hard to consider since it sounds like a crazy woman.. just saying
also i'm no mechanic but what does .25" more than stock get you if the turbo exit is just 2.75 at max ?
It's all about removing as much back pressure from the exhaust system as possible as quickly as possible. A larger diameter pipe allows the turbo to spool more easily, and thus more power sooner. With turbos our size the reduction in spool time isn't all that great, though it is there and it leads to a legitimate gain, just not a huge one. Maybe 10 HP or so. But, once you start moving into larger turbos or hybrid aftermarket solutions, larger compressors or variable geometry vanes on the turbine side, ect, you can find some really interesting power potential in a pretty small package. But ideally you want the entire system to be equally sized.
Just to explain a turbo or boosted engine really quickly , the turbo is in effect an artificial engine displacement enhancer. Engines are designed to move a certain amount of air. In our case 3.5L per full rotation of the crankshaft. With a turbo delivering 14.7 PSI at sea level that engine is now effectively 7.0L based on the amount of air it's moving. Beyond that say at 30PSi it's roughly 10 Liters ( Someone check my math I might be off). But there comes a point of diminishing returns based on how the turbo is designed. You couldn't necessarily strap a single Garret T76 Turbo onto our engine and expect to run 25 or 30 PSI. There is no telling where the boost would come on or what the spool characteristics would be like. The turbo could take it for sure, but the engine might not. Ultimately you have to read the compressor map to properly choose a turbo; plus you'd need supporting mods like a fuel pump,injectors, map sensors. custom tune, and a stronger intake depending on exactly how strong the stock intake is. Plus you'd need to at a minimum put in some head studs. I've gone this route before. It was worth it, but it's was a huge PITA .But I'm way off topic now.
Anyway, that relatively small change in diameter can make drastic differences in power delivery. Additionally, there are some benefits in reducing exhaust gas temperatures as well. But overall the less restriction you have in any boosted engine leads to an engine that has less overall stress on engine components, more power potential and in my opinion a better sound.
