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This is a discussion on got an 02 wrx last week within the New Member Hangout forums, part of the Community - Meet other Enthusiasts category; Originally Posted by MainFrame No, e85 requires around 33% more fuel, therefore you need injectors that flow at a minimum ...

  1. #16
    Registered User djuhnk's Avatar
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    Quote Originally Posted by MainFrame View Post
    No, e85 requires around 33% more fuel, therefore you need injectors that flow at a minimum 33% more than your stock injectors. I would get at least 750cc injectors to run e85.. but I hardly feel it's worth it on a tiny td04.
    Thanks, Didn't we just discuss you can achieve nearly 300 whp with 91 on stock injectors? That must mean they have room to spare.. I know of the whole 33% thing that's why I asked.

    I'm still looking at getting a vf39. Would a 05+ legacy gt turbo bolt on?

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    Master Baiter EJ257's Avatar
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    Quote Originally Posted by djuhnk
    Thanks, Didn't we just discuss you can achieve nearly 300 whp with 91 on stock injectors?
    You'll get nowhere near 300whp on an '02 WRX with stock injectors. STIs have a tough time hitting that, and that's more based on the dyno in question than anything.

    Quote Originally Posted by djuhnk
    I'm still looking at getting a vf39. Would a 05+ legacy gt turbo bolt on?
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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by djuhnk View Post
    Thanks, Didn't we just discuss you can achieve nearly 300 whp with 91 on stock injectors? That must mean they have room to spare.. I know of the whole 33% thing that's why I asked.


    My 565cc STi injectors were over 90% IDC on 93 octane at 256whp.. You always want to go bigger on injectors. If I was looking to run a vf turbo and possibly e85 in the future I would just get ID1000s and be done with it.

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    Registered User djuhnk's Avatar
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    Quote Originally Posted by MainFrame View Post
    My 565cc STi injectors were over 90% IDC on 93 octane at 256whp.. You always want to go bigger on injectors. If I was looking to run a vf turbo and possibly e85 in the future I would just get ID1000s and be done with it.
    point made, i guess my options are boost controller and max out the stock turbo, or new turbo, injectors and boost controller. ~$1000 vs. ~$100 and about 50-75 whp

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    Quote Originally Posted by MainFrame View Post
    You always want to go bigger on injectors.
    Not true. If that were the case everyone would be running 4000cc/min injectors.

    You have to consider your fuel pump capability and sacrifice to injector resolution.
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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by zax View Post
    Not true. If that were the case everyone would be running 4000cc/min injectors.

    You have to consider your fuel pump capability and sacrifice to injector resolution.


    Well obviously you don't want to get injectors that are known to not run right (I don't know of anyone that makes 4000cc Subaru injectors), but it's not going to hurt to run ID1000s on a stage one or stage two setup. As long as the injectors can achieve a short enough pulse width to idle properly then they aren't really too big.

    I don't think fuel pump has anything to do with injectors in this sense. You need a pump that will flow enough fuel to match the demand that is created by the turbo. You could have 2200cc injectors on a stock turbo and the stock fuel pump would be fine (as long as it was sufficient in the first place). Installing higher flowing injectors has no effect on how much fuel you need.

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    Master Baiter EJ257's Avatar
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    Quote Originally Posted by MainFrame
    Well obviously you don't want to get injectors that are known to not run right (I don't know of anyone that makes 4000cc Subaru injectors), but it's not going to hurt to run ID1000s on a stage one or stage two setup. As long as the injectors can achieve a short enough pulse width to idle properly then they aren't really too big.

    I don't think fuel pump has anything to do with injectors in this sense. You need a pump that will flow enough fuel to match the demand that is created by the turbo. You could have 2200cc injectors on a stock turbo and the stock fuel pump would be fine (as long as it was sufficient in the first place). Installing higher flowing injectors has no effect on how much fuel you need.
    If you have no intention of running E85 or upgrading the turbo, there's no benefit going with ID1000s vs ID725s with an STI.
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    Subaru Newb MainFrame's Avatar
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    No benefit, but also no harm (except maybe pocket book).

