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Old 10-27-2018, 07:18 PM   #1
wwentzler

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Timing Theories

So the further I find myself in trying to understand every aspect of how these things work, I find more questions. I know this is probably coveted secrets among tuners, but how is everyone determining optimal injection timing?

I know there are a few schools of thought on how this should be done, but basically I'm looking to make a little more advanced calculator than just punching in rpm and duration and giving you a timing based on splitting the pulse roughly 50/50.

Just kind of looking for a little better justification for how to calculate timing than yeah should be about 50/50. Looking to make a little bit nicer calculator for use in HPT or MM3.
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Old 10-27-2018, 09:38 PM   #2
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A calculator like you mentioned will get you very close. In order to find that "sweet spot" for your particular combination, that takes dyno / track time, as well as tracking fuel mileage (if you care)
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Old 10-28-2018, 02:33 PM   #3
wwentzler

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Alright let me a little further explain my thinking. What were really after is ignition timing, which lots of variables play into exactly when this happens after we start to open the injector right? Things like rail pressure, rpm, EGT, head swirl etc etc etc. Now the earlier this ignition happens along with at what rate we start to inject fuel determines the curve of cylinder pressure in relation to crank degrees. Too early and we start having bottom end and head gasket issues, too late and we rob potential power and a few other minute things. So although the 50/50 theory would get us close, I am shooting for more exact. So I guess my first question is how much variance typically are we seeing in that 50/50 on a truck that has been optimally setup? Are we seeing that accross the whole band were within +/-10%? 20? 30?

Im not trying to reinvent the wheel here, I have been tuning for about 3 years and have had zero issues building, given my equipment access and inexperience, what I believe are fairly happy tunes with nothing more than careful intuition. But there is always room to improve through understanding, which is what I am after.
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Old 10-30-2018, 11:13 AM   #4
Dom_c13b

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I usually end up at about 75% for max effort cummins tunes. That's typically only above 3000rpm that I give it that much timing and on big injector trucks. On stock injector trucks I typically stop at 60%. I've gone up into the mid 80%'s before on a truck. One thing you really need to consider is end of injection. If you keep the injection even going too far past top dead center you will end up with a melted piston.
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Old 11-02-2018, 08:42 PM   #5
apples24v

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Dom and Jwhite are both pretty smart with this stuff ,here's a link to another thread about this where they both chime and Dom explains the formula behind the calculator a Jwhite gives another quick rule of thumb he uses sometimes. But like they said best way to really tell what the truck likes is a dyno

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Old 11-08-2018, 07:50 PM   #6
wwentzler

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I appreciate the responses guys. I suppose without logging cylinder pressure on a dyno it would be hard to tune based on ignition timing like makes mathematical sense to me. EOI is deffinitely a good point as well. Also what kind of injector delays are typical? I know flowsheets generally show them, but roundabout how long is typical?
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