Aftermarket: There are basically three aftermarket
exhaust systems: Borla, Modern Performance, and the local muffler
shop. Borla has a 2.5" stainless steel cat back system with
an option of 1 or 2 pipes out the back of the car. Borla also
has a 2.5" downpipe. Modern has both 2.5" and 3"
cat back systems and also sells hi flow catalytic converters.
Three inch downpipes are available from Modern Performance. Muffler
shops can bend up 2.5" and some can work with 3". Both
2.5" and 3" mufflers and CATs are widely available.
Other: Testing has shown the Borla single pipe system to flow better than the Borla twin outlet system.
Testing has also shown the 3" downpipe to decrease turbo lag and increase power even with the OE catalytic converter and pipes. This is because the exhaust gas coming off the turbo is spiraling; in fact depending on impeller speed the direction of gas rotation will change. By using a large chamber coming off the turbo the gas will change from spiraling to a more linear flow. This reduces the dynamic flow resistance. It just happens that 2.25" is not large enough but 3.0" is; on higher flow engines the 3" may not be large enough to reduce this phenomenon.
A large pipe on the last few inches of the exhaust has no performance effect on the engine, only on some drivers.
Replacing a functional catalytic converter with a high flow one may be illegal in some areas. It is not illegal to replace a non-functioning cat. In practice it is nontrivial to determine if a catalytic converter is functioning completely so muffler shops are more than willing to replace them.
|OE: The OE system is 2.25"
The flow potential of an exhaust system is only as good as its biggest restriction, no matter how big of pipes you're using. In a typical exhaust system, either the muffler or the converter will have a higher pressure drop than all of the piping combined. Mufflers are like a cork in the system, at least "stock" type mufflers. A single 2.5" system will flow enough for almost any street 2.3 turbo if the right converter and muffler are used, along with fairly smooth bends(they don't have to be mandrel bends). A dual exhaust on an XR makes no sense from a performance standpoint. The exhaust starts out as a single pipe at the turbo right? Well splitting it into two causes a substantial restriction at the Y-pipe if not properly designed(and they usually aren't). You want proof? My friend's '88XR ran a 13.99 @ 100mph with Nick's 2.5" system and an SVO ECU and VAM. That's it, no other mods. Sure, he was running 20 psi of boost and 98 octane gas, but it still shows what a good 2.5" system can do.
> > Unplug the knock sensor? That is real sound advice.
That is the only way you can run the SVO computer. The knock sensor is too sensitive and will severely hurt your performance(See the dyno sheets from my car on Dave Compton's web site to see what happens when you leave the KS connected). If you run good gas and use your ears, detonation won't be a problem with a good intercooler.
> > Why not take your car to the track and actually time it in the 1/4?Your > "results" would be much more credible..
I have basically the exact same mods to my car as Mohamed does other than having a different intercooler and my car recently ran a 13.6 @ 104.5 mph. My motor has 130k miles, has never had the head off, and has unported stock manifolds and turbo. There's nothing strange or unbelievable about Mohamed's results, these cars are easy to make fast if you concentrate on the areas that need improvement and leave the parts that already work well alone. An XR could easily be made to run 12s in the 1/4 without ever taking off the valve cover if you could get the rear to hook without blowing up the diff.
Have fun, Chris