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    Quote Originally Posted by MainFrame View Post
    I don't think fuel pump has anything to do with injectors in this sense. You need a pump that will flow enough fuel to match the demand that is created by the turbo. You could have 2200cc injectors on a stock turbo and the stock fuel pump would be fine (as long as it was sufficient in the first place). Installing higher flowing injectors has no effect on how much fuel you need.
    The turbo is irrelevant!! It's just a power adder! The point is that if you are running 1500cc/min injectors, and you have a stock fuel pump, the fuel pump will NOT be able to achieve target fuel pressure. Think of it this way... the fuel pump is flux of a flowing river. Now you add a dam (injectors). 1000cc injectors are like two big holes in the dam. 565cc injectors are like one small hole in the dam. When you open the hole, the water will drain MUCH more quickly from the two big holes vs. the one small hole. Now suppose you use the dam-holes like you would an injector: open and close them according to a duty cycle. If the drain rate of the holes EXCEEDS the flux of the flowing river, you will not be able to achieve the pressure required to push the water through the hole at the desired rate. THEREFORE! If the fuel pump is not matched to drive the injectors, you will not be able to achieve the desired flow rate of the injector!

    But you've missed the point. Clearly there are benefits to running smaller injectors, Robin. Otherwise, manufacturers would simply slap on 5000cc/min injectors and call it a day. Being from a precision measurement and manufacturing employ, I can assure you that it does not cost any more to drill 5 LARGE precision holes vs. 5 SMALL precision holes in the cap of an injector. The reason larger injectors are more expensive in the aftermarket is a simple study in market demand: People are willing to pay more for larger injectors because they don't understand the benefits and risks of running said injectors. One of these risks is that, by increasing the flow of fuel per injector duty cycle (larger injectors), you effectively decrease the resolution of your fuel system control. Think of it this way: you take a 1000cc/min injector and you fire it for 100% duty cycle, you flow X amount of fuel. Similarly a 500cc/min injector will flow X/2 amount of fuel in the same time frame. If the computer can accurately control a minimum duty cycle of 0.3% (I'm not sure the actual number), the 1000cc/min injector will inject X/333 fuel, while the 500cc/min injector will inject X/666 fuel. Therefore, as you see, the smaller injector has a finer resolution and can control the fuel system to a higher degree of precision.
    Last edited by zax; 08-29-2012 at 06:23 AM.
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  12. #26
    Registered User Heide264's Avatar
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    Quote Originally Posted by zax View Post
    The turbo is irrelevant!! It's just a power adder! The point is that if you are running 1500cc/min injectors, and you have a stock fuel pump, the fuel pump will NOT be able to achieve target fuel pressure. Think of it this way... the fuel pump is flux of a flowing river. Now you add a dam (injectors). 1000cc injectors are like two big holes in the dam. 565cc injectors are like one small hole in the dam. When you open the hole, the water will drain MUCH more quickly from the two big holes vs. the one small hole. Now suppose you use the dam-holes like you would an injector: open and close them according to a duty cycle. If the drain rate of the holes EXCEEDS the flux of the flowing river, you will not be able to achieve the pressure required to push the water through the hole at the desired rate. THEREFORE! If the fuel pump is not matched to drive the injectors, you will not be able to achieve the desired flow rate of the injector!
    I think it's harder to realize this with a fuel return as opposed to a return-less system. I can see both trains of thought regarding the matter having some merit, but I would definitely lean towards zachs point here.

    MainFrame is claiming that as long as you keep the rails pressurized by the stock pump... It won't matter whether you have 1000ccs opening for a small amount of time Vs 100cc opening for a larger duty cycle. That's clearly an exaggeration, but just using it for the sake of devil's advocacy. Either way the FPR (Fuel pressure regulator) retains pressure in the rails assuming your fuel pump can keep up with the volume requirements.

    Even with a fuel system where the fuel is returning once passing the injectors, each injector opening pulls some volume away. With larger injectors, it will allow it to pull more volume away in a shorter time - despite the duty cycle (which will be harder to adjust now). Even though there should theoretically be fuel in the lines at all times, when you start to increase the rate of fluid flow through them... especially in a sequential rail system like ours... you will have a lower rail pressure most likely by the end.

    It's commonly like the argument of turbo output in terms of CFM instead of psi. While the pressure of the fuel system may be constant still for injectors of various sizes, the flow through it will change based off of the volume/time distributed per injector - which will be higher with 1000ccs. Just as if 18psi of air from a large CFM turbo does not equate to 18psi of air from a small CFM turbo.

