Your little 4 cylinder hasn't given up because it's a stock iron block BMW engine with very little power.
Trust me, I looked into it. You might make 250whp.. You could probably make that on a ghetto RRFPR ebay setup on a M50 and not even break a sweat.
91 318is - M50NV (gone)
02 WRB Subaru WRX EJ207 forged, 320 AWHP stock turbo.
85 - 325
02 325i Sedan