Zcar Turbo Information

Intro

This page has been compiled from internet sources, especially from  posts on zcar.com thanks to all! It is presently a work in progress and at this point, it simply is a lot of cut and pasted info. I am in the process of streamlining the presentation.

Newbie Turbo Info is here. It is highly recommended that you read it first before continuing

Turbocharger

• A stock '82 280ZXT has Garrett Air Research T3 Turbo (60 comp trim and .63 turbine trim) installed. 
• A common upgrade is to go with a Garrett T3/T04 Hybrid unit. 
• Details and specs for a T3 are further down near the bottom of the page.
  
  Basic air flow through a turbo. Intake is blue and exhaust is red
  
  
  
  Exploded view of a generic turbo
  
  
  

Fuel

• Since a turbo adds more air, the fuel injection system must also add more fuel. Two common ways to do this are to increase injector flow or to increase fuel pressure. Increasing injector size by swapping out the stock with higher flow injectors is the more common route. When doing so, the fuel to air metering system (part of EFI system) must be recalibrated for the new injectors to prevent a lean or rich burn.
  
• Stock 280zxt has 265cc/min or 25.2lbs/hr injectors . (FYI Non-Turbo 280z has 185cc/min or 17.6lbs/hr injectors) 
  
• Stock injectors start to lean out at 10psi boost at nominal fuel pressure (engine consumes 1761lbs air/hour (click here to see calc) thus at 12:1 air/fuel ratio, 147 lbs-fuel/hour is consumed. Dividing this among 6 injectors gives 24.5lbs/hr of fuel for each injector.
  
• Here are some ball park injector sizes and HP for an inline 6 turbo
  
  

  Engine HP	RWHP	cc/min	lbs/hr
  200	160	268	26
  250	200	335	32
  300	240	402	38
  350	280	468	45
  400	320	535	51

  
  

Boost

• the turbo on a stock 280ZXT usually hits full boost at ~3,000 rpm
• Normal (Stock) Boost is ~6psi
• Safe (Stock) boost without intercooler is 10psi
• Max (Stock) Boost is 15psi (efficiency drops to ~ 60% at 6000rpm) (click to see map look at efficiency where blue line terminates)
• BEWARE! over boosting can cause detonation which can damage your engine (click here for an L28 that suffered detonation)

Compression:

• Standard compression with F54 w/dished pistions, standard HG and P90 - 7.38:1
• F54 w/dished, HKS 1MM and P90 - 7.52:1
• N/A engines typically have compression ratios from 8.5 to 10.5
• Low compression AND inter-cooling AND timing control all interact together to determine the high boost potential of a turbo motor. Regardless of the motor being from a N/A with high compression or from turbo with low compression, detonation occurs at a certain temperature and pressure.
  

Block Diagrams:

Stock 280zxt airflow diagram

L28ET Air Flow (click to expand)

Modified 280zxt airflow diagram

Typically modified 280zxt turbo flow/part diagram (click to expand)

Overview of air flow in the above diagram

1. Exhaust air spins the turbine
   
2.  Air is sucked into intake (due to spinning compressor and piston suction on down stroke).
   
3. Air flows through the AFM and its flap moves proportionally. The AFM sends a signal to the "computer" to tell how much air is entering the engine and the "computer" tries to add enough fuel via the injectors to maintain an approximate 15:1 air to fuel mixture.
   
4. Air passes through the turbo's compressor section where it is compressed. The process of compressing heats the air (talk to Sadi Carnot). The amount of compression is called "boost" and is usually measured in PSI however a BAR is often used. 1Bar = 14.7PSI which is the average atmospheric pressure at sea level.
   
5. The hot compressed air flows internally through the intercooler. Wind from the car's motion passes externally over the intercooler and carries away heat. Thus the internal pressurized air is cooled.
   
6. The cool pressurized air passes over the throttle body, through the manifold and into the combustion chamber via the intake port.
   
7. The cool pressurized air is mixed with injected gas upon entering, compressed by the piston, then ignited by the sparkplug.
   
8. The expanding gas from the explosion pushes the piston down then exits via the exhaust port.
   
9. The exhaust gas passes through the turbo's turbine, spins it and starts the cycle again.
   
   

Overview of feedback and protection devices/functions in the above diagram

1. The waste gate is a door to allow exhaust air to bypass the turbine. It is opened and closed to modulate how much exhaust gas flows over the turbine and in turn modulates how fast the turbine spins the compressor which, in turn, controls how much pressure is built up in the manifold in area "A". Too much pressurized air entering the combustion chamber can cause premature detonation as the piston compresses the hot high pressurized air. So-called "detonation" can push the piston backwards against the engine's momentum and destroy it. The waste gate is mechanically opened and closed by the waste gate actuator.
   
   
2. The boost controller modulates the waste gate actuator. The input line (lime green colour) to the boost controller can be connected to monitor the pressure at "A" in the manifold (as shown) or alternately to monitor the pressure at "B" at the outlet of the compressor. The boost controller is set to limit the maximum boost pressure. For a stock ZXT the boost controller's set point is fixed at ~6psi but for an intercooled zxt it is typically set to between 10 and 16psi.(more boost means more power from the engine but also increases the risk of detonation).
   
