Will right angles (straight 90 degrees) instead of bent angles (swooping 90 degrees) in a exhaust system cause more back pressure? Think of it like a "Z" pipe instead of a "S" pipe.

You mean like this? This is before polishing

That would be two 45s +/-

Yes, but with 90's. I'm trying to find bent 90's but can only find straight ones. Anybody know where I can find some.

I don't know where you are located at but me living in the dairy state have found allot of stainless steel piping at creameries, milk plants, cheese plants, Etc. they can only use there piping for so long and then have to discard it. I have never paid for my stuff. even if you do they prob will sell it for cheap.

good luck

Yes, remember that air is technically a fluid and fluid flow principles apply. Think of a garden hose with water running and kink a 90 vs a smooth radius. It also may cause more reversion if in an exhaust application. The fluid(air) along the edges is stagnant or actually moving in reverse slightly. The more radical the bend, the more stagnant fluid there is. There are some pretty sharp engineers on here who can explain it better than me.

Imco makes a smooth bend 90 in aluminum.

We carry stainless smooth 90 degree bends also
http://www.cpperformance.com/detail.aspx?ID=4536

Do not use sharp 90° bends if you can help it. The longer, more sweeping you can make the turn, the better.

Think of a Winston Cup car trying to make a sharp 90 degree turn at the end of a straight a way.

Ooops. A Nextel Cup car too.