Yeah I can see that. Series of events: WOT with high turbo RPM -> throttle off (drastic flow reduction) but turbine still spinning due to rotational inertia -> turbine creates upstream vacuum (until it spins down or throttle is reopened).

 

My first attempt looked something like this. It cracked at the weld to the flange as did a few other manifolds I had seen of similar geometry. My reasoning was the difference in thickness between the flange material and the exhaust tubing. The uneven distribution of thermal mass caused the flange and the pipes to heat and cool at different rates thus they can be at different temperatures for short instances. This causes a huge stress concentration at the interface between the two different thicknesses of material.

I will post a picture of mine

Bob

 

Here is a picture of what I did. The studs holes are countersunk well into the welded on round bar stock that multifunction as nuts for the studs, gussets for the flange, and transitional thermal mass to give a more even mass distribution up to the joint.

The diving board thing attached to my turbine outlet actually holds the turbo positioned up to the manifold flange even if all the studs are loose it is spring loaded a tad when everything is assembled. The first one made of mild steel cracked this one was made out of 4130 and has not cracked.

Bob

 

The inconel studs have proven not to stretch. Extra safety, do the following...
Stephanie

 

Thanks Bob

 

did yall actually use set screws to hold the studs in the mani?

luckily my mani / turbo is holding up well after 3~4 track days, but muh dang turbo / DP nuts keep falling off...

 

How much for the inconel studs? Do you guys have them? How long are they?

Interesting that in that picture you show 8.8 studs. Just like the last set I had that stretched. Doesn’t matter what you do to keep them from turning if there still going to stretch, loose clamp. or fatigue crack like mine did after enough extreme heat cycles.

Bob

 

The set screws prevent the problem of the stud backing out and disappearing through a safety-wired or otherwise restrained nut after it stretches and loosens. But having a fastener stay working the way God and Carroll Smith intended would be better.

Are the threads on the Begi Inconel studs machined or rolled?

 

Yup. It would be awesome if everything worked the way it was supposed to from the get go, but when it doesn't we will make it work. That is just how we did it.
Stephanie

 

That picture was pre-inconel studs. And no, bolting the studs in won't do a darn thing if the stud is stretching.

The inconel studs are $148.60 for a set of four. We do have them. They are 47 mm in OAL. If you do not need that length, they can be cut down some. They are so expensive because they are machined by hand. This grade of Inconel cannot be cut with a normal die. I will get them added to the website in a few!
Stephanie

 

I wondered what those gussets were for on the 323 manifold you built for Ethan. Genius.

 

I will be ordering a set in the next few weeks. I think the next time mine comes loose I will need to Helecoil because the threads get damaged some every time you take it apart after hard thermal abuse. I could put in the set screws as well.

Bob

 

The threads will get damaged with repeated install and uninstall. Taking a die to it, gently, will be OK.
Turbo Mounting Hardware - Inconel BEGi

Stephanie

 

Were these repeatedly tested? Or just 1 hard track day?

 

So far one hard track day. The car goes to another track day again in a few weeks. The previous studs stretched on this car after one track day, but the Inconels did not.
Stephanie

 

https://www.miataturbo.net/showthread.php?p=554872

TSE inconel stud kit GB, $105 shipped.

 

Just an aside because I know its been mentioned in the past.

I noticed today while browsing M-Carr at work that they now carry high temp inconel Belleville washers (1100F). Not sure on the load ratings, but it seems the concern with using these washers in past discussions was their inability to retain their form at high heat.

 

We talked about it like ~600 posts ago, I think the consensus wasn't that heat was an issue, but just that they would bottom out once you put enough torque on the nut to actually seal them up. We'd need a stack of them to get enough clamp force and still have them be a little springy, and there's just not room for that on T25 flanged turbos.

 

Shame on me for not reading through the whole thread. I'm a bit surprised considering how high some of the flattening load forces are on the 1mm+ washers vs. the torque specs on a turbo flange, however, I haven't bothered looking into load maintained at say 30% deflection verse 80%.

oh well.

-zach

 

Epic failure of the Dorman 10.9 studs. I was a little nervous when the box they came in said made in China. Day one at Laguna 3 out of 4 studs broke just under the nut. Surprisingly my little spring loaded turbo support keeps it from having a massive exhaust leak thankfully. Day 2 replaced all 4 studs with 4 new ones of the same kind 2 held one kept loosening and the third broke off.

Grade 8.8 definitely better than 10.9 for the turbo But still not acceptable.

Bob