The solenoid valve is an appliance valve for water commonly found in automatic dish washers.  The solenoid needs to be 12 volts DC rated.  The one in the picture is actually rated 24 volts AC.  However, the RMS value comes out around 14 volts DC so it works well in this application.  The valve I used for a "pre-set" is a brass needle valve for water often used for coffee machines, humidifiers, etc.  The exhausted air blows through a sintered brass muffler to reduce hiss and turbo whine, although I found that it really wasn't a problem.  This configuration will raise your boost to about 15psi at 75° ambient temperature.  Boost drops inversely to temperature increase.

The boost controller switch is a simple SPST (single pole-single throw) mini switch with an over size paddle.  Any switch will do.  I ran a wire from the fuse panel on the driver's side to the switch and the other wire through the fire wall to the solenoid coil.  The other lead from the coil is then grounded to a screw in the mounting bracket of the controller.

To tie this into the turbo waste gate hose, cut the hose and insert a 1/4" brass Tee.  Don't use plastic.  Heat, oil and pressure could eventually destroy plastic.  Attach a 1/4" fuel hose to the Tee and run it up to the booster controller.  Also, use good hose clamps.  To find the waste gate hose, jack up the car and ALWAYS support it with jack stands.  Look behind the left front wheel, next to the engine side of the frame.  There you will find a small space up in which you will find the hose.  It's a tight fit and you will have to work with one hand but it can be done.  You can also remove the big dust cover under the engine to get a little more room to work in.

Venting to atmospheric pressure will give you about 15psi tops.  At this you are pushing your luck if you have a stock engine.  However, if you want to get more boost, first install a good intercooler.  Then run a hose from the vacuum canister to the exhaust side of the solenoid valve.  This will produce a greater pressure drop keeping the waste gate closed longer.  Boost pressures up to and even beyond 25psi can be achieved depending on which turbo you have and what condition it is in.  Don't get stupid and power happy.  Believe me.   Do this only after you have an intercooler.  The excess heat will cause detonation and blow a hole in a piston.

There is a 1/4" NPT plug in the intake manifold behind the throttle body that is an ideal location to tap into the manifold to monitor boost presure.  I fitted a female Luer fitting (used in medical devices) in the manifold (see image above) and attached a male Luer to the end of the tube that goes to the pressure gage.  I used the Luer fitting as a "temporary" set up, having planned to remove the tube and gage and seal the fitting with a Luer cap.  This gage has worked so well that I decided to keep it for a while.  Of course,  you don't have to get so elaborate.  Use a brass hose barb.  Inside, I attached a small 0-30 psi gage to the steering column.  I used a "wet" gage.  This is a gage that is filled with glycerin or oil to dampen needle vibration caused by the "pulse" caused by each intake stroke.  This "pulsitility" can make a needle vibrate so bad at times it appears as just a blur.  The fluid allows the needle to transition smoothly and accurately across the scale.  These gages are available from most industrial suppliers under $20.  By the way, the camera angle above was very low when I took the picture.  In a normal sitting position the gage does not block the view of the tach.

You can modify the stock emergency relief valve and save 80 bucks or more over an aftermarket adjustable.  It is considered a "sealed unit" and unmodifiable, but I found the way in and have a clever way of installing a supplemental spring to raise the pressure threshold.  The hard part is finding the right spring.  I am looking for an off the shelf spring and part number so anyone can order the right part and do this mod with confidence.  The following are some basic instructions and the pictures will be here shortly.

BENCH TESTING
You should do this before and after modifying your valve so you can see specifically how your valve progresses.  To bench test your valve you need to visit the hardware.  Get a 1 to 1/4 inch reducer, a 1/4 TEE, 1/4 close nipple, inexpensive pressure gage 0-30psi range and of course you'll need a pressure source (compressed air).  Screw the pop off valve into the 1 inch end of the reducer.  Use Teflon tape on the threads if you have it.  Screw the close nipple into the other end of the reducer and screw the TEE onto the nipple.  Screw the pressure gage into the TEE.  The last remaining port on the TEE will attach to the pressure source.  Additional hardware may be needed to make this last connection.  Bring up the pressure slowly and listen to the pop off valve.  When you begin to hear a SHHHHHH note the pressure on the gage.  This is the minimum venting pressure.  Keep raising the pressure until it does not increase significantly anymore.  Copious amounts of air should now be venting from the valve.  Note the pressure.  This will be the maximum venting pressure.  There should be only a 2 to 5psi delta (difference) between the two.  You now have characterized the pressure limits of your pop off valve.

MODIFICATION PROCEDURE
 The first step is to open and lock open the valve.  Find a screw with a head that is about a 1/4" or 5/16" wide.  Hold the valve upright (threads down), place the pointy end of a screw driver up in through the bottom of the valve with the handle on a hard surface.  Push down on the valve body (not the cover plate) to open it (fig.1).  you will see the cover plate raise up.  When it is all the way up slip the head of the screw inside so it props the valve open.  You won't damage anything.  Now push the cover back in a little and rotate.  The idea is to move the three point bracket away and around the punched tabs on the housing.  The core of the valve assembly should come out in 2 pieces plus the valve body (fig.2).  The smaller piece will be the valve face.  Set that aside.  Under the valve face is the spring retainer disk.  At this point you should have the supplemental spring ready (fig.3).  What you have to do is get the spring inside the original spring and under the retainer disk.  At first look it may seem impossible since this is a welded assembly and won't come apart.  Here's the trick.  Drill an 1/8" hole in the spring retainer disk just inside the spring, about 3/8" from dead center of the post (fig.4).  Once you have drilled through twist the drill sideways to make a slotted hole.  This will make it easier to get the spring in.  Here comes the cool part. Start the end of the spring in the hole and rotate the spring.  You will hold the spring in the same orientation as the original spring and slowly "Screw" the spring in (fig.5).  The spring will gradually feed through the hole.  When you get to the end of the spring you will have to finish it with a pair of needle nose pliers.  Once the spring is all the way in you are ready to reassemble the valve (fig.6).  From this point it the reverse of the disassembly.  Drop the valve face into the body.  Insert the valve core into the body, rotate and pull it back ensuring that the brackets lock into the three perimeter tabs.  Pull the screw out and your done.

