Why is my car slow..?

You can have a good combo (which you do) but it all comes down to the driver. Traction is usually the time killer....also, like dinomite said you may be shidting to early or too late (every car is different and everyone has to know what works best for his/her car). I think you should be in the mid 13's. Get a nice set of heads and you'll be there for sure.

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'90 LX 5.0; 12K original miles (no sh*&); 3.55 gears; pulleys;Edelbrock Performer Heads; BBK shorties; MSD 6AL box w/ blaster 2 coil; Motorsport E303 cam; Pro-M 75mm MAF; BBK 70mm TB; Eibach spring kit; Southside welded subs; K&N cone filter charger; Hurst shifter; fiberglass turbo hood; A/C-less; rear seat-less; cat-less; 2 chamber Flos; Corbeau racing seats (fronts);
30# injectors; JMS Chip; 190 lb fp; TFS track heat Intake (12.299 @ 113)