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Discussion Starter · #1 · (Edited)
With all the talk about “timing rattle” I thought I put something out here to get everyone on the same page. My explanation is very brief and huge papers can be written on this subject. The constant talk of timing rattle is taking up too much time and space on these pages and needs to be better explained.


Before I get started there are a couple of terms that need to be defined.
Injection timing: The time measured in degrees when the main injection BEGINS. This is usually stated as BTDC(before top dead center) or ATDC (after top dead center).
Ignition timing: The time measured in degrees when the fuel self ignites stated at BTDC or ATDC.
Ignition delay: The time difference between Injection and Ignition. Diesel fuel needs to evaporate after it is injected before it can self ignite. This delay varies with temperature in the cylinder.


To explain the diesel cycle as it applies to our engines, I am going to use some numbers that I pulled out of my Bu**, for explanation purposes only.


Lets take a typical truck cruising down the road unloaded on a typical day with the ideal timing map installed. Fuel is injected at lets say 6deg BTDC and continues to flow throughout the pulse duration. The engine is at operating temp of 200F and fuel starts to evaporate and by 5deg BTDC it ignites. When it ignites, the rest of the fuel ( 1 degree worth) in the cylinder also flashes causing a rapid increase in cylinder pressure. There is a knock associated with this rapid increase in cylinder pressure but it is hardly noticed.


Now lets see what happens when this same truck is parked overnight and fired up in the morning.
The driver takes the truck up on the same road, same speed, same load, but the engine is not up to temp. The fuel is injected at the same 6deg BTDC and continues to inject throughout the pulse duration. The engine temp is 60F and the fuel starts to evaporate but slowly because the cylinder is so cold. By 1deg BTDC the fuel ignites and all the fuel in the cylinder, 5 deg worth (6deg- 1deg) flashes over. This rapid increase in pressure causes the cylinder to ring loud enough to be heard in the cab! This is what everyone here has been calling timing rattle.


Ignition delay is what is causing the noise. The time between when the fuel is injected and the fuel actually ignites.


So how do you get rid of the noise? Inject later in the stroke when the air in the cylinder is hotter. This is what is done from the factory. Another way is to use pre-injection. A small amount of fuel that is injected early and allowed to flash but is such a small amount that it doesn't make noise. Just before the pre-injection has finished burning the main injection is turned on creating a smooth transition and much less noise. I think advancing pre-injection with the main would work with our engines but I have never seen an adjustment for pre-injection.


The issue is, as far as I know, there is no cold timing adjustment in the 6.7 computer. Even EFI doesn't have a temperature dependent timing adjustment as far as I know.
So you are stuck with an engine that rattles when cold or gets good mileage when warm.


So does rattle hurt my engine? From theory I would say no at small throttle. Even though there is a pressure spike associated with the rattle there is so little fuel and total pressure that I feel it is highly unlikely to cause damage. Empirically it think it is pretty obvious that nobody I know has ever blown a head gasket at part throttle. But...what about full throttle on a cold engine!!!


There have been many cases of 6.7 engines blowing head gaskets when not allowed to warm up. So lets look at what is happening.
By looking at a H&S timing map, you see at WOT your timing could be as much as 32 deg! So on a cold engine you inject at 32BTDC and have ignition at lets say 15BTDC and you have 15DEG of fueling going off at once! Oh, did I mention you are also at full boost when all this is occurring!:shock:
Well this explains the blowing head gaskets on cold engines.
I was never satisfied with the excuse that the head studs needed to be heated to reach full clamping force. Just didn't make sense to me.


So, light throttle till warm and enjoy the sound of MPG!
Any questions:confused013:





Credits,
I have read so many papers on diesel injection that there is no way I could remember them all but one does stand out,
Michael Patton at KillerBeePerformance provided much to my understanding and with the exception of being a Dmax fan, he is a pretty smart guy.
 

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Very good and straight forward explanation. Nice work!
 

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I like it, nice thread.

