I hoped I would see a change after changing my O2 sensors that had over 200k miles on them. Nothing. I even went in and cleared all adaptations on the truck. Still no change. Oh well.

Exactly. I would even argue that the computer doesn't know how much fuel its putting it. It's just modulating an injector pulse width to hit an O2 sensor value that it deems to be correct. All of the sensors and devices involved drift over time: the airflow meter, injector flow rates, O2 sensor, and even the wheel speed sensor due to changes in the tire diameter. The computer isnt concerned with making sure your MPG meter is right, its concerned first with maintaining emissions, then AFRs. That's why logs are the only truly accurate way of measuring changes in MPG.



You're also exactly right andrewwynn, that the computer learns how to react to changes we make when we modify or maintain the engine, and sometimes it takes a little time.

All I can say is that the replacement of the stock manifolds, which look like a terrible design to me for performance considerations (outside cylinder charges run directly into each other and then make a sharp 90-deg turn downward), netted me a not insignificant increase in power in the mid to upper RPM range (SOTP measurement) and a ~10% increase in highway mpg as measured by the trip computer.

I've always found my trip computers to be pretty accurate, and I don't buy that it will all of a sudden drift off like that.

BTW - I had a C4 corvette with the 6 speed manual and easily got 28 mpg hwy and could top 30 if I kept the speeds 60 or below. This was measured with fuel/mile log. The L98 TPI motor is tuned for low-end torque and turns about 1600 rpm at 60 mph with it's 3.33 rear gear and 0.5:1 OD ratio.

Do some full tank mpg mpg measurements. It would be awesome to get an actual measured increase


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I'll do that next time I take a longer drive absent heavy traffic.

Yes. Quoted for posterity.

UPDATE: Decided to stay with Catlytic Converters. Mine had a massive crack in them which would explain the "puff puff" noise. First set i ordered on ebay. Learned my lesson and I cannot be mad at it. Nothing Lines up.. Returning and another member mentioned he used these (Links below) with is a performance brand that MangaFlow bought out in 2017. He said they bolted right up. So with the free time waiting for shipping, i did my Valve Cover Gasket, and replaced all the studs.

For anyone doing this on the 3.0 X5, you DO NOT have to remove the stabilizer bar, just remove the bushing and gently adjust it out of the way.

BOSAL is the brand..
Front
Rear

I thought I was going to light the fires again last weekend, but when I chopped off the OEM flanges into the resonator, I found one to be 1 7/8 OD and the other to be much closer to 2". So my 1 7/8" ID adapter from my 2" cat fit over one pipe, but was useless on the other.


Is that a design feature, or a manufacturing defect?

I'd be surprised if it was a manufacturing defect. I believe the 4.6is had different pipe diameters L to R(by 5mm IIRC), and they are both metric - something to keep in mind. Maybe the 3.0 was the same for packaging reasons.

My exhaust manifold gaskets were kind of flimsy and when I pulled on them they separated.
After some research I found this to be a common problem, with people usually experiencing rattling.
I unbolted almost all the top bolts and I'm planning on changing the manifold gasket.
However with the manifold off, I was thinking of maybe installing headers and found this thread.
The russian headers come to around $800 CAD which doesn't seem sane.
Are there any cheap ebay headers that would fit properly?
As I don't have my engine out, I can't drill and tap extra holes and I want to avoid this situation with exhaust leak
https://youtu.be/Cc3DxEE-0pc

From 6 pages of this thread looks like the only person who actually installed the headers is e39_touring
And another person paid around $1000 for headers and catalytic converter.
Let's see if anyone else was able to successfully replace headers for cheap with a proper fit??