Frequently Asked Questions

You’re interested in getting into HPA but don’t know where to start? Here you will find the answers to the most commonly asked questions regarding our systems and HPA in general to help you get started in the world of HPA airsoft. This section will be updated regularly based on feedback from customers like you so, if you don’t see the answer to your question, please email us at and we will be happy to help.

HPA 101

We currently offer two different lines of HPA conversion systems. There is the Fusion Engine line which is a complete drop-in replacement for an AEG's gearbox. So, you just remove the whole gearbox and drop the Fusion Engine into the rifle in its place. Then we also have our line of cylinder conversion kits which are cylinder replacement systems for an AEG's gearbox. These reuse the original gearbox shell and simply replace the gearbox's internal components.

No, our systems are designed to work with the AEG's original barrel, hop-up and magazines so nothing needs to be changed.

Three different components are needed to run our systems. You will need a regulated air rig capable of regulating output pressure to 120psi and below, a HPA (High Pressure Air) tank and a battery to power the on board FCU (Fire Control Unit).

All of our systems require minimal maintenance as each system only has at most 2 moving parts. We recommend disassembling, cleaning and greasing each system after every 30,000 to 50,000 shots.

The PolarStar HPA conversions can be separated into two broad categories: Dual Solenoid and Single Solenoid systems. Dual solenoid systems use two solenoids to move the nozzle and fire the round. This allows each action to be controlled separately and precisely. Single solenoid systems have only one solenoid, which controls nozzle movement from it's retracted idle position to the forward firing position. The firing air is not directly controlled by the solenoid, but by the position of the nozzle as it moves forward to chamber the round.

Fusion Engine™: A complete drop-in kit which replacing the entire gearbox. It is the most adjustable system of the group with the widest range of adjustment in both velocity and rate of fire. It is a dual solenoid, closed bolt system so you are able to adjust the poppet valve (firing air) and nozzle (feeding) independently for unparalleled control over both the loading time and air volume used. Being a true closed bolt system it is easier to achieve a high level of accuracy since it is not as critical of the hop-up configuration as an open bolt system. Additionally, it does have the best feeling trigger since the trigger micro switch is custom fit to the frame so the trigger has a distinct break, hard stop and short reset.

F2™: The F2 is basically a Fusion Engine that has been miniaturized into a drop-in cylinder conversion kit like the F1 and JACK in which it replaces the components within the original gearbox shell of the AEG. Like the Fusion Engine, it is a dual solenoid, closed bolt system for complete independent control over the poppet and nozzle and delivers the performance you can only get from a closed bolt system. The F2 has a more air efficient design than the Fusion Engine and is comparable to the JACK in terms of efficiency.

F1™: An open bolt conversion kit which replaces the components within the original gearbox shell of the AEG. It is the most air efficient and uses a flow regulating check valve as well as "2-stage" firing. The unique flow control system uses an integral check valve to regulate the forward speed of the nozzle while maintaining the fastest retract speed possible. When the nozzle is traveling in the forward direction the check valve is sealed, limiting the speed of the nozzle for smoother chambering and consistent BB placement. After the nozzle travels a certain distance, it begins to allow airflow through the bore to begin the acceleration of the BB. When the nozzle reaches the forward position the check valve opens to allow the maximum flow into the barrel. The additional flow is maintained throughout the rearward travel of the nozzle, allowing it to retract without limiting its rearward speed.

In addition to the flow control and 2-stage firing, F1 also uses a custom solenoid designed specifically for this application. This allows the solenoid to become an integral part of the system rather than just a switch to control airflow. The advantages include a more compact system and a more efficient system due to the reduced internal "Dead Volume".

JACK™: Like its sister product, the F1, it is another open bolt conversion kit which replaces the components withing the original gearbox shell of the AEG. It has a simple, straight forward design which is very reliable and delivers solid performance. It has better air efficiency than a stock Fusion Engine and is only slightly less air efficient as an F1.