   
3. The pop-off valve is simply an emergency pressure release device to prevent over-boosted air from entering the combustion chamber. It functions by venting the air to the engine compartment when the pressure rises above the maximum boost threshold. For a stock zxt with boost set at 6psi, the pop-off vents at ~9psi. For an intercooled zxt with the boost controller's maximum boost set to 12psi, the pop-off valve would be set to ~ 14psi. The pop-off valve can protect the engine in the instance when the waste gate or waset gate actuator fails. If this were to happen, boost levels could go well beyond 25psi without a pop-off valve present.
   
   
4. The blow-off valve is opened when the throttle valve is abruptly shut to prevent back-pressure on the compressor.
   In normal operation, air flows though the compressor then freely into the engine, thus the pressure on each side of the open throttle valve in "A" and "B" is nearly identical. However, immediately after the TB valve is abruptly closed (foot off the gas pedal), the air flow through the engine is nearly brought to a complete halt at the TB. Exhaust air and self-momentum of the turbo wheels continues to spin the compressor. The compressor then continues to pack air into the now sealed area "B". The packing of air rapidly increases the pressure in "B". This high pressure air in "B" causes backpressure on the compressor and slows it down. (a bad thing for performance and it also causes an unusual strain on the compressor). Since the blow-off valve is monitoring the pressure in "A" (lime green colour), it can tell when the pressure in "B" is different from that in "A". If this pressure differential is too high, the blow-off valve will open and allow the high pressure air in "B" to vent back in "C". This equalizes the pressure on each side of the compressor thus it spins freely and does not suffer the back pressure problems.
   A common mistake is to vent the blow-off valve into the engine compartment rather than re-circulating to the front of the compressor. By venting the air that was "metered" by the AFM, the air to fuel mixture changes and the engine runs rich.
   
   
   

Twin Turbo vs. Single on L28 engine:

• Twin (SEQUENTIAL) Turbo set-up. You have one smaller turbo and one larger turbo. This is done to eliminate turbo lag. The smaller turbo will spool up first, so it starts making boost, as you start making boost and climbing higher into the power band you start making more exhaust, which allows you spool up the larger turbo, making more boost and allowing better exhaust flow while at the top end of the RPM range. Having two turbos doesn't mean more power in a Sequential set-up. In fact the big dogs running Supras, etc, are all running single turbos.

L28 with twin turbo

L28 with twin turbo

L28 with twin turbo (Twin Mitsubishi TDO5 16G's w/ 7cm housings) click here for more info

L26 260z with twin turbo (click on picture to expand). click here for more info

 

Block diagram for a Dodge Stealth Twin Turbo Set-up

Protection Devices:

The primary purpose of protection devices in the turbo system is to prevent detonation. Detonation is caused by too much heat or too much pressure (which causes heat) when the piston is compressing the intake air during the engine's compression stroke. The combined heat from the turbocharged air AND the heat from the compression stroke can ignite the gas in the cylinder prematurely. The resulting early explosion pushes the piston in the reverse direction (against the rest of the engine) which causes extremely high pressures on the piston, rod, crank arm, rings, head gasket, valves. It is easy to understand the principal of pressure causing detonation if you think of how a diesel engine works: The diesel has no spark plug to ignite the fuel, the heat from the pressure does it!


red hot exhaust side of turbo charger

• Intercooler  
  
  
  
  An intercooler cools the hot air coming out of the compressor and entering the manifold. When a turbocharger compresses (squeezes) air into your engine, it causes the air to heat (to  check this yourself, find a biker friend (Lance Armstrong Type not Evil Knievel), and pump up his high-pressure road bike tires using a frame pump ). Also,  thermal coupling from the turbos's exhaust side also heats up the intake side's air charge. The intercooler is placed between the Turbocharger's output and the intake manifold. The bigger the better and also the best flowing the better. Unfortunately 280zxt's do not come with an intercooler. This limits the maximum boost (air pressure increase due to compressed air) to ~ 10psi. Above this, detonation will probably occur due to the extra heat. BTW the extra air also has extra oxygen which also assists in the detonation (just like blowing on ambers to re-ignite a camp fire). You can find out more on intercoolers in the how to section.
       
  Typical Intercooler (input and output ducts are for cooling the pressurized air charge flow from turbocharger)
  
  What is the purpose and/or advantage of an intercooler?
  The purpose of the intercooler is to remove the heat in the air charge that the turbo/supercharger puts into the charge when compressing it. There are two advantages: Reducing the heat in the air charge increases the charge density (more molecules of air per cubic foot), thus increasing the potential for making more power. Reducing the heat decreases the tendency of the combustion process to knock (detonation).
  
  How does the intercooler affect the power output of the engine?
  Power is dependent on the density of the air charge. By decreasing the temperature the intercooler increases air charge density, therefore, the power is increased. Typically, the magnitude of the increase will be between 10 and 20% for the average (street) boost pressures.
  
  Is some intercooling better than no intercooling?
  No. It depends on the design of the intercooler, and there are two factors involved; efficiency (how much heat is removed) and the flow restriction (lost pressure) created by the presence of the intercooler. Regardless of the efficiency, if too much pressure is lost, then the intercooler is either useless or can actually decrease performance .
  
