135mm rods

[slow95tsi]

I need a set of 135mm rods. What will be the best set to get on a budget? As far as lightweight and strong, going in a m50

[Jszy25]

Is this for an NA engine or are you planning on going forced induction?

[slow95tsi]

NA gonna do a high compression 2.8

[Leo_328i]

Best ones are either the M52 B28 ones which are forged or the the M54 B30 ones which are slightly lighter but also slightly weaker, they are fracture split.

[nando]

ALL RODS ARE FORGED!!!

;)

S52 rods are a little bit lighter than M52 rods, but either would be fine.

[10Toes]

I'm using the M52b28 rods on my 2.8 stroker.

[Jszy25]

NA gonna do a high compression 2.8

You said budget and you said NA...I'd stick with stock rods. Possibly pick up a used set in good condition for dirt cheap.

[Leo_328i]

ALL RODS ARE FORGED!!!

;)

S52 rods are a little bit lighter than M52 rods, but either would be fine.

Nando,

You are 100% correct. All rods are forged - I too hate it when people for some reason think that some connecting rods are cast instead of forged....

However, not all forging is the same. M50, M50TU, M52 and M52TU all use conventionally forged rods but the M54 uses 'fracture split' forged rods which is a mildly compromised style of manufacturing that saves on costs.

Not saying that M54 rods are weak, its just that they are not as strong as M52 ones.

Proof here:

http://www.forging.org/FIERF/pdf/FatigueBehavior.pdf

Skip to the summary and read the conclusions. When the writer is refering to powder forging, he is referring to what BMW calls 'fracture splitting'.

One key finding is that traditionally forged rods were found to have 37% higher fatigue strength. I am not an engine builder or a physics student but that to me sounds like the traditionally forged rod is the way to go.

[Bimmerman325i]

Nando,

You are 100% correct. All rods are forged - I too hate it when people for some reason think that some connecting rods are cast instead of forged....

However, not all forging is the same. M50, M50TU, M52 and M52TU all use conventionally forged rods but the M54 uses 'fracture split' forged rods which is a mildly compromised style of manufacturing that saves on costs.

One key finding is that traditionally forged rods were found to have 37% higher fatigue strength. I am not an engine builder or a physics student but that to me sounds like the traditionally forged rod is the way to go.

Are M52 rods stronger? Certainly, but does it matter?

Just before people start freaking out about misquoted engineering principles, a bit of perspective is in order.

Certain materials, like steel, have a fatigue limit. Below this limit, the material can cycle between its max/min stresses indefinitely. If it is designed improperly, the loading conditions will not always stay below the strength limit, and will eventually fail.

Certain other materials, such as aluminum, do NOT have a fatigue limit. They will eventually fatigue and fail, and there is nothing that can be done from a design perspective to stop this from happening; it's a material property.

However, the number of cycles is typically in the tens of millions, and there are significant safety factors on the relevant parts so that the calculated fatigue limits or #cycles to failure are conservative.

Main point: don't worry about fatiguing rods in an NA engine. More strength doesn't necessarily mean more better.

S5x engines with original rods are known to go for 250k+ miles without issue. You will NOT have an issue with any oem rod.

[RobertK]

Don't forget you can also use M50 non vanos rods which are thicker but can't say from experience if that makes them any stronger. I do know that the turbo guys used to prefer them but they are heavier.

That said I have 5 good M52 rods and a full set of NV M50 rods if you need them.

[Wanganstyle]

for a NA build, pick the Lightest Oem rods.

street engine builds of M50 family dont get into the RPM range to stress an OEM rod to break.

the Fracture split rods are very light and cap fit to support bearing structure is PERFECT..... I don't see NA engines killing rods- just valves and rod bearings.

[Leo_328i]

Thanks for the correction guys.

I was always under the impression that the higher strength forged M52 rod would be a better choice for high revs due to its increased strength with only very slight increase in weight.