Resistor Mod continued...

Ben Swann benswann at
Fri Feb 20 22:21:56 EST 2004

On your questions.

1. There are a large number of pressure regulators available in a range of quality and pressures - I'm sure the Sears one will work fine.    Input would be the manifold pressure, and output would be the regulated pressure, toward tha wastegate.

Basically you need to set the opening pressure of the regulator to something like the 12 PSI that you will be regulating to.  When the regulator opens, it will allow pressure to be forced into the wastgate port, opening the wastegate to bypass exhaust gas away from the turbine.  You have it correct.  You need to start with a setting that you think is right, then adjust by trial and error.  It usually takes many runs to get it set right.  Start conservatively and by all means MAKE SURE YOUR INTERCOOLER IS STRAPPED!  Oh carry a roll of duct tape in case something else blows during your testing.

What I did was adjust the pressure reg. to control boost to the desired level.  Then I used the adjustable resistor mod to get the resistance set to just below the value that causes fuel pump cutout at the desired boost setting.  You want the value to be as high as possible without permitting the overboost protection to kick in.  I'm guessing you'll be able to safely dial in around 1.8 -2 Bar without detonation.  It will be interesting what resistor setting you finally end up with - I'm going to make an educated but silly guess it will be around 1300 ohm.

2. Don't use a bypass valve as a blowoff valve - it will mess up your mixture and cause lost power.  Plumb the output side of the bypass back into the tract somewhere before the turbine.  Plumbing into the large metal tube is good and not too difficult.   Take the time to do this right, as it will be worth it in the end.   I made a special adaptor to aim mine right at the blades of the turbine, but this probably took way more effort than is actually worth.

Good luck with all of this.  I'm looking forward to hear success!

  ----- Original Message ----- 
  From: Ivan Tuma 
  To: Ben Swann ; quattro at 
  Sent: Friday, February 20, 2004 12:38 PM
  Subject: Resistor Mod continued...

  Hey Ben,

  Your help is much appreciated ... If I get this working right, do you have a donation system set up through paypal?

  We had some weather in Denver, so I haven't really had the time to finish the resistor mod as specified.
  I don't know which spring I have yet ... I'll have to take it out to tell you the color. I plumbed by boost guage
  off of the line on the intake manifold that gives me both vacuum AND boost. I think this is good as without the resistor mod (turned off), the guages in the dash and aftermarket match exactly (except in the rare case where my in dash says 1.3bar and my boost guage says 8.75# and again, this is without the resistor mod in place ... I know it's a mystery and it happens very rarely now that my check engine light isn't coming on all the time from a mis-alligned distributor)

  I have a couple more questions, I hope you don't mind me asking.

  1. Which way does the air flow through the regulator? Manifold to Wastegate correct?  I bough a relief type 5-125# (smallest one they had) at Sears. It's the kind you would use on a paint gun to regulate pressure. My thinking is that I hook it up to a compressor first to find a 1.8 bar ~12psi setting. Is a "relief type" correct? It basically doesn't allow for pressure larger than the setting to get through.

  2. Looks like you are using the 20V Bypass valve which bleeds back into the intake between shifts. Can I use an aftermaket (or even the 20V bypass valve if available) and vent to the atmosphere for the "kshh" sound in between shifts? 



  Ben Swann wrote:

    I'll add my comments below, but sounds like you are getting somewhere - I'll expect you'll only be able to boost to a conservative level, perhaps around 1.8 bar, expecially if the maps are not changed.

      ----- Original Message ----- 
      From: Ivan Tuma 
      To: Ben Swann ; quattro at 
      Sent: Wednesday, February 18, 2004 4:04 AM
      Subject: Re: 200tq check engine light, but no codes!?

      Hi Ben,

      I'm afraid I'm not having much luck with the MAC-14 resistor modification.... I got most of my information about this mod from your website. Last time we "talked" was around six months ago, so allow me give you a rundown, along with some links to pictures of what I've done:

      I think your resistor mod setup will work fine by diabling the frequency valve and pluming in a pressure regulator.  I think you will be limited to around 1.8 maybe as much as 2 bar.  The reason is, as the resistor mod skew the p.t. output voltage, the timing is advanced.  So you get more boost, but timing will never be optimal.

      Here is the diagram for the regulator:

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