IW STi Forum banner

1 - 8 of 8 Posts

178 Posts
Discussion Starter #1
this is a great post. Basically its a free mod where you can cut the tops off the stock injectors and get them to flow more :) No need to buy new injectors now :)


A recap :

Its a tough choice on which injector to buy for your WRX. Buy STI Pink injectors? $300-$550 and you get an extra 100cc worth of fuel. PE 650 or 850's? $600-$800 a set and because these are mechanically slow you need tuning or a flash to make them work correctly. Other companies sell an option for $450 and another that requires you to solder in a resistor pack. You can skip all of this and make your own 740CC injectors for free.
The Stock Blue 440CC injectors and the STI pink 540s are identical other then the end cap and the color so either injector will work for this mod. There is no advantage to having the Pink STIs once modded so sell them, Make some cash and mod your stock injectors. There are a few ways to accomplish the mod but what needs to happen is the end cap of the injector needs to be removed. You can grind it off or cut it off. Does not matter really. You can use a band saw or a dremel tool and a vice. Take your time and cut the silver end cap off where the nozzles (holes) are.
Since you are removing the end cap you dont need to send these off to have them matched. There will be no adjustment a injector shop can do anyways since they are sealed units. What you are doing is removing the flow restrictor (end cap). The biggest advantage is that the stock ECU is calibrated for the correct opening time compensation for these injectors (dead time). PE's are twice as slow and need more dead time. All of the issues that the PE's bring to the table are absent with these injectors.


How much will the injectors flow once modified?

What about the spray pattern?
You end up with a find disk pattern

What about the idle?
Rock solid idle as stock once AFR's are tuned.

What about reliability?
They are stock Denso injectors and you are just removing the end cap containing the sized holes.

Do I need a fuel pump?
Yes, Pressure at the injector is reduces since the injector body can now flow more fuel. You need a fuel pump capable of handing 740CC injectors. Walbro is fine.

What if I dont have engine management will these work on a stock ECU?
NO, You will nearly double the fuel flow so you will need a Utec, SAFC, Link, Gems, Flashed ECU or other EM to calibrate these injectors.

I am still worried about equal flow. How Can I be sure this works?
You can send the injectors To Venom performance in California or RC engineering for testing once the mod is complete. Venom will even do the mod for you and test each one but the cost is nearly $300.

Why are you posting this information and not selling modified injectors?
Because it costs nothing to do this mod and its better to give back to the community. If you do not want to perform this simple mod yourself there are companies out there that will take your $200-$300. Maybe one of the members will try this, report back and offer to mod your injectors for a reasonable price? $50? This way its worth his time to do it and you save a ton of cash in the process by not having to spend $450+ on injectors. Work as a community guys

How many cars have you tuned with these injectors and how long have people been running them?
Kingpin has tuned 7 cars with these modified injectors. All idle smooth, All run fantastic and all have low IDC's One customer has 5000 miles on his set now with out issue.

Hope you guys try this and report back. I would not suggest something that I do not believe it. Some of you can flow test them if you wish and let us know the results. Let the flood gates open!

5,261 Posts
hmmm sounds interesting... might be a cool mod once utec is out. Got any pics?


357 Posts
This is only for WRX or STi JDM spec motor. USDM STi uses yellow side feed injectors. We do not recommend modifying the injectors although we hear of a company doing that. Fuel system is very critical and we want as minimal failure rate as possible with fuel both for motor safety and also for fire hazard purposes. I am using the 850cc injectors on my STi and with E-Manage they idle like stock once the car is warm. I don't think any of you would be able to tell I have 850cc's in the car if yuo drove it. Also, even with 850cc I am at 59% duty cycle at only 322 whp so if you plan on 400-500 whp, you have to go bigger than 720cc anyway.
1 - 8 of 8 Posts