Rough Guide to Injector Sizing

    I have recently been rambling about injectors in a couple of different threads and I wanted to add some clarity/links to this subject. This may help people understand what injectors do, and their importance to an engine running correctly after making modifications to boost power.

    FROM - https://www.injectorsonline.co…-to-choose-fuel-injectors
    Nothing will have your engine running rough faster than the wrong choice of fuel injectors. Not only that, you risk ruining your engine altogether if you make the wrong decision. So it’s important to understand the factors you need to consider when deciding which fuel injectors are right for your setup.

    Bigger is not always better
    It’s easy to fall into the trap of thinking that bigger fuel injectors equal more power, but unless your engine is set up to deal with the extra fuel, you’ll be on a fast track to trouble. The excess fuel will flood the engine and it either won’t run at all, or will be running too rich. This can lead to fouled spark plugs and damage to the engine.
    There is a whole range of things you can do to increase your fuel capacity to accommodate bigger fuel injectors but the main ones are:
    - increasing the airflow – this can be done by installing a turbocharger or supercharger, or fitting a bigger throttle body
    - installing a bigger exhaust
    - remapping the computer
    - fitting a bigger fuel pump
    - adding an adjustable fuel pressure regulator.

    Running too lean
    At the other end of the scale is where you’ve modified the engine but the injectors aren’t delivering enough fuel to keep up. If you’ve added a turbo, a higher flowing fuel pump, a bigger exhaust and remapped your computer, you’re going to need bigger fuel injectors. Otherwise your car will be running too lean and you’ll have a situation where your it is sluggish and lacking in power at best, or at worst, you’ll burn out the pistons and destroy the motor.

    Size does matter
    Both of these scenarios highlight why it’s important to have the right-sized fuel injectors. The best place to start to work out the right fuel injector size for your needs is to get an understanding of how much horsepower you expect to achieve with your modifications. We recommend you enlist the aid of a reputable car tuner to determine this.
    The other critical factors are:
    - whether the car is turbocharged or supercharged, and
    - whether you are using E85 or standard pump petrol.


    Once these factors have been established, you can use the charts below to help you select the correct injector. The following considerations should be taken into account when using these sizing charts: horsepower is at the engine (you lose roughly 30 percent if calculating from the wheels); injector duty cycle is 80 percent; and fuel pressure is 43.5 psi (3 bar).





    Flow Rate Conversation Website
    https://www.deatschwerks.com/flow-rate-conversion

    Fuelinjectorclinic.com Flow Calculator
    https://fuelinjectorclinic.com/flow-calculator

    The stock Slingshot injectors are 35 lbs/hr models. The above information and Flow Calculator tool are great resources in understanding what injector should be chosen for your power goals. They are used as a rough guide and final validation comes from dynoing or street driving the machine while closely watching the injector duty cycle (software is available that does this). In almost all cases, tuners don't like to see injector duty cycles over 90% during desired AFR conditions.

    Example for most forced induction kits making around 300whp...


    Owner of Slingshot #263 that has some stock parts left on it. :D

    I am.running 850cc injectors. After this weekend I'm moving to 1000cc injectors. This also has required a bit of help to the fuel pump delivery of a constant 58psi.


    @TravAZ chart is pretty close. If you look at the power levels and fuel requirement. If you all remember I had to move to 80 pound injectors when I was pushing 500hp and now i am needing 95lb injectors. That is a long way from 24lb stock

    The post isn't specific to anyone or any one thread. There has been a lot of confusion around this subject for awhile and I wanted to try to add some clarity. There are people running boost with stock injectors, stock injectors + running 60lb secondary injectors, running 60lb only, etc, etc. It is all over the place... Hopefully this gives people some general knowledge so they can better understand injectors and what they may need for their power goals.

    Owner of Slingshot #263 that has some stock parts left on it. :D

    Question on my side of things...would this reduce the amount of fuel (or is it controlled by the ecu?)...essentially reducing fumes? One of the local guys here running a DDM supercharger is looking to go to smaller injectors because he thinks he is getting too much fuel.

    Actively looking for another sling...It is time...

    The reason the slingshot fuel mileage read out is wrong when you go boosted is a combination of the larger injectors and the ecu. The ecu does not know how big the injectors are, so the calculations are off. Plus when the ecu gets reflashed, the injector pulse width (time the injectors are activated) is changed from stock. So changing injectors without any other programming change (IMHO) is NOT the proper way to go. Plus you risk the chance of major engine damage having a too lean a/f mixture and ending up with a 300 pound boat anchor

    Nobody gets outta here ALIVE

    @rabtech couldn't agree more , Only been boosted four days now but have found enough spare time in the evenings to run through three tanks now , I cannot believe How much info can be gained from watching A.F.R through all boost scenarios . My eyes are glued to that A.E.M. Failsafe constantly ! And I took Inewtons advise and configured it with boost in the background arch and F.R. in the main digital display, works great . I,ve ran boosted equipment many times in my past but I,ve got to admit, that one little gauge has certainly aligned a lot of stars for me in the past few days , Its like this huge light just came on in my head , it's even got me thinking , what the hell was I thinking before !

