Julian@MRC

iTrader: (28)

Preface:This past week has been a bad one for 350z/G35 owners at our shop. we had to people come in with blown motors on Turbo charged cars.One was a Twin turbo set up and one was a single turbo set up, we will leave it at that. The Twin dynod at 380whp and a perfect A/F ratio, where as the single turbo dynod at 380 whp with a 13.0-13.5 A/F from 6000 to redline, way lean.
Long story short the twin threw a rod through the block on the second dyno pull with the same perfect conditions and no signs of detonation.The knock count was monitored with an aftermarket knock sensor,and there was zero knock leading up to failure.
The single turbo came in a week after the dyno with an engine knock after racing an evo on the street. We installed the Twin turbo kit, flawlessly as we have dozens already, and we did not install the single turbo kit nor work on his car in any way...That out of the way here is our findings and conclusions...

Motor one: This one was from the single turbo car with the engine knock. This guy was VERY luck for what happened. The diagnosis was a bent rod Very lucky he got off it when he did and the rod did not snap..We inspected the piston for visible signs of detonation, there were no signature knicks and pings associated with a detonated piston, and all the rings and ringlands were intact.. The rod end bearings were in good condition and shown no signs of lack of oil, or damaged caused by severe detonation..

Motor two: different story on this one..the rod on this one actually did snap and go through both sides of the block. Inspection of this rod showed a bend and twist to it similar in motor one. Inspection of the piston shown no signs of any detonation, but did show damage from hitting the valves, due to the rod snapping in half. Othewise, same scenario as the motor one, no signs of any abnormallies indicating detonation or oiling/heat issues..

Conclusion: based on the condition of the pistons we were able to reach only one conclusion in both failures..The respective combustion chamber had some sort of liquid introduced during the combustion process, making it impossible to compress the liquid, therfore bending the rods in both cases leading toward failure. We think either an injector stuck opened and filled the combustion chamber with fuel or the combustion chamber filled with antifreeze, cause by head lift. It only takes a small amount of liquid, about 4 oz's to cause this type of failure, and we are leaning toward the head lift issue as being the culprit, and would shed light on alot of blown motors under low boost or no boost conditions, with no signs of detonation or oil starvation. This scenario can play out in a split second with no warning..
Basically the liquid in the cylinder chamber is impossible to compress and combust normally, therefore the rods have no where to go but twist and bend sideways..

Here are some pics to enjoy...

Fairlady_z33

iTrader: (56)

Nice but one question dont we normally get headlift at more then 12 to 13 pounds or r we getting headlift at 8 pounds.

MI 35th

iTrader: (16)

interesting write-up so would you recommend a HG and ARP hardware on stock block to alleviate some of this risk for the 400hp range?

sentry65

iTrader: (11)

so what exactly causes head lift?

is it caused by huge sudden bursts of torque or just general useage or something?

Julian@MRC

iTrader: (28)

Mostly head lift has been an issue on the built motors running higher boost, but with a car with a static compression ratio of 10.3:1 running stock head studs, anything is possible..

Julian@MRC

iTrader: (28)

Combustion chamber pressure coupled with weak fasteners on the heads..

sentry65

iTrader: (11)

would running aftermarket cams help in anyway by letting the exhaust gas out faster or earlier?

Nismo350ZRT

iTrader: (4)

Which cylinders did the rod failures occur? I wonder if they are the ones that OverZealous had identified as prone to headgasket failure.

Julian@MRC

iTrader: (28)

one was the front drivers side, the other was the passengers side front.

Julian@MRC

iTrader: (28)

no..The only cure is a really tight seal between the heads and block, or less combustion chamber pressures/boost..

sentry65

iTrader: (11)

wouldn't there be traces of coolant leakage present then on the engines that have blown up inside of the cylinder chambers possibly - though I know it's a mess as is when a rod breaks

could you tell if you have headlift if your coolant is gradually going down? What if you used a mixture of 75% water? Would it be more likely to vaporize in the combustion?

