Another myth exploded?

Blue · Post by **Blue** » Mon Dec 17, 07 6:57 pm

This is interesting reading...

http://board.moparts.org/ubbthreads/sho ... ost4026803

latil · Post by **latil** » Mon Dec 17, 07 7:28 pm

They haven't worked in bike engines on the street for about the last 60 years either.

Never have seen the problem.

Kev · Post by **Kev** » Mon Dec 17, 07 9:24 pm

Cool read. Why do Fuel Teams swap out their rods so frequently? Is it due to the environment they have to live in?

GTXJim · Post by **GTXJim** » Mon Dec 17, 07 10:25 pm

Many moons ago I had bought some second hand ally rods, and run them in me race aspen until I cracked the block (left plain water in it over winter) then used them in my street moter in the GTX and drove it about no problems

Anonymous · Post by **Anonymous** » Tue Dec 18, 07 7:34 pm

Kev wrote:Cool read. Why do Fuel Teams swap out their rods so frequently? Is it due to the environment they have to live in?

Because they bend, after several runs in a fuel motor you start to see the maybe one or two rods starting to bend. At that stage it is safer to change the lot.

This is all based on the materials theory surrounding aluminium. Steel follows an elastic strain path under the yield strength of the particualr material. As long as that yield strength is not surpassed then the material should be able to withstand those strains forever. This is quite old theory by the way, recent research on creep and fatigue stress in steel have shown a slightly different story.

Aluminium however, does not have a yield point as with ferrous metals. The stress / strain curve follows an altogether more smooth path. As a result a stress resultant called the 0.1% proof stress is used to caculate the yield stress limits of the material. But aluminium even then does not follow this in a linear fashion, requiring a further safety margin. Again this is quite old theory and recent aluminium alloys display much better properties.

http://en.wikipedia.org/wiki/Stress-strain_curve

But that is probably where the myth comes from.

Anonymous · Post by **Anonymous** » Wed Dec 19, 07 4:22 am

Well, they are lighter for the same strength and any move to reduce reciprocating mass is good. The trade off on original thinking is that they need frequent inspection and changing.

Recent 7000 series alloys using forging methods have resulted in aluminium parts displaying similar strengths to steel. I have no experience of using these in engines where dynamic forces are extreme and sometimes very hard to predict. But they have been most succesful in chassis structures where a few years ago only a steel part might have been used.

The guy to ask about this would be a very clever F1 or Nascar engine designer. Of which I am neither.

Kev · Post by **Kev** » Wed Dec 19, 07 10:55 am

The guy to ask about this would be a very clever F1 or Nascar engine designer. Of which I am neither.

Yet!

Ivor · Post by **Ivor** » Thu Dec 20, 07 1:02 pm

I would be concerned with the effects of work hardening on an aluminium or aluminium alloy con rod, particularly where there are high combustion temperatures present such as a gassed engine.

However, work hardening can be beneficial in some cases, such as when aluminium is forged, also some aluminium alloys are heat treatable.

One of the most commonly used aluminium alloys in racing is 7075 (I think) and due to it's strength and stiffness, I know it can be used safely because I had a flywheel made from the stuff in my old BRM motor and that revved to about 7,500 and I've still got both legs.

Sorry to be an old school fart but I'll stick with forged 4340 steel rods!

Anonymous · Post by **Anonymous** » Sat Dec 22, 07 11:58 am

Nice one Blue,

Where would we be without myths???

Another gooden?

Page one of the Mopar Performance Bible,

1976-1978 400 and 440 blocks are thin wall castings and should not be bored more than 0.020 thousanths??? Therefore unsuitable for racing purposes???

I bored a 76 440 block out, to over 200 thousanths oversize to fit a liner, and it still didnt break through the wall.

Baccy.