Fusion Engine

Basically the way the system comes as stock, the poppet valve allows the maximum amount of air flow through the system and it is the nozzle which restricts the flow into the barrel which varies the velocity. This gives you a wide range of velocity adjustment by simply changing out the nozzle to shift the velocity range of your rifle up or down in large increments.

The low flow poppet valve does what the name suggests and restricts flow at the valve itself so it allows less air through. Therefore the main restriction is back further in the system instead of right up front at the nozzle. Normally, with the nozzle being the restricting point, there is air backed up in the section between the poppet and the nozzle which will only be able to vent out as fast as the nozzle's restriction allows. If a more restrictive nozzle is needed to keep the muzzle velocity within field limits, there will still be air pressurized in this section after the poppet valve has closed and the BB has left the barrel so that air is wasted. Since it is the poppet valve which turns the air flow on and off, having it being the restricting point conserves air since less air is allowed through at once.

The low flow poppet also helps improve how smoothly air flows down the barrel since it has time to expand slightly in the system after it goes through the valve so there is less turbulence behind the round which can improve accuracy slightly. With the poppet being the main restrictive point, the nozzle doesn't have as much of an affect on restricting flow although it can still restrict if the flow reaching the nozzle is still greater than the nozzle can let through at once. This is why you typically use a higher flow nozzle in conjunction with the low flow poppet such as a Black, Silver or Red nozzle. With the Red Low Flow poppet installed along with one of these higher flow nozzles, a Fusion Engine can achieve up to 50% greater air efficiency than a stock engine producing the same muzzle energy.

With the low flow poppet valve the rifle will be optimized for air efficiency when operating at standard field limits. While it may not be able to hit as high of a velocity in the maximum pressure range as it would have with the standard poppet valve, it will still be able to achieve rifleman and DMR velocities and with greater air efficiency.

Here are some examples of how a Red Low Flow affects muzzle velocity. Testing was done at 80psi and 120psi using a VFC M4 with a stock hop-up and 6.05x380mm barrel.

Red Nozzle: 400fps - 485fps w/ .20g 355fps - 415fps w/ .32g
Silver Nozzle: 390fps - 450fps w/ .20g 340fps - 390fps w/ .32g
Black Nozzle: 370fps - 430fps w/ .20g 325fps - 375fps w/ .32g


Most likely, yes. F1 was designed to fit virtually every AEG gearbox available, including those with offset nozzles. The only things to determine are the correct nozzle and switchboard.

For most applications a JACK/F1 switchboard is already available to drop directly into the gearbox. If no switchboard is currently being made for that model, plugboards are available for custom wiring.


Single solenoid systems are required to choose between closed bolt/fixed volume or open bolt/adjustable volume operation. Dual solenoid systems like the F2 and Fusion Engine allow for the nozzle and poppet valve to be adjusted independently of each other so you are able to achieve the best possible air efficiency while getting the performance advantages of a closed bolt system.

Being able to fine tune the air volume which the system produces makes it possible to optimize air efficiency for a particular barrel length so it will only release the precise volume of air needed to fill that barrel. Fixed volume systems release a set volume of air so their air efficiency suffers in comparison.

The difference between closed bolt and open bolt is the position of the nozzle when the system it is at idle (waiting to be fired). With an open bolt system the nozzle is back and a closed bolt the nozzle is forward. When you fire an open bolt system the nozzle is driven forward so that it chambers the BB and then fires at the end of its travel before retracting back to the rear position. When you fire a closed bolt system the system will fire first, then cycles the nozzle back allowing a BB to enter the chamber before the nozzle returns forward to push it into the bucking.

A closed bolt system is more forgiving of the hop-up system since the BB is in the chamber and everything has stopped moving before the system fires. An open bolt system is more critical of the hop-up system since it is pushing the BB into the chamber and firing in the same motion so the BB never actually stops. This is why open bolt systems typically require a bit more tuning of the barrel group than closed bolt systems in order to optimize accuracy.

