The T3/T4 turbo is a hybrid. It uses the turbine from a T3 turbo and the compressor housing from a T4 turbo. This allows the turbo to spool up quickly, and run more boost, which gives you more power than a typical T3 turbo setup without the typical turbo lag that you see with T4 turbos..
It's a great setup
for anyone who is serious about power, but also likes to drive the car around town.
For power, most people run these at 12-20psi. With around 25psi at the track. If you want to go all out, these turbos are able to handle as much as 30psi of boost. Usually you'll be looking at gaining ~300whp with this setup after a little bit of tuning.
Where to Buy Cheap T3/T4 Turbos:
Getting custom turbo kits is the way to go if you want to avoid headaches with your install. There are a number of T3/T4 turbo kits available for sale. There are turbo kits from major turbo manufacturers like HKS, Blitz, Greddy etc and these kits use Garret turbos and exhaust manifolds, and tend to come with everything you'll need to instlal the turbo kit. So you don't need to waste time deciding which oil line or blow off valve to use, and can just focus on getting the power. Prices start at ~$1,600.
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If you are a looking to save money on a t3/t4 turbo kit, and think that cheap turbo kits are still expensive. You can opt to buy just the T3/T4 turbocharger itself. Then all you need to do is just put together all the other turbo kit components and you can get your T3/T4 turbo kit on sale at a big discount. Prices start at around $900