in a N54 / N55 is very close to the S65 / S85, as those M-engines trade stroke for RPM.

S65: stroke 75.2 mm rev limit 8,400 RPM
MPS = (2* (75.2/1000) * 8400)/60 = 21.05 m/s

N55: stroke 89.6 mm rev limit 7,000 RPM
MPS = (2*(89.6/1000) * 7000)/60 = 20.90 m/s

So why is there all this hubbub about rod bearings in higher mileage S65 / S85 engines, but not as much discussion for our engines? I will provide a few guesses off the top of my head, but I'm looking for informed opinions as mine are only amateur speculation:

1) S65 / S85 have been around for longer, so more examples have reached high mileage (maybe really marginally longer in the case of the S65 vs N54, though...actually, which was out first?)
2) On average, owners of S65 / S85 cars are probably more likely to do track days or other high impact use
3) S65 / S85 need to be revved high to make power so they encourage use at high RPM, N54 / N55 don't encourage this as much
4) Maybe S65 / S85 pistons are slightly heavier as a result of the larger bore? I have no idea if this is the case or not. What about the weight of the rods?
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      07-07-2017, 04:11 PM   #2
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Interesting. I believe the 7k rev limiter on the N54/55 is set due to CFM limitations of the turbo. It's possible that turbos with efficient high rev flow could run to an 8k limit with the bore stroke, but then would see as many or more bearing failures?

Probably even more issues with HPFP and lean conditions leading to popped pistons and worn rectangrings.

I haven't seen any weight specs on pistons/rods/crank/camshaft/sleeves etc for N54/N55/S65. Thoughts on the weight of the pistons alone, the S65 could be heavier, but then there's detail on the amount of material on the piston skirts, so they could even be lighter! Anyway...
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      07-07-2017, 04:15 PM   #3
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Quote:
Originally Posted by ShocknAwe View Post
Interesting. I believe the 7k rev limiter on the N54/55 is set due to CFM limitations of the turbo. It's possible that turbos with efficient high rev flow could run to an 8k limit with the bore stroke, but then would see as many or more bearing failures?
Absolutely! For shits and giggles the car could physically stand to rev higher until you run into things like valve float and injector window limitations (not sure where those come into play for this car as it's not a VW or Honda so people don't do that kind of stuff for the most part) but as the stresses increase exponentially with increased RPM reliability would decrease fast the higher you raise it, and not just the bearings! Who knows what would fail first in that scenario. And of course with stock turbo it would make less and less power as you go up, due to flow limits.
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      07-07-2017, 04:46 PM   #4
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Quote:
Originally Posted by The Wind Breezes View Post
Absolutely! For shits and giggles the car could physically stand to rev higher until you run into things like valve float and injector window limitations (not sure where those come into play for this car as it's not a VW or Honda so people don't do that kind of stuff for the most part) but as the stresses increase exponentially with increased RPM reliability would decrease fast the higher you raise it, and not just the bearings! Who knows what would fail first in that scenario. And of course with stock turbo it would make less and less power as you go up, due to flow limits.
Well. So at least on the N54 the high rev airflow limitations have been resolved with replacement of the front and rear turbo inlet pipes and using slightly larger turbos, less heat, more flow. Small amount of E85 to drop combustion temps helps.

I've seen some built N54s with raised rev limiters. It's starting to come along. I'm not sure there's anyone out there that wants to try a 7500-8k rev limiter without reinforcing the bottom end. The crank on the I6 is too long compared to the V8

Couple threads on BB for you (replace the **):
http://www.***********.com/showthrea...-N54-to-9k-rpm
http://www.***********.com/showthrea...top-end-recipe
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      07-07-2017, 05:39 PM   #5
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While engine speed is a factor, cylinder pressure is the main design factor when it comes to rod bearings. With the N54/55 being turbo engines, engineers knew that these bearings would typically be exposed to higher loads from the higher cylinder pressures of turbo engines. Because of this, the rod bearings were most likely designed with a more conservative safety factor. What high engine speed really causes problems with is wear at the piston ring/cylinder wall interface.
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      07-07-2017, 05:45 PM   #6
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Wow that's fascinating on those BB threads. Interesting info in both, but the first really got shitted up hard, so I'd need someone to tell me if he actually reached 9K lol.

Quote:
Originally Posted by BimmerAg View Post
engineers knew that these bearings would typically be exposed to higher loads from the higher cylinder pressures
Very interesting, but you'd figure safety margins with respect to stock power would be similar between the M and N engines...if anything you might expect the M series to have more margin! Of course, totally different deal if the engineers expect everyone to flash tune the turbos and increase torque by 30-50% or more...perhaps that's what's going on with our motors?
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      07-07-2017, 05:55 PM   #7
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Thought it was well documented the s65 and s85 bearing issues are down to a design problem. Too tight a clearance causing the oil not to film properly in the bearing, this is compounded by the use of 60 weight oil vs 30/40 weight on the n54/55.
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      07-07-2017, 05:56 PM   #8
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Quote:
Originally Posted by The Wind Breezes View Post
Of course, totally different deal if the engineers expect everyone to flash tune the turbos and increase torque by 30-50% or more...perhaps that's what's going on with our motors?
Yeah that's a possibility...heck, BMW themselves released the PPK flash tune so they obviously knew there was plenty of margin to work with!
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      07-07-2017, 06:08 PM   #9
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Quote:
Originally Posted by titium View Post
Thought it was well documented
More info / source on this please!
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      07-07-2017, 06:14 PM   #10
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Quote:
Originally Posted by The Wind Breezes
Quote:
Originally Posted by titium View Post
Thought it was well documented
More info / source on this please!
http://www.m3post.com/forums/showthread.php?t=892838

one example but a Google search brings up heaps of results. Can't trust everything online but the general vibe is that there is an inherent issue with the s65 and s85 motors.
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      07-07-2017, 07:03 PM   #11
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Quote:
Originally Posted by titium View Post
That's amazing, thanks for providing that link. Although it doesn't really address the S85's issues, it's interesting to note that the S85 has a more favorable journal size to clearance ratio.

