M54. Let the foolishness begin.

[Wanganstyle]

The charts I posted have a good amount of trig in them to define the relationship between the rod and crank. Its honestly been a numer of years since I was really trying to dig into the subject, so I am a bit rusty. Here are some articles that I think are good at explaining the subject and the relations to harmonics better than I could ever rattle off from the top of my head:

piston motion basics

Crankshaft design

Crankshaft torsional absorbers (aka harmonic dampers)

I also like this paper on torsional dampers: here

Regarding the S54 damper, its method of attachment is different. Also the unit itself is tuned for different frequencies. The S54 is designed for higher revs where the M54 clearly is not. Even though the S54 can hit a much higer rpm, it is key to note you cannot spend a lot of time there as the system is not designed for that. Anyone who races an S54 with factory damper should be able to tell you that. This is the confusion most people have, the rev limit is for a maximum rpm NOT an rpm that should be included in normal operation. The same can be said for any engine rev limit though.

Discussion and being challenged is all part of learning. I wish I knew more than I do, but unfortunately its not the case. Im more than willing to have an intelligent discussion on the topic though! Id like to pull people out of the weeds who know more on the topic.


Digger, excellent post! Heaps of good info.


Chris, BMW consiously chose cast iron for the S54 block due to the higher cylinder pressures and other factors. I think part of that decision is the lack of volume needed to develop a proper alloy block to meet the needs. Unfortunately "better" a lot of times is cost based so who knows if its really the best for the application.

The reference to race teams is because that was the intended use of the motor. The US S50/2 certainly cannot say that and neither can the M54. Most classes allow brake and suspension upgrades, but very few engine upgrades.

The US S50/2 morors have the shafts shear as well. Its not only an M54 issue by any means.

I think I didn't make my point well. The reason I beought up the S54 is because it has similar curves. The S54 was designed more for the high revs and the oiling system and damping system reflect that. The S54 is leaps and bounds better than the M54, no question. Its foolish to think the bottom end is not subject to a lot of the same issues though. Looking at how the S54 dealt with the issues can give a good direction of how to solve the M54 issues.

I don't think anyone is saying the M54b30 is designed to be a high rev motor. If it was, there is no way BMW would use such an awful crank stroke with such short rods and horribly insufficient damping.

I think the biggest mistake with the M54 block from a performance perspective is it does not come with a bedplate or main cap girdle. The reasons they did not do this are obvious (cost) but it would have been really nice if they had done this.

If I was building a performance moror, it would be based off a smaller stroke than the 3.0L.

Motorsports DNA of the small 6 has always been iron engine based. Alloy is the new thing and the small 6 alloy blocks - i.e. N54. N55 and magnesium alloy blocks are HUGE compared to the older M54/m50/m20 based items.

S54 and S50b32 euro share 4 bolt damper mounting. Both have been extensively designed for motorsports; I personally did not have stress issues with S54 harmonic damper- This item has stresses when fitted in full racing applications with a multiple plate super light clutch. If fitted with a dual mass flywheel the DMF eats much shock. S54 damper does not commonly die in street applications, factory redline is 8k and longmotor can handle 8500. I've done this at high cost in my own street car for fun

[digger]

the 93.8mm crank used in the alpina is about 6-7lbs lighter than a M54 89.6mm crank the counterweights are much slimmer and no bigger in radius and it is a much nicer piece. it seems to come from the some manufacturer as the the other BMW cranks so not done by an aftermarket place.

i dont know what size rods are used or how heavy the reciprocating mass is but the revlimit is set at 7200rpm from what i saw for the B3S atleast. it appears compared to regular M54 some of these issues may have been improved and it is more capable of these rpms.

does anyone have pictures detailed the different harmonic balancer setups between M54 and S54? i am not familiar

[wazzu70]

The rev limit on the Alpina engines are still low (7000).

Iron is a superior block material in a "dumb casting" thats just a generic shape. Aluminim can be just as good with proper reinforcements. You guys keep talking like the aluminum block is a replica of the iron block, which its not.

The area supporting the main caps is significantly bigger and the block has a lot of webbing for strength. The alloy block is not the same in regards to stiffness, but it does not mean its WAY behind either.

I design stuff all the time that takes some crappy heavy iron casting and turn it into a cast aluminum piece that is much lighter and has better properties in every area. Its not that difficult, it just takes some thought and the right design priorities.

