Downpipe tech... FM Turbine outlet vs 3" elbow vs BEGI Separated - Miata Turbo Forum - Boost cars, acquire cats.

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Old 12-04-2014, 09:37 AM   #1
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Default Downpipe tech... FM Turbine outlet vs 3" elbow vs BEGI Separated

I am having a 3" downpipe and exhaust fabricated for my car locally and I am wondering what is the ideal way to solve my problem.

Specifically this discussion is in reference to an internally gated turbo like the GT2650 or 2860. MY main goal is reducing time to boost target. Peak HP/Torque is but a bonus.

One method is the BEGI method where the wastegate gasses are routed via a separate tube into the exhaust down stream.

Another method is the Flyin' Miata Turbine outlet which uses a separate cast elbow and from there the downpipe bolts onto the bottom. I am unsure how large the outlet of the casting is.

The last is the big open elbow method which is taken by some people here.

Is there any benefit to separating the gasses until downstream? How does the "big elbow" method compare with the BEGI and FM methods? IF I decided to just use a FM cast outlet and have the rest fabricated in 3" from that point on, will I be losing any measurable power or boost latency?

If this were a smart person discussion of downpipe design, what would we be talking about?

Last edited by Chiburbian; 12-04-2014 at 09:51 AM.
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Old 12-04-2014, 09:58 AM   #2
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In theory the separated gases setup is better than the bellmouth which is better than most cast elbows. But the difference on the dyno is hard to find between a lot of separated gas and bellmouth setups IME.
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Old 12-04-2014, 10:06 AM   #3
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Get the FM. BEGi fitment sucks, and they will likely get it to you a year late.

If you're getting neither and just copying the design, the one that separates the gasses and re-introduces them as far back in the downpipe as possible is the one that should make slightly more power
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Old 12-04-2014, 10:45 AM   #4
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Thanks. The question is though, will the added complexity of the dual tube design cost me too much in money to be worth it?

FM housing costs $220 plus $550 "three inch conversion" kit. Add a cat and complete the midpipe... Yikes!

BEGI version goes for $550 + $85 three inch conversion. The separated gasses thing looks pretty complex to fab up correctly. I imagine the reason the BEGI design costs what it does is that they solved the problem already and can jig it up for additional builds.

All together this is looking expensive. :(
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Old 12-04-2014, 10:48 AM   #5
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just get the elbow and fabricate the rest. that's what I would do. I have the elbow, and its a great piece

is it worth it? no.

if you're really looking to make some serious power you shouldn't even be messing with this configuration. you should be looking at a top mount tubular or bottom mount or absurdflow style
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Old 12-04-2014, 11:04 AM   #6
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What spurred this on was Blaen99. He is using the same manifold I am using but with a 3" exhaust.

Honestly I am totally happy with his numbers. I haven't even built my motor yet so I can't even push it, I just figured this upgrade would give me a bit of spool and would not mean replacing tons of stuff later after I built the motor. You are making me question my thinking. My end goal is "only" mid 300s...
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Old 12-04-2014, 01:47 PM   #7
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So what's your current exhaust? BEGI's 2.5" dp and exhaust?

Have you considered going with Artech? That's what Jared has if you were happy with his results.

He also says there is some warping on perhaps both manifold and turbo surfaces from before he installed the TSE studs, so that will only hurt build up of boost.
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Old 12-04-2014, 01:51 PM   #8
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What the hell is an FM "3 inch conversion kit?"

Am i missing out on something?

Data point: My car used to have a Begi downpipe. It didn't fit, so now it has an FM cast elbow with 2.5" FM downpipe. No complaints.

FM downpipe "system" is $439.
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Old 12-04-2014, 01:54 PM   #9
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Quote:
Originally Posted by concealer404 View Post
What the hell is an FM "3 inch conversion kit?"

Am i missing out on something?

Data point: My car used to have a Begi downpipe. It didn't fit, so now it has an FM cast elbow with 2.5" FM downpipe. No complaints.
Flyin' Miata : Turbochargers : Parts and upgrades : Hybrid to full 3 inch exhaust upgrade

This upgrades the piping to 3" from the turbine elbow
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Old 12-04-2014, 01:56 PM   #10
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Quote:
Originally Posted by Chiburbian View Post

Oh. So that still goes on the same cast turbine elbow? I wonder if you can get just the 3" downpipe and not the cat. Pretty sure i have that in a box in the garage.
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Old 12-04-2014, 02:00 PM   #11
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Even Abe's are 2.5" necked out to 3", and I think it happens after it bends down. No turbo outlet is actually 3" AFAIK.
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Old 12-04-2014, 02:03 PM   #12
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Quote:
Originally Posted by curly View Post
Even Abe's are 2.5" necked out to 3", and I think it happens after it bends down. No turbo outlet is actually 3" AFAIK.
The bellmouths are, actually you cant normally make them just from crushing 3" tubing you have to cut and splice.
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Old 12-04-2014, 02:13 PM   #13
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Fitting a full 3" downpipe at the back of a t2 turbo that is not on a low mount manifold is next to impossible without cutting up the rear shelf.
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Old 12-04-2014, 02:44 PM   #14
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Great, that makes sense. Looks like I will be getting the FM elbow and either buying their 3" kit or having the rest of the downpipe fabbed.

I DO want a catalytic converter. Maybe not that specific item though so I will talk to FM and see what they say.
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Old 12-04-2014, 03:02 PM   #15
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Quote:
Originally Posted by shuiend View Post
Fitting a full 3" downpipe at the back of a t2 turbo that is not on a low mount manifold is next to impossible without cutting up the rear shelf.
and completely pointless since the turbine outlet is 2" or less (unless you have a mammoth turbo which wouldn't be t2 anyway) and you want it to taper gradually and not just step to 3"
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Old 12-04-2014, 03:14 PM   #16
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76mm downpipes can fit without modifying the shelf with axial logs. It depends on how the axial log is made and how the turbo is positioned.

On typical cast logs the t2 flange is set higher than the center of the manifold by an inch or so. Is this significant? Yes as it reduces the clearance available for the downpipe as can be seen in the following sketch. (red line is shelf/firewall)


Also in this case the turbo is rotated by 10 degrees. This means that the downpipe angle leaving the turbo is 55 degrees as opposed to 90 degrees and more in some of the other types.

It is possible to fit 80mm downpipes by keeping the T2 flange at port center and rotating the turbo 10 degrees.

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Downpipe tech...  FM Turbine outlet vs 3" elbow vs BEGI Separated-downpipe_zps0f08e6a9.jpg   Downpipe tech...  FM Turbine outlet vs 3" elbow vs BEGI Separated-p1050615_zps1497e068.jpg  
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Old 12-04-2014, 03:16 PM   #17
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Neat William. Your downpipe is what I was thinking of as a true 3" from the flange style actually.

Just got off the phone with FM. Looks like I will be going with their stuff unless my fabricator puts the kabosh on it.
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Old 12-04-2014, 03:35 PM   #18
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Quote:
Originally Posted by curly View Post
No turbo outlet is actually 3" AFAIK.


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Old 12-04-2014, 03:45 PM   #19
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OH hai.

I sold this pair of bad boys a couple years ago.





750ml bottle of gin.
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Old 12-04-2014, 04:22 PM   #20
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Oh you get my point.
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