Melted turbine wheel?
 

So after Laguna, my car started making a howling sound whenever it got into boost. After screwing around looking for a boost leak today, I pulled the turbo off and found this:


http://www.codrus.com/miata/turbine/turbine1.jpg
http://www.codrus.com/miata/turbine/turbine2.jpg
http://www.codrus.com/miata/turbine/turbine3.jpg

Oops. Definitely not a boost leak. The ends of the turbine blades are gone, they're chewed up and bent over, and there's some kind of pitting happening on the plate that the turbine wheel sits against. There isn't a much play in the turbo bearing, either axial or radial, and there's no evidence of it scraping on either housing.

So... why? What causes this kind of damage? Poking around at various other sites, it kind of looks like FOD, except I'm reasonably sure the engine hasn't ingested anything. I haven't pulled the head off to check for damage in there yet, but I did do a leakdown test and had 95% or better in all four cylinders.

Heat? The engine gets a steady diet of 95 octane, except when it's on the track where it gets straight 100. I'm running lots of timing, but it'll go all the way up to MBT without pinging. I'm in the process of switching from a Hydra 2.5 to a Reverant MS3, I've been having some issues and it was putting the Hydra back in for Laguna. After doing that, it was misfiring a lot (a LOT) when trying to get it running for the track. The Hydra turned out to be unhappy with the crank angle sensor, once replaced it ran fine. There was no howling before the track day. It's conceivable that the howling started on the track and I just didn't hear it with helmet on/etc, I dunno. It came home on the trailer and was making the howl the next time I drove it.

It's a 99, built bottom end (Carillo A beams, Supertech pistons), ported/polished head with +1mm intake valves, FM2 kit, 2560 turbo, FM 3 inch exhaust, making 270 at the wheels. The 2560 has about 60K miles on it, and probably 30 track days.

--Ian

I'd say the bend on the blade circled clearly indicates something passed through. I had the same thing happen. I assumed it was slag/scrap that broke loose inside my long tube manifold. After swapping EWG flanges I found more evidence internally that might have been the case. Lots of slop inside. Damage pic below looks like yours. JKav also suggested this was the cause. In aeronautical applications, Turbine Intake Temps are typically 1500-1600f, so a turbine wheel should be capable of withstanding temps above that. My peak TiT measure with a single EGT probe peaked at just over 1500f for only a brief moment- right before rev limit.

https://www.miataturbo.net/attachmen...9-59-34-am-png
https://www.miataturbo.net/attachmen...ine=1411308190

Were you running an egt probe in the manifold ? If so maybe the tip melted off an ate your turbine wheel

I have EGT senders, but I checked and they're intact.

--Ian

What were the egt temps while at the track?

They aren't data logged, so I don't have a complete history, but when I glanced at them there wasn't much out of the ordinary. Peaking between 1300 and 1500, depending.

--Ian

will direct thread pic to my turbo rebuild guru, works turbo chargeers and see what he thinks. sorry for typs - intocicated

+1 to foreign object damage and not melting

Definitely FOD. Tearing down to look inside the motor I found this:

http://www.codrus.com/miata/turbine/tb1.jpg

The infamous throttle body screw. I'd thought those were high-RPM failures, but I've always had my rev limiter set at the factory 7200, so apparently not.

I haven't quite pulled the head off yet, hopefully it's not too badly chewed up in there.

--Ian

Ouch. Stupid screw...

my TB screw didn't even do that much damage. but it was enough to just unbalance the wheel enough that it affected performance.

They also fail is the TB is fully open before hitting the stop- and spends a lot of time at WOT with the throttle plate assembly loaded up.

Is the shaft cracked or did the head just come off the screw? I jbwelded my tb screws so hopefully i dont ever have to deal with this. However im still really considering getting one of the fancy skunk2 TBs 949 sales so i dont have to worry

Debunked long ago, this failure has nothing to do with an improperly adjusted throttle.

