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Injectors for more power?

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  • Injectors for more power?

    Hi I have a my10 forester xt, I have had the intake, intercooler, exhaust, fuel pump and upgraded to a vf52 and am putting down 192kw my dyno sheet is uploaded. I've been told my injectors are holding me back now. My question is after getting them done will I get gains over the hole rev range or just at the top where the power plateus on my dyno sheet? Thanks for the help.

  • #2
    TLDR: Yes, you will make more power. The point in the line where it starts to drop away at 3500-4000 before picking up again later on will become steeper and have a larger area under the graph at that point and that plateau will disappear and provide you with a nice big peak power rating.

    You won't make any gains off boost down low over the stock injectors but once the turbo spools up you will make more power because you will be able to run more boost than before. Basically, the amount of fuel injected is dictated by how much air you are supplying to the cylinder each cycle and this is represented by your air fuel ratio which is around the 12.8 to 13.2:1 for best power gains. Generally you upgrade both injectors and turbo at the same time because the stock setups are matched together pretty well and changing one without the other upsets the balance. To put it simply your new turbo is able to supply more air than your injectors can EFFICIENTLY and reliably supply at lower RPMS. The stock injectors are having to work harder to keep up with the amount of air supplied by the new turbo and are operating near their limit once the air volume supplied starts to increase as the RPM increased. When you upgrade the injectors, for the same time period that the injector is firing, it will supply more fuel efficiently and reliably and as a result you have more power making potential.

    for an example with imagination numbers Ive pulled out of thin air, say your old turbo needed 10mls of fuel per revolution to keep the air fuel ratio at the ideal level on wide open throttle, your stock injectors were sized to provide that amount during a designated injection time window of say 10 degrees arc and the whole system worked in harmony together. The turbo supplied a certain amount of air, the injectors provided the exact right amount over a certain injection period to burn that air completely and X power was made. But the new turbo is able to supply a larger quantity of air than the old turbo per intake cycle and now the engine requires 20mls of fuel instead of 10mls to keep the air fuel ratio correct, and now at wide open throttle your stock injectors are having to stay open through a longer arc period to provide the correct amount of fuel to keep the mixture from leaning out, say their now having to be open for 20 degrees of arc instead of 10. The injector is working harder to provide a larger amount of fuel to keep the air fuel mixture at the 12.8:1 ratio. Running out injector means that you are now supplying so much air that it doesn't matter how long the injector stays open for, it can't physically supply enough fuel in the time frame it has available to burn all the air completely before the injection window is over and combustion begins. By upsizing your injectors to a larger cubic centimetre capacity, you are now able to inject the same amount of fuel that the old injectors took 20 degrees of rotation arc to inject which in our example was 20mls in just 10 degrees of rotation like it was when the turbo and injector were sized correctly for each other at stock levels.

    Now, what this means for power is that you can start to turn up the amount of boost that your engine can actually use through the rev range. Normally when we put a bigger turbo like yours on and its boosting, it will very quickly start providing more air than what the engine can use, the waste-gate opens and it continuously bleeds off the excess that it doesn't need. What happens when we turn up the boost is we say 'no i want you to use that air and inject more fuel to match it' up until we start to run out of injector capacity and the engine runs begins to run lean and we are producing the maximum amount of power at that point in the rev range. Generally at lower RPMS before the turbo is really spooling hard, the injectors can supply more fuel than the turbo can use. As the RPMs climb, the turbo spools faster, provides a larger quantity of air and is matched with the injectors, waste gate is still closed and were still making max power for that point in the rev range. As the RPM climbs even further and the boost increases exponentially, the amount of air supplied quickly out strips what the injectors can burn and the wastegate opens and we would dump unusable air. To make use of this air, instead what we do is raise our wastegate opening pressure on the boost controller, increase our injector opening period and supply more fuel and make more power. Eventually we get to the point that we can't keep the injector open any longer and now have to finally open the wastegate and dump air out that we can't use without running lean. With bigger capacity injectors, you will be able to run higher boost levels all the way through the rev range and keep the wastegate closed longer before you need to start bleeding off the air. The end result is you will pick up a few KW's down lower at around the 3500 point where it flattens out slightly but but don't expect a huge amount, this is lag and is what you get when you fit bigger turbos. You'll notice a big increase later on in the rev range at around 4000-4250 where it will very quickly get a steeper line as the boost starts rising and the engine is suddenly able to make use of that boost pressure that its creating. at the top end where it plateaus, prepare to make whole hive of killerwasps as you keep going up in the rev range until you eventually hit limiter or boost cut if it has one. You might also run out of either head stud and start lifting the heads from combustion pressures or run out of con rod and start getting personal with Rodney and his friends when they redecorate your block with windows if you get greedy with the gains. Or both. Or you run out of turbo and injector again and have to upsize both to keep making more power.


