is the size (diameter) of stock TBI and TPI fuel lines. im running lean on nitrous and thought maybe that my fuel lines were maxed out since my car used to have TBI now it has TPI and is running alot more pressure. any ideas?

They're both the same size lines. Those fuel lines are good to about 500 HP so you're most likely not maxing the lines. Your fuel pump might be maxed out though. You can put in a 255LPH pump in the tank then you're fuel system will really be able to push 500 HP on motor no sweat. You might just need a booster pump for the nitrous, look into those.

I agree with Mark. We installed an in-line pump on my brother's 87 T/A (in addition to leaving the stock one in the tank running)with a stout Minirammed 409ci small block and it holds constant fuel pressure under all conditions where the stock pump wouldn't hold pressure past 4000 RPMs at WOT.

well let me clear up some misnomers:

1) they are both not the same size (at least for TPI, im TBI illiterate but they should still be the same)
2)depends on the fuel system you build and 500hp is no where near the max

The feed line is 3/8" and the return line is 5/16"

If you run a dual intank pump setup, you can use the stock lines to about 700-750rwhp without problems with fuel pressure about 65psi max

i have a holley/walbro intank 255 lph pump in it right now. it has about 1000 miles on it, so i guess that the lines and pump arent my problem. thanks for the info

Procharged - I know the feed/return lines are different sizes, I meant TBI vs. TPI lines.