The primary vf25/26 is a plain bearing design, the secondary vf27 has a ball bearing core.
I will show you how I used the vf27 BB core to create a hybrid vf25/27 ball bearing primary turbo.
The key to all this is the fact that the vf25/26/27 all have identical compressor and turbine wheels. This basically means you can mix and match exhaust housings and compressor covers however you like. This is great for people with jdm cars, but the audm turbos use different compressors, so it doesn't work.
Anyway, here's how its done:
Starting with the vf27, remove the oil & water lines, compressor cover, compressor wheel, compressor mount plate and exhaust housing. Make sure you mark the relationship between the compressor wheel and the drive shaft so you can re-assemble it exactly into the same postion. If you get it back to exactly the same spot then you won't need to get the turbo balanced.
The little nut holding the compressor wheel on is a REVERSE THREAD. don't make the same mistake as me and try and turn it the wrong way, you will shear it off and that will be game over for that turbo.
Also, be carefull removing the compressor cover, you will probably need to tap it off with a hammer. Just make sure it comes off straight or you can bend the compressor fins very easily.
Now take of the exhaust housing of your vf25/26. You can bolt this straight on to the vf27 core.
You will end up with something resembling this:
Now for the tricky bit: The compressor wheels are physically identical between the vf25/26 and vf27, but the ball bearing vf27 compressor sits about 1.25mm 'higher' than the plain bearing 25/26. This means you can not just bolt the compressor cover from the 25/26 straight on, as it will not clear the compressor wheel.
If you look in the vf27 compressor cover and compare it to a 25/26 cover, you will see that the machined profile to fit the compressor wheel is identical, but it is machined about 1mm deeper into the vf27 cover.
VF25/26:
VF27:
To get around this, you need to make a 1mm spacer to fit under the vf25/26 compressor cover.
I made this from a sheet of 0.95mm galv steel, using the compressor backing plate as a template to cut it out. It needs to be made fairly accurately as this will become part of the air pressure seal of the compressor cover.
Clean up the vf27 compressor backing plate and apply some sealant around the middle, then you can bolt it back on.
Bolt the vf27 compressor wheel back on, making sure you get the alignment spot on.
Put sealant around the top of the backing plate and on the compressor cover also.
Now you can bolt the vf25/26 compressor cover back on along with the spacer plate. You will need to drill 2 new holes, as the vf27 bolt hole spacing is slightly different to the vf25/26. Make sure the compressor is free to spin after you have tightened it all up. I also pressure tested the compressor cover to make sure it was still airtight with the inclusion of the 1mm spacer plate.
You can now bolt on the oil and water lines from the vf25/26
So, my final recipe for this turbo was:
VF26 P14 exhaust housing
VF27 BB core
VF27 compressor backing plate drilled to accept a VF25 compressor cover
1mm compressor cover spacer
VF25 compressor cover
Not sure how it goes yet, i've run out of time to actually install it into my car, hopefully next weekend i'll see how it goes.