On the M54 Im sure weight reduction was a higher priority than having completely equivalent block stiffness. They only needed enough stiffness to meet the design goals for the intended operating range and use.

[wazzu70]

The charts I posted have a good amount of trig in them to define the relationship between the rod and crank. Its honestly been a numer of years since I was really trying to dig into the subject, so I am a bit rusty. Here are some articles that I think are good at explaining the subject and the relations to harmonics better than I could ever rattle off from the top of my head:

piston motion basics

Crankshaft design

Crankshaft torsional absorbers (aka harmonic dampers)

I also like this paper on torsional dampers: here

Regarding the S54 damper, its method of attachment is different. Also the unit itself is tuned for different frequencies. The S54 is designed for higher revs where the M54 clearly is not. Even though the S54 can hit a much higer rpm, it is key to note you cannot spend a lot of time there as the system is not designed for that. Anyone who races an S54 with factory damper should be able to tell you that. This is the confusion most people have, the rev limit is for a maximum rpm NOT an rpm that should be included in normal operation. The same can be said for any engine rev limit though.

Discussion and being challenged is all part of learning. I wish I knew more than I do, but unfortunately its not the case. Im more than willing to have an intelligent discussion on the topic though! Id like to pull people out of the weeds who know more on the topic.


Digger, excellent post! Heaps of good info.


Chris, BMW consiously chose cast iron for the S54 block due to the higher cylinder pressures and other factors. I think part of that decision is the lack of volume needed to develop a proper alloy block to meet the needs. Unfortunately "better" a lot of times is cost based so who knows if its really the best for the application.

The reference to race teams is because that was the intended use of the motor. The US S50/2 certainly cannot say that and neither can the M54. Most classes allow brake and suspension upgrades, but very few engine upgrades.

The US S50/2 morors have the shafts shear as well. Its not only an M54 issue by any means.

I think I didn't make my point well. The reason I beought up the S54 is because it has similar curves. The S54 was designed more for the high revs and the oiling system and damping system reflect that. The S54 is leaps and bounds better than the M54, no question. Its foolish to think the bottom end is not subject to a lot of the same issues though. Looking at how the S54 dealt with the issues can give a good direction of how to solve the M54 issues.

I don't think anyone is saying the M54b30 is designed to be a high rev motor. If it was, there is no way BMW would use such an awful crank stroke with such short rods and horribly insufficient damping.

I think the biggest mistake with the M54 block from a performance perspective is it does not come with a bedplate or main cap girdle. The reasons they did not do this are obvious (cost) but it would have been really nice if they had done this.

If I was building a performance moror, it would be based off a smaller stroke than the 3.0L.

[Wanganstyle]

Cool, thanks for responding.



More about this S54 damping system? Anything special?

I think the general information that's propagated is that S54s have no problem with revs, whereas M54B30s have a huge problem. So given that context, your graphs seemed contradictory to your assertion.

Gotcha. I was just pointing it out. We had another (but pointless) discussion recently where a bunch of people mixed up the orders of differentiation...

I don't think it would be THAT tough to calculate the intrinsic torque pattern on the crank... just a bunch of trig. Having said that, I am no practiced automotive engineer, I just think really hard about stuff. :) So if you insist that it's indicative in practice, I guess I can just take your word for it.

Ah yes compression/combustion torque. It would be nice if we could measure that directly, wouldn't it? ;)

I am very interested in the exact effects of rod ratio on combustion torque pattern. Any more pretty graphs? :) Seems that a lower RR would shift the torque pattern to the left and (maybe) compress it a bit.

Again, thanks for responding. I always like talking engine stuff, and it's rare to find someone who really knows their shit through and through and is willing to take the time to spill the beans.

Yes; S54 system is special.

4x bolts hold the damper to the crankshaft through a double sprocket turning key'ed pulley.
4x smaller bolts (S54 and s50b32 euro) vs 1x giant bolt of M54/S52/M50


the Alpina E46 B3.3 and B3.4 use a S52 engine that is stroked/bored to 3300cc and 3400cc.

