Cummins 855 Injector Timimg Accuracy

[Steve Shover]

I’ve been working on my sons dump truck engine which is a Cummins 855 NT400 Big Cam trying to set the injector timing. This is an FI code and believe the timing should be set at .069 fast .070” nominal, .072 fast. The problem I’m having is in the accuracy of setting this number, I’m using the Cummins dial indicator set up and have even switched to larger indicators to increase accuracy. The problem is detecting top dead center, it seems to have what I think is a large band width for TDC and within that rang I can see the injector lift change by .005/.007. At first my injector setting was all over the place due to this, once I started to always wait for the piston to start down and would see dial indicator movement then re-zero the injector indicator it became very repeatable. Is this typical and does anyone have any suggestion for the best method of setting this and how critical is a few thousandth off. I know it will be advancing or retarding the timing I just don’t know exactly how critical it is. Any help would be appreciated.

 

[catserveng]

I haven't worked on any of these for quite a few years, but a couple of things I found,

Older the engine (more run hours) more slop in the gear train, harder to get consistent reading.

Always turn engine in direction of rotation, if you turn past your target, go around in the same direction, don't turn it the other way, at least how I was taught.

Getting precise TDC is tough, your numbers seem close to mine in the day, but honestly never seemed to make any real difference. This engine is not what I'd call "precisely timed", especially compared to newer electronically controlled engines.

Cummins engines were not my area of expertise, but did work on quite a few, maybe someone else has better info.

Hope that helps, MikeL

 

[Hoxton]

I agree with catserveng.

1) Always turn the engine in the direction of rotation, this avoids any errors due to backlash in the gear train.

2) Getting a piston to TDC is difficult. See

http://www.animatedengines.com/diesel.html

If you think about the operation, the piston speed is highest when the conrod / crank angle is 90[sup]0[/sup], and thus slowest when the angle is 0[sup]0[/sup]. Thus, the piston slows as it reaches the top (and bottom) of the stroke. This explains why you find it difficult to determine actual TDC.


If you could see inside the engine, it might be easiest to see when the crank / conrod are in line, ie at 0[sup]0[/sup].

As catserveng says, this probably does not matter so much on an ‘old technology’ engine. Modern engines are electronically controlled and are more accurately controlled, because they have to be for emissions and fuel consumption optimisation.

 

[MikeHalloran]

This works very very well on a SI engine.
You should be able to rig up something similar on a Diesel.
{ finding TDC
- Get an old spark plug and remove the side electrode, center electrode, porcelain, everything but the shell. Use epoxy or otherwise install a pipe coupling or nipple or something similar, so you end up with a hole through the center, and a hose barb for ~1/4" hose.
- Position the engine before but near TDC on #1 compression stroke. On some engines, this can be done by letting the piston push on a wooden dowel in the plug hole. Mark/note where the dowel stops rising, and rotate in normal direction just less than two revolutions.
- Fill the combustion chamber with ATF.
- Install that adapter in the #1 plug hole.
- Connect ~3 feet of ~1/4" transparent hose, and suspend the free end above the engine.
- Now rotate the engine very slowly in the normal direction.
- The ATF will rise in the tube.
- TDC is very close to where the ATF either stops rising or stops spraying out of the tube.
}

What the procedure above does is turn some simple plumbing into a very sensitive indicator.
The indicator magnifies the piston travel by the ratio

diameter of piston ^2
--------------------
bore diameter of tubing ^2

Because of that magnification, you will need a very long handle extension on a socket wrench on the crank nose in order to turn the crank slowly enough to not overshoot TDC.
... or more likely, to just creep up on the meniscus reversal after you clean the discharged ATF off of the ceiling and the engine and everything else. ;-)





Mike Halloran
Pembroke Pines, FL, USA

 

[Steve Shover]

I appreciate everyone’s post, I think I found some of my problem in the dial indicator defecting a small amount. The Cummins fixture I’m using has a strong spring on the injector side to take all the slop out of everything and that was causing some defection error. After correcting I got the error down to about .002/.003” if I could detect down into the tenths (.0000) I’m sure I would see exactly TDC.
Thanks All

 

[Hoxton]

Well at least we got some thanks from the OP, we don't always get that!

 

[enginesrus]

You will not find TDC with just an indicator, reason there can be crankshaft movement with no piston movement when at TDC, all depending on actual engine component dimensions. That is why a degree wheel is needed.
And for what you are doing accurate crank TDC is not needed. You just need piston at the highest point, and then of course zero the indicator then back the crankshaft opposite engine rotation for about 45 degrees then go in normal direction to the BTC dimension you want. When finding the max piston top position you can rock the crankshaft CW and CCW and the piston should not move if it does just note the position that represents top high point, and go back to it and zero the indicator. Oil film and bearing clearance can introduce some inaccuracy's too.

 

[TugboatEng]

A dial indicator is the correct way to find top dead center. The technique is called tramming. First insert a rod through a hole in the head that touches the piston and roughly find TDC. Next, lower the piston 0.005-0.010", mark the flywheel, and zero the indicator. Rotate the engine through TDC and continue to rotate until the indicator again reads 0. Mark the flywheel again. A line drawn exactly between the two marks is the true TDC.

 

[enginesrus]

TugboatEng, yes basically a simple degree wheel of sorts. For what he is needing finding actuall TDC is not needed.
Since he is setting timing as a dimension before max piston travel.

 

[TugboatEng]

You said you can't find TDC with a dial indicator. I said that's the correct way to find it. None of the engines I work with actually require you to be at TDC. Injector height is set anywhere on the base circle.

 

[enginesrus]

I said with JUST an indicator. Meaning you can't move the piston up till it just stops and call that TDC. And yes I agree you mentioned the correct way.