Injector Sizing

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  1. #1

    Default Injector Sizing

    I have a chance to get a set of ID1050X injectors for a steal. What would you say a safe limit for power would be on these injectors with a PD blower (3.2 LC Whipple) on a 4.6 cobra running pump gas?



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  3. #2



    Sizing injectors based on anticipated power levels can be misleading. The idea of BSFC provides a wholesale correction for parasitic drag, pumping losses, power to drive the blower etc. Not surprisingly those numbers move around a fair amount depending on how the engine was built and who built it.

    A far better way to estimate nozzle sizing is The Fuel System Calculator. In stead of guessing at all the possible variables that will put a drag on the power output the calculator focuses exclusively on the engine's air consumption and the AFR you specify for the engine. if the safe AFR you want to run is 11.8 (suitable for non-oxygenated fuels) and you know your engine's using x amount of air, the air fuel ratio tells you how much fuel you need for that volume of air. Whether a portion of the power created by the combustion process is consumed in parasitic loss or not you know with absolute certainty that the air and fuel consumption is defined by the ratio of air to fuel you have commanded.

    Check out the calculator here => Fuel System Calculator or in the TToC. The calculator is the fourth entry down, under the Fuel System Section. There is a downloadable Windows and also a Mac version along with a pdf file that is the instructions. If you are running E-85 PM me there is a modification needed for E-85.

    If you want to pursue the HP approach to injector sizing then this is how you would go about it;

    Your injectors are 1065 cc/min injectors. At a maximum 80% duty cycle they would flow 1065 x 0.8 or 852cc's /min per injector. Your engine uses 8 injectors so the consumption would be 852 cc/min x 8 injectors or 6816 cc's/min of fuel. The specific gravity of gasoline varies depending on brand and octane but a good number to use is 0.755. That means 6816 cc's per hour weighs 6816 x 0.755 or 5.146 kg or 11.34 lbs/min of fuel consumption. Using an 11.8:1 AFR you need 133.8 lbs of air/min to burn the fuel. To make 100 hp you will need ~10 lbs/min of air. 133.8 lbs of air per minute will support a maximum of 1338 HP.

    It is important to remember that the 1338 number does not account for pumping losses, parasitic friction loss or power to drive a roots blower. At 6500 rpm a 4.6 engine has about 85% mechanical efficiency — it looses 15% to parasitic drag. That drops the 1338 number to ~1137HP. Pull out another 100HP or so to drive a blower and you are at essentially 1000 HP. A water pump is worth 7.5 to 10 HP depending on the pump same thing for an alternator and we haven't even gotten to the other interesting power consumers or parasites like pumping losses yet. 1000HP is potentially an optimistic expectation depending ...

    Do yourself a favor and use The Fuel System Calculator. It will not only save you all the math it will give you the correct answer. If anyone intends to run a turbo instead of a PD blower just increase blower size until you get the boost number you intend to run with the turbo's. The injector size calculation will be correct for the boost.

    Don't forget lower octane numbers require reduced timing that in turn reduces maximum brake torque and therefor horsepower.


    p.s. The correct AFR for a fuel with 10% ethanol or other equivalent oxygenate is ~11.2:1.
    Last edited by eschaider; 05-17-2019 at 05:12 PM. Reason: Spelling & Grammar

  4. #3


    Ed, thank you for the help. After using the calculator, it looks like 1065cc will not be enough to support my power goals. Thank you!

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  6. #4


    It is a handy little gizmo, Robert and it whisks away all the black magic,

    BTW when buying injectors buy big. If you just squeak by at an 80% duty cycle you are going to be in the market for injectors again and sooner than you think. If you size for a 50% duty cycle (or less) you will have reasonable head room for growth and no trouble idling. We have one member on the site here with dual 2200 cc injectors per port and the engine idles smooth as silk at 800 rpm (using both injectors) with off idle torque like a Massey Fergusson farm tractor — and that's with big cams.

    Don't cheat yourself go big, you won't be sorry.


  7. #5


    WOW! Dual 2200cc and an 800rpm idle. Common knowledge would tell you that isn't too easy. What would ID's mentality be to market their 1300cc as the largest injector with good idle characteristics? They make the jump from 1300cc to 1700cc as being the not really a good idea if you want a clean idle.

  8. #6


    Can't explain their reasoning, Rob.

    I can tell you the engine uses a Haltech Elite ECU, a Pro-M 117 MAF, dual Bosch 2200 cc injectors per cylinder that are not staged and run concurrently with a gigantic cam and tons of overlap. A big part of the engine's idle performance is the use of the PRO-M MAF and the 40 or so point air flow map that it came with.