Nismo350ZRT

iTrader: (4)

Both front cyclinders...that's interesting. I wish OZ would chime in here.

Julian@MRC

iTrader: (28)

one would think there would be traces, but like I said only about 1/4 of a cup of fluid could cause it, and it could have been an instantanious leakage due to lift at that point. one motor went at 4500rpms at peak TQ, the other went at 6000-6500rpms, where he was lean.He could have knocked a bit and lifted the heads at that point or he could have been running a 380 cc injector that was overworked and stuck open at that point.we honestly do no know for sure what happened at this point..

sentry65

iTrader: (11)

is this strictly related to boost pressure or engine power - as in, say you're running 8 lbs boost with aggressive timing vs 10 lbs boost with a lot of pulled timing? Would one setup be worse than the other or not really if in the end they're producing the same power levels?

Julian@MRC

iTrader: (28)

Timing has alot to do with it also IMHO. We run a very flat timing curve on cars we tune. The way I look at it, the extra 10-20 whp is not worth running very agressive timing unless you plan on running race fuel all the time.
But with head lift, cylinder pressure is cylinder pressure. Less boost or less timing helps, but if their gonna lift, their gonna lift..

georgec

That amount of fluid would tend to bend a rod not break a rod imo, I still believe that rod failure is due to the big end bearing spinning in most rod failures.

Do you have a pic of the failed rods, that would tell the story quickly.

George

thawk408

iTrader: (16)

Did either one of these cars have cams? Or has the head ever been off the car before the rod went?

georgec

Very Interesting. By the looks of that bent rod you'd have to say you exceeded the mechanical limits of the con rod - the likely cause is very high cyl pressures at peak torque.

Wired 24/7

iTrader: (2)

Call me an idiot/noob/naive or whatever. But where are all the people claiming that the stock block is good to 400+whp with a good tune? I am waiting for them to come in and raise the BS flag and tell us what they think REALLY happened to the TT motor.

I've heard more stories about bent or broken rods than i'd care to repeat. For my money I'd spend a little extra and do the minor engine buildup, even if you are going for ~400whp.

Subscribed so I can link to this thread next time someone says 400whp on a stock block is safe.

taurran

iTrader: (18)

Blown motors in the 300-400whp range are nothing new. No form of FI is "safe" on a non-factory forced induced motor. However, steps have been made in the right direction, allowing for stock motors to run this kind of power consistently with no issues.

Trust me, you've "heard" less stories than many folks on this board have dealt with first-hand. As you can see, even those that work with these issues on a daily basis are still trying to pinpoint the trouble areas involved in boosting on the stock motor. Unfortunately, this is no simple task. Due to the complexity of the system and a large amount of possible contributing factors, the reason for most blown blocks never obvious.

Talk is cheap. Built motors are not. The expense involved does not outweigh the risk at these power levels for most. Besides, there's no reason to limit yourself to 400whp even on a basic built short block.

Do that, but remember that some people's idea concept of acceptable risk vs cost may be vastly different than your own.




MRC - It's a good though, but wouldn't this realistically bring you back to step one. What I mean by this is that by your theory, only one element in the string of circumstances must be present for failure to take place. For instance: Maxed out injectors > Lean Condition > Detonation > High Cylinder Pressure > Headlift > Liquid in Combustion Chamber > Increased Stress on Connecting Rods > Bent Rod. I'm just saying that while this theory may be entirely valid, it doesn't necessarily help determine the root cause of the failure as it could be a number of problems and the resulting chain of events.

I also think that Sentry brought up a good point in questioning if either of these cars had displayed signs of headlift previous to failure. While its definitely possible that this was a sudden case of headlift due to other contributing factors, its hard to place blame when there is no physical evidence other than a bent rod.

Regardless of the outcome, it is good to see that you're actually willing to share your findings with the community. Thank you for that.