I was considering putting a TB48 into my GQ
I went and bought a TB48 to do just what you are proposing which I still have and it is for sale. It needs a full rebuild and I would suggest you are better rebuilding an engine, rather than buying a so called going concern. A wrecker will only ever give you a short term limited warranty generally 3 months and by the time you get organised and get the donk in yr 3 months will most likely be up. They will only ever give you a replacement donk which then means you have to do it all over again. At least if you rebuild a 4.8 from scratch using a good engine builder you know it is all good, you can run it in properly, etc, etc. The 4.8 donks had an issue with failing oil pumps occasionally as the one I have has happened. So if you buy a second hand one you need to check this before you buy. Oil pump comes as an assembly with the timing cover and is expensive new from Nissan.
Don't know what the most suitable turbo for 4.8 would be but there is a guy in Smithfield who rebuilds Turbos and is good to talk to. He may be able to recommend the appropriate Turbo to use. John at 'Precision Turbos' 0297565757
You should be able to buy a rebuild, get it all machined, etc and if you know what you are doing put it together yourself for 4-5K I would think. I went through all the pricing of machining and parts. I don't know where you are located but I do know a couple oh machining shops around Sydney who are suppose to be good. I also know a guy on the sunshine coast who I have known for a very long time and he knows his stuff, but he might be a bit expensive.
A 4.8 is much better and more efficient engine over the 4.5. Double overhead cams, 4 valves a cylinder, etc. All my research at the time to fit the TB48 was that it will fit straight up using the 4.8 engine mounts but as to fitting to yr GQ auto, I don't think so. I am guessing the 4.8 Flex plate is different/ bigger as they are with a manual set up ( manual 4.8 runs a 300mm clutch versus a GQ4.2 uses 275mm clutch. If you do this conversion it would be great result but I would also definitely fit a 5 speed tiptronic auto as these are much better than a 4 speed auto and you won't have any adapting to do. Shifter may be in slightly different position but that shouldn't be too hard to adapt. Also if I am not mistaken the 4.8 runs 2 oxy sensors but maybe yr 4.5 does too?? Obviously you will have to use/change to a 4.8 ecu and if you go with the 5 speed titronic you will have to use the right ecu for the auto.
I decided in the end to fit a Turbo to my TB42 on injected lpg as the 4.8 rebuild cost for me was not worth it even though I may have my GQ for a long time yet. I am very happy with the torque and driveability I now have. I am not wanting the power you are looking for. But I do understand why you want it.
Anyway let me know if you might be interested in the 4.8 I have or if I can help further. Also I spoke to a guy who is in QLD who did 4.8 into GQ conversion and he was very helpful at the time.
Michael 0417591746
