gleem wrote:I was thinking why does it need to be alloy or steel when ill be running 15 psi.
to answer your original question, people generally use alloy because its lighter, doesn't rust, and it dissipitates heat alot faster than steel.
steel will rust unless you get it coated, then you'll likely end up with rusty air and probably bits of rust off the walls going into your engine/turbo. also absorbs heat alot, so you'll end up with a higher inlet air temp.
also, hydraulic stuff is likely to be more expensive than anything