I could be completely wrong on this, but it might be because the turbo uses backpressure to drive it, and the 3" pipes allow too much flow for the turbo to spool up on a car with less than 300hp. Using 2.5" pipes restricts the flow a bit and allows more backpressure to build up.

 

According to Bernoulli's principle, a faster moving fluid (air) is associated with a lower pressure. This is what causes lift on an airplane wing. If it's true that a 3" pipe allows for faster exhaust speeds than a 2.5" pipe, then the pressure of the ehaust moving across the turbo blades would have less pressure. Imagine putting your finger over a water hose. The hose with the greater pressure will get the "fan blades" moving quicker. Of course, like most of my logic, it could be completely wrong.ô¿ô
Doug

 

as big of an exhaust you can fit under there, put it. The larger the better. I run a 3"mandrel bent, turbo back w/ no cat/silencer etc. The turbo will spool slightly faster with a smaller exhaust , but you will lose out on top end horsepower and increase backpreassure causing the torque to jump up as a biproduct of this.

 

Comming out of the engine you don't want a 3" pipe to the turbine because of this reason but after the turbo you want the exhaust as free flowing as possible. Just think if you shrunk the exhaust and turbo down to the size of a straw and you blew into it to spin the turbine. If you had a smaller "turbo back" on it, it would be harder to spin the turbine than if you had a bigger "turbo back".


Also, Mike is a nice guy and back in the day he was one of the only subaru sites but he is extreemely conservative.

 

If you have a car that already doesn't have much torque to get you moving then keep in mind you will sacrifice more of your torque to gain the hp for the top end. Its all in how you want your car to drive and what you will use it for. If all you care about is top end or drag racing then go for it cause in both those situations it won't matter since you can either rev the hell out of the motor or you will want better trap speed. If its stop light to stop light daily driven all around car then it might be more of a concern. I think it also comes down to a matter of overkill. If you don't intend to go beyond 300hp then would it really be needed to spend the extra money on an exhaust that will make little difference on a car that won't be pushing the limits of a 2.5in exhaust.

 

I agree totally here, many people believe in total horsepower, which is great in a high reving racing situation, but for us street drivers and road racers [the track kind] torque it the "holy grail".

But then you start breaking gears, horsepower is great on the open highway, but those are in very short supply here in New Jersey.

A 2.5 inch system just maked sense for a daily driver and eliminates all sorts or clearance problems, dif protectors, hitches.

IMO

 

2.5 is better than 3 for low-end torque

 

Yes, it is better for better torque. It is also relative to your setup. If you have a stock car, and the factory boost setting is 7psi, and it comes stock w/a 2" exhaust, then yes. A smaller size piping will do just fine. It is when people start raising the boost, putting larger turbo's on etc when that rule no longer applies. Just cause it will produce good tq #'s doesn't mean it's good for the car.

 

You guys are thinking of N/A cars. We get all of our torque from the turbo and you need to spin it quicker to get that torque. The least resistance after the turbo will get it to spool quicker.

 

Your car still gets its torque from the engine itself since off of idle your turbo won't be spun up to speed and making positive manifold pressure. The turbo will add to the hp and somewhat add to torque but not in the way that you think. Try taking off your exhaust completely and you'll see what I am referring to. The car will be a gutless pig without spooling up the turbo.