T25 Turbos are on the smaller side of turbo applications. They are great for 4 cylinder cars. Usually they are set for 7psi boost, but it's safe to use them at 13-15psi. The max boost is usually 17psi.
The T25 Turbo is one of the best options for daily driven FWD 4 cylinder cars...it gives you plenty of power without causing major traction issues, and you don't suffer any of the turbo lag that you'd get with the bigger turbos. The HP gain is usually 60% which translates into about ~70+whp gain. Which is great bang for the buck, since at this level you don't need to beef up your internals.
Where to Buy Cheap T25 Turbos:
For those looking for a full package, you'll need to buy complete turbo kits. These come with Garret turbos and manifolds, and come with everything you need for a turbo install. From stainless steel oil lines to a high flow intercooler, the kit comes with everything you'll need to turbo your car. Prices start from about $1,600.
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For those who want to create something better than the stuff from Turbo Kit manufacturers. You can buy just the turbo and piece everything yourself. These are usually the same turbos that you'd get from Turbonetics, HKS, Neuspeed, Apexi, Blitz, Greddy and other high end Turbo Kit manufacturers. When you get to select the turbos yourself, you can create a unique way to turbocharge your car to get just the properties you need. Prices start at around $1,000