While going through my old photos (ironically looking for pictures of Veilside style rear spats/mudguards on my old car) I found these photos.

Now its been a few years ago since this happened, but in a nutshell I had stock twin turbos, and decided to turn the boost up on my old JDM 1992 R1.
From memory, I turned it up to 14psi and ran it for 4-6 months. Then upped it further for a track day. On the second lap, there was a noise from the engine similar to an angle grinder, and the boost stopped. Immediately began smoking , heavy oil smoke.

Towed it home, took the car apart. Found aluminum shavings all through the intercooler and in the throttlebody. Looked at the primary turbo and this is what the wheel looked like:







It had sheared its shaft under high boost, more than it was designed to output. The sheared wheel jangled around at high compressor wheel speed, smashing into the alloy snail housing, abrading it and sending chips of alloy through the intake. Once we replaced the turbos, the engine lasted a little bit longer, but ultimately it blew the rear rotor a short time later.

If you look at the shaft at the point it sheared, you can see that it is barely (probably not even) 1/4 inch in diameter. This is why twins like the Knightsports set and BNRs, etc all have thicker turbine shafts and the shafts are made of a high grade alloy steel, called Inconel, which can handle high heat and is high strength (they use it in jet engine components due to its durability)

This stock compressor wheel is a paperweight now

*Not sure if the abradable-seal 99' spec turbos are as fragile in the shafts. Never had a set apart to look yet.

 

What was boost turned up to for track day if you recall?

 

what he said LOL

 

Damn...pretty neat to see though

 

If I recall, something like 16 psi. But that was only for about 60 seconds of boost before it failed. I have good reason to believe that it was the longer stints as a daily driver at 14 psi which fatigued the weak little shafts.
But its easy to see how things and fragile the shafts diameter is. It really is smaller than 1/4 inch. It isn't made to take much load. I guess Hitachi went for light weight for quicker spool at the expense of strength.

 

lol, my friends turbo did that, the rest of the shaft and turbine wheel were missing, so it will fit thru a 3" exhaust and N1 dual...

 

Stock twins should be capped at 12 psi if you want them to last...we usually get a few months out of them running 16-18 before they **** oil

 

And make sure you do a boost leak test and eliminate all leaks.

Its the excessive shaft speeds that cause harmonic vibrations and shear the shaft.

 

Only had that happen to one of my stock turbos haha, but it was not that bad and I think I remember the nut spun off and was found in the air filter, after running about 17 to 21 psi on a few different sets for a few years. I have recently been wondering if metal shavings went into the engine and caused all or some of the groove marks in my rotor housing that were seen about a year later after the engine blew from detonation with more power and single turbo and not enough ai.

 

does anyone think this would this happen to a set of 99 twins? a friend of mine is running his 99 twins at 1 bar and its been a few months. i told him that 12psi is the limit he should be at but he doesnt listen.

 

It should be very unlikely at 1 bar for any stock turbos, based on my experiences

 

yea it is that looks terrible

 

Cool Just to be crystal clear, this happened to my old R1 approx 5-1/2 years ago. They were old photos I happened to come across the other day, and I just wanted to share the destruction, and the tiny diameter of the turbine wheel shafts, so everyone could have a chance to see.

The turbine wheel nut didnt back off though. Its still locked on the wheel (wherever I put it in a drawer somewhere)

I'd agree that 12psi would be the safest option above 10psi. Looking at the small shaft cross section and the way it shears, I'd tread lightly around stockers that pumped 14psi or anything above that. I'm no metallurgist (I did do a tribology introduction course at tech college) but there's too many factors in it.

No idea if the 99 spec twins were beefier in the shaft or not. Dont know if Hitachi manufactured those as well?

Semi-related side info for anyone who may care/is choosing a set of stock turbos:

I know there were 'revisions' to the twins over the production run, the ones I'm aware of are to do with the problems of the heat cracking in the main cast iron hot side casting. Dont quote me, but different raised letter/numbers on the iron castings ('C1/D1/E1' etc) represent revisions to the cast iron center section, specifically to do with the amount of nickel content in the cast iron. To alleviate the cracking, they added more nickel, to make the cast iron more durable. By 1999, I think they settled on a nickel content, and the 99 twins dont crack nearly as much as the 1992/93 twins. I used to have the specific info on my hard drive, but I cant find it, so I cant give a verifiable link But in my own rack of twins, I tested the C1/D1/E1 casting thing, and found it to be accurate- the later the year of the car, the more advanced the casting number was on the turbos that came off it. (The higher nickel in the cast iron, along with the abradable nylon seal in the compressor turbine is the reason the 99 spec twins are the pick of the stockers for me)
Had talked to Father Grumpy (Allan Horsley- Mazda Australia racing head tech guy responsible for the RX-7 SP, son of the RX-7 M2-1020) when he came down & we were doing Targa Tasmania one year alongside him (the year Mazda entered that RX-8 SP thing whenever that was, 2008?) and he confirmed the 99 turbos were the ones to have. Not very talkative, but very straight to the point :p

 

From everything I've read, the Japanese don't run more than 0.9 bar/13 psi on the 99+ twins. That seems to be the generally accepted level for tuning, quoting power outputs, etc.

P.S. My original twins lasted maybe 5,000 miles (90,000 total) after I turned the boost up to 1 bar. After that I developed a bad oil leak from the rear turbo

 

13 is about right on '99's for track use. You don't really need any more anyway, and higher boost on (any) twins, and you'll start to generate a lot of heat. I don't like to see much more than 60C AIT's. I run higher (1 bar) on the street, but that's usually short bursts.