Author Topic: Help me select a VNT-15 spring from vacuum to boost conversion.  (Read 181 times)

February 05, 2020, 01:09:04 am


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  I'm converting a VNT-15 turbo from vacuum
  to boost controlled.

 Pulling it apart,
 flipping the diaphram
 using a different spring.

 A picture just for fun:

 Just noticed I didn't flip the diaphram in that one.
  On the next one the rod will be pointing down,
 and the spring will be under the cup.
 So coil bind has to be less than 11/16".
  This will allow the cup to bottom out,
 and have full travel. There is a metal cup under there.

  I used both of those springs in the last can conversion.
 Found the big one in a 020 5-speed trany.
 It smashed from 2 3/8" to  1 3/4" with 4 pounds.
 .089" wire.
 Not sure where the small one came from.
 But they work great together.

 My ability to pull springs out of dark places has diminished.
 I'm going to have to buy one.
 Some info about the spring for which I search:
 Minimum length 2"
 Since coil bind is under 11/16"
 Full travel is around 1 5/16"
 Width from 1 3/4" to 1 7/8" O.D.

 I guess the big question is
 how many pounds per inch of travel does this spring need.

 In the picture, the part with the washer is about 2" across.
 Not sure if that's the dimension I should use
 The boost maxes out at 18psi.
 What kind of pressure would that put on the spring?

 I think the area of a circle formula is  pi  x   r squared
 3.14 square inches.
 Should that be multiplied by 18? psi

Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #1February 05, 2020, 01:11:13 pm


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Re: Help me select a VNT-15 spring from vacuum to boost conversion.
« Reply #1 on: February 05, 2020, 01:11:13 pm »
Yes, multiply the max boost pressure by the area of the diaphragm that the boost is acting on in order to find the pounds of force that is being applied to the rod.  You will get best boost regulation the closer you can match that pressure across the range of motion of the rod. 
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