i've heard of people putting flex pipes in their exhausts but i have no idea what they are or what they're used for. :dunno:

 

It's a braided or spiraled steel section that can be bolted or welded into an exhaust system. The purpose of it is to maintain a leak-free exhaust system while accounting for engine movement that usually occurs in higher output applications.

Here's an example of the braided style:

And here's a shot with both styles in one photo:

 

my car came stock with a flex pipe i thought all cars did

 

Not all cars. Most just come with a spring and donut gasket joint that combines the exhaust manifold and the under-car exhaust system back to the rear bumper.

 

so will things break if you don't use one?

 

If your engine vibrates a lot, that vibration will get transmitted tot he exhuast, and it may rattle. I don't have a flex section, and my downpipe goes straight to my cat with no donut gasket or anything, and it's been fine for over 2 1/2 years. My engine doesn't really vibrate that much though, since the urethane mount inserts hold it in place pretty well. I think more of my engine vibration is transmitted to the chassis thru the mounts than thru my exhaust. If your engine moves around a lot, get a flex section.

 

I had my downpipe crack when I eliminated the flex joint.

 

You need a flex connection if you bolt the exhaust system to the car. That includes pretty much any transverse motor setup, unless you're using solid motor mounts & torque strap mountings (read: "not for street use"). Don't confuse braided flexpipe with the cheap corrugated type! Corrugated is for half-assed installs, and can't handle continuous flexing. Use the braided kind.