Thrown a rod- why???? (pics included)

After 13 months building my mk2 20VT track day car it lasted FOUR miles!!

Went like stink, scared the hell out of me, then blew a hole in the block the size of my head!

Stripped it all down today and tried to find what had failed. Absolutely nothing obvious- plugs were the perfect colour, there was no witness marks showing seizure on the piston, the big end shells were unmarked, it doesnt appear to have been running lean and at the time of failure oil pressure was totally normal.

The gudgeon pin does appear to be slightly tight and the conrod snapped 1/3rd of the way from the top, having been bent I would surmise on the upward stroke.

The spec was: AGU block and head with APY manifold and K04 turbo and oil pipes. MBE ECU mapped by QPEng running 1.4bar boost.

Am building a BAM to go in next weekend, but before I fit it all up I'd feel more comfortable knowing why it failed. APpreciate your thoughts.

Nick

 

Did you run it on the road with an AFR meter first? My Qpeng map was a country mile away from what is safe. Over fueled and pinking it's bollox off.

 

Blimey!

Where they standard agu pistons and rods?

 

was it raining or maybe there was a bit of water some in the system, engine sucked it up from some where ?

 

AFR meter was still in the box (I am so ashamed) but wont do that again this time - that said if it was overfuelling that would not be a problem surely. Looking at the plugs (from what you can tell after 4 miles) the mixture looks fine, and there was no signs of anything running hot.

Yeah the block was entirely standard AGU. I had started off intending to make a nice 200bhp AGU conversion, but then got carried away and bolted a K04 to it. However I was led to believe (and maybe I was naive to believe this) that the bottom end of the AGU and the 225 spec engines was the same.

And finally, no it wasnt raining- bloody car was scary enough in the dry!!

 

STD rods do not like in excess of 300lbft. On a 20vT with a K04 at 21psi you will breach 300lbft at 4000rpm+ and it may cause the rod to compress and bend. Get yourself some H sectioned rods if you plan on going above this limit.

 

21psi is 1.5bar?

So running 1.4bar was pretty much on the limit then- makes sense.

Best i order some new rods!!

 

I agree, been there, done that.

 

lol looks like i need to re think my engine mods

 

The mind boggles! It's clearly true that the rods are the weak point then! I'm kind of glad this happened....makes me re think my plans too!

 

beer for the original poster me thinks

 

i thought it was common knowledge that these snap

 

OH

Guess it is now!!

 

ah at least you can provide evidence for others to see so that it is taken seriously

 

When you say it blew up after 4 miles... was this a rebuilt bottom end? If so, it might be a good idea to run it in a bit before giving it the big welly... once you've got stronger rods...

 

For any substantial power increase in a 1.8t I must highly recommend uprated rods. I was lucky as my conrod bent but did not snap but alot of people have not been so lucky.

 

Lack of research, scoolboy error

You've learnt by a very costly mistake, such a shame to see a good engine die.

Better luck next time, plug the AFR meter in too as a safeguard

 

Are they not?

What different?

I know the AGU has forged crank, pistons and rods, and 20mm gudgeon pins, but no-one ever discusses any other differences...

 

ok so it may not have been raining, but had the block been stripped aprt before putting in the car, if not it may have collected some water, ive seen this before in a tdi block and a 2.8 24v block, engines can`t compress the water and throw a rod out the side of the block

 

21psi on standard internals? Did you not expect failure?

And you say it lasted four miles? Well usually after a rebuild you have the boost on a low setting and run the engine in for around a thousand miles, then give it a remap to set it up to be driven properly..

Was it its first real run? 21psi, standard internals, on a newly built engine is asking for trouble imo..