are the Borla catback systems 3" tubin?
any loss of back presure by goin to 3" from 2.5"?
It's common to think that a larger pipe will reduce backpressure because everyone knows that a larger pipe flows more air. However, in many cases this is a false assumption.
When we speak about exhaust pipe diameters (for bikes, cars trucks etc.) I would say 90% of the discussions involve diameters between 1.25" and 3" and it's very hard to imagine how fluid dynamics are affected by such small diameter changes. However, if we use an extreme example it becomes much easier to visualize and understand. Imagine a constant and consistent stream of water flowing through a garden hose. The water flows through the hose, moving away from the source and out the other end of the hose. Now imagine taking that same constant and consistent flow of water but this time move it through one of those 5' concrete sewage pipes. What will happen? The water will pool up and build upon itself and take much longer to make it out the end of the pipe. As you can clearly see from this example, a larger pipe will not always outflow a smaller pipe, it depends on the amount of air (or in this case water) being moved through the pipe. Another real world example would be putting your thumb over the end of a hose that isn't flowing much water. You decrease the diameter with your thumb and the water exits more quickly.
So, you can actually create backpressure with a larger pipe because you slow down the exhaust gas velocity, making it take longer to exit the tailpipe which inhibits the efficiency of the engine.
Another contributing factor to this phenomenon is friction. Larger diameter pipes have more surface area which creates more friction and friction slows down the exhaust gasses even further.
Pipes that are the wrong size, too big or too small, inhibit the efficiency of the engine. Bigger is not always better.
The one big caveat to this is that at high RPM, when the engine is pushing a lot of air, larger pipes can (not always) create more high end horsepower. However, it's not worth losing all that torque down low to pick up a few HP up top.
Another aspect of exhaust velocity is heat. Exhaust gasses cool very quickly as they move through an exhaust system and cooler gasses have a slower velocity than hot gasses do. If your pipes are too big this just exacerbates the efficiency issue. In fact, many competition race car exhaust systems become smaller in diameter as they get further from the engine (heat source) to maintain a high exhaust velocity.
There's so much more but let's start with this for now.....