Now the hard part.  I took out the original spring and measured the free height, working force, ID and spring rate.  The rate is about 0.5kg per mm.  The ID is 14mm.  The height is about 30mm, and the working force is about 9kg @ 20mm.  What you need is a new spring with about the same spring rate, free height and working force but with a 12mm OD so it will fit inside the old spring.  I will be looking in some industrial catalogs to find 2 or 3 specific springs so a person could select a range of pressures.  When I find part numbers and a source I'll post it here.  If you are somewhat mechanically inclined you should be able to find a spring locally that will be suitable.  I found one this one at an ACE Hardware and cut it to size.

 As you may have noticed, there is often a significant drop in power available horse power once the engine is hot.  Especially on hot days.  This is typical for turbos but NA's experience this also to a lesser degree.  This is because the 300ZX intake draws it's air from the engine compartment.  Once the engine compartment heats up, the hot air has expanded or has thinned, which bares less oxygen by volume.

To quantify the effect in thermal terms, I did some simple data collection and analysis.  I setup a dual channel Fluke Digital Thermometer to monitor the temperature variations under varied conditions.  One probe was placed in the air intake. The other in ambient air (outside). When the car was started the intake was the same as ambient, 70°F. After the engine heated up the intake was up to 136°. When the ambient was 90° the intake was up to 154°. That's a 64° difference.  Add this to the adiabatic temperature increase after turbo compression and you have some really hot air entering the engine.  Aside from robing power outright there is a serious risk of "detonation" if you are running increased boost.

Sloan gave some of us inspiration for setting up our own cold air intakes. His is a bit more elaborate than mine and looks pretty sharp.  What I did was far less expensive and much more simple to do.  I removed the entire intake and filter box and resonator assembly.  Then I cut some grey polypropylene foam into various sized blocks about 100% larger than the space it will be fitted into.  I wedged the blocks into all the spaces around the edges of the filter compartment where hot air comes in from the engine compartment (fig.1).  I also applied 1/2 by 3/4 inch high weather stripping on the frame around the compartment. The idea is to create a 100% sealed compartment where the filter will be isolated from the hot engine compartment air. But now you need a source of relatively cool air from outside for the intake. All the way in the nose of the bumper is a long metal bar (fig.2) with a rubber strip on it. It is fastened at each end with a bolt. It's hard to get at but worth the trouble. This will open a large space in the nose to allow copious amounts of cool outside air into the sealed filter compartment. You have to take out the headlight washer reservoir, 2 mounting brackets and the radiator hose to get the bolts out. Or try your luck with an extension and universal.  Once you have the bar unbolted you need to remove 4 black plastic "button" fasteners that hold the rubber strip in place.  Lift the radiator an inch or two and slip the bar out. It takes a little dexterity but it's pretty easy.  It would also help to be double jointed but not necessary.

Put the hose back in and leave out the reservoir and brackets (a small sacrifice).  If you really want the headlight washer to work you can tap the hose into the windshield washer reservoir and it will work just the same.  Seal the space between the hood and frame with weather stripping to complete the "box" and, whammo!  You have a "Cold Air Intake".  It took me less than 2 hours.  No big deal. I also installed a 12" K&N filter so it would breath a little easier.  I adapted this to the 90° rubber elbow with something unusual.  I took a 1 liter polyethylene laboratory bottle (fig.4) and cut the top and bottom off leaving a 6 inch long, 3.5 wide "pipe" segment.  This slipped over the filter neck and the rubber elbow (fig.3). The bottle is ductile enough to give under the clamps and make a nice tight seal.

One last important item.  If you install the weather stripping under the hood it may be necessary to readjust the hood switch for the security system.  The night after installing the weather stripping I was woken up in the middle of the night by honking when the wind whipped up.  The next day it went of in a shopping center parking lot.  It seemed odd that all of a sudden there would be a problem with the security system.  I had to think what changed recently that could have had an effect.  Then I realized that the weather stripping being rather stiff may be holding the hood up just enough to trip the hood switch located in the front left side with a rubber boot on it.  So I set the security system and pulled up on the hood and sure enough the alarm went off.  I readjusted the switch up about an 1/8 of an inch and the alarm problem went away.
 

To disable the limiter finction remove the kick panel on the passenger side to expose the ECCU.  Wire #29 is a purple wire.  Cut it.  That's it.  Your done.  What I did was to connect a male/female insulated spade connector if, for some reason, (and I can't think of anything other than old age perhaps) I decide I need to re-enable the limiter.  The insulated spade connectors also protect the free ends of the wires from shorting out on anything.

Now Be Careful!  Watch you red line.  Your guardian angel is no longer watching over you.  Don't send Mr. Engine to automotive heaven. 

 I purchased replacement 300ZX sockets for brake lights.  There is a plug in the hole for the socket that has to be removed.  One of the tabs on the socket has to be notched with a knife or side cutters so it will fit the reflector hole.  Wire the cooresponding wires into the brake/parking light wires above (brake to brake, park to park, ground to ground.  Insert a fresh 1157 bulb.  Bump the brake light fuse up 5 more amps from 15 to 20.