Dual solenoid systems allow you to change the sequence in which the system fires. Traditional "closed bolt" operation means that the system will fire air down the barrel first and then cycle the nozzle to chamber the next round. This is how all single solenoid, closed bolt systems operate. The disadvantage here is that if you run the magazine dry and load in a new magazine, the first shot will be a dry fire since the system will fire first and then cycle the nozzle to chamber the first round.

With dual solenoids you can control the nozzle and poppet valve independently so you can choose between running the F2 in traditional "Closed Bolt" mode or reverse the firing sequence for "AEG Mode". In AEG Mode the nozzle will cycle first to chamber a around and then the system fires. Therefore even if you run the magazine dry and load a new magazine on an empty chamber, the first shot will not be a dry fire.

Yes, an offset version is planned and is currently in development!

Yes, we plan to eventually have the same switchboards available for the F2 as we do for the F1 and JACK.

No, the F2 is simply another addition to our product line. The Fusion Engine will remain as our flagship product for the foreseeable future.

We will release different low and high flow poppets for the F2 however there will only be one nozzle for each model. Different levels of restriction are not necessary for the nozzles on the F2 due to the much wider operating pressure range. This allows the F2 to achieve a much wider velocity range simply be adjusting input pressure.

Yes, the F2 uses the same input fitting as the F1 so it can use the same IGL as the one the sell for the F1. The IGL is a very close fit to the lower solenoid though so it can be a bit tricky to install. They recommend keeping downward pressure on the IGL to avoid contact with the solenoid while threading it in. Or, remove the solenoids before installing the IGL and then reinstalling the solenoids afterwards.


JACK is a centerline AEG cylinder replacement. If the gearbox uses a standard piston and cylinder assembly and the nozzle is centered in the cylinder there is a good chance JACK will fit.

In order to install JACK into an AEG, the correct nozzle is required. PolarStar manufacturers a number of different JACK nozzle lengths to cover a wide range of guns. If the nozzle length you require is not already in production, custom nozzles can be made upon request.

In addition to the nozzle, JACK must interface with the selector and trigger of the host gearbox. This is accomplished using various switchboards which install into the gearbox shell.

For more information on the compatibility of specific models please see our Compatibility Guide in the support section.

FCU / Settings

A 7.4V LiPo battery is recommended for powering the FCU. Since the battery voltage is be regulated and reduced to 5V additional voltage is unnecessary.

9V Alkaline batteries should not be used as they do not provide sufficient current to power the solenoid.

All PolarStar FCUs may be set to a semi-only mode through the selector settings.

In instances where the replica must be permanently converted, semi-only firmware is available for each PolarStar FCU model (Standard, Mini and Bluetooth). This restricts all selector positions to a single shot per trigger pull and the ability to adjust it has been removed.

Control units may be ordered with this firmware pre-loaded. Users may send in their current FCU to be re-flashed.

Unfortunately, no. The Mini FCU was designed to work with single-solenoid systems like JACK and F1.

Although it shares the same connectors and 5-position wire harness it is only able to control one solenoid. The "unused" solenoid wire has been repurposed as an additional input in order to allow the Mini FCU to work with 4-position selectors.

To correctly set the dwell on either the F2, F1 or JACK you would first set your dP to the maximum setting of 99 and then adjust your air pressure until you are shooting at the desired velocity with the BB weight you will be using.

Then start reducing the dP by 5 at a time until you see the velocity decrease or become noticeably inconsistent. This is a sign that you are under voluming the barrel so that air flow is being shut off before the BB reaches the end of the barrel. Increase the dP by 1 at a time until the velocity and shot consistency returns to where it originally was and then set the dP 2 higher than that point.

The "iS" is the anti-stiction timer. Each number has a value of 10 seconds (i.e. 01 = 10sec, 02 = 20 sec, ect.). The timer will begin after each trigger input and once it ends it will add the value you have set in "iP" to your "dP" for the first shot.

