I would think the downpipe bracing would see less thermal movement than the turbo, but honestly I'm not sure what's best myself. I'm using the default inconel kraken studs and had good luck for a whole weekend turning laps with stage 8 hardware. I believe I did use Cotronics Resbond 907TS threadlock (which came with my miataroadster replacement exhaust manifold studs) on all the turbo studs. Not sure if that helped but nothing seemed to back out. I am running less boost (and thus generating less heat) than you, so that could certainly be a factor. Hopefully you can get it all sorted man, car looks good!

 

I didn't realize the M10 studs weren't Inconel. Still kinda blows me away that the manifold failed with all of that bracing, but like Teg said, who knows the exact thermal dynamics that are happening with the downpipe vs the turbo.

I'm starting to get the feeling that I got real lucky running my 2560r/Kraken kit on track with no bracing and minimal hardware issues.

 

He said he removed all the bracing a few posts up because he was concerned it would cause cracking. I was wondering why he removed it..? I would think a downpipe brace should be a-ok at a minimum.

I was showing my manifold and downpipe bracing to an old s2k acquaintance that was an engineer for garrett and he said it looked good, and the thermal expansion of cast iron is much less than typical welded stainless manifolds (304 or 321). I think on a welded pipe manifold there's more expansion and a bit more concern about cracking (I've been thinking about making a brace for my honda this winter). I'll be interested to see how my car fares with more track days but I gotta think the bracing is helping a fair bit and honestly it wasn't THAT hard to make.

 

I've got some leftover inconel M10 studs that are brand new if someone is interested. They came from Full Race. Been sitting in a drawer. $25 plus actual shipping, which should be peanuts.

 

I removed it because bracing is most likely not going to solve the stud issue and will only add more stress to the manifold. The downpipe has a flex pipe, which absorbs a significant amount of stress from the manifold. Any additional hardware will constrain its movement and increase internal stress. Cast iron is brittle, not ductile. Bracing a stainless manifold makes more sense because it is more ductile.

I did the math behind the stud problem:

The issue is that turbo exhaust housings are made from a cast iron/silicon/ni material with a low coefficient of thermal expansion from 70°F to 1400°F—around 7-8×10⁻⁶ in/in/°F.
The Full Race studs are not Inconel; they are A286 stainless steel, which has a thermal expansion coefficient of around 10.2×10⁻⁶ in/in/°F over the same temperature range.
Assuming our exhaust gases are 1400°F, the studs need to handle that temperature. If you calculate the expansion of the studs, they will expand at a faster rate than the turbo flange:

• From 70°F to 1400°F, the studs will lengthen by 0.00515 in, and the flange will lengthen by 0.00389 in.

If you torque the M10 studs to, say, 45 ft-lbs, you will stretch the stud in the elastic region to 0.000708 in at 1400°F. To check if the bolt will remain clamped, you can calculate:
(flange expansion - stud expansion) + stud stretch from preload.

This results in a negative number, meaning the fastener won't stay clamped.

However, if you use a material like 718 Inconel, your studs will match the thermal expansion of the turbo flange. Both will expand together, maintaining preload at the joint. However, concepts such as creep and fatigue stress come into play, which might explain why others in the Miata community still experience issues with M8 Inconel hardware.

Unfortunately, there are no M10-size 718 Inconel studs available on the market. The only company that truly advertises 718 Inconel is FM with their M8 kit. I contacted some vendors who advertise Inconel, and they indicated it’s an "Incoloy" alloy. I ran calculations for this some alloys that fall under this category, and they still do not meet the requirements at 1400°F.

Currently, I can't find a company willing to produce M10 studs in 718 Inconel, except for ARP, who quoted $400–500 for a set of four studs which is nuts.

However behind my math for A286 if the studs reach 1000F and you torque to 60ft/lbs. The fasteners will stay tight, but math doesn't govern reality.

(These calcs assume lots of assumptions and neglect various aspects, its not exact but just gives you an idea)

 

Cat given for thoroughness and math that I'd never attempt myself!

That's incredible regarding the price of the ARP studs. I'm surprised there aren't more M10 options available on the market given how many turbo manifolds utilize larger studs. I'm only going off of OEM stuff I see at work but all the 4cyl Ford EcoBoost engines utilize M10 studs for the head to turbo connection, although there's obviously many different mechanics at play vs a turbo BP (exhaust manifold integrated into the head, factory bracing, etc etc.)

Hope you get to the bottom of it!

 

I'm no mechanical engineer, but OEMs wouldn't use braces on their turbos if it added more stress to the manifold and caused cracking (and they all use cast manifolds). The OEM MSM turbine housing has a brace to the block right from the turbine housing (that threaded bung).



I'm not sure what the alloy used on Kraken's manifold is but I sent him a message and will find out. I'm guessing it's similar to Ni-Resist D5S, which is what garrett uses on their turbine housings and has a coefficient of thermal expansion of ~7.2. 304 and 321 SS has a coefficient of 9-10, and Inconel 718 has a CTE of ~7.2. I think if anything bracing is more suited and easier for cast manifolds due to the lower CTE and less movement of the metals. I've also never seen an OEM turbo setup that didn't have the downpipe braced to the engine somehow, not sure why you think it's a bad thing..?

Sorry to hear about that stud issue. That's a big pain in the butt for sure. Obviously, I would say it's worth reconsidering your bracing. I'll be sure to report back how my car fares next year with more track time.

 

damn dude, that was impressive math!

Was that quote on a full threaded 35mm ish stud?

