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I ran the Ebay headers on my old E46 M3. I paid $175 and they probably provided more power than any other mod on the car. I got 12whp with the stock ecu, OEM tune. That was way more than I actually expected from them. I used better exhaust gaskets than the ones that came with them and there was no issue with fitment. However I think that most vendors sell the E46 M3 header for the Z4M. I think there my be a couple of fitment issues with these on the Z4M. I haven't looked into trying to put them on the Z4 yet.
I don't think a more expensive better build header would have provide much more power on my car. I don't really care if they break. For $150 I'll try throw away and buy another set. I beat on them for 36k miles on and off the track and they were fine.
But I would not pay $1000 for them.
I change cars about every 2 to 3 years. If I was going to keep the car for a long time then quality/durability would be more important.
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