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3" downpipe

 
Old 03-24-2008, 11:02 AM
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Default 3" downpipe

So my parts collecting is going nicely and I finally have a Begi/ DP combo to mount my SR20 T28 on. However looking at this DP (2.25" outlet), I want to fab up something bigger, like 3" directly out of the turbo. The car is an 02 SE. Right now I'm trying to figure out how much clearance I have to play with between the turbo outlet and the "shelf" and trying to see how in the world to design this piece in such a cramped space. I'm thinking I may have to settle for 2.5" out and flare up to 3" later downstream. Just wondering what you guys have done and any pictures would be great. Thanks!
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Old 03-24-2008, 11:06 AM
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Y8s did exactly that. Flared it out like 8" after the turbo using a smooth transition piece. You could cut the end off your begi pipe and weld on a 3" transition. That wouldn't be all that bad.
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Old 03-24-2008, 11:21 AM
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its pretty common practice to match the turbine exit and then slowly step up teh exhaust.

for example, my outlet is 2.25", then steps to 2.75" then 3"
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Old 03-24-2008, 11:29 AM
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yep - in theory a gradual transition from the turbine outlet size to the dp size is best for flow.
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Old 03-24-2008, 11:37 AM
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here, ive posted about it before:

The goal is flow; you want to keep the exhaust gases moving. A rapid decrease in temperature or rapid expansion and turbulence should cause the gases to slow. Starting with a matching port will keep the temps high and exhaust moving, as the exhaust cools the second it leaves the cylinder. This is also why we don't tend to port the exhaust on the head, but we do port the intake. Again, to keep the exhaust moving. It's really all a compromise. Throwing the 3" pipe on a 2" turbine outlet will cause both turbulence and a rapid decrease in EGTs, slowing the flow. But if you keep the exhaust moving out of the turbo with a matching port size the back pressure should drop more, since the area behind the turbo wants to keep moving rapidly into the stepped exhaust.
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Old 03-24-2008, 12:19 PM
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bell mouth 3" FTMFW!!! a la Turbo Tim






i got your turbulence right here... hangin
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Old 03-24-2008, 12:34 PM
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Whoa! Sexy!! now that's what I'm thinkin. I'm not sure I've got the clearnace for that though. Once I get the turbo/ manifold mounted, I'll do some test fitting to see what works.
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Old 03-24-2008, 01:07 PM
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you can measure the diam of the outlet and get a transition piece to go directly from that to 3" and that's going to be the best bet. The bellmouth is OK but you'd have (slightly?) better results to divde the wastegate portion out. somewhere someone posted that 11 degrees divergent angle is the optimum but i can't back that up with any data. maybe corky can jump in here.

anyway, chances are you'll be more limited by the angles of reducers available more than anything else. I picked up a 2 to 3" and cut it where the diameter matched the turbine exit.

anyway like chad said, i did the expansion a bit after the turbo because there was just no space to fit it with my turbo placement.

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Old 03-24-2008, 03:17 PM
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my next downpipe will be v-band, looks so much easier to take off when you need to.
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Old 03-24-2008, 09:58 PM
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Originally Posted by zoom2zoom View Post
my next downpipe will be v-band, looks so much easier to take off when you need to.
trust me it is a must have. easiest removal ever.
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Old 03-26-2008, 08:51 PM
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Well the DIYer in me took control and I started to figure out a way to mod the DP to improve flow. Two big things bugged me about the DP. The fact that the wastegate gases didn't have a smooth exit and the pipe size was only 2.25". So here's my solution.

http://www.flickr.com/photos/[email protected]/2365386726/

http://www.flickr.com/photos/[email protected]/2364553833/

An improvement I'd say I need to actually mount it to see how soon I can flare to 3".
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Old 03-26-2008, 09:03 PM
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Originally Posted by rcc-miata View Post
Well the DIYer in me took control and I started to figure out a way to mod the DP to improve flow. Two big things bugged me about the DP. The fact that the wastegate gases didn't have a smooth exit and the pipe size was only 2.25". So here's my solution.

http://www.flickr.com/photos/[email protected]/2365386726/

http://www.flickr.com/photos/[email protected]/2364553833/

An improvement I'd say I need to actually mount it to see how soon I can flare to 3".
Very good work there. Making that cutout piece is one of the things I really hate doing. You just gave yourself 5hp and 100rpm quicker spool, for sure. And maybe a little better boost control.
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