Crosman detunes their b18s in various ways; longer piston, thin spring, and a hole in the piston, including any combination of the aforementioned methods. You will have to open it up and find out what you're starting with.
My own experience was with a 495fps .22 phantom. It had the standard (short) piston, no hole in the piston, and a very weak spring. With a full power spring, velocity was 720 fps with 14.3 gr pellets. While the gun was accurate, it was very pellet picky, and very hold sensitive. It wasn't fun to shoot. I changed the spring to the 12fp spring (can't remember the Crosman part #) and velocity is currently 630-660 fps. (About 13fp) Hold sensitivity has disappeared, cocking force is considerably less, and it will shoot almost any pellet well, even Grizzly domes! I sure don't miss the 80 or so fps lost with the slightly weaker spring.
Since my gun was factory detuned by a weaker spring only, I was able to use the stock piston, seal, spring guide and spacer.
I recommend a full lube-tune when using a stronger spring. The higher velocities tend to magnify any inconsistencies in the overall tune of the gun. Interestingly, my most accurate springers are all detuned to 12-14 FP.
Just a reminder that you will require a PAL if you modify the gun to exceed 500 fps.
Edit: If you order a piston, they do not come with a seal. When you order a spring, add a few piston and breach seals to your order. They're cheap and if you have spares, your originals will last forever.
Good luck!