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It's the turbine
wheel that failed, not the compressor wheel. On each occasion the weld broke
right at the base of the turbine wheel, the wheel came off and blocked the
exhaust outlet.
My money is on a combination of overspeed, and
overtemp. I'm sure the rotary exhaust isn't hot enough to melt the weld,
but it might be getting in the neighborhood of that temp, depending on
what the stem, and wheel are made of. One of the things that
Turbonetics told me was that street turbos never get anywhere near the max
exhaust temp, because you just can't sustain that temp long enough to heat soak
the metal. That changes when we sell one to John :-)
Let's assume we're running the turbo faster, and at
higher temps than you ever could in a stock RX-7 on the
street. We know that all metals lose strength as they
are heated, though some metals have better high temp characteristics than
others. That's why we can run thin wall 321 SS exhaust
pipes, but wouldn't try that with mild steel, or even 304 SS.
Without having any idea what the stem and wheel are made of, I would
venture to guess that it's not the best material money can buy. Remember,
this is a mass produced item, what was engineered to be as cheap as possible,
while barely surviving the warranty period.
Under our conditions, the metal isn't as strong as
expected, because we're running it at higher temp than designed. We're
also spinning the wheel faster which causes more stress on the weld joint.
I'd be willing to bet that it's just that simple.
The OZ mod won't do anything for the temps, though John
can add some fuel to keep the EGT's lower (what was your EGT BTW?). The
real benefit will be lightening/slowing the wheel by removing some of the blade
area, as well as slowing it with a better porting job on the wimpy
wastegate. If they do a super balance job on the rotating mass, that will
help too. Will it be enough? I guess John will let us know
:-) I do believe it's a huge step in the right
direction.
Cheers,
Rusty (turbo mode
off)
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