These are the biggest OEM manufacture BMW small 6 engines I know of- the Alpina B3.3 and B3.4 use a special harmonic damper:




Alpina Engine Parts catalogue for 3.3 and 3.4L;

B3s 3.4 engine uses a 93.8mm stroke crank - compared to s54b32: 91mm crank

http://www.thealpinaregister.com/downloads/completecats/e46%20b3s.pdf

as far as mass/rotation/speeds/stress is concerned; the simple physics:


S54 rods are 137.5mm and HEAVIER than M54 135mm rods
S54 pistons are 87.0mm bore and heavier/thicker material than M54 84.0mm bore or s52 86.4mm bore pistons
S54 crankshaft is a heavy beast; heavier than S52/M54b30 units.

The S54 and S50b32 euro blocks are special TALL DECK BLOCKS. Taller than M54 or S52 blocks - they are TALL DECK IRON for a reason. MORE IRON for more ridgidity

When compared side to side you can see the difference - easy to measure also.

the Small 6 family of engine is still based off M20 DNA; Originally designed as an IRON block with enough strength to be run in DIESEL FORMAT; M21 and M57 diesels are based off petro blocks and have compatible spacing; Diesel cranks are used to make strokers often.

[digger]

what we have is the forced response of a system. if you force the system (i.e a force applied at certain frequency) at close to the natural frequency of the system the amplitude of displacements go up compared to the same force applied statically. the amplification or magnification depends on how much damping there is.

which damper has more damping in it, the S54 or M54?

The natural frequency of the system (crank, rods, pistons, flywheel etc etc) comes from the mass of components and stiffness of them. the S54 crank has less overlap between mains and rod jornals and was designed for higher rpm so it is probably a stiffer crank dont know about the mass. the mass of components in S54 may be lighter as well? if this is true the torsional natural frequency is higher and can therefore operate higher speed before resonance becomes as bad.

aluminium has roughly 1/3 stiffness of steel (i.e. elastic modulus) and 1/3 density so if the alloy block mass is lighter then it is highly likely less stiff if both iron and aluminium blocks are designed by equally competent people and the basic envelope is the same. the block stiffness is what supports the crank as it is not rigid in the main bearings. the mode that causes issue is probably a combination of torsion and bending (coupled mode)

the force comes from the pressures, friction and inertia forces. the forcing frequency comes from the rpm (or harmonics of it depending on firing order/crank configuration etc) so at a certain rpm the engine will be operating at the natural frequency of the drive train and this is where problems will be worst if the damping is not sufficient. it is important to note that there is quite a large spread of rpm where the ampltide of vibrations is still magnified due to operating close to the natural frequencies so the engine does not need to operate at the exact natural frequency of the drive train to cause problems

if the components were made from very very stiff materials and very light weight then there is no chance of resonance and the forces could essentially be assessed as a static system.

by looking at the acceleration of the piston you are only getting part insight into the force magnitude (still need mass i.e. F=ma) and then there is so much more to it to work out the actual response of the system. think mass, stiffness and damping of the entire system of components

[nando]

I would buy the argument more about the blocks, if the blocks were exactly the same dimensionally but of different material. This is not the case though. The aluminum cases have reinforcements cast in, and there are also reinforcements bolted in. The areas supporting the mains are very different. The blocks will differ structurally, however I highly doubt they are as different as people think. I would love to see testing on this though to know for sure.

Also, there are plenty of failures of the oil pump shaft on S50/S52 engines. Most people just say LOL my oil pump nut failed even though its still welded/wired to the pump gear.

that's exactly it. aluminum blocks are simply not as strong as steel/cast iron. they *have* to add extra gusseting, and it's still weaker.

BMW specifically chose cast iron for the S54 - the engines were developed in similar time frames. BMW could have cut a lot of weight with an aluminum S54, but the performance wouldn't have been there.

yeah, pro race teams had some trouble with the motor - but so what? it's a street motor. Pro race teams would also destroy stock brakes in a couple laps and chunk the factory tires in an afternoon practice section.

The S50 and S52 had the pump nuts back off. the M54 *shears* off the pump completely. a totally different issue. the S54 doesn't have oil pumps explode normally - some bearing issues on early motors, in heavy track use the crank dampener wears out, and a design flaw in the vanos tabs - but otherwise, it takes the stock 8000rpm redline in stride. The M54 can't even outrev an M20. I have my limiter set to 7400, I cringe to think of an M54 reving that high. :(

don't get me wrong, it's a nice motor - light, effecient, gobs of torque - it's just not a high RPM motor. It's like a modern ETA but with more breathing room, and twice as much power.