  What are the differences between short tubes and long tubes?
  The longer the tube the greater the pressure loss accompanied by a slight increase in efficiency
  
  Does the length of the tube affect efficiency?
  Very little. The most heat comes out of the tube where the temperature difference between the inside and the outside is the greatest. That exists in the first couple inches of the tube. The last inch of the tube, wherein the charge temperature is rapidly approaching the cooling media temperature, will transfer very little heat, thus being of minor use.
  
  What factors affect efficiency of an Air-to-Air Intercooler?
  Frontal area: This is a rapidly decreasing function. If the proper core size is used, then doubling it will definitely not double the efficiency. More likely, doubling the core would raise the efficiency about 5% and cost twice the necessary amount and add substantially to the weight.
  
  Plate area: Plate Area (the sum of the Core-Plate Area which is exposed to the Atmosphere) is directly proportional to the frontal area and the thickness. Thickness, however, is a double-edged sword. With the greater thickness, the plate area increases but less ambient air can penetrate the thicker core to offer cooling.
  
  Ambient air quantity: It is very important to insure that air coming in the snout of the car will actually go through the intercooler.
  
  How does one measure the efficiency of the intercooler?
  The efficiency is defined as the ratio of the temperature removed from the air charge by the intercooler relative to how much temperature is put into the charge by the turbo/supercharger.
  
  For example: If the turbo/supercharger puts 150 degrees F into the charge when compressing the air, and the intercooler removes 110 of those degrees, then the efficiency is:
  
  Eff = 110 / 150 = .733, or 73.3%
  
  
  What factors affect the pressure or flow loss?
  The internal flow area is the major controlling factor. Tube length is the second biggest consideration, as a tube twice as long as another will have nearly twice the drag at the same air velocity. Tube entry and turbulator density play small roles and can be considered insignificant. When configuring the orientation of the core in a given space, always position the core to offer the shortest length tube and the most number of tubes. Clearly, this optimizes the internal flow area.
  
  
  What ranges of pressure loss can be expected? And what is acceptable?
  For good solid performance, the pressure loss across the intercooler ought to be kept to less than 1.0 to 1.5 psi. If any pressure in excess of 4 psi is measured, then the intercooler is not suited for the job and certainly harming the performance.
  
  If the boost is raised is it necessary to increase an otherwise proper intercooler?
  Very seldom. While the loss through the intercooler is proportional to the flow (CFM) squared, unlikely the change will be of a magnitude that requires a bigger intercooler. If dramatic changes in flow are created, say 50%, then the flow loss would increase by 1.5 squared, or 2.25, and that would prove excessive thus strongly suggesting a larger intercooler.
  
  

Typical example of power gain from an intercooler is sometimes an added bonus (~10% to 30%) 

 

 

• Waste Gate 
  
  External Waste Gate                                Internal Waste Gate

  
  External Waste Gate Installed

  A waste gate is essentially a device that bypasses some exhaust flow around the turbine section of a turbocharger to control maximum boost. A waste gate is usually controlled by a pressure actuator that is connected to manifold pressure. The waste gate is normally closed, held shut by a spring inside the actuator canister. When preset pressure limits are exceeded, the actuator progressively opens the waste gate, allowing exhaust flow to bypass the turbine, thus regulating manifold boost pressure. On the surface, it sounds like a simple premise, and in fact, a waste gate is a simple device. The problem comes from the pressure in the exhaust system, called turbine inlet pressure that can bear against the valve, overpowering the spring in the actuator, and forcing the waste gate open at lower than intended boost levels.

  Original equipment turbocharger waste gate actuators are selected or engineered for a specified boost level and turbine inlet pressure. To keep costs down, such actuators are usually just big enough to do the job at the stock boost levels. If the turbocharger boost is increased for additional airflow and performance, the stock waste gate actuator is frequently incapable of holding the waste gate fully closed until the higher boost level is reached. This happens because turbine inlet pressure also increases as boost pressure rises. The fix is to use a bigger spring in the waste gate actuator to hold it closed until the desired peak boost is achieved, however, that also requires a bigger actuator diaphragm to override the heavier spring when the desired boost level is reached.

  The net affect is that the turbocharger comes up to peak boost more quickly and then maintains that boost level throughout the engine’s RPM range for optimum mid-range torque and top end performance. 

  
  Mechanical Waste Gate Actuator

   

• Waste Gate Controller AKA Boost Controller
  
  Mechanical Boost Controller             Electronic Boost Controller (computer, hose, solenoid valve)        
  
  
  The boost controller sets the pressure at which the waste gate actuator operates. An electronic boost controller is better at holding the waste gate shut and the level of boost can be controlled from inside the car.
  
