My understanding from looking into all the different options is that it really comes down to where you want to spend the time/money.
CA18DET parts support is waning so despite being a nice enough engine it is a worse option with each passing year simply due to being old. The RB25DET is a total bruiser of an engine but heavy and requires oiling system mods + machine work to not devour its own bottom end on track. RB20DETs are novel but have the unfortunate trait of making less power and torque than the SR20 while also requiring the PITA mods a RB25 needs. DIY KA24DE turbo setups are a lot of bang for the buck but a big time investment with the "nickel and dime" potential...for what I want to do the SR20 is worth it to me as something I can more or less put in the car and incrementally build up.
As far as what it takes to put a SR20 in a Z32, here's an overview from one of the first guys to do it:
Crossmember: the crossmember requires some modification to clear the sr20 oil pan. The vg is rear sump while the sr is front sump. There is not enough space or material on the crossmember to allow it to be modified to run an aftermarket lower oil pan. Modification is not complicated nor costly and most moderately experienced performance or welding shops are capable of the job
Engine mounts: I currently offer custom cnc machined mounting brackets that adapt the oem or nismo sr20det engine mounts to the z32 crossmember. That will allow users to choose their stiffness level preference, either oem or stiffer Nismo, to allow the sr20 engine to drop into the z. Cost is $350/pair and each pair has an individual serial number. Message me for purchase info.
Intercooler piping: I had custom fmic piping fabricated for a direct swap but will not have it manufactured due to lack of demand. You will have to have your own piping fabricated.
Exhaust: I had a custom exhaust from the turbo to the catback fabricated but will not have it manufactured. The exhaust I ran consisted of a PBM Cobra 3" divorced elbow pipe modified for the z32 that runs to a dual 2.5" y-pipe and connects to any 2.5" z32 catback.
Wiring: this part is not complicated for the electrically inclined, you will utilize the original sr20 engine harness and modify it similarly to how it is modified to work in a 240sx. I have completed a wiring diagram that I have uploaded into the documents section that can be used and I may also offer a service to complete the wiring for customers.
Cooling: any 240sx sr20 radiator drops right into the na z32 core support. I ran a koyo with a custom efan setup, but the stock clutch fan can be utilized with a custom shroud. I used sr20 radiator hoses and the top hose will need to be trimmed down since the radiator sits closer to the motor than in a 240sx.
Transmission: a mazworx type transmission adapter bracket for vg to sr is utilized with the z32 transmission. Machining of the bellhousing edge is required for the adapter. This allows you to keep everything from the bellhousing back stock. Oem or 1-piece z32 driveshaft can be utilized. It will use the sr20 starter and flywheel along with any clutch setup you prefer. Na z32 and sr20 clutches are interchangeable, so if you already have an upgraded clutch in your na you can keep it.
Fuel pump: the sr20 chassis in Japan utilize a dual stage fuel pump controller, exactly like the z32. Wiring will allow you to utilize the stock z32 fpc with the sr20 which is controlled by the ecu. This retains oem function and prevents any further wiring headaches.
Fuel lines: the z32 has fuel lines that run down the left frame rail, while on the sr20 chassis the exhaust runs that direction and the fuel rail is on the right side. You will remove the fuel hard lines from the left frame rail and move it to the right frame rail, where there are already did threaded holes for the brackets. Use soft fuel lines in the rear to connect to the hardlines from the tank hardlines and soft lines in the front to run to the rails. Slight bending of the hardlines in the front will be necessary. I recommend Gates Barricade for the soft fuel lines as it is durable, better than oem and is rated for e85.
Power steering: you will utilize the sr20 power steering pump. The reservoir can be run any number of ways, I located mine by the brake booster but it can also be under the nose panel or somewhere else in the engine bay. The high pressure line will be the same aftermarket braided lines offered for the 240sx. Low pressure lines will be the typical cut-to-length heater hose.
Ac: while it is possible to retain ac, custom lines are required and somewhat costly. You will need access to a shop with an ac fitting press, plus all of the original fittings, braided (or rubber) lines, and -AN fittings welded to the original stock fittings plus -AN fittings for the braided lines. If using rubber lines, a local ac shop should be able to use your existing lines to fabricate custom lines.
The outcome: expect a car that is very nimble compared to a vg-powered z. Mine weighed in just over 2800lbs after just the swap, so you could easily get down below 2600lbs by removing interior, stereo, ac components, half the exhaust system, etc. Your fuel efficiency and reliability increase significantly while maintenance costs drop with a drivetrain that is far easier to work on. Parts are readily available and sr engines have been known to produce over 1,000hp, so the performance potential per dollar is extremely high. You will have the edge in road course or autox racing since your brakes will be more efficient and g-forces will have less of an impact. Imagine a BRZ/FRZ with a turbo, and that is how your car should feel after the swap.
