Jonnny, have you thought about getting a second car that has punches of power?
This way you have cars for what ever mood you're in.
For the money you're looking to spend on putting a V8 into your Z4, you could easily afford a decent V8 car with plenty of power.
I've got a few and they all give me that hit for what mood I'm in. The Z4 gives me roof down fun. For when I want to go for a hoon with more power, I've got a Jag XKR with 5.0 V8 supercharger and tuned to over 600 BHP. If I want full on hands on craziness I grab the keys for my Corvette Z06, this has 7.0 V8 and tuned to nearly 800 BHP.
When I take the family out, I have a customised van!
Being in a lucky position and having owned and currently own a few cars, if I had the funds to put a V8 into a Z4 I wouldn't do it. A V8 is a heavy engine going into a very small car, it would be completely unbalanced. It would take months of expensive tweaking trying to fight the handling, its completely unknown. Both my Jag and Corvette's 12" rear wheels often struggle to plant the torque of those V8's, you'd have a nightmare on the Z4.
The Z4 chassis, in my opinion, is too small to handle so much power and I would feel I'm driving an accident waiting to happen. I feel safe in the Jag, Corvette + all the other V8's I've had. The manufactures spend millions testing these chassis, suspension, brakes etc testing for hours and hours. The car feels safe, if the back end goes I'm still in full control.
If the Z4 chassis was made for a V8, it would have come from the factory this way. Don't put your life in risk if you're not a trained driver.
I have far more fun grabbing the keys to a car that suits my mood. the Z4 puts just a big smile on my face as does the Corvette.
You can pick up some seriously good V8 cars for your proposed engine change money.
If someone mocks you about having a 2.0 engine, show them a pic of your other car...soon shuts them up!!
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