Ceramic Stockies

[bogcock]

One of the HPI dvds had their R34 GT-T project car blow one up on a track day but they were sort of expecting it to after all the mods they did. Was funny though.

On guy who lives near me has a NOS system on his GT-T and the stock turbo is still hanging in there.

nos wont affect the turbo, as long as hes running a good boost controller all the nos will do is create more zorst gasses, the wastegate will redirect the gasses.

[MsG_ChRiS]

Ceramic is very heat resistant but it is also very brittle and in know way malleable like steel ... it doesnt put up with pressure or etreme force acting upon it very well...

It also fractures when its placed under extreme conditions of thermal expansion and contraction.

Just like floor tiles when they crack same deal =)

Been lighter in weight; ceramic is a better choice for linear power delivery. On a relatively low capacity engine running low boost transition onto boost is smooth.

Me personally i would not run more than ten psi of boost on the standard turbo...

But then theres people like 2rismo running 11's on the standard turbo

End Rant =P

[bogcock]

But then theres people like 2rismo running 11's on the standard turbo

hes obviously running wih an exceptional tune, and/or an auto.

drag racing is wot for hopfully 6 seconds

constant heat and stress causes failure

[MsG_ChRiS]

i reiterating that =)

but you can also f**k your turbo by running it at 1 bar boost; like say around a race track then switching you car off straight away without letting it cool down ...

the change in tempreture the turbo experiences is able to cause fractures due to contraction ... thats all im saying ...

low remp =

high temp =

gooooooo the soccerooooosssssss =)

[bogcock]

i agree

Edited June 19, 2006 by bogcock

[Cubes]

i reiterating that =)

but you can also f**k your turbo by running it at 1 bar boost; like say around a race track then switching you car off straight away without letting it cool down ...

the change in tempreture the turbo experiences is able to cause fractures due to contraction ... thats all im saying ...

Sounds like speculation to me.

[bogcock]

cubes i was thnkn the same thing, but i couldnt be f**ked typing it HAHA

[MsG_ChRiS]

sounds like everybody needs a hug =)

and yes the last comment could be veiwed as speculation ... thanks for pointing that out cubes ...

[GTRgeoff]

I think someone should ask for their money back for their engineering degree, or go back and study rotating dynamic forces and ceramic material properties.

What you are claiming is the turbo shaft heats faster than the ceramic turbine at the interface, causing the turbine to break at the hub. If this is so then the rate of thermal expansion between the shaft material and the ceramic must differ greatly. Not a bad theory but not fully thought through. A root cause analysis would require that this would happen soon after startup every time, otherwise the adhesive material must be considerably flexible and the turbine over shaft tolerance must be very loose.

Ceramic is more tolerant of thermal shock than other materials, hence its application in many aerospace roles as well as piston crowns and innumerable other useful high temp locations. Also, ceramics are not just ceramics. A floor tile will not be the same material as a ceramic turbine or an inconel turbine which is also a ceramic (the stuff used for an R34 GTR N1 turbine), and is usually temperature relieved in order to allow the item to become a single grain through high temperatur grain growth, especially in gas turbines in military jet aircraft.

Having said that let me say again as I have mentioned in other threads that the most likely causes of ceramic turbine failure are FOD (foreign object damage) such as a loose piece of material like carbon dislodging in the exhaust and striking the blade(s), often after a turbo R&R or severe overexpansion due to excessive RPM. Ceramic is not tolerant of tensile loads so a situation occurs where the centrifugal loads increase due to the weight of the blades themselves causing one or more to separate. The resulting imbalance either tears the turbine apart or severely fatigues the shaft causing it to snap off at a stress concentrator like the turbine attachment or the bearing region.

So, after a refit of stock turbos, play the lottery and hope for the best, or never run excessive boost which causes high turbo rpm.

Cheers

[Cubes]

Good thing I don't have an engineers degree then. lol

I've always thought it comes down to 2 things that break the ceramic turbines.

Excessive EGT's (i.e track use, been known to have a turbo let go at stock boost)

Foreign particles (spark plug bits due to detonation or as you state carbon)

High turbo's RPM, well yes, with excessive shaft speed the turbo will be spinning out of its efficiency, back pressure is high as are actual inlet temps. Both of which contribute to high egt's and detonation.

I'm not sure the boost is really such a good example as its airflow that really matters.

A good example of this is how an RB20DET has a ceramic turbo spinning at 1bar for years yet do the same with an RB25DET and its ceramic turbo will let go sooner rather than later.

I believe thats basically what you have said Geoff.

[paulr33]

this is likely to be as both engines make different power given the same boost pressure. that is the rb25det turbine will be spinning lots faster at 1bar boost pressure than the rb20 turbine

[GTRgeoff]

High temps will be a factor of high boost on a turbo not suited to that level of boost as the small exit area forces the unburnt fuel to linger longer waiting to exit the turbine and so it continues to burn. Boost is a result of turbine rpm and at higher rpm the blades are subjected to increased tensile load until eventually they rupture. In Jet turbines I have calculated for up to 2mm of blade growth due to tensile loading.