TB failures are from the garbage 1980s motors vibrating like crazy, the TB shaft vibrates like a guitar string. Science 'n shit.

AFAICT the shaft is fine, it's just that the screw head came off. The throttle still works fine, doesn't stick, idles at the normal level, etc. I had to specifically go look at it to know that it had failed.

Also, I lied about the rev limiter. I checked the Hydra settings, and it was at 7500, not 7200. Doh.

--Ian

After breaking two 1.6 throttle shafts within 6 months I epoxied the throttle plate and screws to the shaft in the replacement TB and it has been fine for almost two years now. I was lucky not to have ingested a screw like yours did. The Skunk TB only fits the 1.8 so that wasn't an option for me.

What kind of epoxy? Ive been running jbweld on mine for the last year, doesnt seem to be coming off but im wondering if there is a better epoxy out there

I used Instant Epoxy from Harbor Freight - I like it because it sets up in 90 seconds which gives you just enough time to get it where you want it, but not enough time for it to sag or run like JB Weld can. It has yet to fail on anything I've used it for.

I pulled the motor out and took the head off this evening. Yes, the bolt went through the motor, apparently in two pieces. #4:

http://www.codrus.com/miata/teardown/head4.jpg
http://www.codrus.com/miata/teardown/piston4.jpg

And #2:

http://www.codrus.com/miata/teardown/head2.jpg
http://www.codrus.com/miata/teardown/piston2.jpg

#4 you can see the imprint of the phillips head in the head, if you look closely. #2 appears to have imprints from threads.

Savington says that when it hits in the quench zone like that it hammers the snot out of the rod bearings, so I pulled the cap off #4 to look at it:

http://www.codrus.com/miata/teardown/bearing4.jpg

Random other things in the motor, a big gouge in #2:

http://www.codrus.com/miata/teardown/wall-gouge.jpg

And scuffing on all four cylinders:

http://www.codrus.com/miata/teardown/wall-scuff.jpg

I'm surprised at the scuffing, the motor only has 10K miles on it.

Any thoughts on the damage? Are these parts salvageable, or is it all junk?

thanks,
--Ian

You can cc the combustion chambers and clean up the bowls and reuse the head IMO
Pistons I'd replace for sure, IMO. Looks like you need to either bore or hone at the very least anyway

Agreed. That scuffing probably looks a lot worse than it really is.

Any photos of this magic epoxy placement?

I realize engine harmonics (vibration at high rpm) cause TB hardware failures, thus all the focus on isolation/bracing the TB in high revving race cars. But this was on a new TB with <5 track hours on it before the failure in 100whp SSC Miata, stock rev limit. Shaft was cracked/split at the screw hole.

Back when I was researching the throttle shaft problem, I was under the impression that everyone was doing this to their TBs unless they had upgraded to the Skunk2 TB - kind of like using Inconel turbo hardware to prevent exhaust leaks. The epoxy not only prevents breakage by strengthening the throttle shaft assembly, it changes the mass of the throttle shaft which changes it's resonant frequency and prevents future cracking.

Here's where my throttle shaft broke, both times:

https://www.miataturbo.net/attachmen...ine=1411489430

My epoxy job:

https://www.miataturbo.net/attachmen...ine=1411489430

How hard is that particular epoxy feel with the thumbnail test? (Some feel more rock-hard than others)

Nobody's seen the magic epoxy break a piece off and get ingested?

What was the data that debunked this?

I have heard from more than one engine/racecar builder that this was an issue for high revving four cylinder engines. Both builders made said isolating the TB from vibrations was done with compressible gaskets between the TB/manifold and manifold/head, along with bracing the intake manifold. Perhaps that's why Mazda put a brace on the manifold - not for weight/load. In my particular failure I suspect it was overloading of the shaft due the fact that the cable was properly adjusted before the race. After the race the throttle pedal was bent when fully mashed to the floor (aka stomper in the SM world) which of course altered the throttle cable tension. Unfortunately I neither have data to prove or disprove what caused my failure. But I can say that in the following 15 years of racing Miatas after that incident it never happened again on that car, nor five others.

