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DIY Converting to Top Feed Injectors on a R33 GTS-t Skyline with a RB25DET

Just about every car enthusiast with their offensively powerful turbocharged vehicle will eventually hit a brick wall with the fuel demands by their motor. Question is, do you stop there? Certainly not! Time to get that fuel system up to scratch and invest more money into your prized depreciating asset that upsets the other half. (In our defense though your pride and joy will probably cost you less than your partner’s wardrobe and will have better resale value so you can use that excuse next time.)

As you may know, Nissan decided their humble R33 GTS-t should use side feed injectors for whatever reason and yes finding ways to upgrade to better modern better flowing injectors is a nightmare. Often people would make the choice of going high flowed side feeds (milling out the stock injector). However this has many issues which predominantly revolve around spray pattern, consistent fuel delivery and failure. All of which the Fridge unfortunately suffered from recently – hence the conversion to top feeds instead.

RB25DET Top Feed 02

So chances are if you’re reading this article you’ve either maxed out your current R33 GTS-T Skyline injectors and are looking to upgrade your injectors the right way OR gone high flowed side feeds and absolutely hate it. Then again you could be an avid supporter and just like reading our articles.

RB25DET Top Feed 03

RB25DET Top Feed 05

Rest assured, this guide should give you plenty of information on how to convert your annoying side feed injectors to top feed injectors and you’re under safe hands.

RB25DET Top Feed 01

Parts Required:

  • 6x Top Feed 1/2 Height 14mm Injectors
  • 6x 14mm to 11mm converters/spacers
  • 6x EV1 to JECs Connectors
  • 1x 11mm Slim RB25DET Fuel Rail & Fittings
  • 2x -6AN to 3/8” Barb 90 Degree Fittings
  • 1x 1/4npt grub
  • 2m of EFI Fuel Hose (preferably E85 compatible)
  • 4x EFI Fuel Hose Clamps
  • 1x Fuel Pressure Regulator
  • 1x Fuel Pressure Gauge
  • Some form of lubricant


  • Socket Set
  • Allen Key Set
  • Spanner Set
  • Phillips & Flat Head Screw Drivers

Duration: 3 Hours (excluding time to remove old injectors & rail)
Difficulty: Medium
Profanity Level: Medium
Prerequisite: Stock Fuel Injection system removed

So now that your engine bay is looking empty with the fuel injection system missing and your throttle body removed, it’s time to get cracking. But before anything, take a deep breath and brace yourself for a few hours of swearing. Upside, look at all that room for your top feed injectors!

RB25DET Top Feed 06

RB25DET Top Feed 07

Step 1:
Clean and fit the male to male fittings to the end of the rail, make sure there’s enough force to crush the crush washers if not you may be prone to having fuel leaks. At the same time, fit the grub screw to the top of the rail (if you’re not using this port as a fuel return)

RB25DET Top Feed 08

RB25DET Top Feed 09

Step 2:
Lubricate the injector bosses and slide the o-rings on, taking as much care as possible to avoid dust/grit between the boss & o-rings

RB25DET Top Feed 10

Step 3:
Clean the plenum injector seats and carefully install the injector boss & o-ring for each cylinder on the plenum. This is best done by applying a thin layer of oil on the seats and using a cloth to gently push in the injector boss. Do not rush this, as it’s air tight it will go down very slowly.

RB25DET Top Feed 11

Step 4:
Start by lubricating the top injector collars and inserting them into the rail. The use of motor oil is more than sufficient. These collars should just slide into place with not much issue once lubricated.

RB25DET Top Feed 12

At the same time lubricate the injector top & bottom o-rings and test fit them. You may admire your artwork if you choose.

RB25DET Top Feed 13

Step 5:
Fit all the injectors into the injector boss for each cylinder, this step might be a little tricky as there is very limited room available – however with the aid of a few beers this may reduce stress levels

RB25DET Top Feed 14

Step 6:
Time to get the rail on, but before that make sure you have slipped on the correct bolts into the bottom row as it will be nearly impossible to slide the bolts on once the rail is on the plenum & injectors. Carefully lower the rail in place and start with applying light pressure on cylinder 6 and gradually working your way to the front. Once complete it should look flat and not touch anything.

RB25DET Top Feed 15

RB25DET Top Feed 16

Step 7:
Align all the injectors so the connectors face upwards and predominantly vertical (not essential). Wriggle the bolts out nearly all the way and slide in the shorter spacers down. Once ready bolt up from the middle up and then the outer bolts lasts using a 90 degree Allen Key (possible to use a Hex Head socket, however there is quite limited room).

RB25DET Top Feed 17

RB25DET Top Feed 18

RB25DET Top Feed 19

Step 8:
Time to get the fuel feed sorted, attach and secure fuel hose to the fitting and attach it to the back of the rail then feed the hose back to the top of the fuel filter. Cut to length and attach.

