Looking for tips on S52 engine post-failure analysis

[93supercoupe]

i just dont see harmonics causing a valve to drop. i do, how ever, believe you have a crank walk issue. If your AC was messed up enough to make that kind of vibration, then it wouldnt take long to wipe out some bearings. BUt even still, i dont see anything like that dropping a valve..

I like the idea of a sheared woodruff key. I thought thats what i was going to find on my messed up m20, but it wasnt the issue.

[93supercoupe]

do s52 share the same key way for the timing chain sprocket and crank pulley/balancer? Or possibly setup like a m20 where the crank pulley fruns off the belt sprocket? because then the AC vibration seems like more of the culprit.

[wazzu70]

^ I agree with your whole post except this part.

The light flywheel does not change the balance of the rotating assembly, which is all i said in my post. If you balanced the rotating assembly with a stock flywheel and a light flywheel it would balance the same. Perhaps you could have worded your first post better...

It certainly affects the harmonics and torsional vibration and i even said that in my post that is "not correct in the slightest"

If you balanced the bottom end with bob weights with the factory flywheel and replaced it with a lightweight flywheel nothing would change? You would not want to change the damper?

Of course it impacts the balance. It has a very large impact.

[wazzu70]

i just dont see harmonics causing a valve to drop. i do, how ever, believe you have a crank walk issue. If your AC was messed up enough to make that kind of vibration, then it wouldnt take long to wipe out some bearings. BUt even still, i dont see anything like that dropping a valve..

I like the idea of a sheared woodruff key. I thought thats what i was going to find on my messed up m20, but it wasnt the issue.

How do you know a valve dropped? Wouldn't he have seen that with a boroscope? Seems a dropped valve was more speculation than a known.

[LJ851]

If you balanced the bottom end with bob weights with the factory flywheel and replaced it with a lightweight flywheel nothing would change? You would not want to change the damper?

Of course it impacts the balance. It has a very large impact.

The fact that you mention bob weights regarding an inline 6 tells me your head is up your ass.

You do not understand what balancing an engine is if you think lightening the flywheel has an effect on the balance. I'm not repeating internet hearsay, i am speaking from experience building many engines.

The harmful torsional vibrations that occur in an inline 6 have nothing to do with crank balance. However, more flywheel mass (or a dampened dual mass flywheel) and a well tuned damper can go a long way toward dissipating the destructive vibrations that certain rpms generate in the crank.



OP if you want to know what has happened, take your engine apart. Guessing isn't going to fix it.

[wazzu70]

The fact that you mention bob weights regarding an inline 6 tells me your head is up your ass.

You do not understand what balancing an engine is if you think lightening the flywheel has an effect on the balance. I'm not repeating internet hearsay, i am speaking from experience building many engines.

The harmful torsional vibrations that occur in an inline 6 have nothing to do with crank balance. However, more flywheel mass (or a dampened dual mass flywheel) and a well tuned damper can go a long way toward dissipating the destructive vibrations that certain rpms generate in the crank.



OP if you want to know what has happened, take your engine apart. Guessing isn't going to fix it.

I stand corrected on the flywheel issue. It impacts TV but not balance like Lorrin said.


Also as mentioned, bobweights are not used on inline engines.


Looks like today was not my day :) Sorry for the misinformstion, and thanks for the corrections.

[93supercoupe]

i think we are assuming that the flywheel its self is balanced correctly. What if its not?

[mda185]

How do you know a valve dropped? Wouldn't he have seen that with a boroscope? Seems a dropped valve was more speculation than a known.

I don't know a valve dropped yet. I only know there was metal on metal contact and lots of aluminum paste in cylinder number 1 from whatever was smashed. I have not had time to pull engine and start tear down with Labor Day holiday and family plans. Agree with other post that says I need to stop speculating and get to work on the tear down.

[mda185]

The harmful torsional vibrations that occur in an inline 6 have nothing to do with crank balance. However, more flywheel mass (or a dampened dual mass flywheel) and a well tuned damper can go a long way toward dissipating the destructive vibrations that certain rpms generate in the crank.

OP if you want to know what has happened, take your engine apart. Guessing isn't going to fix it.

Agree I need to take engine apart. Finding time is the challenge. I am going to prep another engine to put in the car before I tear this one down. (I have another S52 in the garage that was intended for my E36 vert.) This was my daily driver and while I do have 2 other bimmers, both have high miles and need some maintenance. I try to have 2 in good running condition at all times. I DIY everything.

