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I have a 525i engine model 1990 and willing to install a turbo system with an ECU.
Can I keep it stock, if I am targeting 400 hp or I should make a rebuild?
First do you have all the stuff you need to make it turbo? Turbo, manifold, wiring for the ECU, injectors, etc (and of course clutch, brakes).
Every stock engine is different in terms of component strength. In some instances everything is pretty close to the limit of what the stock engine produces and any real power increases will potentially result in failure while others can often cope with double the stock power with no problems. I'd recommend asking on a few BMW enthusiast forums and see if you can find out if there is a known weakness. I'd be surprised if 400 hp would be an issue for the stock internals but I simply don't know enough about that engine.