N12 Turbo Forums

have a t-boost controller, that ball was removed so shouldn't be hill effected.

The overboost valve is 10psi right? well those kg/cm2 figures means it was hitting...7 psi back straight, 12.8 on front straight, spiking to 17psi.

remember though boys, this was only a $250 second hand motor that was supposed to last til I rebuilt the original.

C

I think, the photos you have resulted from the following, in this order:

Wastegate slightly seizing when hot, resulting in the moderate overboosting.

Mixtures going south with the overboosting, causing leanout.

Because your not running masses of boost, it started melting bits in the engine, plugs, slugs and probably valves.

#2 finaly shat its pants and melted a river of alloy down its side.

Then the sump was pressurized which pumped oil past the comp wheel and into the inlet tract, causing the plugs to take on the 'Elvis' hair look.




Take a couple of pics of the engine destruction. I'm betting on a melted #2, and #1 to have a rounded top edge.



Damo

all sounds plausable...

interested in why the wastegate would 'stick when hot'...it was freezing day the engine bay would have been relatively cool, when compared to the 40 days I driving it briskly before, normally in traffic to maake sure it hot then blast up the freeway boosting it down the on ramp...didnt normally stick then.

might take a bit more investigation, think I'll start by removing the silly T boost controller - it was set to for leak anyway, with the idea of turning it up latter when I knew what the engine was doing.

Just hoping the turbine wheel is ok

C

Finally got round to pulling down the blown engine.

Found that it had been ported:



The failed cylinder is fairly easy to spot from the leaking oil:



Pretty obvious that somehting went wrong:





When the piston was droped out the real damage became more apparent:




Remember boys and girls this engine drove home after it failed!

Seems number two wasn't the only one having troubles, number one:






and for the keen eyed out there, you may already have noticed that the valves and the pistons liked each other so much they were getting a bit close....


the valve strike marks were on most pistons, both intake and exhaust sides (intake dents were larger). some looked old but at least one looked new. Hard to say when this had occured and harder yet to believe it hasn't bent valves so that the no longer worked! Either way, that cam isn't going in the new engine unless I can confirm the cause....

C

ohh, and there was about half a litre of oil in the cooler and a quart still in all the pipes!

C

not a bad effort. through the rings and all.

Callumgw wrote:ohh, and there was about half a litre of oil in the cooler and a quart still in all the pipes!

C

Shyt thats nasty! I recon you keep that piston 3 for memorabilia.

Dex

desk monument!

C

ROFLMAO!!
Oh man that thing had detonated itself to death. Im almost surprised you didnt hear it!

Looking at that "port" job, valves clouting pistons, the entire thing was pretty much a grenade with the pin pulled! Just waiting to go BANG!

Still, some surprisingly good numbers can be had from that kind of port work tho. I'll stop short of calling it butchery!!!
Its just offensive to look at mostly. A very ametureish job.
Gasflow does not really know the difference between a professional and beginner hand if the basic port shape is not altered from stock.

I think heres is a lesson learned for many of you, that if you even slightly modify and engine and place it under conditions that are not just day to day with the odd traffic light squirt then this is the likely result.
The money spent even just doing a ghetto rebuild with mostly used parts would MORE than justify the cost of a decent adjustable ECU!

L8r
E

well, for me it was a $200 secondhand filler engine....so no real loss. I never ran as well as the one in it from stock, which I had ported (intake and exhaust) by a mob that race e15's in buggies. That engine was very nice to rev and its that head with a standard cam that going back on the rebuild.

I also measured the pistons in the detonated one, they were 75.25mm (the 7 is right and the .25 is right, but the 5 might be a 4, can't remember, but not important) so they might be a first oversize, but they are nissan pistons.

The valve springs also look a bit lighter than I recall....are NA ones lighter than turbo ones?

C

Hear what UR saying C, Just using ur case as an example really.

Ive not known NA springs to be any lighter given both engines run the same camshaft so i couldnt see it.

L8r
E

Being that the engine was limited to 6.5 can't see were the valve bounce can from unless the springs were a bit light.....
I think it was valve bounce because some of the marks were fresh, others were definately old, which I take to mean the interefance wasn't consistent.....

anyone got better ideas?

C

Oh darn, that looks nasty... poor little thing.

WOW, thats pretty graphic!

Spot on to what I was expecting. It must have been #3 that showed up zero on the comp test rather than #2.

It seems very strange that the inlet valves hit, they are nowhere near the pistons when they close (bounce)
The exhaust valve episodes are quite obvious. What you will probably find is, that when the contact first happens, the top ring gets pinched between the deformed crown and the top ring-land. If this is a big enough hit and the rings are cast, the ring will fragment and be left to flog around in the groove, quickly making a mess of the ring-land. With the now busted top ring, the combustion gasses will use this as the easiest path to low pressure in the sump. Couple this with extreemly hot leanout exhaust gas, and it doesn't take long to melt the piston. Once you are pressurising the sump and adding crap loads of oil to the inlet charge, detonation starts across the other 3. Again, as your not running stacks-a-boost the detonation will be 'mild' and you will just melt things.

I wonder if the history of that engine is a little colourful! If the previous engine build had jumped a few teeth on the belt, it 'may' have caused the damage to the pistons that you have. Then that 'ported' head was added later.

If you are going to use a bigger cam, get some yella terra 186 valve springs for it. Any engine builder should have a bunch of them kicking around their workshop somewhere.

Damo

nice summary Damo,

I never remember which is 1 and which is 4. The second from the dizzie, third from cam belt is the excessively damaged one and the burn crown is right next to the dizzie.

The first ring is a long way down from the crown, I'd be surprised if it pinched. There is spalling on the skirt of one of the other pistons and that ring had failed there, so that still consistant with your approach. That piston looks clean and nice on the crown though....but had a valve mark

Could the cam be badly ground? with too much height and too sharp an open radius?

C