ALFAGTV6.COM

[MD]

I broke a con rod in two places in number 2 cylinder on a 1600 Guilia Super motor while in a club rally and revving up to 6700 for about 2 hours.

It developed a strong vibration but I drove it home (about 150 kms )without any further hassle, just underpowered.

When we pulled the motor down, I noticed that the piston was stuck at the top of the cylinder, the middle part was in the sump and the remainder was still around the crankshaft !!!

Don't tell me Alfa engines aren't tuff !!

[Mats]

Having 1k to stretch in some places is not wrong.

exactly...

Another piece of trivia re. rods breaking: almost always, 4-stroke engine rods break during shifting gears at very high revs, when the throttle is closed...there's no "cushion" over the piston at those instants and the pulling force the ascending piston exerts is much higher (a condition that never happens in 2-stroke engines where all cycles are 'hot'). That's exactly where a lighter piston is of benefit, having less inertia.
Jim K.

Isn't that what happens every exhaust stroke?

[Jim K]

MUCH worse when the throttle is closed, think about it, no residual gases to go out since no new mixture comes in. Most of the exhaust gas energy is expended during the early phase of ex valve opening (if properly timed!) the piston ends up pushing the rest out and this 'effort' is not insignificant.
Jim K.

[benski105]

does de-acceleration of the crank come into that equation too?

ie the crank slowing but a heavy piston with too much weight and inertia wanting to go up that little bit further? therefore putting extra load on the rod?

[Jim K]

Of course piston weight has a major role in this, as well as rpm (translated to piston speed here)and stroke length (same rpm with double stroke length = double piston speed). To give some relative numbers, rod tension during the exhaust stroke may have a ROM (rough order of magnitude) of 15,000lbs, while the load on the compression stroke can be 11,500lbs in the same engine. The loading on the rod changes from tensile to compression and this happens a few million times during the life of the engine. Tests have been carried out re. rod fatigue with very interesting (as well as expected) results, concerning aluminum, iron and steel rods. Consider that they evaluate rod life in millions of cycles before failure. An average race engine is torn down and rebuilt after ~400 race miles which is roughly 1.5million cycles. This rebuild interval has a lot to do with rod life! Some std production rods don't even make it to 1million cycles, proper race rods can exceed 15million! In between the two are many rod quality categories.
Jim K.

[Zamani]

What is the best way to test rod fatigue. The reason why I'm a little worried is, a few years back my car's engine was hydrolocked. As far as I know in most cases the hydrolocked cylinder's rods should be bent. Mine was not according to my mechanic. But it is worrying, I should have put in a set of carillos then and won't have to worry about rod fatigue at all now.

BTW, Jim what about rod ratio, wouldn't that be part of the equation too?

[Mats]

measure them, c/c, straightness, twist, ovality and so on.

Rod ratio really doesn't have a huge impact on acceleration because you need to change it pretty radically to make a difference, i.e. a 1mm pin offset won't do much. Better leave it alone, there are other issues to consider as well when you play with the ratio.

[Jim K]

Fatigue testing is different than tensile testing. It involves specialised equipment and takes a much longer time than just measuring ultimate strength.
It's about push/pulling at certain preset load values and keeping track of the number of cycles until the component ultimately fails (kind of like bending a length of wire back and forth in the same point until it also breaks, something all are familiar with).
No matter what type rod you have, hydrolocking is a serious event. Fluid is not compressible and results are usually catastrophic. If your rods weren't bent, it means (a) engine speed was very low (cranking) (b) the fluid volume was less than the combustion chamber volume (c) weak starter/battery combination or (d) initial lack of spark for some reason, until engine stopped. Any other ideas?
Jim K.

[Mats]

Sry, you're right. I was thinking Rp02, i.e. the bending bit from the lock.
Micro cracks can be found with "paint" and "developer". Apart from that it's cheaper to buy Carillos (probably several times over).

[Barry]

My big dog just pee`d on my little dog while she was taking a pee....mmmm

Hey,we ALL have rev`d an engine way beyond its power making revs and such...In my case it was to keep a Chevy v8 behind me with my little turbo...

Jim,Corky Bell`s book explains the cushening effect very well..but I believe Mats is also correct about the exhaust stroke...what you say alsomakes sense though..

Thats why my little engine got so eerily quiet at those revs..the more ,the quiter and silent it got...

[Jim K]

I must have missed something here...nobody's arguing with anyone on this...hey Barry, close one eye and walk a straight line....can ya?? I guess not!
Jim (hiccup) K.

[MD]

Hey JK, Bazza doesn't know what his stroke to rod length ratio is cause he is still a virgin...

[KingDom]

If you want to see a bent Alfa Nord rod have a look at Mike Valant's web site. His site is very interesting.

http://engr.smu.edu/rcam/cpm3v/2lrebuild/2lrebuild5.htm

Zamani, these could be what your rods look like

[Zamani]

Nope. I think Jim was right. I remembered how it happened. The car wouldn't crank and prior to that I had some over heating problems. So my guess is, the rod was saved. My mehcanic is very experienced and he told me, he was surprised it was still straight as he checked it out when we did the rebuild.

The car has been running well ever since though. But in hindsight I should have at least installed Pauter forged rods as they are cheaper than Carillos. OK they have questionable shape, but at around $150+ I can't really complain. I think even stock rods are not much cheaper, especially if you include the labor to shot peen and polish. If it takes 1 hour to shot peen 1 rod, than that would be about $600-$700 per set for the V6. At which point you start to wonder, why not just go with something like Pauters?

[Zamani]

Interesting, Eagle rods which are quite famous with the Honda/Mitsubishi crowds are forged in China, I never knew that!.

BMW rods from Eagle are $100 each.
Carillo rods for Alfa is $353 each.