Here are the parameters:
1. A generic internally wastegated T28/T25/GT2xxxR
2. Running external wastegate, so IWG is being blocked/welded/etc. This particular turbo has the turbine outlet separated from the wastegate pocket.
3. No space limitations directly behind the turbine outlet (This is not a Miata chassis)
4. Downpipe still needs to make a 90 degree turn downwards relatively quickly (Just like most Miata downpipes)

So the turbo flange is 2", what is the best way to get it to 3"? Expand quickly to 3" straight out of the turbo and then turn it downward, or make the turn with a 2" diameter pipe and then flare to 3"? In the latter setup, should it flare quickly, or slowly?

I was also toying with some unorthodox ideas for creating a bend that tapered continuously. Like making pie cuts of a megaphone, or maybe even some diy hydroforming

 

Also, does anybody know where I can find a flange like this for a good price?




There are a few on eBay but they are described as being cast iron with a zinc coating. I think Id like a nice properly cut flange to start with.
All the quality flanges (ATP, Columbia River, etc.) I find have the large oval cutout, which just creates more for me to seal up.

 

double post

 

You want to do the bend in 2.25", you can achieve this by porting the turbo rear housing. Then once the initial bend has been done go up to 3" with a conical reducer/expander.

I can post a billion photos if you need.

 

Check out "Corky's Area Rule"
BEGi - Articles - Corky's Comments - Corkys_Comment_AreaRule

A gradual taper from the outlet diameter to the largest exhaust diameter is ideal.

"The intersections should all match with no diameter changes. Smooth flow of the exhaust gas requires no sudden changes of cross section area. Never/ever/never hook a 3.0" tube up to the 2.0" exit of the turbine. If a section change is required or desired, try to keep a gradual taper in between the two sizes."

 

That's simple fluid dynamics.

 

Nitro speaks with straight tongue.

 

I bought the 5 bolt flange that's totally open like an oval.
Then I bought a 120* 3" bend and cut it almost 90* and heated it and bent it to an oval so it would fit the oval on the flange. Weld that bad boy on there and you're golden.
I think I bought my parts on eBay or APTurbo I think? It was a while ago.

 

Here are a couple of design variants I have come up with.





There are defintely some space constraits because the downpipe has to make that turn back under the oilpan, but its pretty opened and 2 dimensional compared the complications of a big DP on a Miata.

Any feedback on which looks best? I think the first looks like it would have the smoothest transition, but the second design gets the downpipe out to 3" sooner.

 

<p>I dont think there is enough difference between those to make a noticable performace difference. Go with the one that is easier to execute.</p>

 

could you combine those two? Like 2.5" 90° do the cone transition, and then the least amount of 3 bend possible rather than doing 2 90s and a bit of straight, Only saves you like probably 10-20 degree of angle, but its slightly better.

 

Maybe something like this?



If I could find a 3" mandrel bend with a centerline radius smaller than 4.5" I could make it tighter.

Were probably just picking nits at this point, but I like the way you think. I would like to make the best possible downpipe within reason.