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  #1  
Old 08-07-2014, 08:58 PM
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Question on the boost/timing relationship

My gut tells me that the presence of boost retards the combustion timing because the engine gets quieter under boost and also, if the injection timing is close enough to the edge of detonation to cause some "pinging", the ping goes away with a little boost.

Anyone have any data to back up this theory? How about a pounds to degrees relationship?

Enquiring minds want to know!

Thanks!

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  #2  
Old 08-07-2014, 09:35 PM
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Quote:
Originally Posted by KarTek View Post
My gut tells me that the presence of boost retards the combustion timing because the engine gets quieter under boost and also, if the injection timing is close enough to the edge of detonation to cause some "pinging", the ping goes away with a little boost.

Anyone have any data to back up this theory? How about a pounds to degrees relationship?

Enquiring minds want to know!

Thanks!
Sounds too scientific for the duty required. Youre right about retarding timing lessening the knock. Timing device on the chain on the IP does adjust timing under load and this is all RPM related so maybe youre thinking its boost, but its really load which of course boost also reacts to. But load would advance the timing to get more power... Is this making any sense?
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  #3  
Old 08-07-2014, 09:54 PM
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Makes sense but doesn't fit with what I'm experiencing. Engine seems "happier" and quieter under mild to heavy acceleration. With light loads, low boost, the engine "pings" like a gasser. Now I understand that with my car, some of this is due to a little air in the system but that's not the only factor.

Now regarding the injection timing device, I'm wondering how it adjusts timing with load. I guess the injection pump becomes harder to turn at higher fueling levels and this retards the timing because more force is applied to the timing device?
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Old 08-07-2014, 09:55 PM
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I wish my gut said such sophisticated stuff to me. All I hear from it is, "FEED ME".
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Old 08-07-2014, 10:24 PM
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I wish my gut said such sophisticated stuff to me.
Be glad it doesn't. Very glad.
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Old 08-08-2014, 06:20 AM
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What car are we talking about? I'm pretty sure injection timing on 61X engines is fixed. Not sure about the newer ones.
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  #7  
Old 08-08-2014, 06:40 AM
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In this case, I'm talking about my 606 but all of the engines have a centrifugal timing device integrated into the pump drive sprocket that works the same way by changing the position of the injection pump shaft to the crank shaft as RPM's increase.

There may be a unique example of a Mercedes engine that doesn't have a timing device but I'm not aware of any aside from the modern CDI engines.
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  #8  
Old 08-08-2014, 09:29 AM
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Evan,
Just a guess, maybe the presence of the extra air is causing the fuel to burn rather than explode. The timing doesnt change under load, just that as you have the stroke on the IP elements increased, it starts injecting sooner & goes on injecting for longer.
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  #9  
Old 08-08-2014, 10:30 AM
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Quote:
Originally Posted by KarTek View Post
My gut tells me that the presence of boost retards the combustion timing because the engine gets quieter under boost and also, if the injection timing is close enough to the edge of detonation to cause some "pinging", the ping goes away with a little boost.

Anyone have any data to back up this theory? How about a pounds to degrees relationship?

Enquiring minds want to know!

Thanks!
Not sure which engine you have (not in your sig) but if your IP is anything like the 617.952, the timing advance is centrifugal controlled by RPM only, no electronic control.

What is your timing set at? To spec or advanced past spec, which may have an effect on the pinging. If I get ambitious, I may set up my RIV Xenon timing light and diagnostic port digital RPM adapter and do a map of my timing advance curve.
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Old 08-08-2014, 12:33 PM
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Quote:
Originally Posted by KarTek View Post
My gut tells me that the presence of boost retards the combustion timing because the engine gets quieter under boost and also, if the injection timing is close enough to the edge of detonation to cause some "pinging", the ping goes away with a little boost.

Anyone have any data to back up this theory? How about a pounds to degrees relationship?

Enquiring minds want to know!

Thanks!
More load and boost = more fuel injected

the bigger amount of fuel takes longer to heat up in the prechamber and reach ignition temperature

This ignition delay retards your timing up to several degrees = retarded ignition sounds smoother

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  #11  
Old 08-08-2014, 02:04 PM
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Originally Posted by layback40 View Post
Evan,
Just a guess, maybe the presence of the extra air is causing the fuel to burn rather than explode. The timing doesnt change under load, just that as you have the stroke on the IP elements increased, it starts injecting sooner & goes on injecting for longer.
X2

I agree it is most likely an air fuel ratio relationship. But a subjective "quietness" doesn't necessarily mean it is doing better.

You can look at the "boost" side of the air fuel ratio or the "fuel" side if you wish but it is really a combination of the two that does the job.

Here's a standard graphic found in most text books



Air

And then there's the ALDA section in chapter 07-010 in the (turbo) part of the OM617 FSM that's worth a read.

This isn't really delaying the timing (certainly not valve timing!) in terms of the point of combustion it is more like the relationship of how well the rate of burning is achieved. I don't think a comparison of a "crap" burn to a late or too early begin of delivery or point of delivery is really going to be all that meaningful.
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  #12  
Old 08-08-2014, 02:50 PM
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From what I found out on My own car at idle speed it will nail if the Injection Pump timing is too advanced and it will also nail if the Injection Pump Timing is too late/retarded.
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  #13  
Old 08-08-2014, 03:13 PM
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The fuel burns rather than actually exploding. This takes time. At higher rpms the piston is moving relatively faster away from it. So the noise of burning may appear less. Advancing the timing does not mitigate this effect. It only allows the burn and its initial expansion to initiate earlier.

In a way fuel milage may be reduced by this effect. Less work might be achieved by the burns attempt to keep up with the pistons speed as well.

The rate of the burns expansion remains pretty much a constant I suspect. The load for the burn is diminishing much quicker at high rpms. I also suspect this is why a taller rear end ratio can produce better fuel milage.

Last edited by barry12345; 08-08-2014 at 03:34 PM.
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  #14  
Old 08-08-2014, 03:24 PM
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Originally Posted by Diesel911 View Post
From what I found out on My own car at idle speed it will nail if the Injection Pump timing is too advanced and it will also nail if the Injection Pump Timing is too late/retarded.
Interesting. Part of the fuel of very late injection may not be usable or burnable until the next compression stroke. If this is so it might point to inadequate exhaust scavaging. In other words there is unburnt fuel left that may combust before the new injection takes place.

Just some thoughts that came quickly to mind. Based on the assumption that once compression has dropped off too far the diesel fuel might be somewhat self extinguishing.

Just because unburnt fuel comes out the tailpipe does not mean all the unburnt fuel left the combustion chamber.

Blower pressure may also help exhaust scavaging leaving less un burnt fuel behind in the combustion chamber. I have no issue with anyone discounting this as I have not really thought it through.

Last edited by barry12345; 08-08-2014 at 03:35 PM.
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  #15  
Old 08-08-2014, 03:43 PM
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Originally Posted by layback40 View Post
Evan,
Just a guess, maybe the presence of the extra air is causing the fuel to burn rather than explode. The timing doesnt change under load, just that as you have the stroke on the IP elements increased, it starts injecting sooner & goes on injecting for longer.
There is something here as well. Depending on the loading of the elements changes the timing somewhat as well I suspect. The elements of the injection pump experience variable amounts of loading.

That is under heavy pedal the element will load more fuel and reach it's discharge pressure in the injector a little earlier. Plus as mentioned a longer period of injection.

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