A few properly adjusted throttle bodies with broken shafts. Search if you want more info than that

It doesn't feel quite as hard as JB Weld, but it's hard enough that my fingernail doesn't leave a mark on it.

With the scuffing and cleaning I did to prep the surfaces before applying the epoxy, I'm far less worried about ingesting the epoxy than I was about ingesting a throttle plate screw.

Did you consider brazing?

very enlightening- thx

FWIW, in my case, the throttle body shaft is fine, the screw broke all on its own.

--Ian

Can someone source the JDM-yo mass dampers?

And I wonder if they need the OE manifold brace to be present to work properly.

https://www.miataturbo.net/attachmen...ine=1411678263

Emilio,

I propose using an accelerometer board to be able to log the G's that the throttle body is being subjected to. This little guy is $37:

EVAL-ADXL326Z Analog Devices | Mouser

It's a 3-axis unit which will measure up to 16 gees. I can build you a little interface circuit to output a 0-5 V DC voltage proportional to peak G's, all 3 axes if you like that you can then datalog. Then we can look at g's vs. RPMs (and MAP or TPS).

This way we can look and see if there is a particular RPM where the IM resonates at, and see if the manifold brace changes the peak value or the resonance RPM, etc.

Jason,
I was thinking the exact same thing, but trying to decide what range sensor to get. Thought 16 might be a little low? Perhaps it's alright... any reason you like that one in particular?

Next step up I could find is 100 G's. Too much?

I also like the analog outputs. You could go and capture a data stream, but it'll be around 3 kHz x 3 axes, and then you'd have to process the data, filter it, and extract the amplitudes and frequency (RPM). And log MAP too. With an analog output and a simple AM circuit, you get a simple 0-5V signal you connect to an aux input of an ECU.

Was the failure that started this thread on a car with the intake manifold brace to the lower part of the engine block removed?

The intake manifold brace was removed, yes. IIRC, there are a number of people who've seen this failure even with that brace installed, however.

--Ian

Heh, that's about where I got - thinking, boy, I need to check into this. I've seen some ~50g ones which look good, but more in integrated units.

I would expect the SHAFT access to be bigger, and the body to be smaller, so what might happen is the board for the TB, and the chip for the shaft, and get that chip in a wider range. The thing is, we're not really concerned with if it's 11.6 or 11.9 G's, we're wondering if it's 5, 25 or 50... So low resolution shouldn't be a problem.

There didn't seem to be anything on the actual card, was there? 3 resistors was all I saw, which means it's just a question of Heroic Soldering to get it as a loose sensor.

If we come up with something quick, I'll go ahead and order it. I'm happy to use your circuits for readout (it doesn't seem like anything crazy, but I'll take the work over someone who does it often over someone who has done 4 in his life), probably save me some development time.

I wonder how many free ADC's there are left on the MS - I can read it out with an o-scope or something, but as you said, it'd be nicer to have it tied at least to RPMs... Actually, the MS can't sample that fast, can it? We'd need at least 600 Hz to get 9k rpm at Nyquist.

Oh, duh! I have a board with like 60 channels or something. :-) Totally forgot. Shouldn't have a problem logging accelerometers on the valve cover, manifold, TB and throttle plate while still recording ambient temperature and real time shoe size updates. :-)

My first order of business will be to measure it with and without manifold brace, with and without damper, and perhaps even over 7k rpm if I could tune my car to get it to go that fast. :-D

(FYI - the feeling I get is that the brace helps but doesn't prevent these breakages. My old FM secondary fuel rail kept falling off, which I attribute to the missing brace.

http://abefm.smugmug.com/Miata/Modif...IMG_0719-L.jpg
FM Provided Alum Tabs Broke

http://abefm.smugmug.com/Miata/Modif...IMG_6383-L.jpg
My Homemade Thicker Alum Tabs Broke

http://abefm.smugmug.com/Miata/Modif...img_8971-L.jpg
Steel Tabs Broke Support Posts

http://abefm.smugmug.com/Miata/Modif...IMG_1766-L.jpg
Triangulated Supports Broke

This was all... spirited street driving, though perhaps some higher revs, I sure didn't spend a lot of time there. It was all with the manifold brace off, though.

