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1,193 Posts
Discussion Starter #1 (Edited)
As some of you might know, I have been researching making a better O2 housing for the stock turbo, or in my case, a modified TKT hybrid using the stock exhaust turbine housing with a Garrett Center Cartridge and a Garrett Compressor Housing. I realized that much of the power of the newer bigger turbo was being choked by the stock o2 housing and downpipe.

So I started off on a quest to make a new BIGGER and better flowing o2 housing. PLUS I thought it was a better idea to divorce the wastegate flow and it all had to fit with my air conditioner compressor..

First, note that the stock o2 housing actually has a standard 2.5 inch 3 bolt flange.
Here is the housing with a 2.5 inch flange placed on it:

Note that the actual ID of the stock o2 housing is TINY, less than 2 inches.

Here is one that someone on MX6.COM made by chopping the stock o2 housing and first welding 2 inch, then 2.5 inch pipe to it. Kinda sloppy but it is better than stock.

The outlet is much bigger, however, it started out from the stock downpipe where there is a significantly small and square restriction in the bell housing, PLUS the wastegate exhaust air is released right there into the turbo exhaust flow.

So you really cant build off the stock o2 housing. You need to start fresh with a good flange like this one:

You can get these at Welcome to BMC Racing Products - Welcome to BMC Racing Products for $35.
However, this wasn't exactly what I needed.
First, this flange basically does the same thing as the stock o2 housing, by the fact that its thicker and there is an indentation that allows the wastegate air to flow into the turbine exhaust flow.

They happen to be in Houston where I am, so I went down there and had them design it like this.

This way, I could weld on 2 separate pipes, the smaller one for the wastegate.
We also made the flange only 3/8th inch thick so we would not have to truncate the pipe so much.
Also note the little notch we added so the wastegate hinge would have no chance of getting stopped by the flange.
Thanks to me, they now sell this one too, and for significantly less, but you need to call them to get the right version and pricing.

The next step was to weld up a fixture from the stock o2 housing. My goal was to make one in the same dimensions as stock for ease of upgrade. That way, I could make more and sell them to anyone that wanted to just bolt it to their stock downpipe (which I don't recommend).

Then we welded a "fence" to the other side so we knew where the a/c compressor was.
This way we could stay clear of it.

This is what we came up with using a 1.25xd 2.5 inch 180 mandrel bend and a 1.5xd 1.5 inch 180 mandrel bend.

It was not easy. It took a good 8 to 10 hours of welding, cutting, flaring, collecting, etc,
to come up with something that would put the wastegate flow back in as gradually as possible and still be
boltable to the stock turbine housing using the 5 studs.

I will show you more pictures but FIRST I want to show you what we did on the OTHER side of the IHI flange.

We took the CNC design of the original thicker flange and determined the
circumferencial length of a piece of 2 inch pipe needed to make a fence that
would just fit into the turbine housing. We measured the depth and made this fence
piece just a few thousanths shorter than the depth and welded it on.
When we test fit the flange, in the case of my cast turbine housing, it was almost a PERFECT match.
We only had to do a slight amount of grinding on the corners.
This fence essentially separates the inside of the stock turbine housing without having to remove it and modify it.

1,193 Posts
Discussion Starter #2 (Edited)
More on the custom o2 housing

Here are some more pictures of the o2 housing with the turbine housing separator fence welded on:

We welded an o2 bung on at the stock location. Later, I will probably add a 2nd bung lower down for a wideband o2 sensor.

Here is a pic of the wastegate tube collected in just above the flange:

This is a MUCH better design than that German thing for sale on eBay for $300:

Note that although theirs does have the fence, its too small, the ports are smaller, the tubes are smaller, the flanges look like 1/4 inch, and the wastegate tube feeds back in at almost a 90 degree angle!!! Not to mention its ghetto chromed.

Here is a side-by-side comparison of the NEW o2 housing versus stock.

Just look at the difference in outlet size!!!

Now tell me... do you think my new design will flow better than stock, or the ugly German one? Lets try it....

Here I'm bolting it to the car. I decided it was easier to just move the a/c compressor than take off the entire exhaust (again).

I had to bend the oil dipstick tube out of the way just a little to accomodate the wastegate tube:

Here it is with the a/c back in place. Note that the wastegate tube does get a little close to the compressor but that area will not be as hot as the main downtube.

I went ahead and bolted it to the STOCk downpipe and drove it around just so I could see if there was any difference.
This is a view from under the car looking at the bottom of the turbo.
The blue thing below is my A/N fitting for the oil drain line, obviously not stock.

I drove it around with the stock downpipe. I first noticed that it sounded different,
an unusual side effect, rather good.
The boost came on considerably earlier and faster. I had my controller set to 8psi
and it got there very quickly and stayed PEGGED on 8psi even and full throttle.
This means, NO BOOST CREEP. So the wastegate was opening and breathing just fine.

I plan to take it off and get it heat coated by the Polymer Dynamics guys, the same ones that make TX-7
They make a new ULTRA COPPER coating that will penetrate the metal and make it a copper/yellow.
The idea is to keep the heat IN the downpipe. Plus the coating will prevent rust since we just made it from mild steel.
I may also nomex wrap it as well since I cannot put the stock heatshield back on.

The next step, of course, is to make a suitable downpipe for the new enlarged housing.
I already have a 3 inch downtube I got from
I may just weld on a 2.5 to 3.0 inch recuder/expander like this one:

Then weld a 2.5 inch 3 bolt flange to the small end of that.

1,193 Posts
Discussion Starter #3
Next generation

I have considered some changes already to this version.

Here are some thoughts on the next evolution:

First, bring the wastegate tube over the top and back of the turbine tube, then down on the front side. This will do several things:
1. You won't have to move the oil dipstick tube
2. It will keep it further away from the a/c compressor.
3. It will make getting the nut on the bottom stud easier. (it was very hard with the turbo and exhaust manifold still attached to the engine).
4. It will lengthen the wastegate tube giving it more volume, allowing it to work better.

Second, use a 1xD (one times diameter) 2.5 inch mandrel bend instead of the 1.25xD mandrel bend that we used. This will mean less truncation of the tube and better flow.

Third, before the bottom flange, weld in a 2 inch gradual 2.5 to 3.0 inch exhaust expander (as pictured above), then go to a 3 inch 2 bolt flange. The wastegate tube will still collect in but part of it will collect into the expander area so there will be even less interruption when the wastegate opens. The less interference there is from the wastegate opening, the better the turbo can maintain spinning. This way, the expansion to 3 inches occurs sooner, and I can actually call it a 3 inch o2 housing.

Fourth, make a 3 inch downpipe with a 2 bolt flange instead of 3. Its easier to bolt on and the flanges we use are plenty strong enough. Then weld a 6 inch flex tube to the other end and/or leave it open on the other end to do whatever with.. 2.5 inch or 3 inch aftermarket exhaust OR even go back into the stock exhaust, although I don't recommend that because you would delete all the advantage you just got with the bigger o2 housing.
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