    Here are two 5 to 8 minute Youtube videos that explain my preference for the Pro-M style MAFs and their contribution to idling with the big injectors. This is the first vid => Pro-M MAF Styles. the voice you hear is Chris Mathews, President of Pro-M Racing. He is explaining the different types of MAFs that are generally available from Pro-M along with their evolution and their different benefits. The second video, click here => MAF Signal Quality, is a test of a good slot style style MAF and then a Pro-M 80 MAF. Pay attention to the quality of the signal trace on the ocilliscope. That signal is what the EFI ECU uses to command engine fueling. The cleaner the trace the cleaner the fueling.

    Big injectors, at idle, need to operate at very small pulse widths, typically near the limits of the injector's flow linearity and repeatable pulse widths. Poorly matched (not flowed) injectors and a poor quality MAF signal translate into minimally a challenged idle. Idle quality will depend on how poor the injector match and the MAF signal quality is.

    As my favorite Knight's Templar, in my favorite Indianna Jones Movie, admonishes Indianna as he prepares to choose the true Holy Grail from many imposters, "Choose wisely, he (the dead Nazi on the cavern floor) did not." When it comes to your Injectors, MAF and ECU, you want to also choose wisely.

    Last edited by eschaider; 05-20-2019 at 03:21 AM. Reason: Spelling & Grammar

  9. #7
    Senior Member Array soap's Avatar
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    The injectors are not going to limit you, the pump gas will. Hell you can probably get away with 80# injectors with pump gas. If you switch to corn then the 1050's will for sure limit your power capabilities.

    I maxed out a set of 1000's last fall with my stock engine/2.3 TVS combo, now I am running 1700's.


  10. #8


    Joe is spot on about fuel selection. As you go up the fuel selection hierarchy, fuels like the alcohols (ethanol and methanol) will require additional fuel volume to get to the optimum mixture for power for the particular fuel. Irrespective of the fuel and its optimum power lambda (or AFR) you want to shoot for two things; #1 do everything you can to keep your fuel system base pressure low — 39.15 psi is a nice number. Ford uses 39.15 psi as their target. #2 use an injector that will meet your target lambda without exceeding a 50% duty cycle at your maximum target rpm.

    Increasing fuel pressure can only provide marginal increases in fuel flow because the relationship between pressure and flow is a second order relationship. Fuel pressure goes up by the square of what you have increased flow by. For example you have a set of 39#/hr injectors but you need a fuel flow of 80#/hr. If you kept your 39#/hr injectors you would have to raise your base system pressure of 39.15 psi by a factor of four to 156.6 psi to produce an 80#/hr rate of flow. To double the fluid flow across an orifice you need to increase the fuel pressure by the square of the increase. If the desired increase in flow is 2X then the pressure rise necessary to deliver the flow increase is 2 squared or 4 times the original 39.15 psi, which works out to the 156.6 psi number.

    While increased pressure is attractive for atomization purposes, a fuel system base pressure of 156 psi is impractical for a variety of reasons. A good place to start with is that while mechanical fuel pumps can easily hit this mark, electric pumps by and large have their internal pressure reliefs (pump savers) set to blow off before 100 psi. When you run 25 psi of manifold pressure then you must add that to the base system pressure of 156 psi and now your fuel system needs to supply an impressive 181 psi of fuel system pressure. Electrical demands aside for a bit, there are no electric fuel pumps capable of that type of performance.

    Yes, you say but I don't want to go that far, all I need is a little 'pick me up' fueling wise. Not True! What you have done is blow through the upper limits of your injectors flow capabilities (because the were too small) and now you are trying to use a fuel pressure fix band aid on a much pricier problem. Fuel system band aids applied on pricey problems for expensive motors come with impressive engine failures and even more impressive repair bills after which, you still have to buy the bigger injectors you should have purchased at the beginning.

    A step up, to either of the alcohols, will demand significant increases in injector size and fuel pump capacity — depending on what you have now. Don't shoot low when selecting injectors, shoot high. Select your injectors to provide for a 50% max duty cycle at WOT, max engine speed. This flies in the face of popular wisdom which says use an 80% or 85% duty cycle. Don't do it! Use the 50% duty cycle. You'll be happy, your motor will be happy and you'll buy new injectors much less often, probably just just once. Look at what Joe needed to use with his TVS to make the engine happy on ethanol. His experience is typical of an E85 fueled engine and remember he maxed out a set of 1000cc injectors with a 2.3L TVS blower.

    BTW if you use the calculator for ethanol set the fuel Specific Gravity to 0.78 instead of the 0.831 that is called out on the Page 3, Fuel Stats tab, Here is a chart out of the DOE's E-85 Handbook that I think might be helpful;

    Name:  DOE  E85 Handbook.jpg
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    Last edited by eschaider; 05-22-2019 at 06:03 PM. Reason: Spelling & Grammar

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