Interesting Math

at idle, 20* might be outside the bowl... at 3000RPM, it could yield combustion dead in the center of the bowl.

combustion and flame travel isn't an instantaneous thing... that's why ideally you have a timing curve. As RPM increases, you increase timing to account for piston speed to "time" the combustion process
 
Forrest Nearing said:
why do you increase timing as RPM increases?
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ignition delay.

you can do the math and get your timing "spot on" but there are too many things that can change it, like RPM, fuel rate, rail pressure. thats the point of testing a tune. if you could do it right the first time doing math, everyone could have a perfect tune.
 
JWhite said:
ignition delay.

you can do the math and get your timing "spot on" but there are too many things that can change it, like RPM, fuel rate, rail pressure. thats the point of testing a tune. if you could do it right the first time doing math, everyone could have a perfect tune.
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exactly, just because the fuel is "out of the bowl" at 12* when the piston is stationary, it doesn't mean it's out of the bowl as the piston is traveling upwards at a high rate of speed
 
Refigured it up, turns out with 03-04 sticks you start to spray outside the bowl between 19 and 20 deg before TDC or about 4.6 mm away from injector tip, HG thickness would be negligible. Does that sound about right? gonna figure for 04.5 also.

Not trying to reinvent the wheel here just get some good round about ideas before playing with the timing. Cheaper than buying an oscilloscope and seeing what the truck is doing with stock ECM.
 
Forrest - exactly, we advance timing because of the speed of the motor vs the actual injection event, precisely because one outruns the other.

How else would one compensate for 5000 rpm in a motor with an injection pump/injection event designed for 600-2800 rpm originally
 
at 5000rpm piston is traveling "on average" 787 in./sec or .00127 sec./in., injector pulse width is in the range of 1 millisec. The injector spraying "on time" is less by a factor of 1000, and if delay time from signal sent to injector to spraying is 100-200 microsec then the various injection events happen so much faster that rpm isn't limiting at all. The one thing I see as limiting the amount of fuel you can get in the cylinder is timing advance, you can only start so early and not be out of the bowl and spray so long at some ungodly r.p like hi 20K's.
 
EPA Violator said:
HG thickness would be negligible. Does that sound about right?
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Depends on how you look at it (*/mm or mm/*) - tip insertion narrows or expands allowable timing advance close to TDC far more than at mid-stroke...
 
XLR8R, near TDC there would be no worry about getting out of the bowl. In that close range would be where you want to match injector spray pattern to piston bowl design, correct? Just how long is it advantageous to spray? All the way to TDC, stop short of TDC, or even spray through TDC into the powerstroke? If my thinking is correct stretching duration out too long (like to TDC or too close to TDC) is where you just get a lot of unburnt fuel in the cylinder ejected into the exhaust and hence hi EGT's. I think this would be the sign that duration is too long. Whatya think?
 
EPA Violator said:
XLR8R, near TDC there would be no worry about getting out of the bowl.

Which is why you can get away with otherwise "excessive" timing.

In that close range would be where you want to match injector spray pattern to piston bowl design, correct?

Synergetic geometry of the tip & bowl angles isn't a function of crankshaft position.

Just how long is it advantageous to spray?

Injector flow rate is finite, so longer spray = more fuel (within reason).

All the way to TDC, stop short of TDC, or even spray through TDC into the powerstroke?

Longer combustion = more power.

If my thinking is correct stretching duration out too long (like to TDC or too close to TDC) is where you just get a lot of unburnt fuel in the cylinder ejected into the exhaust and hence hi EGT's. I think this would be the sign that duration is too long.

Again, depends on your POV... duration-induced high EGTs = more turbine drive HP, which is handy for making more boost.
(One of the reasons why CRs spool so well - passive EGR designed into the OEM cam lobes creates high TIP)

Whatya think?
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...... I think it'll be more fun when Cummins Live allows us to melt down our junk on purpose. :evil
 
Big Swole said:
Kind of a personal question there isn't it!!!


LOL




Sorry, nothing useful to add.......All I know how to do is blow them up. lol
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Easy now Swole...don't you guys call that "sexual harassment":rules:

:hehe:

I was referring to his motor..:doh:
 
Eliminated everything except ECM, instead of buying stock ECM I am waiting on the aftermarket ECM. Putting together parts for a new build. How bout you Gus, what's going on.
 
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