How can you make the 20b street legal? Also if having a force inducted 20b, how big would the pipes for the exhaust have to be?

 

There as nothing non-street legal about a 20B as long as the installation passes emissions. What "passes emissions" means depends on local laws. Some areas just send you through a sniffer and if tailpipe emissions pass, the car passes. Some jurisdictions require you to run all the original factory emissions equipment to stay legal. This is more of a challenge than just trying to pass a sniffer test.

The size of the exhaust will be director proportional to the power you intend on making.

 

^ As above, "street legal" depends on the local regulations. For example, no 20B is street legal in California.

The RXX-7 was street legal in Maryland, and made 373bhp @ 12psi with stock porting.
http://www.cris.com/~Asam/Dyno1.html
http://www.cris.com/~Asam/index.html

In states with no emissions testing and no visual inspections, I suppose you could run up to 1400bhp.

Technically, a 20B is not street legal inside the USA because it has never passed USA importation standards, but the feds never check things like this.

Also as stated above, the optimal exhaust size would depend on the hp level of the engine, but like anything else anybody could install a size that is larger or smaller than optimal. I guess I don't really understand the question.

 

oh ok, and I always thought 2.5 was to be used for N/A engines, and 3 for force inducted engines. well that clears it up.

 

The piping diameter has nothing to do as to whether the car is NA or forced induction. The diameter of piping required is simply a consequence of the amount of exhaust you need to pass through it. Typically, this means that lower HP engines have smaller exhaust pipe diameters, while higher HP engines have larger exhaust pipe diameters.

A 20B at stock boost on the stock turbos would be fine with a 2.5" diameter pipe. However a 20B with a GT45 would want more like 4" diameter.