My testing proved that anything below 5.5 degrees BTDC would eliminate the cold engine "rattle" on a 2012. The 2010 is a little more fussy. I recently learned that if you taper in the timing chart very smoothly that the sound is greatly reduced. I asked Ryan from H&S just the other day if he could design a work-around on this "issue". My thoughts were to have the truck pull and use a map with only 2-3 degrees of timing until the coolant temp sensor reached above 175 degrees F. Once that temperature is reached, logic activates the main map you wish it to use at 6-8 degrees BTDC. Ryan and Bentley working together can pull off some magic, so lets see if they can dial this in and make that "rattle" go away forever in their next software update. :thumbsup:

I think they already have this figured out, it just requires a full calibration flash and much more than a box tune can provide at this point in time. Things will only get better as they dive into the 2010-2012 trucks in the next few weeks.

Listed below is some great info that was shared with me and I'd like to share it with you also:

The CM2200 (2010-2012 trucks) can accomplish as many as 6 injections per combustion event.

2 Pre main
1 main
2 post main
1 emissions (regen)

The biggest hurdle with the CM2200 right now is the logic fueling algorithms. In previous models the fueling was mainly controlled by a user input (throttle position). Now, instead of controlling fueling from throttle, that throttle position is interpreted as a desired RPM value, and the ECM uses a combination of algorithms and software gain values/limitations to compute a final fueling value to be delivered. It becomes very complex trying to tune these trucks.

Thanks for the thread, its a great topic.
 

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Discussion Starter · #6 ·
Norcaler,
There is a good possibility that the ECU uses some filtering to smooth transitions between points on the map. Depending on the type of smoothing you could get overshoots. So any large changes between points on the map, like a step function could generate higher timing that is listed.
 

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Very interesting ... most straight forward explanation I've come across here.
Thanks!
My still-stock 2012 rattles pretty good first thing in the morning some days, always has ... -30C to +30C, doesn't seem to matter. Some days it purrs.
Same thing after it's warmed up after a good run ... it's a toss up.
 

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Very interesting ... most straight forward explanation I've come across here.
Thanks!
My still-stock 2012 rattles pretty good first thing in the morning some days, always has ... -30C to +30C, doesn't seem to matter. Some days it purrs.
Same thing after it's warmed up after a good run ... it's a toss up.
my truck also rattles regardless of outside air temperature but -20 and lower its noticeably louder.
 

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Discussion Starter · #9 ·
It should sound like a diesel but not like a 2003 Cummins!
With a NC tune you should have a quite idle but notice some noise when driving till it is warm.
If you can hear it in the cab after it is warm then something is wrong.
 

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my truck also rattles regardless of outside air temperature but -20 and lower its noticeably louder.
I would imagine at these extreme low temps the fuel is slower to atomize for a proper combustion?


Sent from AutoGuide.com App
 

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I like it... Now I know what the annoying rattle is and it does not bother me as much anymore.
 

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Very nice!! What would you say is warmed up I never get on it until it clears 185 but sometimes that takes along time depending in the weather and so on.. Just wondering what temp is the 6.7 considerably safe at? Thanks
 

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Since I'm towing a toy hauler, should I use high idle for a few minutes prior to leaving? Asked another way, do you guys go through any particular warm up other than just taking it easy on the throttle until the engine comes up to temp??
 

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Discussion Starter · #16 ·
No special warm up for me. I just idle for 20 secs to get the juices flowing then drive easy.
Now if I am towing heavy and/or taking the short cut to the hwy, then I will high idle with the e brake till full temp before leaving the driveway.
The short cut is 1/2mile with a steep grade, second or maybe third gear.
 

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Good idea on the high idle, as I can see the 70 mph zone from my house!!
 

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Thanks for the info, I love learning.
 

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You have to remember its not just the engine that needs warming up. The trans (auto or manual...auto more so) and the differential also need some warm up. Especially in winter.

Regardless of engine warmup I always go easy for the first few miles or a bit more to give the other parts time to get lube moving a bit.
 

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You have to remember its not just the engine that needs warming up. The trans (auto or manual...auto more so) and the differential also need some warm up. Especially in winter.

Regardless of engine warmup I always go easy for the first few miles or a bit more to give the other parts time to get lube moving a bit.
This driver needs warming up too, as the people around tend to drive like idiots!!! LOL
 
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