Hello,

I have an 09 RA with about 70K on it and I just recently had my timing chain replaced under warranty. This kinda got me scared because I don't understand how this could have happened.

I take good care of the car, don't really drive it very hard (i.e. abuse it), and have kept up with all the maintenance work..

Upon some further research I found that my car had the old style MN183891 chain on there which apparently is known to break and/or stretch, hence Mitsubishi released a new stronger 1140A073 chain to replace the part. I now have the updated chain installed on my car.

I was still kinda skeptical about this happening though so I looked up other causes for timing chain stretch and one of the common responses I got was due to downshifting too quickly. This brings me to my question: when I'm driving my car in manual (i.e. using paddle shift) I manually gear down as I'm slowing down or braking, and sometimes if I'm dropping speed quickly those gear-downs are not too far apart. Now, is that detrimental to the car? Should I just let the ECU gear down on its own when slowing down or dropping speed? Is there is a desired method of driving on paddles that is good for the longevity of the car is what I'm really asking?

 

It shouldn't.

On decell you 'load' the engine (versus making power) and reverse the load on the timing chain, but you do not put 'more stress' (which stretches things) on the chain.

It was/is a metallurgy issue - the material is not strong enough for the loads (driving the cams). Unless we have 'high resistance cams' (and i doubt it) then it was either a bad batch of metal or it was specificed incorrectly (bad product sourcing or bad engineering - pick one).

 

Makes sense, so as far as the driving controls goes.. not much that I can do, I guess? I mean as far as letting the ECU gear down or gearing down manually on my own

 

You might want to reconsider that. The timing chain is ALWAYS driven by the crankshaft. So, the "load" on the chain cannot possibly "reverse" itself in a 4 stroke engine, EVER. Even in engine "over run" or decel or engine braking situations, the crankshaft drives the timing chain. The camshafts never drive the timing chain.

Without a doubt though, the load on the timing chain varies greatly with engine acceleration, but there is no significant amount of time where the load on the chain is reversed in a normally operating engine.

You can have situations where the transmission drives the crank (engine braking/decel) but on the timing chain side of the engine the crankshaft is still driving the timing chain.

The only time that the cam shafts would drive a timing chain is when moments of inertia are greater in them than in the crankshaft. Given that the moments of intertia of the crankshaft is orders of magnitude larger than in the cam shafts (thanks to the mass of the crankshaft itself and the fly-wheel bolted to it (opposite the timing chain)), the situations in which where the cam shafts can drive the timing chain are going to be limited to outside the scope of normal engine operation.

 

D'oh! ><

You're right! I had to draw stress riser diagrams on the whiteboard and animate them in my head... When i did, I could think of no circumstance where the cam gears 'pull' the chain up (drive) from the crank pulley which would reverse the stress load on the chain like you stated.

So, answer is still the same - no increased load stretching the chain from allowing the engine compression to slow the car by downshifting (i just understand better now *why*)