  
  

• Blow Off Valve (BOV) (Vacuum Control modulator valve on 280zxt)
  
  Blow Off Valve (BOV) (inlet is in bottom, outlet is on right side) 
  
  
  Many factory turbo installs have their BOV venting into a "Tube-inside-a-tube" so that it's flow remains perfectly in-line and parallel to the inlet stream. Remember, this thing doesn't just vent excess pressure, it's also flowing sonic waves and pulses. It makes perfect sense to vent this straight back into the turbo to help keep it spooled-up, instead of just wasting it by venting that energy into the atmosphere
  
• Pressure Release Valve aka Pop Off Valve aka Emergency Relief Valve that is screwed into the rear of the manifold TBD
  This is basically just a backup BOV set to vent ~ 2 to 4psi above maximum boost pressure 
  
  

How to: (The basic mods and tips)

• Your first step to raising your boost is getting a boost controller. This can be had for cheap if your not picky and just want to raise boost. There are a few manual boost controllers out there for around $50.00. I'm using TurboXS dual boost controller...that set me back about $200.00. But when I first started playing with my motor, I started out with a cheaper manual boost controller...I think one company is Granger boost controller. You will also have to buy a boost gauge. The stock one is useless when you raise your boost and getting a new gauge is the only SAFE way you can see what your turbo is actually doing. I got my Autometer boost gauge for about $45.00 on ebay.
  
  
•  Get a boost controller, you can get a manual one for $35 at www.dawesdevices.com
  
  The turbo (OE & most aftermarket) have a provision for boost signal on the compressor of the turbo that goes right to the waste gate. Your boost controller goes inline with this, i.e.: the hose off the compressor housing goes to the inlet on the boost controller and then the outlet of the boost controller goes to the waste gate actuator inlet. Some systems with IC's can run a boost signal in the IC piping. Last resort would be to use a signal from the manifold (as you described). I say this because by time the boost builds up and gets signal to the waste gate, you're probably over boosting by several pounds. The only benefit is that waste gate creep is kept to a minimum, but your set-up will have to be able to handle a slight over boost.
  
  counter: If you are modifying your car you DON'T WANT the boost signal from your compressor outlet. You want the boost signal from your INTAKE manifold.
  
  TIP: ANY manual controller is best kept under the hood, with the lines as short as possible.
  
  TIP: The dual stage XS controller is CHEAP compared to what you would pay for an electronic boost controller.
  If you are not running boost high enough to cause serious traction problems, stick with a single controller.
  Using a two stage to allows the use 15~20psi for launching, and 30psi+ for acceleration at speed in fourth and fifth gear...
  
  Tip: Manual controllers works great. The only disadvantage of the device compared to an electronic solenoid is that the air vacuum takes longer to travel through all that hose causing potential over-boost problems before the waste gate opens. Not a problem if your boost gauge is accurate and you raise your boost slowly. 
  
  
  
  "Dial A Boost" controller info on this page
  
  
  DIY Manual Boost Controller
  Use an air regulator valve from an air compressor. However a regular run of the mill air regulator is not accurate enough to control low boost in small increments. If you have the means, find a regulator that is super precise as far as low pressure. A place called Mcmaster-Carr has them (almost like Grainger units but better) Ranges are  0 to 20 psi out and 0 to 100 in for  $45.00. For running 10 psi, you can adjust from 3 to 5 psi over 3 full turns. Accuracy and precision are critical!
  
  
  

Electronic Boost Controller 
For a stock ZXT, the boost is controlled by the waste gate actuator. It has a ~7psi  spring in it holding the waste gate shut.....its also has a diaphragm in it that's hooked to the intake manifold. So when you get to 7psi+ of pressure (boost) inside the intake the pressure behind the diaphragm is greater than the 7lb spring holding the waste gate shut so it starts to open up the waste gate...
An electronic boost controller it performs the same task, the same way; but instead of just letting the intake manifold pressure do the actuator. it has a valve between the intake and the actuator and it computes how much pressure gets to the actuator after it has computed what would be best ......it monitors a bunch of things going on in the engine and figures the best way to do it (optimally). ...and costs you a lot of money.

• Get a boost gauge to see what your boost controller is actually doing. (The stock one on ZX's is reported to be inaccurate)
  
  one bar = 14.7psi of boost
  
  
  
• Get an  intercooler, you can get a used one off a starion or others. Its what's going to help protect your engine. Intercooler pics
  
  NPR Intercooler
  GOOD CHOICES are. Isuzu NPR, Porsche 944, Toyota Supra, SVO Mustang, Thunderbird SC
  BAD CHOICES are, Volvo, Starion, Mitsubishi, some Audi's
  
• You can run up to about 10 psi with the stock T3, you will need a new boost gauge and you need to either block off (not recommended), modify, or replace your pop off valve with an aftermarket unit. Stock injectors start to lean out at 10psi
   
  Blow off valve location venting to engine compartment (not efficient)
  
  
  blow off valve 
  
  TIP: Surging the compressor sounds like a dying goose right? It's because your compressor wheel is getting slowed down real fast from real high rpm. Not good for your new turbo. Get  a BOV!
  
  
• The turbo from just about any Z31 300zxt will fit with no problems. It is a Garrett T3 just like on the 280ZXT however the 300ZX t-3 is water cooled and the 280ZXT one in my garage is not. (water cooled is better than just hot oil).
  
  
• If you have a stock ZXT manifold, and are not running some ridiculous pressures or flows, the stock BOV from the 280ZXT is more than enough.
  Even with an intercooler and the added volume, the stock unit works fine for me up to around 12psi.
  
