Forum » Practical Engine Building » Engine failure analysis

Engine failure analysis

Practical Engine Building

Discussion and questions related to the course Practical Engine Building

Page 1

Mitsubishi 4B11 turbo charged engines completely stock block never opened running 0.7 bar of boost tuned like 9 months ago and running very well I just tried to rev the engine while the car was stationary the engine started to smoke ( blown engine) the con rod broke with the shape I've attached and the con rod broke the engine block all the pistons are clean and no indication for knock the car was running well the last monthes before blowing the engine i was enjoying WOT i just stopped in the gas station and while i was stopping this happened what could be the reasons for this and how not to do this again

Attached Files

That's a pretty serious failure! The stock 4B11 rods aren't overly strong but they are more than up to the task of supporting 0.7 bar boost (which is significantly less than stock). There's something else going on here that's caused this failure. Either the engine has had a massive overboost at some point or it has had water or fuel make its way into the cylinder and hydrolocked which will bend a rod quite easily. What do the other rods look like?

There are marks on the piston crown however I'll assume they have occurred post failure.

The other rods and pistons are fine with no problem or bends this happened with this rod only the thing which I can't figure out is the car was tuned like 9 months ago and it's running with no problems this happened while i was reving the engine while the car is stationary ( no load on the engine )

It's not going to be the free revving that has damaged the rod. Something else has caused the damage but it's hard to know what this would be.

There's something I forgot to mention the rom was being tuned for over run crackles by retarding timing on low load to -30 and inject more fuel on the same load after doing this and trying free revving engine the rod broke

I can't see how that would contribute to a broken rod. The retarded timing doesn't put any load on the piston so there's no real reason it should be able to break a rod.