I have a 91 f-150 w/ a 302. Does anyone know what I can raise the compression ratio to and still be legal. Also what else needs to be changed when raising compression? All responses are apreciated.

 

You can raise the compression and be legal, but it usually raises the temp and the pollution, so you might end up having trouble passing smog tests. Before you do this, I'd see how much advance your timming can take before it pings on a warm/hot day under load. If your having pinging with a few deg of timming advance, I'd forget about raising the compression, otherwise I'd look to milling the heads as the cheapest way to increase compression.

You might be able to do a head swap with an older head or change the pistons but that's more work and money. I milled down a head on another car to get more compression in an effort in increase the mpg and it helped with that and power was up a tad too. A cam change can help, but that usually means going to a smaller cam.

 

1) Why do you want to go to higher compression? Power? Performance? Fuel economy?
Things to consider when raising your C.R.
1) Anything over 10.0:1 generally requires high octane to prevent detonation. Super, or turbo charging is basically a dead issue now, unless you want to put extra machining costs in for better cylinder sealing.
1a) Higher compression yields higher concentrations of NOx (formed from heat and pressure in a cylinder)
2) Rule of thumb for raising your CR --- 15 hp/point (i.e going from 9.0:1 to 10.0:1 = 15 hp).
3) Raising the CR is not the optimal way to increase performance.
4) Better cam timing (i.e. duration and lift) enhances the change in CR.
5) Speed density systems (yours) are not very compatible with larger lobe centerline angles/degrees of duration.
I could go on for a long time, but the bottom line is this:
If you change one thing in your system, it affects many other components. Changing one thing, may make the system work worse. You may need to change several items to take advantage of your original change.
An old, but true, statement: Horsepower costs money, lots of it.