no low end torque ,need help

[hotstang111]

i have a 98 ex and i just put a brand new 4.0 ohv in it (dont ask how i got it) and im lacking alot of torque for taking off, i have to ride the clutch or it will bog out and then it has a hiccup around 1k rpm, i thought it was the fuel timing so i repositioned the syncronizer withe the correct tool from ford, it got slightly better but still very little torque. its a 1999 engine so would the computer(98') have the wrong fuel mapping from that year? any suggestion on what to look at

 

[2000StreetRod]

fuel pressure?

Your 1998 stock was a return fuel system. In 1999 Ford switched to a higher pressure returnless fuel system. Did you use the fuel rails from the 1998? Also, I seem to recall there were engine wiring changes in 1999. Did you use the 1998 engine wiring harness with the 1998 PCM?

 

[hotstang111]

yea i switched everything from the 98 so all i changed was the shortblock and the heads, intake and plenum, switched the fuel rail and injectors, it doesnt set any codes, the last code i got was the p0340 about a month ago

 

[hotstang111]

how much would the backpressure effect it, i have a 2 1/2" flowmaster 40 with a turndown after the cats.