Therefore if you have "iS03", "iP10" and "dP50", after the system has been idle for 30 seconds it will automatically increase your dP to 60 for the first shot and then lower it back to 50 for the following shots until the system has once again sat idle for 30 seconds.

Note that if you have nothing set for your iP (i.e. iP00) then it will not add an extra plus to the dP regardless of what you have iS set to. The iS is simply a timer.

To program a fire mode to each selector position you would adjust the S1 and S2 settings in your FCU programing menu. On most common rifles such as an M4, the S1 is your semi auto selector position and S2 is your full auto selector position. By default the S1 should be set to S101 for Semi Auto, and the S2 should be set to S200 for full auto. Changing the number will change the fire mode set to that selector position.

00 = Full Auto, 01 = Semi Auto, 02 = 2 Round Burst, 03 = 3 Round Burst, etc. on up to 09 = 9 Round Burst.

The semi auto delay mode is available on FCU's that have firmware revision rE1.0 and higher. To program it, enter programing mode and scroll to the "Sr" menu. The default is Sr0F (off). If you press up on the joystick you will change it to Sr01 which is a 0.1 second delay in-between shots. You can continue to increase that number in 0.1 second increments up to Sr99 which is a 9.9 second delay in-between shots.

To view the shot counter you simply enter programing mode so that you can see the revision number (e.g. rE1.2) and then hold the joystick in the down direction. It will display a 4 digit alphanumeric number. Type that into an online hexadecimal converter and it will give you the number of shots that FCU has cycled. If you have a "high mileage" engine and you have reached the maximum number for those 4 segments, the count will continue to another 4 segments which can be viewed by pressing up on the joystick. Those are simply placed in front of those displayed on the "lower" display.

For example, the down direction on the joystick displays 3bcE and the up direction displays 0006. You put the 6 in front of the 3bcE so you have 63bcE which equals 408,526.

Air System

We only recommend running our systems on HPA or Nitrogen. While liquid propellants such as CO2 technically can be used, it is required that the air delivery system be specifically designed to use CO2 and properly utilizes an expansion chamber to safeguard the system from pressure spikes.

The SLP rigs require that you use an HPA tank that has a SLP (Super Low Pressure) regulator. All tanks have an integral regulator that takes the tank's raw pressure (e.g. 3,000psi.) and reduces it to the output pressure of the tank itself (i.e. the pressure that comes out of the tank and goes into the air rig). This pressure is then reduced further by the regulator on the air rig which the tank attaches to. The SLP tanks produces 300psi or less while "traditional" HPA tanks produce over 800psi. Therefore you can only use a SLP air rig with a SLP tank so as to not over-pressurize and break the air rig while an air rig like the Redline SFR can use either SLP or traditional HPA tanks.

There are two basic types of HPA tanks on the market. The 3,000psi tanks are typically made from aluminum and are the least expensive of the two. However, the higher pressure 4,500psi tanks are carbon fiber so they are lighter and will last for more shots per fill than a 3,000psi tank of the same size.

MRS Regulator

The standard MRS is adjustable from 40psi up to 140psi depending on the input pressure of the tank.


Before being able to address the issue you will first need to determine if the issue is related to the system's electronics or if the selector pate or cut offer lever of the engine/gearbox is simply not interacting with the micro switch correctly. However, before diving into the inner workings of the system, you will want to check that the selector positions are in fact programed correctly in the FCU.

For the Classic FCU with the LCD screen running firmware rE1.0 or higher, you would go about programing a fire mode to each selector position by adjusting the S1 and S2 settings in your FCU programing menu. On most common rifles such as an M4, the S1 is your semi auto selector position and S2 is your full auto selector position. By default the S1 should be set to S101 for Semi Auto, and the S2 should be set to S200 for full auto. Changing the number will change the fire mode set to that selector position.