  
• '82 FSM gives the waste gate operating range as 41.9 - 47.2 kPa or 6.1 - 6.8 PSI and the relief valve operating range as 50.7 - 53.3 kPa or 7.4 - 7.7 PSI
  
  
• Well if you are planning on a significant upgrade, then I would suggest a TO4E compressor side with a 50 trim, and a stage 5 exhaust turbine wheel. That turbo will get you to 500hp.
  Now here is the good part: You can run that turbo on a stock application, what you will want to do is leave the waste gate actuator alone, so that the turbo will maintain it's stock boost level of 7-8psi of boost. Use the stock exhaust turbine housing and machine it for the stage5 exhaust turbine wheel. Very realistic build, and when you start upgrading you will not need to mess with the turbo. All you will need to do is get a boost controller and you are in there.
  
  
• The 82-83 F54 blocks are the same for NA and turbo cars. Crank and rods are also the same. The only major difference is pistons: dished for turbo, flattop for NA. Both are cast aluminum. The ring lands and grooves are the same thickness. The turbo piston has a slightly higher pin height. Consequently, the higher compression ratio of the NA block will not allow as much boost (non-intercooled). But with proper intercooling and an efficient turbo, you can still turn up the boost, and make tons of power. There are people doing it to NA blocks. Whatever block you use, stock turbo injectors & ECU are limited to about ~270 crank HP. The car with the higher static CR will be a little faster on the low end though. I think the reason why converting an NA to turbo is discouraged is because of the added complexity. It's just easier to start with a turbo block, because all of the parts are there.
  
  
  
  T3/04 Installed in L28 with mechanically adjustable waste gate controller.

VW Turbo Block Diagram


BOV and POV Info

Stock 280zxt airflow diagram with special items highlighted

L28ET Air Flow (click to expand)

Emergency relief valve = Pop-Off Valve
Vacuum Control Valve + Auxillary Air Control Valve = "Pseudo" Blow Off Valve

• A "blow off valve" and the "pressure relief valve (also referred to as a "Pop Off Valve") are two different things. 
  There is no stock BOV on the 280zxt, however there is a stock pressure relief valve. If installing a BOV, it should go before the throttle.
  
•  A pop-off valve. It's on the intake. The BOV is typically aft of the intercooler but prior to the intake. The pop off dumps all charge over a certain pressure and the bov dumps when the foot is lifted on the accelerator.
  
  A BOV installs on the i.c. piping or j-pipe (close to the throttle body) and vents into the cold air plenum between the afm and turbo.......depending on what valve you get, the install will be different. some have flanges that have to be welded, some have a pipe w/a fitting for the valve on it that's coupled into the j-pipe or i.c. piping . mine is actuated by seeing vacuum in the intake.
  
• If you dont want to spend an arm and a leg for a blow off valve. find one from a 1st gen eclipse.I have one that I will use. I hear that they can hold up to 15lbs of boost fine and they are all metal.
  
• I'm using the Eagle Talon 1G BOV and it works as well as some high priced aftermarket ones. If you look you can find one for around $25-50. They will hold up to 22psi boost and more if they are modified. If you run the factory ECU you will have to route the outlet back into your intake tract between the AFM & turbo. This will quiet down even the noisiest BOV's. If your running an aftermarket fuel injection system you can vent it to atmosphere and make lots of noise.
  
• Under vacuum, the BOV should lift and relieve pressure in the manifold before the throttle plate to allow the compressor to keep spinning at high spool-up speed allowing quick recovery into boost when the pedal is depressed. It keeps the compressor section from going into stall under static pressure. If you plumb the blow off line back into the inlet with a pipe pointed straight at the impeller wheel, when it blows off, it actually will pre-spin the turbo up to speed! Correct design routes the bov that way, racer-boyz like the quiefe sound, so they vent it to atmosphere. Take a look at where the Porsche turbos vent the blow off pressures...
  The only time a BOV is giving you problems is when it lifts under boost, when it doesn't seat and leaks boost, or doesn't open quick enough---causing a boost spike in the intake tract possibly stalling or semi-surging the compressor.
  Vacuum-actuated units do act JUST like you describe when functioning properly---it is also referred to as a 'compressor bypass valve" in some design circles, as the valve opens under vacuum, allowing air to bypass the compressor under lift throttle, and until the compressor has sufficient boost built up in the intake tract (lets the compressor spool-up free-flow...)
  These hard pop-offs sound neat, but that's not how a properly designed system should sound. Basically a properly adjusted unit will let you hear turbine spool up, and with the lightest lift of the pedal give what sounds like a loud sigh from the engine.
  
  
• An easier and cheaper way to do this is to re-route the air comming out of the blow-off valve back into the intake before it hits the turbo.
  
• A BOV (Blow Off Valve) or compressor bypass valve, main function is to relieve pressure in front of the throttle body plate when it is closing ie: deacceleration .
  
  The BOV is placed after the turbo but in front of throttle body plate . It uses to pressure lines, #1 in front of the throttle body and #2 behind the throttle body .
  Normally the BOV is closed, pressure from a spring seats the valve and keeps it closed .
  
  Situation 1 : Full throttle or gas pedal being pressed down . Turbo begins making boost, pressure in front of throttle body is 10 psi, #1 pressure line acts against spring pressure of BOV trying to open the valve . But #2 pressure line behind behind the throttle body will be a little less than 10 psi . #2 pressure line helps with spring pressure to keep valve closed . Therefore valve stays closed during gas pedal being pushed down .
  
