240sx with a 96 RB25DET engine

I have added a RB25DET engine on my 1995 240sx runs pertty good but I am not able to floor it... it seems like it backfires and wants to die so i let out of my foot....

what would cause it...
I went to have it dyno and they toll me that I am running up to 20 PSI but only getting about 136 hp..what is the deal with that???I don't have a gauge yet but will add it, also don't have a boost controler but will be adding those soon... I did change the spark Plugs and the exhaust from the stander 240sx to 3 inch pipes from the down pipe to the end. also added a magnaflow catalic converter....

Also only getting about 14 mile to the gallon is that normal???

if you have come accross any of this issue and fixed them let me know something... thanks...

I think that It could be the exhaust or the injectors and the fuel system but, Im about to but a RB25DET in my 91 240SX. And what you just posted is making my think twice about it.

FUEL PUMP...
You need a R33 fuel pump fitted to supply more fuel pressure and flow...
With the factory Nissan turbo, you cannot run more than 14psi or the exhaust wheel will eventually shatter and destroy your turbo...
I previously owned a Nissan S14 Silvia with a RB25DET conversion, plus done this conversion for friends so either of you PM with questions.