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Old 07-04-2020, 01:14 AM   #1
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Equal-ish length headers and GT25 turbo

This is a belated thread for work that happened between May 2018 and January 2019.

I had previously done a ground-up Trionic 5 conversion on this 1992 900 Turbo (as well as a laundry list of other modifications). I wanted more power, but not a ridiculous amount. I'm looking for a dyno-verified 200 whp and that's it, because I don't want to get caught in an endless vortex of blowing up the next weakest link. There's no shortage of paths to this modest goal, but I decided the easiest one, for a certain definition of easy, was the stock GT25 from a ng900.

Hear me out. The ECU, injectors, harness, and every other piece of the conversion that matters is from a late 1990s ng900 turbo donor. I have a 1997 turbo road course car that I've gotten to a good place, tuned (for a lazy definition of tuned) for reliability at ~190 whp with everything stock but the exhaust, which is what I'm using as a working example. My guiding principle for this project is, imagine what Saab might have done if they'd kept upgrading the classic 900 for another 5 or so years alongside the Opel platform ng900. It would have probably gotten backported hardware from the b204 engine, T5, the more modern turbo, etc., right?

I had a crunchy GT25 already lying around, and rebuilt it with a kit.

The GT25 is slightly smaller than the old T3 pattern turbo that came in the c900 stock, but more importantly it has different flanges and mounting bolts and needs to be clocked differently. The best place I could figure to start is to make the new support bracket and suspend it in a plausible place in 3d space, then build towards it.

A French tuner used to make a sweet-looking header for these cars, but it's no longer available. I used that picture to visualize what I needed to build.

I got a lego-like modeling kit from ICEngineworks. Not worth the money for making a one-off, but I expect I'll build a lot more headers before my life is over.

Played around with rough tape measurements, came up with a shape that looked right.

Taliaferro sells a flange with blended transitions which skips a ton of tedious work.

Modified the collector to fit the turbo. Then modeled the tube mockups on the car.

Transferred to stainless steel. Forgive me, my welds are better in mild...

Applied paste and wrapped. These present substantially more surface area than the factory log-shaped manifold.

Mocked and then welded up a 3" downpipe. The transition flange is another Taliaferro part.

It's a 3" downpipe and center section of my construction, that transitions down to a 2.25" straight-through glasspack and rear end which was originally a Swedish Dynamics bolt-on bought back in the early 2000s.

The water plumbing for the turbo was straightforward. The oil inlet was kind of a pain since the factory supply is tapped off a banjo bolt buried under the intake manifold. The tricky part is the turbo oil drain, because it's only bolted to the turbo and not the block.

The T3 has a hard pipe section bolt directly to the turbo and tightly fit into the block with an o-ring.

The GT25 has a flange and pipe nipple bolted to the turbo, expecting a soft drain hose to make up the distance to another metal nipple returning to the oil pan.

I used a combination of stainless tube sections, welded-on detent ridges, and silicone hose to make a drain adapter between the factory B202 block and new-to-me GT25 turbo. This works but I'm not thrilled with it. I expect to make a tighter-fitting hard line someday.

The other tricky part is clocking the 3 parts of the turbo. The intake and exhaust halves are self explanatory. The center section has a narrow range of possible orientations dictated by the oil inlet and outlets, which need to be within a few degrees of vertical. That would be fine, except the wastegate actuator mounts to the intake housing and expects a certain clocking between intake and exhaust which was specific to the B204 in the ng900. Not only is that clocking not the same between the two sides, but the center section is clocked differently and its hoses are in the way of the wategate actuator arm. I had to make tiny adapters to work around this, which introduces more slop. It works, but I'm not sure I ever got this adjusted perfectly.

I did a bunch of drives around the block with my t5suite laptop and a wideband O2 sensor tweaking things until it ran right. It likes to run super rich even unloaded, and it won't take as much boost as it should before misfiring no matter how much I fatten it. It wasn't perfect before this swap either so I think I've got an unrelated timing issue to track down.

In any case, it's back out of the shop and on the road again. Bonus material: I put these Braid rally wheels on it.

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Old 07-13-2020, 11:25 AM   #2
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Join Date: Jan 2007
Location: Granby, CT
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Wow - great fab work! Love to see this kind of stuff being done to c900's. Almost a lost art... good luck with the rest of the build.
2011 9-3: Current
1983 900: Sold
1996 900SE stage 3: Sold
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