Considering your engine engine and electrical problems are in good order, explosions(backfiring) is ALWAYS an indication of wrong air-to-fuel ratio. The cut-off at 4000rpm to my mind is simply fuel starvation at higher rpm's. Imagine a swoosh of extra air from the turbo where there is just too much air compared to gas injection. Engine will burble out.
In practice, simply putting a turbo on top of an engine WILL NOT WORK unless mods are made to the compression ratio etc.... If I remember correctly there are several topics regarding turbo modifications in this forum if you will take the time to research.
Additionally, you cannot exchange Jetronic & Motronic parts where it is no cross reference specified. They are completely different systems and the sensors which act as inputs to the processor have different parameters & specs.
Last edited by darkhorse737; 08-15-2009 at 10:59 PM.