  Situation 2: Letting up on the gas pedal . The turbo is still spinning quickly making 10 psi boost in front of the throttle body, #1 pressure line sees 10psi . When you let off on gas the throttle body plate creates a restriction, #2 pressure line gets like 5psi behind throttle plate . Since #2 pressure way the only thing holding the BOV closed before and it is less now the valve opens . Now the BOV opens and relieves the pressure between throttle body and turbo compressor . This pressure in there would have caused the turbo to slow down due to backpressure on compressor blades . When you go to hit the gas again turbo is still spinning very fast, which gives you better acceleration .
  
  
• Yes, the POV will vent anything over about 7-8psi. You should know if its venting though, I haven't ever set mine off but I understand it makes a loud shrieking sound when it vents. A blow off valve (BOV) is a completely different animal, it releases excess boost during engine deceleration, like when shifting gears, to prevent damage to the turbine, and is not a stock item but a performance upgrade. Much more useful on a manual trans car in my opinion.
  For the POV, you can shim it with some washers (use the search feature for details) to increase the max psi, replace it with a plug (maybe not the best solution, there would be no safety release), or get an aftermarket one set at a higher level of boost.
  Just a note, if you don't have an intercooler, don't boost over about 10psi and be very careful at anything over 8, detonation will kill your engine in short order. Some guys are running as much as 13 without an intercooler, but personally I wouldn't recommend it.
  
• You don't need to waste money and time and effort to install the BOV. On the stock intake there is a Vacuum Control modulator valve. This valve is the BOV but it works on vacuum. When you slam the TB shut, there will be vacuum in the intake which will suck this valve open. This valve is the little valve that screwed into the 4th runner of the intake and the J pipe. When this valves sucked open it will release pressure built-up by the turbo.
  
• Why install a BOV on a stock ZXT? The ZXT comes with a BOV from the factory, (NOT referring to the "Emergency Relief Valve" that is screwed into the rear of the manifold). There already is a BOV (Surge Prevention Valve) in the design of the stock ZXT manifold, you don't need another one if you are only running the stock turbo, or a non-intercooled (increased captive volume system) set-up.
  
• The Engineers at Nissan were not stupid. The factory BOV is indeed set in the runner for #4 cylinder, and is plumbed from the stock J-Pipe in the hose that also supplies air for the AAC and the fast idle bypass circuit. It is all debatable whether it is a vacuum or pressure operated device but the function of the valve is to relieve anything over 2 or 3psi in the intake plumbing when the throttle is slammed shut.
  Vacuum alone will not open this valve, and neither will blowing on it with 2psi of air. BUT, when you slam the throttle shut, and get a 19" Hg spike in the intake manifold (-8psig) and have another +7psig in the factory J-Pipe, you have an easy 15psi differential there, opening the valve, leaving the excess pressure upstream of the throttle plate to relieve to the inlet manifold.
  
  
• I don't know what the difference between the original bypass valves and the blow off valves are, other than engineering and money...That is the big difference.
  I agree, they are compressor bypass valves---but the stock unit does no function in that fashion--it is a dump valve. It does not open under vacuum to flow air around the compressor section (like the old HKS and Cartech units did). The stock valve is basically a dump or blow off for drop-throttle conditions keeping the metered air in the metered part of the system.
  
• BOV off of a 1G DSM (first gen eclipse/talon/laser) was also reviewed in that article, and performed just about as well if not better than almost every aftermarket unit they tested. I picked one up used for about $50 off ebay, probably could have got one way cheaper from a junkyard. I also recirculated mine, car wont run right if I don't, runs way too rich any time the BOV is open, which is pretty much any time I'm not accelerating! I never ran the car w/o a BOV, it was my first 'mod', but I've heard/read enough to know you really shouldn't run any turbo car w/o one... not only does it benefit in that the turbo doesn't slow way down between shifts, allowing it to come back on boost faster after a shift (some people will debate this) but it takes off a LOT of strain from the turbo bearing, and can greatly increase the life of said bearing, especially if you are a lead foot.
  
• "BOV" it installs on the i.c. piping or j-pipe (close to the throttle body) and vents into the cold air plenum between the AFM and turbo.......depending on what valve you get, the install will be different. some have flanges that have to be welded, some have a pipe w/a fitting for the valve on it that's coupled into the j-pipe or i.c. piping . mine is actuated by seeing vacuum in the intake
  
• You may be confusing a blow off valve with a pressure relief valve. The prv is preset to vent boost when the boost level exceeds the preset amount. When raising boost on a ZXT, some simply remove the prv and install a pipe plug. You can also modify the prv by removing the top and adding washers to increase the pressure required to vent boost. A blow off valve's purpose is to vent boost pressure when you let off the gas. Imagine, you've got your foot down, you've built boost, now you need to let off on the gas to shift. As soon as you let off the gas, the waste gate opens and you subject the turbo to vacuum. Not good. The blow off valve vents boost pressure so the turbo isn't fighting itself.
  
• Setting a BOV:
  Author: ase mat of www.Zcar.com
  Date:   Jul 6, 11:50am
  
  Once you understand the exact "How's and Why's" of a BOV,
  you'll know exactly how to adjust any BOV.
  The main purpose of any BOV is to protect the intake-side of the turbo from shock-waves.
  These shock-waves are created when the throttle is closed during boost, especially high boost.
  The more boost there is when the throttle is closed OR the more abruptly the throttle is closed...The stronger the shock-wave that is reflected back into the intake side of the turbo!
  