Paul the R33 gtst turbine is a larger diameter and as such would rupture at lower rpm than the R32 gtst turbine. It is a combination of the rpm and the actual mass in each blade. GTR turbos are often run at 1 bar with less incidence of failure because they are smaller again.

[bogcock]

its not so much the wheel heating quicker its the wheel COOLING quicker than the steel shaft, causing the damage

[bogcock]

anyway, i have had my say as previous in this thread, so read it, make your own decision. this theory has been tested though, so accept it

time for work:(

[GTRgeoff]

Actually I won't accept it unless you provide the physical evidence and a significant enough sample.

You said the shaft expands, not the shrink fit of the turbine onto the shaft causing excessive hoop stress.

Methinks you are not an engineer at all and have decided to run and hide.

I have not heard of one rupturing off boost.

[djr81]

..an inconel turbine which is also a ceramic (the stuff used for an R34 GTR N1 turbine), ...

Beg pardon?

[GTRgeoff]

Yeah you're right to question. It's still a carry over for me from early uni days when it was strongly pushed by the materials guru as more a ceramic structure, even though almost all of the elements are metallic and is precipitation hardened. I argued the toss but decided to keep my averages up rather than piss him off by arguing further.

Consider MMC's (metal matrix compounds) that are often held to be ceramics by some as well but I think quite different.

Some articles so others can make their minds up. I will point out that all use the term alloy so I should adjust my terminology as well.

http://www.burnsstainless.com/TechArticles...el_article.html

http://www.suppliersonline.com/propertypages/Inconel706.asp

http://hcrosscompany.com/metals/inconel.htm

Cheers

[Cubes]

Paul the R33 gtst turbine is a larger diameter and as such would rupture at lower rpm than the R32 gtst turbine. It is a combination of the rpm and the actual mass in each blade. GTR turbos are often run at 1 bar with less incidence of failure because they are smaller again.

In the case of the RB20, r33 Rb25, r34 rb25 and vg30 turbo's I really think shaft speed is the very last thing to kill a turbo, I believe high egt's/detonation is the likely cause.

Raise boost you raise airflow, actual outlet temp raises (especially if its running in to choke), backpressure within the turbine housing builds. Compounded you have one awesome receipt for detonation. So what do we do? Reduce ignition timing to prevent detonation and we raise EGT's. Are we better off running it a little richer so we can keep some decent timing in there?

I would assume so however It would be awesome to have a play on the dyno with an egt guage to see the benifits of tuning in this manner.

Why R34 GTT's running stock boost have been known to let go when you hit the track also supports the EGT theory.

Another mention that 'may' support the EGT theory and not so much the shaft speed is:

As you increase boost you dramatically increase shaft speed, improving VE also increases shaft speed but no where near as dramatically as higher boost levels.

The R34 GTT turbo runs the larger VG30 OP6 style turbine housing, larger turbine housings reduce back pressure resulting in higher VE that results in lower boost required in order to flow x amount of air in and out of the engine.

But do consider, as you make more power you make more exhaust gas, as a result backpressure 'may' be the same as the smaller housing just at a higher power level. The R34's do make more power stock and are generally known to make roughly 10rwkw more than the r33 rb25 turbo. I'm not completely sure on this maybe disco could share his thoughts.

As all RB turbo's share the same turbine wheel (same size, trim everything); one can rule out the turbine wheel weight differential between the turbo's as to why one would let go before another.

When setting up my bodgy actuator some time ago I accidently pushed 18psi through the stock turbo through second gear.

IF it was the case that the wheel has growth surely that squirt through second gear would have been enough time to grow as a result collecting the turbine housing.

So to conclude....

Turbine shaft speed is directly related to airflow not boost. Boost is definitely comparable on a given motor/turbo setup, not to be used when comparing different turbo's on different motors. i.e rb20 vs rb25.

The question remains... Is it turbine shaft speed or high egt's that will get to the turbo's ceramic wheel before the other.

I believe egt's.

[bogcock]

Actually I won't accept it unless you provide the physical evidence and a significant enough sample.

You said the shaft expands, not the shrink fit of the turbine onto the shaft causing excessive hoop stress.

Methinks you are not an engineer at all and have decided to run and hide.

I have not heard of one rupturing off boost.

i never once said they will throw a wheel off boost, i said the damage occurrs during that period. the wheel will throw once spooled.

thankyou for the comments, but not good enough im afraid

[bogcock]

and if anyone has a spare ceramic wheel and shaft lying around, send it to me and i will put it into my machine and spin it up to 300,000rpm and leave it for 10 min at that rpm, then i will apply heat for 10 min with the assistance of an oxy torch with accetelene bias so as not to melt the thing.

open to offers. i will record the findings on video