I expect the fundamental of the vibration signal to be equal to the cylinder firing rate. At 7200 RPM that's 240 Hz. If you digitize that in practical terms you need at least 8x sampling that or 2 kHz sampling. Then you need to post process the data and extract the amplitude. PITA.

If instead you use an accelerometer with an analog output, and feed this output into a peak detector circuit (an 8 pin opamp chip with a few R's D's and C's), then you just have a slow 0-5V signal that can be logged at 20 or 50 ms intervals (typical ECU sampling rate). Said peak detector just needs to have a 100-200 ms decay rate so the datalog can see the peaks.

And baby steps. Read just the IM vibes near the TB first. Don't worry about what the shaft is doing yet.

Arduino.

--Ian

My old FM aux rail brackets cracked even with the brace installed. I wondered about the fact that the aux rail was connected before the pulse damper in the OEM fuel line -- vibrations in it coming from the pump? I dunno.

--Ian

Yay, new turbo arrived today.

http://www.codrus.com/miata/teardown/new-trubo1.jpg
http://www.codrus.com/miata/teardown/new-trubo2.jpg

--Ian

That's not a 2560, is it? :party:

Nope. GTX2863R.

--Ian

That's a lot more work than building my peak detector circuit and using a 0-5V MS datalogging analog input.
And it's a lot more work post processing the data.

I suspect that the problem with the broken screws and throttle shafts on turbocharged cars is not vibration, i think that is due to the forces that the butterfly is subjected to.
On a normally aspirated engine when you close the throttle the butterfly gets hit by the differential pressure of intake 25" vacuum on one side and atmospheric pressure on the other side, on a 2" butterfly that would be 120# of force on the throttle shaft, if the engine is turbocharged at 15PSI when the throttle is suddenly closed there will be a brief moment before the blow off valve dumps boost that the butterfly and throttle shaft will be subjected to 267# of force.
I think that after 10,000 miles and thousands of boost to closed throttle slams the shaft will become fatigued and start to bend putting a lot of shear on the screws and break them like what happened to your car.
Just my two cents.

In my case, I replaced my first broken shaft TB with a used TB from a normally aspirated bone stock wrecking yard car with 120K miles on it. It lasted less than 4k miles and 6 months before breaking the throttle shaft in the same manner as the first one. While boost pressure/closed throttle may be a contributor, there have been plenty of non-turbo engines that have had shaft breakage problems as well.

Plenty more for sure. The Spec Miata and Miata enduro racing guys have been fighting this for far longer than its been discussed here on the turbo forum.

There is a reason I ponied up for the Skunk2 throttle body, and it was not for their reputation or customer service.

Hmm I should really make it a priority to check and possibly do preventative maintenance with those screws.

Not sure why you'd need 2k sampling, seems like 1k would be plenty, you're not going to draw a pretty picture but you should have an idea for the amplitude. You're right that (we expect it to be) at 2x engine speed - I want to test till at least 8k, I run my own motor there - though it's not really firing speed so much as "when pistons are at the top" - I suspect you'd see just as much or worse on a motor being spun by an external power source!

Anyway: What does your circuit do? Count the number of pulses? Just give you the highest value? I like that later one, it'd be useful no matter what - like you said, if it's fast enough.

Draw me up one! :-) I'm still looking at chips, there seems to be a lot at the 8g range, the 2g range, and the 100+, but very little in the middle. Well, we'll know after one pull. Once it's characterized, I could run it on a shaker table without risking anyone's motor, and instrument it better for sure.

Ian: I love the wastegate mounting.