I will add a little bit more to the discussion about crank harmonics. I actually have some experience measuring torsional vibration in helicopter drivetrains for Navy applications. Not everyone contributing to this thread seems to understand exactly what they are.

Tutorial time:

The crankshaft is not a rigid part. It can and does bend and twist under load. When it twists, it reacts like a spring and twists back and forth until the force causing it goes away or the engine hits an rpm range where the twisting oscillation is damped out naturally. This is torsional vibration. The crankshaft will have some resonant frequencies where the amount of oscillating twist can be particularly bad. The frequencies are directly related to engine rpm and the length of the crankshaft. At a resonant frequency, it takes much less force to excite a vibration mode.

These twisting vibration modes travel down the length of the driveshaft and depending on what is attached to the ends, the vibration mode may be literally reflected back down the other way. This is what leads to destructive resonant modes in the axial direction. Remember that at the same time, the connecting rods are trying to bend the crankshaft and this just adds to the problem. The length of the stroke on the S52 makes this problem worse than on shorter stroke variants like the M50/M52. The harmonic damper and dual mass flywheel are designed to absorb some of these twisting vibration modes instead of reflecting them back down the length of the driveshaft and that is how they contribute to damping out critical vibration modes. A heavy flywheel also contributes to damping out these vibrations.

Balancing an engine has very little to do with controlling this kind of vibration other than reducing the force on the crankshaft from connecting rods and pistons if lighter than stock parts are used. It is a good idea to make each rod and piston assembly weigh the same and this is part of balancing an engine. Balancing rotating parts like flywheel and clutch assembly will not dampen out this kind of vibration but it will keep these parts from contributing their own vibration forces to what is already going on.

My speculation right now is the a failing A/C compressor caused forces to act on the end of the crankshaft that excited some resonant torsional vibration modes and the lightweight flywheel was not capable of helping dampen out this vibration. Maybe the harmonic damper was also a part of the problem as it has high miles on it. What is not clear to me, is how this torsional vibration caused something to hit the piston or pistons. I suppose if the vibration was bad enough, the piston may have been too high in the cylinder bore and it hit a valve. the weird part is that it does not seem to have failed until I shut down and did a warm restart. Another theory is that the vibration was bad enough to cause a valve keeper to fail.

Thanks for all the comments so far. I always thought this forum had a better body of technical expertise and you are proving me right.

[LJ851]

If your crank has noticeable end float, then you have

1) a broken crank or

2) a bad main bearing, the one with the thrust surfaces on it. Was your clutch acting weird?

[wazzu70]

Good tutorial! As Lorin mentioned I was mixing balancing with torsional vibrations which is incorrect. Your summary does a good job explaining which is which to help others from repeating my same mistakes :)

[mda185]

If your crank has noticeable end float, then you have

1) a broken crank or

2) a bad main bearing, the one with the thrust surfaces on it. Was your clutch acting weird?

I won't be able to answer that until I get the engine on a stand. There is nothing I can detect with it installed when I push and pull on the harmonic balancer. The clutch was not acting weird prior to the failure and felt fine when I drove it out of the inspection bay after the failure.

[Wanganstyle]

Thrust position main bearing
+
Non standard vibrations from aftermarket flywheels/heavy duty clutches

= not happy in 2 well known engines: 4g63 (DSM) & 2jzgte (supra twin turbo mk4)

Both will have crankwalk at extreme; the thrust position bearings can spin in a 2jz.

The metal inside cylinder 1 would not be a valve; when heads from valves drop the wealth is shared among the cylinders and you would have pieces of broken scary in other cylinders (been there done that cost me alot of pain)

Recently I inspected a s52 torn down to naked with similar issues and came to an interesting conclusion; the pistons all left alu in the iron bores; all worn and slightly smeared.

Prior owner had:

Supercharged+overboost+possibly wrong chip map and removed s52 to sell and replace with stock non super charged setup


Sent from my GT-N7100 using Tapatalk 2

[mpowerful]

You mention it was a hot day. I wonder if its possible it knocked its way to failure.

[wazzu70]

The engines with dropped valves I have seen at the racetrack have massive destruction which would be obvious. The also impact multiple cylinders as the debris from one cylinder destructing gets ingested by the others through the intake. If a valve dropped at idle, it could be a lot less severe....but I still doubt that is the problem here.

My guess is the deposits in #1 are from the piston motion mashing it against the wall due to torsional vibrations.

Until its taken apart, we can only speculate. Speculation isn't really helpful at this point...but it is fun.