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dap930 dap930 is offline
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Join Date: Dec 2011
Location: Virginia Beach, VA
Posts: 417

There was a lot of discussion of this feature when it was first introduced with MS2. The theory is to inject fuel before the intake valve opens. I have SC cams which start to open about 400* BTDC. I use "end of pulse" for injection. I set my low load areas to 400* and increased to 440* as RPM;s increase to account for injection lag as engine speeds increase. But at high boost, high RPM's things change. Assuming the intake valve starts open at 360* BTDC, for an injector duty of 80% at 6000 RPM's there is only 16ms available for injection; 80% of 720* is 576*; at 6000 RPM, 360* x 6000 = 2160000 degrees/min =36000*/sec = 1/36000sec/degree = 0.0000277778 sec/degree = 0.0277778 ms/degree; .027778 x 576* = 16ms. There is even less time once you approach 7000 RPM, 13.7ms. Of course if your injector DC is over 80% you have a longer injector pulse which would carry the injection into the intake stroke.
So if you have a 13ms injection pulsewidth @6000 RPM, the engine travels 13/0.027778 = 468*. If injection starts @TDC of the compression stroke the end pulse would finish at 468* BTDC well before the intake valve opens, so if you have the injection timing set at 400* it would get all the fuel in no problem. But, I just read the tool tip in TS that says this is based on the spark event so I assume injection start for the next cylinder combustion event would start at the spark advance point of the current cylinder? If true that would add 10* - 3x* more for injection to occur.
At any rate, if your injectors are big enough there is no issue getting all the fuel in. My injectors are little too small. At 6800 RPM full boost my pulsewidth is 18ms (94%DC). So I have my pulse ending with the intake valve open in that region (180* BTDC).

As for small pulsewidths, you need that information about your injectors, which is not provided by most manufacturer's. If you have ID1000's they provide that info. I don't have any experience with this.
1977 930 Slant, MS3 EFI, Carrera intake, Twin plug, Powerhaus headers, Magnaflow muffler, Garretson intercooler, GTX3071R
Old 09-07-2017, 09:37 AM
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