 

I did attend Miata reunion and was able to speak with Keith at FM on this matter. He mentioned that when they braced the downpipe and they has issues with cracking and he highly encouraged me to remove it. He then really sounded confident with the 718 Inconel hardware. They did do a dp brace back in the day and there were instances of it cracking.

My main concern bracing the manifold is how much the manifold is trying to expand. If it does in fact reach 1400F(exhaust gas temperatures), that thing may be expanding roughly (0.070in) 2mm (away from the engine). If you got some brace keeping it from expanding, your are introducing lots of stress. There are numerous OEMS with bracing, my old WRX had bracing and that's where I got the idea from but a BP engine with high vibrations and cast iron, I do not thing bracing is the right application here unless you incorporate some sort of rubber bushings allowing things to expand and contract.

Hiem joints may also be better as they will allow rotation reducing imposing manifold stress, but I still strongly believe bracing is not the issue but stud expansion is.

 

It was a rough quote but yes. 4 studs, gentleman said its gonna run like 3-400 dollars

 

Personally, I wouldn't count on exhaust piping to bear any weight. Between the thermal stress, vibration, and thin wall, it's just not a fair ask of the material imo.

I haven't run the numbers, but I worked on a higher strung turbo 4 like this in my last job at one of the big 3. From memory, the post turbo exhaust had a flex pipe right next to the next underbody exhaust mounting point, which was several feet away from the turbo.

 

https://www.tillix.com.au/product/in...di-8-stud-kit/

These are inconel 625 10 1.5 and 1.25 on turbo side. Not sure on the math of expansion vs 718 tho.
Also, "We offer our Inconel studs in various sizes to suit a wide range of applications … please contact us if you need something different."

Didn't Kraken offer a 10mm version a while ago as well?

 

Very interesting, thanks for sharing that. Ya it's a tricky problem for sure. The issue I have with zero bracing on the manifold or dp is the first hanger in the system is all the way just forward of the diff. That means the manifold is bearing the weight of the downpipe and some of the exhaust which is a tall ask imo. I understand thermal expansion is a concern, but now you're talking about a lot of weight and leverage on the manifold/turbo with vibrations/etc. I'm honestly not sure what's "best" per say, but letting it all hang off the turbo seem a bit much. You mention your WRX but I'm pretty sure most OEM use cast iron for manifolds right? Obviously the OEMs have engineers calculate CTE rates and design the braces accordingly.

I will be readjusting my turbo brace and tensioning the heim joint a bit with the engine warm. This seems like the prudent thing to do for sure. Regarding stud expansion.. you'll have to refresh my memory but the issue was you couldn't keep the nuts/bolts tight with the original inconel 718 8mm studs? The thermal expansion rates of the turbine housing, manifold, and studs should be nearly the same, so I don't think it's an issue of CTE and the studs expanding but vibrations working the hardware loose. OEMs use standard hardware and don't have an issue with harware backing out constantly, but they also aren't pushed as hard (and run as hot) as a turbo track car. I understand the concern regarding CTE, and there's a tradeoff between appeasing CTE and vibrations from working fasteners loose. I think given the tradeoffs, if I end up with a cracked downpipe or manifold at some point, hopefully it'll be after a long while and I'll just replace/fix them. If it means not worrying constantly about loosening hardware maybe that's the tradeoff I'll take.

Ya that's my concern with leaving it unsupported till the first hanger forward of the diff. That's interesting.. I try to look at oem setups and mimick them as there's usually quite a bit more thought put into them compared to aftermarket stuff.

 

The interwebs says ~7.3 so virtually the same as inconel 718. Maybe those would work.

 

As Z_WAZ mentioned, look for OEM solutions. The trick back in the day was to buy some M10 studs from the FD turbo setup that were long enough. IIRC some studs in the system were Inconel and some were not, so you might have to do a little digging. I'm strapped for time, but there are a few threads on this forum about this very topic.

 

Original studs backed out with the copper hardware. When I retorqued 1 stud snapped inside the manifold. Another one would not tighten or come out. This was the studs provided with the kraken kit, we do not know its exact spec thou. If you are worried about that the far hanger maybe incorporating a hanger closer to the flex pipe would be better than bracing the dp to the transmission.

 

According to specialmetals.com, @70-1400F--> 718: 8.91 and 625: 8.5. It actually has less thermal expansion than 718. Good find! I am going to run these studs.

Purchased: https://www.tillix.com.au/product/in...ange-stud-kit/

Material Properties:
https://www.specialmetals.com/docume...-alloy-718.pdf
https://www.specialmetals.com/docume...-alloy-625.pdf

 

Jeez that really blows. Good luck man hopefully those inconel 10mm studs will do the trick! Another idea is maybe safety wiring the manifold nuts? Of course if the studs themselves back out it doesn't help, but maybe some resbond or something on the studs as well will help?

I will think about my bracing some more but will probably just run it as is for the time being. I think the heim joint should definitely help allow some movement so hopefully the manifold will be ok. Regarding the downpipe, hopefully it lasts a while if/before it cracks.

 

The M10 Inconel studs from Tillex Performance studs passed the spark test. No doubt it is a Inconel alloy. I gave the wrench several taps with a hammer to help get some torque on the nuts then tack welded them for good measure. Nuts always loosened within an hour of use.
It survived 2hrs with 0 mercy. Hopefully many more

 

Hell yeah dude, glad to see the car didn't need any attention after this go-around! Nice lap as well, looks like you're getting good with proactively counteracting oversteer.