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Injector sizing for low power engine

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  • Injector sizing for low power engine

    I have a low power (at the moment) Chrysler 440 that I would like to add a pro-flo 4 efi system to .... for mileage drivability reasons. My guess is that the motor can’t be making making more than 250 HP ... it’s a standard 1975 smog motor with low miles. Looking at the install instructions It say’s that even a basic 440 (timing) should run 35 lb/hr injectors. That seems an overkill at this power level and I would have thought that 29 lb/hr injectors would be a better starting point..... but if try the smaller injectors which start up base map would I use? Perhaps one for a smaller CID motor?
    Last edited by Jacbjacb; 08-13-2019, 12:04 PM.

  • #2
    I moved your post from the other thread and put it here in the Pro-Flo 4 section. Just a general forum etiquette reminder but you typically want to post in the most relevant forum section (Pro-Flo 4 vs Pro-Flo 2) and you typically don't reply to very old threads. The thread you replied to was from 2009. Now it's in the right spot so here we go...

    Not to get too long winded but when you have two different displacement engines that both have the same power rating, they both have roughly the same volumetric efficiency and fuel consumption rate. That's to say that a smaller motor with better VE (airflow) can make the same power (or even more) as a larger displacement motor if the bigger motor has lower VE. We use CID as a rough guide to use a particular fuel map for different calibrations but mechanically, an anemic 440 is probably more comparable to a basic 350 in VE and power. So, as a starting point, I'd look at cals for a stock cammed 350.

    Now as far as getting 29 lb injectors in a kit that isn't sold with 29 lb injectors, you have two options. One, if you're in the US, you can call up and order a Pro-Flo 4 system directly from Edelbrock. At time of purchase, mention that you want to change the injectors and that can be done for a small fee. Your kit will ship to you with the different injectors already installed. Option two is to order from your preferred retailer (Summit, Jegs, Amazon, local race shop, etc) and remove the injectors that come in the kit (before installing or using the kit) and send them in to Edelbrock with a note to exchange them for whatever size injector you want. There is a small fee so include your contact and payment info. You can call Edelbrock to make purchases at 1-888-799-1135. You can call Edelbrock Tech Support at 1-800-416-8628.

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    • #3
      Originally posted by Nate S View Post
      I moved your post from the other thread and put it here in the Pro-Flo 4 section. Just a general forum etiquette reminder but you typically want to post in the most relevant forum section (Pro-Flo 4 vs Pro-Flo 2) and you typically don't reply to very old threads. The thread you replied to was from 2009. Now it's in the right spot so here we go...
      Thank you Nate S for your enlightening me regarding modern forum etiquette …. Maybe Debrett's should be informed as well? LINK

      So if I sum up what you have said correctly …. Use the injectors that are appropriate for the HP of the engine in question.

      Use the Pro-Flo 4 EFI Setup Wizard Map Matrix in the install instructions, select an engine that uses the same size injectors (that you have selected) and has the same camshaft duration. The HP level will be a guide and the fuel pressure required by the tune/map will be shown. Then use those details when you setup the handheld device prior to first start … this will set the base map?

      My guess is that getting an oversized injector will loose resolution by using very little of the available duty cycle?

      Thanks again
      Last edited by Jacbjacb; 08-13-2019, 05:30 PM.

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      • #4
        Ha that's funny. Thanks for being good humored about it.

        Yes, size injectors to match engine's expected power output - even if that means having to change injectors from what is included in the kit. I would start with cal #5006. This cal was tuned on a bone stock 350 that's probably comparable to your smog 440. As luck would have it, #5006 is the cal that's uploaded to the ECU by default - you don't even have to run the wizard. Just fire it up, set the timing and go drive it.

        And yes, you're right. By properly sizing an injector to best match an engine's power output, you're using more of the injector's "dynamic range" and gain granularity giving you better fueling and AFR control. This gets very important at low duty cycles as this can sometimes get the injector to operate in a non-linear range which is bad for fueling and AFR control. Thus the general rule of thumb is that injectors should always be as small as possible while providing just enough fuel to support the expected power level.

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        • #5
          Got It …. Thank you.

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