[Wh33lhop]

Cool, thanks for responding.

The S54 has a completely different damper system to deal with its harmonic issues. Even then, an S54 with factory damper cannot sustain high revs for long. The Grand Am teams that were forced to use to stock damper die to rules replaced them regularly as maintenance items as damper failures and then engine failures would be common. If you dig up some pics you will see massive cooling ducts there to help the damper live through the race without melting the elastic.

Bottom line, yes the S54 sees a lot of nasty forces working against it.

More about this S54 damping system? Anything special?

I think the general information that's propagated is that S54s have no problem with revs, whereas M54B30s have a huge problem. So given that context, your graphs seemed contradictory to your assertion.

Deceleration was the wrong word choice, correct. Your rate of acceleration decreases, then increases ect. I got lazy with my wording. It still is the major contribution to these cranks having issues and the crank is much happier if the acceleration curve does not have the dip.

Gotcha. I was just pointing it out. We had another (but pointless) discussion recently where a bunch of people mixed up the orders of differentiation...

Of course its not the pistons resonating. The piston motion as measured at the pin is incredibly indicative of what is going on in the system. Its also something you can easily calculate. Its not a gnats nuts assessment of what goes on, its an insight. There is pretty much no way to calculate an answer better than this without tons of specialized equipment which unfortunately I do not have access to.

I don't think it would be THAT tough to calculate the intrinsic torque pattern on the crank... just a bunch of trig. Having said that, I am no practiced automotive engineer, I just think really hard about stuff. :) So if you insist that it's indicative in practice, I guess I can just take your word for it.

Combustion torque is improtant to get a more accurate assessment, very true. Your rod ratio is going to give a lot of information on how that combustion torque acts on the crank, which is why the data I have is relevant and commonly used to investigate this type of problem.

Ah yes compression/combustion torque. It would be nice if we could measure that directly, wouldn't it? ;)

I am very interested in the exact effects of rod ratio on combustion torque pattern. Any more pretty graphs? :) Seems that a lower RR would shift the torque pattern to the left and (maybe) compress it a bit.

Again, thanks for responding. I always like talking engine stuff, and it's rare to find someone who really knows their shit through and through and is willing to take the time to spill the beans.

[wazzu70]

The S54 has a completely different damper system to deal with its harmonic issues. Even then, an S54 with factory damper cannot sustain high revs for long. The Grand Am teams that were forced to use to stock damper die to rules replaced them regularly as maintenance items as damper failures and then engine failures would be common. If you dig up some pics you will see massive cooling ducts there to help the damper live through the race without melting the elastic.

Bottom line, yes the S54 sees a lot of nasty forces working against it.


Deceleration was the wrong word choice, correct. Your rate of acceleration decreases, then increases ect. I got lazy with my wording. It still is the major contribution to these cranks having issues and the crank is much happier if the acceleration curve does not have the dip.

Of course its not the pistons resonating. The piston motion as measured at the pin is incredibly indicative of what is going on in the system. Its also something you can easily calculate. Its not a gnats nuts assessment of what goes on, its an insight. There is pretty much no way to calculate an answer better than this without tons of specialized equipment which unfortunately I do not have access to.

If you think everything has to be directly related to give you a very good answer, you have a lot of learning to do :) Very rarely is that data ever available....even in OEM designs. Correlation is king!

Combustion torque is improtant to get a more accurate assessment, very true. Your rod ratio is going to give a lot of information on how that combustion torque acts on the crank, which is why the data I have is relevant and commonly used to investigate this type of problem.

[Wh33lhop]

I see some inaccuracies here.

Here is some of the data for the M54 bottom end (S54 for comparison).

According to this graph, the B30 and S54 forces are near identical rev for rev, meaning the s54 will undergo much worse harmonic fuckery since it's seeing nearly 25% more revs, though some of those may be past the dreaded threshold of resonance.

The dip signifies the piston accelerating, decelerating, accelerating again and decelerating in a short time period. Compare that to a smooth motion and its easy to see why the crank does not like this!

