- 60degreev6.com Forum
 

08/31/2003

*Turbo Gus Mahon, unfortunately, is no longer with us. He was killed in late Summer 2002 in a motorcycle accident. He was quite an inventive person and was able to propel his 2.5L Turbocharged Dodge Caravan into the 13's at the dragstrip, frustrating many musclecar owners to no end. We will be reprinting some of his more generic articles mainly due to the great information they provide. We are extremely sorry to hear about the tragic accident that took his life, but will remember always the contributions he has made to the hobby. The following is the article in full along with how the results were derived.

    When you are manually raising the boost on a Mopar turbo car (and many other brands, too!) you quickly discover that at some point above stock boost levels the engine will SHUT clean off (or "CUT OUT"). It's very annoying, to say the least. In our beloved Mopar FWd turbo cars cut out is most often at 14.7 psi. The computer can't recognize boost levels above 14.7 psi, so it shuts your motor off if you try to go higher.
    We can fool the MAP sensor into telling the computer that we're still below 14.7 psi. We can keep the pressure down in the MAP sensor line with this device which raises the cut-out point, or eliminates it (if you want to go that radical).

    Because of legalities, I can only suggest this as an off-the-road mod. Attempt this at your own risk.

  McMaster-Carr sells the un-modified valve  as part number: 48935K25

Modify as follows:
First take the stock valve, disassemble it by unscrewing it, and re-assemble it with the steel ball moved to the OTHER side of the spring. The steel ball will now be on the red-threaded end of the valve.

Then get a screw-on barb (McMaster-Carr part# 5346K51) and screw it onto the red-threaded end.

...and put together a couple of short pieces of vacuum line, a tee, and a restrictor orifice (brass piece with pinhole in it).
I pick up welding tips at a local welding store. That's one in the picture above. Below is a completed valve.

WARNING!!  The restrictor orifice (or welding tip) should be .040" to .050" A small orifice, like some stock ones, will cause the "check engine' light to come on. The illustration above shows .035" as the smaller limit, but actually .040" to .050" is better.
 

     If you have the old style logic-module-in-the-kick panel setup, then you put this device in the car, or underhood.

 In the car: splice into the vacuum line that goes to the MAP sensor - logic module. Pull back the passenger's side footwell carpeting, or remove the kick-panel. Splice in the cut out raiser with the white nylon connector facing the MAP sensor. Of course the brass restrictor points the other way!

 Under the hood: Splice into the vacuum line close to where the vacuum line for the MAP goes through the firewall. Splice into the line near the firewall with the white nipple facing the firewall, and the brass nipple towards the barometric solenoid.

 If you have the newer style barometric solenoid/MAP sensor together under the hood, think of the baro/MAP combo as one device. Cut the vacuum line close to the baro/MAP sensor, and splice in the cutout raiser device with the brass nipple towards the intake manifold, and the white nylon nipple towards the MAP sensor/barometric solenoid combination.

 After installation, screw the knurled knob almost all the way in, and the car will still cut out around 15 psi. Loosen the lock-ring, and unscrew the knurled brass knob a half-turn. Re-tighten the lock-ring. After you do this a few times, you�ll notice that the cutout point gets higher. You can stop at 16, or 18, or whatever you want, BUT, the higher you go, the more fuel you have to add.

     Set it as low as you can get away with, because your computer will give you NO MORE fuel past14.7 psi. The computer will believe that you�re never going over 14.7 psi!

     If you have a rich-lean gauge, back off when the gauge indicates that you are NOT rich!
 If you have no gauge, run a wire from your O2 sensor wire (the one that reads .1V-.9V at warm idle, bouncing up and down) through the firewall, into the car. Hook the O2 wire to the red probe of a digital voltmeter; hook the black probe of the digital voltmeter to a clean ground. When you floor it, you want to see .87V to .97V. Less than .87V means you�re not safely rich. Back off, and add fuel.

     You can add fuel by raising the fuel pressure, adding an extra injector, or switching to slightly larger injectors. If you don�t add any fuel, don�t run high boost! I can usually run 15 psi on the stock setup.

       Here's a simpler MAP bleed without the Grainger valve. You need a needle valve, and a restrictor orifice instead.

       The Grainger valve set up will leave you at full richness until you hit14 psi or so. Then you'll begin to lean out.

       This needle valve set up will lean you slightly all the way through your boost curve. If you're running too rich from big injectors, this needle valve set up is better for you. If you want to raise the boost  just2 or 3 psi past cut out, this will be good for you, too.

       NEITHER set up will affect idle, gas mileage, or part throttle cruise. They only come into play when you go into boost.

Do NOT open the valve much to start with. Just open it enough to bring your cut out point a little higher. The brass needle valve can be replaced with another simple restrictor orifice. The smaller the orifice, the less raising of the cut out point; the larger the orifice, the more raising of the cut out point.

       If you are running computer controlled boost, this needle valve mod will *also*raise your boost level!

       I designed this set up for manually boosted cars. Computer controlled cars will see a rise in their boost. Will make a fun experiment. Just go easy on the throttle 'til you see where your new boost level is. If you don't bleed much, the boost won't go up much. And you *don't* want to bleed much...