I wonder how many people from the S65 design team got fired?
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      07-07-2017, 09:59 PM   #12
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There is way way way way more to the failure of s65 and s85 bushings than mean piston speed.

For the love of god please don't spread paranoia about rod bearings on the n5x. I had enough of it when I had my e90 m3!
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      07-07-2017, 10:27 PM   #13
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Quote:
Originally Posted by WhatsADSM View Post
There is way way way way more to the failure of s65 and s85 bushings than mean piston speed.

For the love of god please don't spread paranoia about rod bearings on the n5x. I had enough of it when I had my e90 m3!
Do a quick google search. N55 rod failures are common enough to make me worry
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      07-07-2017, 10:47 PM   #14
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Quote:
Originally Posted by WhatsADSM View Post
There is way way way way more to the failure of s65 and s85 bushings than mean piston speed.

For the love of god please don't spread paranoia about rod bearings on the n5x. I had enough of it when I had my e90 m3!
Look at the 2 posts directly above yours. As a corollary, consider not replying to threads like this with that kind of comment without reading the whole thing.
Quote:
Originally Posted by HyeWarrior View Post
Do a quick google search. N55 rod failures are common enough to make me worry
I did a quick google search and didn't find much. I've seen a couple spun bearings and freak rod issues. The rods can bend with bad tuning, the bearing issues may be a freak accident or oiling related, maybe due to the pump sucking air under hard cornering with sticky tires. After all the car does have a wet sump. I don't think those are common failures on the N55, though.
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      07-07-2017, 11:51 PM   #15
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Quote:
Originally Posted by The Wind Breezes
I did a quick google search and didn't find much. I've seen a couple spun bearings and freak rod issues. The rods can bend with bad tuning, the bearing issues may be a freak accident or oiling related, maybe due to the pump sucking air under hard cornering with sticky tires. After all the car does have a wet sump. I don't think those are common failures on the N55, though.
oil pickup is so low on the n54/55 I recon you'd need to be a good driver on sticky r comp tyres for oil starvation to be an issue in the corners.

I personally don't think n5x bearing failures are any more common then any other make or model (Google honda rod bearing or suburu rod bearing and you probably get more results)

biggest cause of bearing failures is oil starvation running the car with next to no oil or oil that's the wrong grade/type and/or passed its use by date. 25k km intervals and a mechanic that says he changed the oil but really didn't is a quick way to kill a motor. Stick to a decent interval, change the oil yourself with the correct grade and type and you'll be fine.
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      07-08-2017, 11:02 AM   #16
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Quote:
Originally Posted by The Wind Breezes View Post
Look at the 2 posts directly above yours. As a corollary, consider not replying to threads like this with that kind of comment without reading the whole thing.
?? I read that thread on m3post years before you even realized it existed. In fact I have posted 13 times in it years ago as well. So there is a bit of irony about you telling me to read threads. As I said there is far more to rod bearing failure on the s65 than mean piston speed. In fact there still is no consensus about the exact reason within the M community.

Quote:
Originally Posted by The Wind Breezes
I did a quick google search and didn't find much. I've seen a couple spun bearings and freak rod issues. The rods can bend with bad tuning, the bearing issues may be a freak accident or oiling related, maybe due to the pump sucking air under hard cornering with sticky tires. After all the car does have a wet sump. I don't think those are common failures on the N55, though.
I do agree with this. Remember there are far fewer cars sold with s65 engines than those sold with N5x. I would guess an order of magnitude. However there were fairly regular reports of an engine dying due to spun bearings (mostly rod but also some main). Usually would see one once a week on average .

However if you really look at the failure rate of the s65 due to spun bearings it was still fairly low, like under 3%. So while there certainly is an issue with those bearings it wasn't as bad as people made it out to be.

In a nutshell, many many times more N5x sold than s65. However forums/Google searches yield less results about beasring failure in N5x. In the end s65 failure rate was still pretty low, ergo N5x rod bearing failure rate is way less than that. So much so I would suggest it is a non issue, and is on the same order of any other failure mode of the engine like a dead oil pump or a snapped rod or something. It's a piece of machinery after all and it's not flawless.

Last edited by WhatsADSM; 07-08-2017 at 11:07 AM.
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      07-10-2017, 09:33 AM   #17
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Good thing to keep in mind is that these cars CAN suffer from oil starvation issues on the track. Off the top of my head I believe most of the major failures have either been linked to oil starvation, or appallingly bad tuning. A couple due to injectors leaking A LOT over a long time to dilute the oil. A few with guys trying to push 800+ on stock internals. Shit happens. Don't think I'd get all bent over it.
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