Modern "fractured" connecting rod use

[AndrisV]

Good evening!

A lot of engine builders around me state, that fractured connecting rods can't be used in any type engine out of factory specs, as those are made from chocolate, etc. Although, almost all manufacturers use this technology even for high reving engines now, including motorcycles and BMW M series engines. So now I'm a little bit confused, although, I always do my own researches and make my own conclusions before I state something.

So, spent whole evening googleing around for this. All manufacturers love this invention as rods come out faster and a lot cheaper. Most interesting part, most of them claims, that using powder metallurgy, rods come out also more durable and has more fatigue strength. Additionaly, there are a lot of usual problems resolved, big end cap shifting for example, and so on.

This leaves me with one conclusion, people are only mislead by fracture surface look, as it looks weak.

And now comes quoestion, in general, does it make sense to use fractured connecting rods in lightly tuned engine? I mean, at least polished or even shot-peened (not sure, how new materials will react to shot-peening), I supopse, it depends on particular manufacturer. At the moment got curious to measure HRC on few different examples.

For example, there is a very good engine for budget rallycar (let's say simple beginners engine) VW 1600 8 valver with crossflow head design and it already produces around 74 kW at the flywheel from factory. Problem is rod lenght, it's not typical VW 144mm or 159mm, it's 149mm and those are modern fractured connecting rods, so there are no cheap options - only special order/design rods. Keeping in mind, that it should be cheap engine, I would imagine it could produce around 90-110kW and not be revved sky-high, let's say no more then 7200-7500rpm and still could use original rods, althogh modified as usual.

Any thoughts, ideas?

[Guy Croft]

They've been used for years in outboard motors and whilst I've never explored using them(why should I?) I can certainly remark that insofar as material and suitablity for racing is concerned and whether_or_not they can be 'fractured (notched and split across the parting line) depends on the rod material.

Some materials can be fractured easily & some can't and a material that lends itself to the process doesn't 'per se' make it a suitable one for high rpm/load use - except to say that - if the material is relatively brittle and thus comparatively weak the rod strength can be enhanced (within limits of course) by making the at-risk sections thicker. Of course, up goes the weight..

Moreover, importantly, you cannot resize a fractured rod which makes overhaul at any interim stage quite impossible. This alone would rule out their use in a competition engine - unless they were so 'cheap' you could junk them, but that would invariably suggest they were unfit for duty in the first place.

The best rods - like Cunningham (maybe I think only Cunningham) are made from split forgings: the cap and rod upper section are formed as two separate parts. This gives optimum material grain flow on both parts. Thos rods hold their shape like no other. You have to ask yourself - what do I want from a race rod? There are rods out there I wouldn't use in a lawnmower and more and more folk are enthralled by the idea of 'cheap' race rods. No such thing. If it's cheap it's not a race rod. Not good.

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