    That being said, the 1000ccs are not so large that they won't work to my (theoretical/arm chair) understanding. They may be touchier to tune (aka, expensive). There are also people on the flip side that claim the ID injectors have a much better fuel dispersion and are actually easier. I've never worked with them personally.

    Lots of forces at play. I'm still a fan of matching injectors to your needs - it maintains the best resolution possible, which is important. Fueling fluctuations are bad. On the flip side, you also want the highest quality injectors - which I can't determine personally.
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    zax
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    Brandon makes a good argument, and delves much deeper into my philosophy. At 1% IDC, it is unlikely that you will inject enough fuel to drop the pressure in your fuel rails by a margin large enough not be be recovered by your pump at the next injector solenoid event. This effect will become more noticeable as your IDCs begin to climb and your fuel pump cannot maintain the FP in your rails. I think we get the point here.
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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by zax View Post
    The turbo is irrelevant!! It's just a power adder! The point is that if you are running 1500cc/min injectors, and you have a stock fuel pump, the fuel pump will NOT be able to achieve target fuel pressure. Think of it this way... the fuel pump is flux of a flowing river. Now you add a dam (injectors). 1000cc injectors are like two big holes in the dam. 565cc injectors are like one small hole in the dam. When you open the hole, the water will drain MUCH more quickly from the two big holes vs. the one small hole. Now suppose you use the dam-holes like you would an injector: open and close them according to a duty cycle. If the drain rate of the holes EXCEEDS the flux of the flowing river, you will not be able to achieve the pressure required to push the water through the hole at the desired rate. THEREFORE! If the fuel pump is not matched to drive the injectors, you will not be able to achieve the desired flow rate of the injector!


    If you take a stock car and change nothing but the injectors and properly scale them in the map, then how is it going to use any more fuel? I guarantee you I can switch a car from 440cc injectors to 1200cc injectors and it will have (basically) identical AFR. So if you are not using any more fuel, then how is there greater demand on flow from the pump?

    Now if you install a larger turbo, you're going to need more fuel.

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    Quote Originally Posted by MainFrame View Post
    If you take a stock car and change nothing but the injectors and properly scale them in the map, then how is it going to use any more fuel? I guarantee you I can switch a car from 440cc injectors to 1200cc injectors and it will have (basically) identical AFR. So if you are not using any more fuel, then how is there greater demand on flow from the pump?

    Now if you install a larger turbo, you're going to need more fuel.
    Overall fuel usage will be the same assuming you can get it idling correct and you have enough resolution to do so. Normally you aren't so lucky with this. In actuality, your main fuel hit (if any) would probably come due to the fuel not dispersing evenly. However, the rate at which the fuel is pulled from the rails will be different - probably 2-3 times the rate given your example. Much like a smaller CFM turbo can't keep up with an engine suddenly pulling in a lot of air, a fuel pump may not be able to keep up with the sudden impulse of fuel pull from each injector opening.

    As per the overall issue with over sizing injectors:
    The bottom line is generally that it will work, its just that it normally isn't ideal. You will have to spend more time fine tweaking them and you generally lose precision in your afrs as you increase injector size. The larger your fuel pump (provided you have an FPR that is capable & appropriate rails for the situation).. the less 'small' variations in pressure you will see in the rails - the variations that may or may not rob cylinders of fuel. It may not be a large issue in a Subaru WRX of year XXX, but its definitely relevant to this conversation.

    Also, traditionally, the larger the injector, the worse the fuel dispersion. With the injector dynamics Vs modded stockers, that is debatable in this situation.
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    Subaru Newb MainFrame's Avatar
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    If there were a river and a dam.. you haven't changed turbos, so there should be no increase in demand for fuel, hence the stream down river stays the same. I think we can agree that if the pump is large enough for the turbo, this means the river up stream from the dam is larger than the stream down river.

    Now if you're letting through the exact same amount of water, why does it matter what size the hole is? If the flow down stream from the dam has not increased, how could the river up stream go dry? It still has the same amount of water in it as before.. If it's going dry, but there's no more water going down stream, where does the rest of that water go?

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