    Quote from Br4hm4

    Question on my side of things...would this reduce the amount of fuel (or is it controlled by the ecu?)...essentially reducing fumes? One of the local guys here running a DDM supercharger is looking to go to smaller injectors because he thinks he is getting too much fuel.

    @Br4hm4
    I would not in any way use the smell of fumes as a guide to reducing your injectors to a smaller size!!!
    YOU WILL BLOW YOUR MOTOR!!!


    Rich is a bitch, lean is mean!


    Rich hesitates, makes less power then optimal, flounders, stalls out, think of your lawn mower and you flood it by keeping the choke on too long, then you have to open the throttle wide open and crank out the extra fuel because it's flooded. Thats rich, thats a bitch


    Lean is mean makes lots of power, but going too lean will burn your pistons, burn your valves etc and blow your engine.


    In my opinion: The canned tune you receive and or load up is running slightly rich. Rich will not blow your motor(within reason), lean will. As @TravAZ states above. Slight changes to your setup can change how rich/lean your engine is running. As far as I know, there has not been any evidence that the ECU tune has blown anyone's motor, because it's tune slightly rich. By tuning that way, if you make a change it is less likely to blow your motor.


    If you or "one of the local guys" want's to get an idea of what is going on, follow the advice from @rabtech and other, get a WBFS gauge and learn it. At which point you can make an educated guess with is driven by actual data and not your nose.


    With that data, you can then decide if you want to have a custom tune done specific to your slingshot.


    I hope this helps


    If I've gone off topic I apologize, I just could not leave that comment out there without responding to use your nose to tune an engine, it will result in disaster.

    John
    2017 SL LE Midnight Cherry
    :COILOVERSS::TURBOSS::MOTOROILSS::FILTERSS::COLDAIRSS::DONKEYSS:

    Injector sizing does nothing without properly tuning your ECM.

    As mentioned above, when tunes aren't custom to each machine, they tend to be programmed to run a tad richer for added safety. Heat, altitude, fuel quality, etc., etc., change from person to person and having this buffer is important.

    The smell issue is more about losing the catalytic converting than the rich AFR difference in running a boosted machine.

    Owner of Slingshot #263 that has some stock parts left on it. :D

    Quote from Casey_SS

    @Br4hm4I would not in any way use the smell of fumes as a guide to reducing your injectors to a smaller size!!!
    YOU WILL BLOW YOUR MOTOR!!!


    Rich is a bitch, lean is mean!

    Oh trust me...I am not doing a thing with mine other than sending the ecu back in to Bob to have the new tunes done. I have a feeling my buddy is just out to blow something up. lol.

    Actively looking for another sling...It is time...

    That chart seems to be a little on the smaller size or leaner side of things not leaving much room for era’s.. ... for example ,my factory boosted ZL1 had 56lbs stock injections @58psi with 580hp..and we jumped to ID850’s @56psi once we turned up the boost climbing above the 600rwhp mark ..so there’s no way I would run a 800hp boosted v8 with 63lb injectors based on that chart..or would I run 47lb injectors on a 600hp boosted motor based off that chart especially at the stated @43psi he was referring to..
    But like was pointed out there are many ways to do it like a boost@pump which increases voltage and boosts pressure, and meth to over come what might be a lean issue at certain points ..although I’d rather just play it safe and Era on the side of caution with larger injectors,as the conversation is primarily about injectors..
    This is the equation we used to teach the kids when I taught at UTI for awhile after being crippled,and what we used at the shop always when we modded customers cars.
    So To be 100% correct and safe Follow this formula to determine injector sizing: (Horsepower x BSFC) / (No. of injectors x .80) So, for a NA 600hp V-8 it would work out as such, (600 x .50) / (8 x 0.8) = 46.87 pound per hour injectors ,which rounded is 47lb. If you want to convert to cc/min: multiply x10.50..
    If it’s a boosted motor then you multiply The power x .60 instead of .50...
    A NA engine has a BFSC of .50 per horse power and .60 BSFC Per horse power for a boosted engine...although if you look at boosted factory engines their injectors tend to be even larger than this equations results if you compare them ...



    Also on most all cars The ECM calculates fuel consumption based on formulas and operating data from the engine and not the amount of fuel injected into the cylinders So swapping injectors wouldn’t matter that much ,plus who swaps injectors without reprogramming it ?..very few cars actually Measure the amount of fuel flowed through the systems fuel lines,the main source is either the mass air,or the map sensor,or both to determine load to calculate amount of fuel being used..I’ve not looked into how the SS system works,but


    Something else and I know it’s a little off track,but you get different mileage from different brands of gas because they have different densities ,and currently there is no way to accurately measure the difference..even summer and winter gas have different densities changing mpg.

    Very good insight @Gnx1234. There is quite a few ways to look at this subject as you stated. Each vehicle/setup tends to be slightly different also and is best verified by a person creating a specific kit or from a local tuner that can see both AFRs and injector duty cycles.

    The simplest rule of thumb my injector guys tells people for 8 cylinder boosted setups (his most popular customer) is the below...
    Desired HP = injector CC IE: 600hp = 600CC injectors

    Owner of Slingshot #263 that has some stock parts left on it. :D