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Old 07-17-2006, 11:54 AM   #1
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Default Modify Greddy DP?

Has anyone used the stock greddy downpipe flange to build a 2.5" DP? Or would it be easier to build a new flange using the stock one as a template?

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Old 07-17-2006, 08:28 PM   #2
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Unless you have access to some pretty serious machinery you would probably be better off just ordering one of the pre-made DPs.

I know that miatanut made the lower flange when he worked at a C&C shop and had access to some nice machines. The flanges are hard to source, the stock flange would have to be seriously milled to open it up for a bigger DP, and in the end I think cost/effort vs. a pre-made larger DP for the GReddy kit is just not worth it. Quite a few guys, myself included, have considered it and just ended up buying a premade one b/c it just didn't make sense in terms of cost/time. Tony makes a really nice 2.5" DP with nice small welds. Hard to beat in terms of price and quality unless you are also an experienced welder/machinist.

However, if you have the tools, you'd probably be better off using the stock as a template or ordering a premade flange. Heck, it'd be cool so see something different with the GReddy kit such as a 3" DP or divorced wastegate setup. Be the first and then sell em'!
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Old 07-17-2006, 10:21 PM   #3
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Been thinking about this for several years now, so figured would toss it out for the collective wisdom. Included two pix from my pages to add some clarity. Still believe the larger downpipe is probably the most expensive item added that produces the smallest amount of benefit or gain.

The exhaust outlet from the turbo has an ID of 1 7/8" .. the same as the supplied Greddy pipe and remainder of the OEM exhaust. I would not change that because I want my exhaust velocity maintained and not lose the low end grunt.

If we assemble the parts will notice a side chamber that houses the wastegate valve. Seems relatively insignificant but that small chamber would cause some turbulance in the exhaust flow. When that wastegate opens, will shoot the bypassed exhaust directly perpendicular into the exhaust flow. Suggest trying to eliminate both of those problems.

The idea: Weld a small flange to the downpipe to cover over the slot. If welding is unavailable, could just drill a few small holes in back of flange and use setscrews. Drill a hole in the flange over wastegate flapper to fit a small segment of 1" threaded blackpipe. Imagine any plumber or pipe fitter would have the proper tap so no need to buy one. From there a series of pipe segments would route the second exhaust down out of the way. For a muffler on it would try using a VW Bug exhaust tip. Never tried this but seemed to work very well as a silencer on my TEK-9.

As far as noise then it should not make any until the wastegate opens. Total investment for the parts needed would be about $10. If it is too loud or does not work, then need to blow another 65 cents for a pipe plug, then remove the small partition. If I ever remove the downpipe again I would try it.
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Old 07-17-2006, 10:30 PM   #4
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pop VII the center of the turbo exhaust is not sectioned off like a mitsu turbo is .it will leak the other and the o2 sensor will freak out & start dumpimg lots of fuel. that downpipe wont work correctly.
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Old 07-17-2006, 10:49 PM   #5
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Do not understand what you are saying. The purpose of the partition mentioned is to seal both chambers from each other. The wastegate has its own seperate exhaust.

Also don't understand what you are saying about the O2 sensor. At the point the wastegate opens you will be in open loop anyway and O2 sensor signal is completely ignored by by the ECU.
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Old 07-19-2006, 12:24 AM   #6
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if you look at the turbo in the picture there is no bridge to seperate the turbine from the wastegate puck . it will leak past it .maybe I'm not understanding you I thought you wanted to dump the wastegate side out of the exhaust system.
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Old 07-19-2006, 02:50 PM   #7
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Sorry, maybe I was not clear or could have chosen different words. The first step is to build that bridge or partition to isolate the valve from the main exhaust flow. When the waste gate opens it will have its own dedicated exhaust system. The biggest problem I see in most aftermarket down pipes and the supplied unit in the basic kit is that when the wastegate opens it blows in at a 90' angle causing turbulance directly behind the turbo. What I am looking for is maintaining velocity with as little turbulance as possible.

Not concerned with velocity with the wastegate gasses. Will act more as a simple pressure valve. My concern is with the noise and what it will take to muffle the noise down to acceptable levels. In theory it should not make any noise at all during normal driving or at lower than preset boost levels.

If it does work, then is a project many Greddy owners can do themselves in their carport with normal household tools. Estimate about $10 and 30 minutes to complete.
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Old 07-20-2006, 11:04 PM   #8
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thanks for clearing that up for me. I've done that to several cars mainly hondas. at normal driving your right you can't hear anything even @ 6psi its not very loud. it sound like a hissing sound but @ 10psi its pretty noticeable.
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