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  1. #1
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    What are the differences between tuning a direct injection vs. port injection motor?

    What's the difference between tuning a DI motor vs PI motor? Are the principals basically the same?

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    Moved to its own thread, I believe Neel@Apex is the most qualified to answer this.

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    I think we had a discussion about this a while ago, but it was in another thread. I don't remember which. I think it would be good to start a new thread on it as we have new members with a lot of knowledge on the subject.
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  4. #4
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    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    I think we had a discussion about this a while ago, but it was in another thread. I don't remember which. I think it would be good to start a new thread on it as we have new members with a lot of knowledge on the subject.
    +1
    The basics on how these engines work is also important. I found this on another thread. For the new booty's, if you don't know now you know...

    "I think most of us on this board understand the basic concept behind the 4-stroke gasoline piston engine. While the N54/N55 are DI, they still fall under the basic concepts of a PI 4-stroke engine, which are fairly easy to understand. On the piston’s decent from the top of the cylinder, intake valves are opened allowing the cylinder to fill with air and fuel (PI engines spray towards the valve which is open, whereas DI engines are plumbed to spray directly into the cylinder). Granted, some DI setups are able to spray fuel well into the compression stroke (haven’t researched the N54/55’s fuel timing to say where during the process fuel is injected under full load), but that’s beside the point. After the cylinder is filled with air, it makes its ascent towards the top of the cylinder in what is referred to as the compression stroke. Towards the top of the compression stroke is where the spark plug fires, igniting the compressed air/fuel, which drives the piston back down (power stroke), and of course the piston’s next ascent pushes the burnt gases out of the exhaust valves (exhaust stroke)."

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    the main difference is the AFR's that you can run under load. DI is much better at preventing uncontrolled ignition partly because of better atomization of the fuel, that also means that you can get away with higher compression ratios, witch can change the timing curve a little but not enough to consider "different" than PI.

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    Click here to enlarge Originally Posted by black e Click here to enlarge
    the main difference is the AFR's that you can run under load. DI is much better at preventing uncontrolled ignition partly because of better atomization of the fuel, that also means that you can get away with higher compression ratios, witch can change the timing curve a little but not enough to consider "different" than PI.
    Yes, but it is limited as to where in the cycle it can inject...

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    DI can yield improved HC emissions and is less susceptible to detonation as it does result in a mixture that is well atomized and well placed in the combustion chamber. A lot of modeling is done with the spray patterns to maintain a dense charge near the ignition source to prevent detonation. High pressure allows for very precise charge timing as well. I've tuned DI motors with standalone ECUs before; having in-cylinder pressure measurement was very important to position the timing of injection accurately. Also, fuel control is done as much with pressure as injection time.

    The issue with DI is that the modeling involved with injection placement and combustion chamber design is very important. When done with OEM resources, you see significant gains. However, its not the kind of thing you can do in your garage....

    -Neel
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    What about when tuning beyond the OEM resources and constraints? Fuel system, etc.?

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    How difficult is it to increase the flow capacity and injection resolution of DI injectors? i want huge power but i also want my car to idle like stock, can DI injectors supply the same gobs of fuel that port injector can or are they not that evolved?
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  10. #10
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    For a ninja quick summary I like this picture

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    Good discussion, but I'd like to understand more of the tuning principals. All of my knowledges is based of off PI, this DI stuff is new to me. What is still weird to me is you don't really see custom dyno tunes per vehicle and does it have to do with the DI system? I guess with how smart the dme is we don't have to be as dialed in on the tuning side Click here to enlarge

  12. #12
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    Click here to enlarge Originally Posted by Blaizon Click here to enlarge
    Good discussion, but I'd like to understand more of the tuning principals. All of my knowledges is based of off PI, this DI stuff is new to me. What is still weird to me is you don't really see custom dyno tunes per vehicle and does it have to do with the DI system? I guess with how smart the dme is we don't have to be as dialed in on the tuning side Click here to enlarge
    You see custom dyno tunes for direct injected cars. You just see more piggybacking on the N54 due to the DME and the difficulty in tuning it.

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