I dyno'd my truck today and I need more boost. How much boost would I gain from swapping the 3.5 to a 2.75? One more question, can you fit a JL 2.5 without grinding the snout down?

 

Why not get a bigger crank pulley, so you have less slip problems.

 

I already got an eight pounder on order.

 

you'll gain roughly 6#

 

If you have an 8# lower I don't think you'll want a 2.5 upper. I know a 10# lower with a 2.7 upper makes 22-24#'s on my friends built truck.

 

My motor is also built.

 

I figured you did, I was just saying I think you will be more efficient and have less problems if you keep the upper as large as possible, which means make your crank pulley as large as possible.
I've got a built longblock on the way and am trying to figure out what pulley combo to go with. I want to run 18#'s on the street and 20 at the track. I'm guessing if I run an 8# lower I can go between a 3" and a 3.25" upper to achieve this.

 

how much boost are you looking to run?

KB says with a

3.50 upper 9.0 lower(6lb) 13.5lb BOOST
3.25 upper 9.0 lower(6lb) 15.5lb BOOST
3.00 upper 9.0 lower(6lb) 18.0lb BOOST
2.75 upper 9.0 lower(6lb) 20.5lb BOOST
2.50 upper 9.0 lower(6lb) 22.0lb BOOST

if your gonna run and 8lb lower i would suggest running a 3" upper

Victor

 

That chart goes out the window on ported heads and cams with full exhaust.