00 = Full Auto, 01 = Semi Auto, 02 = 2 Round Burst, 03 = 3 Round Burst, etc. on up to 09 = 9 Round Burst.

If both S1 and S2 are set the same then the rifle would operate the same regardless of the selector position. (e.g. having S101 and S201 would cause the rifle to only shoot semi auto)

If the selector switches are programed correctly you would then go on to check that the electronics are working correctly. To check this you'll first need to remove the engine or gearbox from the rifle so you can see the micro switch. If it is a Fusion Engine, or a V2 cylinder replacement kit such as the JACK, then you would be able to see the micro switch from outside of the gearbox. If it is a V3 cylinder replacement kit then you would need to open the gearbox to clearly see the micro switch.

With a Fusion Engine or a Version 2 cylinder kit the micro switch is located on the back side of the switchboard and is activated by the selector plate as it passes over top of it. The switch will be a little square block with a black fin sticking up. The black fin is depressed by the plate as it passes over top of the switch.

With a Version 3 cylinder kit the cut off lever depresses a micro switch at the bottom of the levers arc of travel as the lever is moved by the selector plate.

To check if the electronics are working you would simply depress the micro switch manually with you finger. If you have a Classic FCU with the LCD screen then you can verify if the FCU is receiving a signal by observing if the LCD display changes between fire modes (e.g. "Shot" and "Auto") when the switch is depressed. If it does then you will know the electronics are working correctly and the issue is simply that the switch is not getting depressed when the selector plate or cut off lever moves. For that you would need to observe how the plate or lever moves and make adjustments accordingly so that it depresses the switch.

If the system does not change fire modes with the button is manually depressed then the issue could be with the switchboard, wire harness or the FCU itself. If the issue is with the trigger board or wire harness, then it should be visually obvious that there is damage with either the switch being broken off or a wire being cut or broken.

If the engine appears to be functioning correctly but refuses to feed then the issue could be caused by the solenoids being plugged into the wrong headers on the switchboard. If the solenoid controlling the nozzle is plugged into the poppet solenoid header and the solenoid controlling the poppet is plugged into the nozzle solenoid header then the nozzle will still retract and the poppet will still fire but the rifle will not feed.

This is because the solenoid is now running on the wrong dwell setting. The dwell for the poppet is much shorter than the dwell for the nozzle so the solenoid is not being energized long enough for the nozzle to retract the proper distance or length of time for a BB to feed up into the chamber. If you've recently had your system apart for upgrade/maintenance and it refuses to feed now that it is back together, checking where the solenoids are plugged in is a good place to start.

To easily determine if the solenoids are plugged into the wrong headers you will simply need to go into your settings and turn the nozzle dwell all of the way down to dn00 (or dn01 depending on the firmware). With the nozzle dwell turned all of the way down the nozzle should not retract when the system is fired but air should still come out of the nozzle. If you find that the nozzle still retracts but very little or no air comes out, then your solenoids are plugged into the incorrect ports and need to be swapped.

The solenoid which controls the poppet will be positioned in the manifold in-line with the shortest of the two clear air lines (left side of engine). Fusion Engines come from the factory with the poppet solenoid marked with a black mark on the bottom of the solenoid connector. Unless the solenoids were psychically swapped in the manifold then the solenoid with the black mark will plug into the bottom port closest to the trigger marked J2.

About Us

PolarStar Airsoft was formed in 2009 as a part of PolarStar Engineering & Machine with the goal of bringing our engineering experience and focus on quality to airsoft. Since then, PolarStar has made innovative airsoft products of the highest quality, designed and manufactured in the USA.

PolarStar products, including the patented Fusion Engine are designed by airsoft players, incorporating airsoft experience with the feedback of the commnity.

Latest News

PolarStar F1Posted 2016-05-13
HPA Speed TriggerPosted 2016-06-23


PolarStar Airsoft
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Newark, DE 19713
P: (302) 449-4866