After numerous searches here and all over the web, i have failed to find a replacement aluminum/titanium or other type of exhaust manifold replacement for the stock cast one.

Why is there nothing from the aftermarket available?

If want to run BNR twins , why would I want to use that weak link- heat saturating, eventually cracking stock manifold?

seriously, no one has thought of making an aftermarket replacement in 25 years?

 

Yes, lots of us have! The problems to over come are high heat, thermal stress (relatively cool on the outside compared to the inside), and manifold shape (thickness changes do not help in stress reduction). As a mechanical engineer I can imagine a HW or HX steel Alloy (Alloy Casting Institute designations) which are typical casting alloys that survive operating temperatures up to 2,000°F, then using special casting techniques and post casting treatments to eliminate internal stresses and covering the outside surface with something that would retard the heat transfer enough to even out the internal temperature differences, but not so much as to overheat the manifold. But, for a low production item it would cost about as much as a new engine. It is more cost effective to use the Mazda standard manifold and replace it when the turbos wear out. Think of it as a disposable item like the turbos. By the way, minor cracking does not affect the performance of the manifold. In fact, a used manifold in good shape is probably a safer bet (since it is a know quantity) that a brand new one that has a small chance of containing enough internal stresses that would shorten its life.

 

your only option is new or [hopefully gently] used OEM.

 

I haven't seen the stock twins exhaust manifold crack.

Plenty of good looking used manifolds on Ebay for $50-100 if your is cracked.

The turbo exhaust housings themselves always crack.

These are available "aftermarket" made by Hitachi who is the original Mazda supplier so they probably aren't going to last longer than the original. Unfortunately, they are only available with the full turbos unit.

You can get stock 1992-2002 255hp regular turbos, 1999-2002 280hp "high power" turbos or "Bathurst" spec high flow turbos from Hitachi.

 

Keep in mind that that "terrible" exhaust manifold you so denigrate is the result of millions of dollars of R&D from a company steeped in racing history. Rarely is an aftermarket part ever the equivalent of the OEM, simply due to the budgets involved.
I mean, just think about it...'Superfast Performance Parts Inc.', basically a couple of dudes working out of their garage, can in NO WAY create something better than an OEM. Oh, it's easy to fixate on "Alabama Flash" and put out some basic air intake, for example and quote some totally unverified claim (gain 22 hp instantly!) and get the uneducated masses to mooingly buy into it, like a herd of cows...but all factors considered the stock intake box is far, far superior.
In other words, there's a reason why there aren't any exhaust manifold options, lol!

 

^That's exactly what I thought as I read the OP's post.

Did I read somewhere that putting a radius on the sharp internal edges of a new exhaust housing was thought to reduce cracking?

 

Not always true, OEMs do often impose budget, packaging and emissions constraints on their engineers.

In the instance of the twin turbo pack I'd definitely agree though. Lots of aftermarket companies out there making twin turbo set ups, none of them making sequential turbos.

 

As stated, the manifold itself rarely if ever cracks. The turbine housing, which is the hot side of the twin turbos, have cracking issues. Mainly the original N3A1 turbos, the later N3C1 turbos have a higher nickel content and rarely if ever crack. Any new turbos out there - 99 spec or the Hitachi turbos - will be using the better turbine housings and shouldn't have near the cracking problem of the earlier twins.

Would an aftermarket version be sweet? Yep. It would have to be an investment cast part which is REALLY expensive to do. Also, there's only so many ways you can get the exhaust gasses from the motor to the turbos due to the location in space the twins need to occupy and the size of the exhaust ports and the inlet of the turbine housings. Which means if you make a new one you probably can't do much to change or improve on the design.

If there was somewhere to improve upon it would be the turbine housings, but investment casting those would be VERY pricey as they are a very complicated design and would require a good amount of true engineering to design. Once all is said and done you'd have a pricey part that would be close in price to a single turbo setup that is far simpler and makes more power. You'll have a REALLY hard sell at that point.

Point is the hot side parts of the twins are good enough - you can make 350hp or so to the ground with the twins and low 400's with the BNR twins. It does the job. If you want more, or want less complexity, go single.

Not saying this isn't a good idea, it would be really cool, but it's most likely not a financially viable project for someone to tackle.

Dale