what is a compression at in a 5.3 with 317 heads on it? im thinking about getting a set when i buy a supercharger. is it worth it?

 

You will be down below 9:1 CR, close to 8.5:1. I personally wouldn't do it because that's going to lose low end torque.

 

so you don't have to lower your compression when you go FI?

I've been reading about FI applications for a little while now.. and it seems people are boltin 'em on with almost stock applications..

 

That's a subjective question. How much boost are you planning on running? If you are running the boost level that came with the kit, you are fine on the stock compression.

If you are thinking about upping the boost (say over 7-8 psi), you need to consider lowering your compression. The easiest way is to get a set of 317 heads. They've got ~72cc chambers which will drop your compression about a full point on a 5.3L. The better route would be upgraded rods and pistons to handle the additional load placed upon them from the boost, but some people don't have the cash (roughly $1500 if you do the mechanical work yourself vs $500 or so for 317 heads; the heads, good springs, gaskets & head bolts).

Put it this way; it's a long road that is very tempting. You boost today, you WILL want more. A lot of guys say they won't, few stick to it. It's just too much fun.