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Old 11-25-2019, 11:42 PM   #1
BarelyRunning
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Default 740 turbo td05-12b cbv delete

My CBV is done for, and I've read plenty of other posts relating to this CBV, they're not serviceable, nor does anyone make a replacement.

All of the posts I can find mention making a block off plate and adding an external CBV or BOV. However they also mention having to "block off the internal portion" or you'll struggle to build boost. I'm not sure exactly what they're referring to though.

Can someone please give me some pointers?


Last edited by BarelyRunning; 11-25-2019 at 11:44 PM.. Reason: Edit
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Old 11-25-2019, 11:54 PM   #2
2turbotoys
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I have a stock Peirburg external setup (760 style) I might be talked into selling, but maybe replacing the turbo with a something else is an option..

They are referring to replacing the blown rubber diaphragm with a solid flat piece of aluminum or steel so there isn't a leak there. I also have a block off plate I made if you need it, it is a very simple thing to make though.

I can post pics if needed
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Old 11-26-2019, 05:16 AM   #3
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Making a block off plate is easy enough, but what's this about bypassing it internally?

Do I just throw on a plate, and add a BOV? Or is there something I'm missing?

I'm going by this thread here, and there's not much info
https://forums.turbobricks.com/showthread.php?t=322899
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Old 11-26-2019, 01:02 PM   #4
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Replace the rubber piece with an aluminum one and add a recirculating bypass valve. A blow off valve will not work well with cars that use mass air flow meters. I don't know what car you have but I'm assuming it has one. People work around that somehow but I never bothered.
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Old 11-26-2019, 10:39 PM   #5
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It's a b230ft in a 240, so you're right, it has a MAF. I'll skip the BOV then. I've attached a pic of the CBV, it's not serviceable, I can't access the rubber diaphragm. I'm still jnot sure what I'm supposed to do. Block it off, and find a way to install a recirculating bypass valve?

I appreciate the help so far, this is my first turbo, and apparently the least common one.

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Old 11-27-2019, 12:21 AM   #6
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It's been many years since I figured this out. Remove the piston from your bypass valve. Cut that bitch out if you have to, I don't remember how it comes out. It will look like this:


Then use the cap as a template to make this from aluminum or steel:




Then I used these 760 parts (obviously powdercoated by me) as an external bypass valve. I used a Greddy valve on that bracket but the result is the same.



So the bottom port on the bypass valve in the pic goes to the port on the right angle pipe in the pic. That is the return. Not pictured is the port on your intercooler pipe that goes to the input port on the bypass valve.

Hope that helps. I did this for a larger turbo setup, the 12B is the smallest turbo available and so no one really messes with it. It is easier to find a decent larger factory turbo in good condition. I would consider that option, a 13C is not a huge upgrade but probably more common and bolt right on in place of your 12B. It should be cheap as well, I just sold a good one for $40.
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Old 11-27-2019, 08:43 AM   #7
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That's the most comprehensive post I've seen so far, thank you so much.

If I have this straight, your turbo only has an outlet, and you've moved that to an intercooler pipe, and then your inlet goes to your intake pipe?

And if I understand correctly, my turbo is a self contained unit as it isn't plumbed into my intake or intercooler, and I'd have to plumb these in? Does this picture make sense (edit: think I got it the wrong way)?


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Old 11-27-2019, 09:29 AM   #8
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I just found this. Block off plate, and plumb in an external valve like so?

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Old 11-27-2019, 08:58 PM   #9
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Exactly what you need.
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