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DIY: convert your TT primary turbo to ball bearing


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#1 tmh983

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Posted 03 June 2012 - 05:31 PM

This will apply only to imported gen3 twin turbos with either the vf25 or vf26 primary and vf27 secondary turbos.

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:
Posted Image

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:
Posted Image

VF27:
Posted Image

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.
Posted Image

Clean up the vf27 compressor backing plate and apply some sealant around the middle, then you can bolt it back on.
Posted Image

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.
Posted Image

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
Posted Image
Posted Image

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.

#2 simon

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Posted 03 June 2012 - 06:09 PM

nice write up. im interested to see how this goes. What improvements should you see from this swap??

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#3 tmh983

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Posted 03 June 2012 - 06:12 PM

i originally had a vf25 with a p12 exhaust housing. I've put the p14 housing onto this turbo which will make it a bit laggier, although i'm hoping the new ball bearing core will overcome that and I should end up with similar response to the original vf25, but with a bit more power available.
Then again, who knows, it may just explode as soon as i put boost through it...
Generally I would expect an improvement in response time when changing from plain bearing to ball bearing.

#4 simon

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Posted 03 June 2012 - 06:17 PM

awesome.......well lets hope it doesnt explode......i will be watching this closely

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#5 RX25SE

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Posted 03 June 2012 - 07:09 PM

Nice write up dude.

Props to you.

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#6 Adam

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Posted 03 June 2012 - 08:01 PM

Nice write up dude.

Props to you.


Agreed.

Thanks for taking the time!

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#7 tmh983

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Posted 03 June 2012 - 08:36 PM

No worries.
I've been wondering why subaru didn't make both turbos ball bearing in the first place.
Is it just a matter of cost? maybe there a problem with reliability using a ball bearing turbo on a car like this, the primary turbo is on boost a much higher proportion of the time than say a wrx? Or possibly its a noise thing? Ball bearing turbos are typically a bit louder than normal bearings. I had an old sr20 motor which I put a BB GT28 on, you could hear there turbo coming a mile off.

I'm hoping that the ball bearing centre will mean less exhaust energy is needed to run the primary turbo, and therefore can get the secondary pumping sooner.
I've just found out that i have to fly out to farkin Port Hedland for work for at least a week. So it might be a couple of weeks before I can give this a run.

#8 Alex

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Posted 03 June 2012 - 08:56 PM

But it's still TT.....

#9 Beckers

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Posted 04 June 2012 - 07:14 PM

Well done, Great info.

#10 sbv

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Posted 11 January 2014 - 11:27 AM

...dead and buried thread revival... Did you get around to installing the BB primary? Thoughts?

#11 KONG

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Posted 11 January 2014 - 06:09 PM

BOOOOOOOOOOOOOOOOOOOOOOOOOM



#12 tmh983

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Posted 12 January 2014 - 08:59 AM

yeah i ran it for about a year before i sold the car. never had any issues with the turbo, didnt blow up straight away as i was worried about... haha
but to be honest, i couldnt detect any difference to the old plain bearing turbo, it didnt come on boost any quicker or earlier in the rev range. it still ran the factory 0.8 bar boost, power output didnt feel any different...
so i guess in the end, it seemed like a good idea on paper, but its not worth the hassle of pulling the turbo out...

#13 sbv

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Posted 12 January 2014 - 09:08 AM

Yes - I thought it sounded like a good idea too. Good on you for giving it a shot!




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