That is not at all what it signifies. It's a graph of acceleration, not velocity. It doesn't stop accelerating in one direction just because the acceleration dips a tiny bit. Moreover, it is not the pistons that are resonating here, but the crank itself (yes they are connected but the pistons are just along for the ride of the crank resonance), therefore this graph is not directly relevant because piston acceleration and torque exerted on the crank are not directly related. Furthermore, and I could definitely be wrong here, but I was under the impression that it was combustion torque that was the problem, not just straight crank/piston intrinsic torque.

[wazzu70]

I would buy the argument more about the blocks, if the blocks were exactly the same dimensionally but of different material. This is not the case though. The aluminum cases have reinforcements cast in, and there are also reinforcements bolted in. The areas supporting the mains are very different. The blocks will differ structurally, however I highly doubt they are as different as people think. I would love to see testing on this though to know for sure.

Also, there are plenty of failures of the oil pump shaft on S50/S52 engines. Most people just say LOL my oil pump nut failed even though its still welded/wired to the pump gear.

[nando]

There isn't any mystery where the harmonics come from at all...its just basic math. The rod ratio using the S52b32/S54b30 crank with 135mm rods is the major cause, its one of the worst options available from BMW actually.

The issue with the oil pump shaft/oil pump nut is from sustained use at high RPM. If you aren't constantly at the top of the rev range you shouldnt have issues with factory hardware.

I am not sure why there keeps being mention of cams in regard to harmonics of the bottom end, its not a contributing factor in any significant way.


Here is some of the data for the M54 bottom end (S54 for comparison). You mainly want to minimise the amplitude of the orders, and you want the acceleration trace to have a flat top instead of a dip. The dip signifies the piston accelerating, decelerating, accelerating again and decelerating in a short time period. Compare that to a smooth motion and its easy to see why the crank does not like this!

if that were the only reason, the S54, S50 and S52 would all have crank harmonic issues - but they don't.

The reality is, aluminum blocks are weaker. The cranks, rods and pistons may be of the same design and material, but the aluminum block is what allows the additional flexure in the crank.

as far as cams, they are being mentioned because typically they allow you to make more power at a higher RPM, which doesn't work if the bottom end grenades above ~6200.

[Wanganstyle]

There isn't any mystery where the harmonics come from at all...its just basic math. The rod ratio using the S52b32/S54b30 crank with 135mm rods is the major cause, its one of the worst options available from BMW actually.

The issue with the oil pump shaft/oil pump nut is from sustained use at high RPM. If you aren't constantly at the top of the rev range you shouldnt have issues with factory hardware.

I am not sure why there keeps being mention of cams in regard to harmonics of the bottom end, its not a contributing factor in any significant way.


Here is some of the data for the M54 bottom end (S54 for comparison). You mainly want to minimise the amplitude of the orders, and you want the acceleration trace to have a flat top instead of a dip. The dip signifies the piston accelerating, decelerating, accelerating again and decelerating in a short time period. Compare that to a smooth motion and its easy to see why the crank does not like this!

From your posted charts the s54 engine has higher velocities and stresses it seems.

What makes it a fair comparison? M54 and s54 are not in the same league of construction although both crankshafts are forged.

Ive spun s54 happily to 8400 rpm and it never had issues with harmonics or oil pump.

Sent from my GT-N7100 using Tapatalk 2

[wazzu70]

There isn't any mystery where the harmonics come from at all...its just basic math. The rod ratio using the S52b32/S54b30 crank with 135mm rods is the major cause, its one of the worst options available from BMW actually.

The issue with the oil pump shaft/oil pump nut is from sustained use at high RPM. If you aren't constantly at the top of the rev range you shouldnt have issues with factory hardware.

I am not sure why there keeps being mention of cams in regard to harmonics of the bottom end, its not a contributing factor in any significant way.


Here is some of the data for the M54 bottom end (S54 for comparison). You mainly want to minimise the amplitude of the orders, and you want the acceleration trace to have a flat top instead of a dip. The dip signifies the piston accelerating, decelerating, accelerating again and decelerating in a short time period. Compare that to a smooth motion and its easy to see why the crank does not like this!

[Wanganstyle]

Major kudos in answering my questions.

Lovely to see these being in the same price category as s52's. Fantastic.

M54b30 is much more common than S52: It should be more avail and less money than a S52b32 setup-

M54b30:

e39 530i
e46 330i

Not a high rpm engine for those on a budget; its costs to modd up to race spec are high. S54 drop in as is would be the high RPM engine of choice - the weight can be handled with suspension tuning.