    • #3
      Whats the IDC at top end? Did the tuner say this?

      I ran with 550cc (STI pinks) and they good for up to 230atw. Dont think thats holding it back, if anything it be the turbo and the tune.


      • #4
        Don't know jack bout your car, like if it comes with small sized injectors from the factory but is the AFR going lean? Have you tried an adjustable fuel regulator and turned it up and it responded well? Not that you'd keep it turned up just to see if it wants more fuel.


        • #5
          Originally posted by Asymmetry View Post
          Whats the IDC at top end? Did the tuner say this?

          I ran with 550cc (STI pinks) and they good for up to 230atw. Dont think thats holding it back, if anything it be the turbo and the tune.
          He hasn't upgraded injectors yet and stock ones are 500cc, so it would make sense that hes running out of fuel at just shy of 200kws.

          the 500's are 90% of the 550's and 193 is 83% of 230. if you factor in age, wear and tear on the injectors, differences in dynos and tire sizes, tire slip on the runners, variables such as temp and heat soak on the dyno sensors and other such things, the numbers work out to be pretty close to each other.

          Hes maxed out his injectors for the amount of kws they can generate and now their him back. He needs to upsize to a decent sized pair, say 750cc or 1000cc to reliably make more power. You always size up your injectors for more power than you want to make so that your not running your duty cycle on them at near 100% because this will cause them to wear out faster from the increased electrical heat load being generated by the relays staying energized for a longer period of time per cycle. So if he needed 550cc to make 230kws which would max them out at that power level, you would buy 750cc injectors to make that level of power safely and reliably and place less stress on the injector components themselves.
          Last edited by Gibbos garage; 03-09-2019, 01:33 PM.


          • #6
            Ej255 06 WRX had blue 650cc injectors, sure that MY10XT have 500cc? Ej205s had 440cc, seems a step backwards


            • #7
              There are many many different types of injectors used in subaru engines and you should always trust what your tuner says and recommends, not what randoms on the internet think.

              its a little dated but use this forum post on NASIOC to get a bit more of an understanding

              pretty much the main ones you'll encounter are

              OEM 2002-2005 WRX 420 cc
              JDM STi "pinks" 550 cc
              OEM 2006+ WRX 565 cc (also known as "blues")
              OEM 2007+ STI 560 cc
              Modified Stock WRX 850 cc

              you can also use this calculator to work out exactly what size you need https://www.rcfuelinjection.com/technical#WORKSHEET

              my mistake, if he has an my10 forester he's going to have the dark blue top feed injectors which are 560cc for that year model. If they're yellow then they are the side feed 535's which only came out in a few year models around 2004.

              you can also use this infomation to work out cc size requirements

              For this calculation forget wheel hp, you need to work from the actual engine hp output. Don't work from your turbo flow either as that is not what your really looking for. Your looking for the amount of fuel the engine must burn to make a specific target hp. You size the turbo to supply the amount of air to make that same hp and you have a balanced system. The hp comes first the sizing of the turbo and injectors both follow from the max engine hp output.

              For quick calculation purposes just multiply your target wheel hp x 1.25 and that will be the approximate amount of crankshaft hp the engine will need to generate to get to your target.

              From there you simply muliply your engine crankshaft engine hp x 0.6 x 10.50 and divide that total by the number of cylinders x .80.

              ((Engine hp) x 0.6 x 10.05 ) / (number of injectors) x ( duty cycle -- use 0.80 ) = cc/minute flow for each injector.


              You want to make 500 wheel hp. Your engine will need to be able to make about 625 hp to be sure you can do that.

              (625 x 0.6 x 10.05 ) / (4 x 0.80) = (3768.75)/ (3.2) = 1177 cc/min for each injector

              The 0.6 is the assumed BSFC (lbs of fuel/hour) required to make one hp in a turbocharged engine.
              The 10.05 converts the lbs of fuel to cc/min
              The 0.80 is the maximum recommended duty cycle of the injectors.



              • #8
                Yep 500cc injectors, the tuner said that's what's holding me back at the moment. I don't often drive at the top of my rev range I want to make more power over the middle of it and if injectors won't do that I probly won't do it.