I just got my 93 240sx hatch up and running with the new diff (stock open diff ) The engine runs like a clock, if I had to guess it's got around 60-75k on it tops. Definately been well maintained, good oil and filters used. My friend with a 93 coupe got his like 5 months ago but the engine blew right after, so he got it rebuilt. they included a slight overbore, said it should have given him 1 cu. in. tops. Also a gutted cat. So I'm pretty used to his car, thing rips through first and second with ease. It's able to keep up with a bone stock Integra GSR easily(7.1s to 60! Stock 240 is 8.33), even beat sometimes if the launch is right. So I somewhat expected it to be a little like his car minus gutted cat. This thing feels simply lethargic through first and second even when taking it up to 6500. Will a gutted cat and a slight overbore do this?? Were decided to race them tomarow just to see where this one stands, but I'm anticipating a train wreck

 

Shifting too high, power begins to drop around 5.5k so shift somewhere around 6k at the latest.

 

Thats exactly whats got me confused, when getting on it with his car the fun doesnt stop till 6500 rpm, he usually shifts around 6700 with pretty good results... I forgot to mention he also has nice plugs with NGK wires... should give him all of 0.005 hp and peace of mind?

 

There are too many factors that could contribute to this to say, especially since he had his engine rebuilt.

I would do diagnostics on my sensors and then check the ignition system. Then I would do a compression test and a leakdown test.