My dad ownes a BMW 523i (2.5 litre engine.) We have done various mods on the engine: Group N Racing Air filter, New ECU which has been tuned to high end accelaration, and it's awaiting a Garret T3 Turbo.
A very nice car, but why on earth go to such expense modding a 523i to go like a V8 when theres the simple option of the 540i ?
I'd sell the 523i and start playing about with a 540i if you really want some results from your investment.
The 540i is in 95% of road situations, right up there with its bigger brother, the M5.
If I were to modify an e39 5 series, I'd start with the 540i and get some M5 add ons for it, both cars are V8's and a surprising amount of tuning parts for the M5 bolt straight onto the 540.
The 523i was never designed to be a tyre burner, so why on earth try to re-engineer it when such a similar but superior platform is sitting in your local car lot, waiting for you to sell your existing car - which in its condition, I'm sure would fetch good money, and add what you would have spent on the turbo conversion to the pot, then go find a 540i, leave some change to modify that if you wanted, but, I'd be surprised if you wanted more performance for a while with it.
Extremely capable and fast motors, I'd consider one when I eventually sell my e38.
Posting drivel here since Jan 31st, 2002. - That long!
"He who laughs last, thinks slowest."