  Obviously...This is not a good thing! Because it eventually pounds the turbo to death from the inside-out...Literally!
  
  The BOV protects the turbo by reducing this undesired reflected-wave.
  It does this by venting some of the super-high pressures (at throttle closings during boost) away from passage between the outlet-side of the turbo and the throttle plate.
  
  Based on the above data...You now know WHEN! you want the BOV to operate, so it vents only during these times.
  
  But the signal-hose (control-line) for the BOV is connected to a vacuum source on the intake-manifold, behind the throttle-plate.
  Because it's behind the throttle-plate, it "see's" a higher-than-normal VACUUM!(not boost or pressure) each time the throttle is closed. This vacuum-signal is the highest when the throttle is closed at super high-rpm shifts...Like the times when you want maximum power.
  
  It is THIS! super-high vacuum-point , during high-rpm shifts, that you want the BOV to operate! And, in general, no other times.
  
  So...Now you know. To adjust the BOV correctly, you don't need any type of testing or measuring devices besides the ones God gave you...
  Your ears!!!
  
  Adjust it so it vents (trust me...You WILL hear it) only during medium-to-high boost throttle closings. If you set it so that it vents at too-low a boost level (so it vents too often and too-early), it will increase turbo-lag.....And we don't want more lag!
  If you adjust it so it vents too-late (so it vents only at psycho-high boost and not often/early enough), it will not protect the turbocharger as intended.
  Other than that...Fine tune the adjustment to your own personal taste!
  
  Please note, I deliberately avoided the ultra-technical explainations here because I didn't think that's what you wanted. I hope I was correct.
  
  Good luck with your new toy.....ase mat
  

Boost

• If you remove the sensing line for the turbo, and put it to the stock manifold you will notice the boost off lift-throttle works much better, as when you slam the throttle shut, the vacuum of the manifold closes the waste gate actuator much faster, and holds it closed tighter during cruise, making for good boost response.
  
• The AAC is the ONLY way for the engine to get air as the throttle plate is 100% closed at idle. You may have a problem with the AAC or it's control circuitry, too. The later units have a screw on the side of them to allow a manual adjustment of minimum air bypassed to keep a minimum speed when the engine is idling, as the original AAC did not have that, and if the diaphragm got a few pinholes, your car would DIE when you let the throttle go back to idle. Very unsafe and irritating! I have always preferred to use the term "Compressor Bypass Valve" or just Bypass Valve. On engines that have AFM's or MAF's, you DO want to route the output back in front of the turbo (after the AFM/MAF) to avoid a rich condition. The term BOV would be appropriate for speed density systems. If one were to syntactically and functionally choose only ONE acronym, I believe that CBPV or BPV should be used by all. It needs to be noted here that HKS originally called these Bypass Valves.
  
• SK made a replacement system for N/A cars with EFI, and it used a compressor bypass valve (I have one on my 73). It was the only system at the time that used one until HKS come out with their system (Cartech was Carbs). It opens under manifold vacuum, and closes under boost. It was adjustable for cracking pressure differential. Very nice and simple unit. Required if you are using an N/A manifold on a turbo conversion.
  Unnecessary on a stock Turbo with the stock turbo pushing 12psi.
  
• I knew that venting in the atmosphere causes a temporary rich/lean mixture if you are running the stock system, that the car will not run worth a crap because of the big air leak in the intake system of the turbo set-up; so if you are running the stock system, and you install the blow off valve, it is open at idle because the vacuum opens it up the car will run very bad if you don't run the valve recirculated, back into the tube to the turbo.
  
• The function of the turbo bypass valve is to redirect turbo boost away from the throttle plate during deceleration. It prevents a shock wave from forming when the stream of air hits a closed throttle plate that would rebound down the induction tubing and disrupt the flow of air across the air mass meter and hit the turbo's compressor blades and stop or slow its impeller. The result of the shock wave is stalling or very low rpms when slowing down after being in a boost situation. The valve is a black plastic diaphragm with two approximately 1 inch diameter 90 degree opposed ports going into the intake tube to the turbo and the intake tube near the throttle plate (connected to the tube for the throttle plate on a 900 by a length of hose) It also has a small vacuum line on it to control its operation. When vacuum is applied to the line, the diaphragm pulls open the passage between the 90 degree ports and allows flow from in front of the throttle plate back in front of the turbo impeller effectively preventing the shock wave from forming
  
•  

What other Zcar drivers have:

• t3/to4b ca, water cooled, external wastegate,stage III turbine,57 trim compressor. its for a streetable daily driver w/a 350hp target
  
• tec II w/ a Garrett T04E w/ 60 trim wheel stage 5 in a .70 a/r housing compressor side and a 76 trim on turbine side w/ .69 housing and a 4" inlet turbo.
  
