Several years ago Prime made the Gen4 swap extremely popular. Ironically they don't work on cars anymore, but the Gen4 and later Gen5 motors are here to stay. Just to quickly touch on both motors, they are very similar to the Gen3 but with more advanced ecu's, and coil on plug ignition systems. The bad news is that they come with smaller turbos (think original CT26 combined with CT20B exhaust wheel), and the weakest rods (and blocks) of any 3S motor.
Don't let that last sentence scare you too much. The block can handle 450whp/350lb ft of torque and the rods can handle about 300lb ft of torque.
The gen4 has a side feed intake manifold. The gen5 has an even more advanced ecu that has an immobilizer built in, extended tip injectors, and surprisingly went back to a center feed intake manifold.
The swaps can be done pretty cheaply; our basic, basic swap starts at $3800 installed. But what we usually do should probably be described as Basic++ We start with the Basic swap, add a side mount intercooler kit, downpipe, exhaust, fuel pump, maintenance package, and clutch package. The total is around $6500. This typically gets you 260whp/320 horsepower. That's enough for low 13's in the ¼ mile and 0-60 in about 4.7 seconds.
So what's the next step? Turbo, ECU and fuel. Let's hope you have E85 available near you because I personally don't like methanol injection, I've yet to see it last reliably. E85 has an effective octane capability similar to C16 race fuel so we can run a lot more boost without the fear of detonation.
ATS developed a plug and play kit for the ecu and fuel system that matches well with the stock gen4/5 pistons and rods on E85.
We paired our popular ECUMaster EMU with a custom set of 875cc high impedance top feed injectors that fit the stock rail. These will support 385-400whp on E85. That's a little more than I like to push the stock gen4/gen5 motors.
Combine that fuel system with our Sweet16/CT16 turbocharger upgrade and you've got a FAST spooling setup capable of 300whp on pump gas and 360whp on E85.Be sure to run a high zinc oil like Valvoline VR1 with this setup because you'll be making almost twice the original power and limited to a stock type thrust bearing in the turbocharger. GT/GTX series ball bearing turbo kits are also possible if you source a Gen3 exhaust manifold.
360whp is ~450 flywheel horsepower. That's enough to push a 2800lb MR2 deep into 12 second ¼ mile times even on street tires. These turbo's spool so fast that torque management in the ecu boost control strategy is a must. Basically use the EMU to build a boost curve that keeps the torque right about 315 max and you'll be fine.