Besides maybe having to plug bypass passages and tighten up the spring any reason a throttle body wouldn't work for an exhaust dump? The brass blades should be fine, and as long as the flange is beefy and square I'd think the body would be ok with the heat. Surely exhaust back pressure isn't much more of a differential pressure than idle vacuum, right? What am I missing?
Idle vacuum ~14psi on the throttle plate.
Not sure what exhaust would be but I would bet it could be significantly more.
That being said, I'd sure give it a shot if I had an old throttle body kicking around.
I don't think you're missing anything, give it a try.
You will let us know!
I'm wondering if there are any plastic bushings or seal or anything of that nature that may not be up to the heat. I suppose the temps in the exhaust go down a decent amount the farther you get from the engine. If there are any plastic parts in the TB, maybe don't mount it right at your header. Give it a shot and see how it works.
Most TBs have brass bushings, so I wouldn't be too concerned about that. I have no idea how much pressure you'd normally see in an exhaust at WOT.
gearheadE30 wrote: Most TBs have brass bushings, so I wouldn't be too concerned about that. I have no idea how much pressure you'd normally see in an exhaust at WOT.
You may end up with one of those auto cut out exaust systems like the high end cars have. Not that that is a bad thing. 
dean1484 wrote:gearheadE30 wrote: Most TBs have brass bushings, so I wouldn't be too concerned about that. I have no idea how much pressure you'd normally see in an exhaust at WOT.You may end up with one of those auto cut out exaust systems like the high end cars have. Not that that is a bad thing.
Meaning it will open up given enough pressure? I agree, not a bad thing!
Aren't TBs usually aluminum?
I would guess that the soot in the exhaust would end up getting into the bushings and ultimately jam the plate closed, but I have no idea if that would take days or years.
Dooo eeeet.
Ive seen this idea crop up a few times, you could also turn the throttle body so the pressure keeps it closed (inlet is the exhaust outlet)
Or one of them heat riser valves from an old V8? I believe the throttles are stainless and should do the trick.
Biggest question.
Can it withstand the heat ? Even at the very end it is hot.
Only one way to find out.
Pretty sure it's been done before, and I think I remember something about it sounding a little like you have an exhaust leak when it's closed, but then it's been years since I did the research for making the wagon's exhaust.
Found this on youtube
Trans_Maro wrote: Idle vacuum ~14psi on the throttle plate. Not sure what exhaust would be but I would bet it could be significantly more.
I've measured back pressures on my turbo Miata. With a high-flowing, 3" exhaust, metal core cat, etc it's about 4-6 psi at the turbine outlet. It's much higher than that pre-turbo, of course, but I assume that's not where you'd want to dump it.
What about a waste gate?
In reply to RossD:
For what even the cheapest ebay wastegates that big go for, especially after factoring in controlling it, you could buy the proper part.
http://www.ebay.com/sch/i.html?_sacat=0&_nkw=exhaust+cutout+3&_frs=1
https://www.youtube.com/watch?v=IXXiGyXl8vE
No end of Youtube inspiration.
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