• Running 12psi (on hot day) 14 psi (on cool day) on stock fuel injectors. I do have Z31 ECU and MAF though.
  60-1/stage III@44psi fuel pressure on stock injectors, I dynoed 253WHP (290crankHP)with 11.8:1 A/F ratio with Z31 ECU
  
• I have been running 10psi on stock 83zxt in my 240z for one year now no problem
  
• I ran 10 psi for a while on stock car with straight exhaust and intake then went intercooled at 15 psi and ran great no detonation or ping ran fine till I blew headgasket
  
• I'm running 12psi intercooled on stock fuel system
  
• ok, I run the stock 83 turbo engine in my 240z and I have added a MBC to get to 10psi. I also fabbed my own 2.5" mandrel downpipe, but everything else is stock, ECU, injectors, pump, regulator, AFM, turbo etc. Mine hits full boost at 2700-2800rpm(in 1st,2cnd,3rd gears), not 3500. It use to only hit at 3500, when I had stock downpipe and no MBC and my fuel pressure wasn't correct. And you can really feel it hit at 2700. In 4th and 5th gear if I'm cruising at 2000rpm and floor it I get about 4psi boost, then by 2500rpm it is 10psi. But that is in the higher gears where the engine is working harder. The more load you put on the engine, the more air/fuel you move through, the faster the turbo will spool. I'm just relaying my results, not making any claims that those two mods fix the problem, because you guys may have something else on the fritz.
  
• I had an n/a with dual SU's it ran great and enjoyed driving it. I just transplanted an 83 turbo with SDS and t3/t4 hybrid. It is a joy to drive. This is my first turbo car but it is very apparent to me now that there are a lot of misconceptions out there about them. I used to believe that turbo would not "kick in" until a certain rpm and at that point it would be an all or nothing deal. This is not true, first off like n/a motors there are lot of factors that go into building them right for your application. The turbo is always kicked in it is simply building up pressure at the same time the motor is naturally making vacuum at low rpm and low load, the motor is winning the battle but shortly by about 1500 rpm in my case the turbo can push the vacuum/boost gauge into positive territory. The response is quick and smooth the dyno on my car shows 150lbs of torque at 1600 rpm and it is up to 310lbs of torque by 3000 rpm and the curve stays nice and fat until around 5500 rpm. I get throttle response. I am sure that there are guys on this site with turbo cars that are set up for 1/4 mile stuff that put up even bigger numbers and that's cool if that is how you are going to use the car- but don't believe that you can't set up turbo car to be great mountain road shoot the apex type go cart. My favourite thing to do with this car now is going up hill as you go thru the turns - its unreal how easy it is to overtake the cars in front of you. Nothing against n/a Zcars I have ridden is some very fast ones but boost is a blast. 
  

• I'd go with the stock turbo. The garret T-3's are capable of up to 15psi. Though I would recommend using a water-cooled center section from a 300zxt or a merkur xr4ti. Its just a better way to go if you want big boost.

• Even More
  

Applied L28ET Air Flow, Calculations, and Turbo Maps

Walk through of calculating Air Flow under boost and translating this to Compressor maps

DIY Links with Great Pics!

Mechanical Turbo Boost Increase

Adjustable Pop Off Valve

Intercooler Installation

Turbo Upgrade

Typical Costs

ECU $50 on Ebay
Harness $50 on Ebay
Fuel Pump $50 on Ebay
Intercooler & Intake mandrel bends from JCwhitney $150
Exhaust $150 - $300 depending on what you want?
Silicon connectors +/- $100
Tune-up stuff - plugs, wires, cap, rotor, fuel filter, hoses, $150
K&N intake filter, since the stock one will be hard to fit $50.
Do you plan on turning up the boost? I assume you do since you asked about the intercooler.
Boost & A/F gauges, boost controller: $150
Does your 240Z have a decent cooling system? If not, spend $200-$400 on new radiator, clutch fan and/or aftermarket electric fans.

280zxt microfiche drawings

L28ET Turbo (Click to expand)

Oil Cooler (Click to expand)

Intake Manifold (Click to expand)

EGR Area (Click to expand)

Garrett Air Research T3 Turbo

A stock 82 ZXT has Garrett Air Research T3 Turbo installed.

• Normal (Stock) Boost is ~6psi
• Max Boost is 15psi
• Safe boost without intercooler is 10psi
•  60 comp trim and .63 turbine trim

T3 Turbocharger
The T3 turbocharger is designed for today's wide speed range medium displacement engines requiring good low speed torque for responsive drivability and tailored control of boost pressures from idle to rated speed. The T3 is engineered to provide an economic performance boost for diesel and gasoline engines. The T3 will effectively cover most engine ratings from 175 to 250 hp, in gasoline/petrol applications. Over 2,000,000 T3 turbochargers have been produced.

T3 Specs
Weight 5.3 Kg
Compressor Diameter 51.4 mm
Turbine Diameter 53 mm
Bearing Span 31.9 mm
Moment of Inertia 2.1*10-5 Kg/m²
Oil Flow 1.2 l/m SAE 30 - 90ºC - 2.75 Bar

T3 Flow Maps

Garrett Air Research T3/T4 Hybrid Turbo

A T3/4 Hybrid Turbo has the Exhaust Side from a Garrett T3 and the intake side from a T4

It is easy to see that the T4 is designed to flow more air. (Opening has 1.48X bigger cross sectional area)

Recommended upgrade for a stock L28ET engine (280zxt) is T3/TO4B  cost ~ $500

T3/T4 Turbo Install (great pics)

CheapTurbos.Com

Z32 Turbo Page