RB25DET Top Feed 21

Step 9:
Now to get the return line sorted, attach and secure fuel hose to the 90 degree barb fitting and attach it to the front of the rail. Cut the fuel hose to length then feed this hose to the Inlet Side of the fuel pressure regulator.

RB25DET Top Feed 22

Step 10:
Using the convertor connectors, attached to the factory harness. Using some silicone spray will greatly help with this process.

RB25DET Top Feed 23

RB25DET Top Feed 24

Step 11:
Connect all the injectors back to the harness, make sure it is in the correct order or else you’ll have issues with your car running correctly. Connect the injector harness back to the main harness

RB25DET Top Feed 25

Step 12:
Attach/extend vacuum hose back to fuel pressure regulator, attach the return line and mount fuel regulator in a sensible location as adjustments will be required

RB25DET Top Feed 26

Step 13:
Re-attach throttle body and all required hoses/lines

Step 14:
Before starting, make sure the fuel system has been primed to remove all the air pockets. This can be done by flicking on the ignition without starting the car multiple times. You should be able to hear air pockets flow back into the fuel tank. At the same time it is wise to roughly adjust your fuel pressure to 3bar OR 43.5psi without the engine running.

Step 15:
Time to rescale the ECU to accept the new injectors, this can be easily done if you have access to your ECU’s programming interface if not time to tow your car to the tuner!

RB25DET Top Feed 29

Step 16:
Start your car, it should fire up instantaneously and purr if the injector scaling is correct!

RB25DET Top Feed 28

Step 17:
Time to recheck the fuel pressure once again, it should ideally show 3bar minus total vacuum at idle, i.e. 2.6 bar OR just disconnect the vacuum hose and adjust the regulator so it display 3 bar at idle.

RB25DET Top Feed 27
All done! Time to enjoy your new injectors and screw in more boost (and a good tune too).
Tell us how you went, comment below!

About Johnny

A typical car nutter that likes to get his hands dirty. Skyline and Silvia fanatic not to mention trak-life's own personal tuner and mechanic. Doesn't mind sparing a few days at the gym to stay fit and definitely likes to eat to keep the balance right.

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  1. hey buddy.

    Your DIY on the top feed r33 skyline injectors is fantastic, however, you have neglected to mention any brand names, is this privacy reasons or are you able to let me know as i need to do this setup for my r33, further more, are these “off the shelf” products or do they require any modifications. your timely response will be kindly appreciated.


    TnT Garage

    • Johnny

      Hi Toli,

      It’s more so we didn’t want to be bias in the guide, however the rail that was used was an eBay special and the injectors used were half height Siemens DEKA V injectors. There’s a mob in the US called Clean Injection which can supply the entire kit as plug & play (the same one used in this article), of course you can build your own kit which may cost a little more but given the options that Clean Injection provide it’s a clear choice for ease to go with them.

      Hope that helps!

  2. Where did you purchase all these parts

  3. Hi, this set up worked with the factory fuel pump?

  4. The previous owner of my infinti m30 (rb25 series 1 swapped) installed mopar top feed injectors, ebay rail, and resistors. They should flow roughly the same as the stock 370cc? injectors. The car however breaks up and cuts off when trying to drive.

    I stumbled on your site while googling the issue, if the resistors are outputting the correct 3.5 (i think) ohms, and the flow rate 370cc (i think) is correct or slightly more, do you think it would cause the car to break up?

    I’ve gone through the wiring, and the car just acts like its not getting gas. There aren’t many options for tuning in my area, but even untuned it should just run rich, not cut off like its doing. Any help is appreciated. Thanks.

    • Johnny

      If they are not the factory injectors the deadtimes will be different which does mean flow will be different at different voltages/load points as well. The only real solution is here is to get OEM injectors/rail/fuel pressure regulator OR get a capable ECU and get the car tuned to suit the modifications.

  5. Hey man you wouldn’t want to sell your of stuff would you? Since I last posted I purchased stock flowing but top feed injectors, the OD was different into the manifold so they didn’t work. I’d like to just find a total stock setup and return the car that route. (Stock side feeds etc.) thanns

  6. Excellent write up. Glad you were able to get things up and running without any issues. Here at Clean Injection we supply injector sizes from 3500cc to 2200cc. We carry bosch, siemens, lucas, delphi, denso, jecs and many other brands of your choice. Every set includes flow data and injector latency info to make tuning a pleasant experience. We focus on providing injectors that are statically and dynamically balanced within 1 percent of each other resulting in a balanced delivery of fuel from cylinder-to-cylinder, even at low pulse widths. All testing is done on the industry leading ASNU test equipment. We can provide larger fittings sizes (-8 or -10) for those hungry fuel demanding setups looking to make over 600whp. We currently have fuel rails available in several colors which are anodize coated making it resistant to corrosion from any race fuel in today’s market. If you have any questions feel free to contact me at [email protected] or you can visit my store at

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