Sizing fuel injectors etc

Discussion in 'Engine' started by Charles Morgan, Dec 7, 2015.

  1. Charles Morgan

    Charles Morgan MB Club Veteran

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    One of the ironies of the internet is that the more information one can find the more one seeks to check it for accuracy, normally online.

    I am planning fitting a Megasquirt ECU and new fuel injection to my W114 Coupe, which currently runs Bosch D Jetronic. This requires me to size new fuel injectors to fit. This site has a handy little calculator to determine the correct fuel rate. My engine, the straight 6 M114 250 was originally rated at 150bhp at the flywheel. My experience of MS conversions on older engines (my Citroen SM) shows bhp similar to original spec, but better torque, so using the original bhp as the input, I come up with needing 6 injectors at a flow rate of c160cc/min per injector.

    Does that make sense to the more technically competent than me?
     
    Last edited: Dec 7, 2015
  2. Bellow

    Bellow Hardcore MB Enthusiast

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    It makes sense to me on the basis that larger injectors are required when there is insufficient time for the injectors to open and deliver all the necessary fuel within that firing cycle (they are still open at commencement of the next cycle and their metering function is lost).
    As Tmax is at much lower rpm then this scenario is avoided.

    (Smaller the better for promoting quality of idle.)
     
  3. grober

    grober MB Club Veteran

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    This is a useful site for Bosch Injector specifications- while most are rated at 3 bar supply pressure you will notice some are rated at the lower 2.5 bar.
    fuel injector flow rates

    Do you have the old injector numbers/ratings as a guide? There may be detailed differences if you intend to run the more efficient individually triggered injectors rather than the two banks of three like the old D-jet setup.
     
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    Charles Morgan

    Charles Morgan MB Club Veteran

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    I had already dug out one of the old injectors to have a look. It's 0 280 150 035, which according to this site works at 3 bar but with a flow rate of 320.6 cc/min, so twice that of the theoretical calculation above. This would certainly explain the very lumpy idle etc of a D Jetronic set up!
     
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    Charles Morgan

    Charles Morgan MB Club Veteran

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    I had taken the idea of using the specs from a BMW M20 325i engine (slightly higher output but similar SOHC 2 v configuration) and that use 0280 155 715 as original (236cc/min) or 0280 155 885 as an upgrade (166cc/min).
     
  6. grober

    grober MB Club Veteran

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    Sorry I am reading that injector having a 2 bar 29psi supply pressure?:confused:
    I agree that is the correct injector mind.
    I can only assume with such a difference in flow rates there is a large factor somewhere not being taken into account
     
    Last edited: Dec 7, 2015
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    Charles Morgan

    Charles Morgan MB Club Veteran

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    Re-reading it and yes it's 2bar. I'll be replacing the fuel pump and the fuel pressure regulator anyway.

    My experience of D jet engines is that they run so rich that over-fuelling is built in.
     
  8. grober

    grober MB Club Veteran

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    The injector rating seems heavily dependent on the Estimated Horsepower figures calculated at 80% duty cycle and also based on a BSFC (Brake Specific Fuel Consumption) of 0.52 which I assume means how efficiently the engine converts ccs of fuel into horsepower. Presumably early engines are poorer at this ----meaning they require more fuel/ bigger injectors for the same horsepower. 3 injectors fired simultaneously in the D jet means that only one of the injectors will be firing into a cylinder on its induction stroke with the inlet valve open. I assume the other two are just adding to an air/fuel mix cloud in the inlet manifold to be used in due course? Its a bit like the later K jetronic CIS [ continuous injection system ] which fires all its injectors all the time acting a bit like a very efficient carburettor :dk:
     
  9. Bellow

    Bellow Hardcore MB Enthusiast

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    Correct assumption G re BSFC - the absolute benchmark of efficiency.

    Hosing in large quantities of fuel was part of the cooling strategy. Cheaper for the manufacturers to let the customer foot higher fuel bills than for them to use more copper and brass in larger radiators. Helps with drag reduction also, not that that was particularly on their radar.
    Less attention to unburned HCs was also prevalent - eg, mixture trapped in ring crevices and released too late to properly burn. Nor were they able to exploit high compression ratios (when they did the octane requirement was high) compared to today's motors with their air-motion and turbulence all predicted by CFD. Ignition systems have also improved.

    There may also be electronic consideration as to why the D-Jet uses such large injectors (albeit at lower pressure) if they are run at short durations ie, recovery time (from heat due to holding injectors open). That is mere conjecture on my part though.
     
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    Charles Morgan

    Charles Morgan MB Club Veteran

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    The cooling issue is an interesting one. As MS will be running mostly at Stoichiometric, over-fuelling for cooling is going to be a thing of the past (except at fully open throttle). I have noticed no difference in the engine cooling of the SM despite the switch in injectors to about a half the previous flow rate (SM D jet to Citroen CX L Jet).
     
  11. Bellow

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    I can't make sense of this as when I look those two injectors are for Ford and Rover fitments and flow 235.9 (260) and 148.2cc/min.

    Maybe more an American thing than European. Have I not seen you write of overheating plaguing older cars though?
     
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    Charles Morgan

    Charles Morgan MB Club Veteran

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    I think the list linked isn't necessarily fully comprehensive. I am currently searching for an injector for another car (from the part number of the removed injector) that comes up on that table for a completely different car manufacturer to the one it's fitted to.

    Over-fuelling for cooling is pretty common on older cars, especially to protect valves etc. Most cooling problems I've encountered are on cars with degraded cooling systems (a silted up flathead block), mechanical wear (seized or inefficient water pumps), inadequately specced original cooling (what's a traffic jam?) and fundamentally poor design. I am likely to fit a lower temp thermostat and might well install an electronic fan, plus renew water pump. But the inherent reality of upgraded systems means you will run more efficiently, so any mapping should only be adding fuelling to reduce the air fuel ratio at points where heat issues are important (full throttle) rather than all the way through the map.
     

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