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Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion > Diesel Performance Tuning

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  #1  
Old 09-15-2008, 01:32 PM
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Prechamber modifications

I have a 240D with 215K. I have turned up the pump quite a bit. I suspect has at least one bad prechamber. I am taking a look today to see. If I have a bad one I am going to replace them all. Has anyone ever opened up the orifices in the end of the PC? People have posted that they were going to try it but never posted the results.

I have seen picks of PCs with holes blown in them and the engine was ok. It smoked but the piston didn’t melt. I can imagine that the small orifices are acting like nozzles that create high speed jets inside the PC for better atomization and to mix in fresh air, but are a choke point for the expanding gasses to get to the cylinder to push the piston down. I also imagine that if one opened up the orifices that the gasses would expand the combustion chamber faster but the PC preinjection high pressure jet effect would be reduced. Plus if one went too far there might be structural problems as there has to be a tremendous amount of pressure in the PC during ignition.

I am just thinking “since I have them out” if there is an improvement to be had sort of thing.
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  #2  
Old 09-15-2008, 02:14 PM
ForcedInduction
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Quote:
Originally Posted by OM616 View Post
If I have a bad one I am going to replace them all.
Thats a complete waste of $300. If they are good, there is no reason to replace them. They are non-moving parts, unless a failing injector melted it or stress cracked, they don't wear out.

Quote:
Has anyone ever opened up the orifices in the end of the PC?
Not that I'm aware of. Even Mauri H. says "No need to modify them."
http://www.mersuforum.net/forum/viewtopic.php?t=44486
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  #3  
Old 09-15-2008, 04:03 PM
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I just checked them and they are all there. My thinking is that, barring a bad injector, if one failed, it would be due to fatigue after 30 years of heat cycles and harmonic vibrations and that the others would not be far behind. Mine are ok as per looking through the hole and poking the balls with a stick so I guess it is a moot point.

I would be curious what Mauri H. bases his opinion on. NOT THAT I AM SAYING HE IS WRONG!!! He may be very correct, so I can learn something, how dose he know? I am not familiar with the name so please pardon my ignorance.
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  #4  
Old 09-15-2008, 04:15 PM
ForcedInduction
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Originally Posted by OM616 View Post
how dose he know? I am not familiar with the name so please pardon my ignorance.
He is a major modder and racer in Finland's "Superturbo" hobby.

The Mersu forum has many great threads, you should spend a few hours looking through it. Even if you can't read Finnish (who can), there are some very good pictures scattered around the forum.

Last edited by ForcedInduction; 09-15-2008 at 04:43 PM.
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  #5  
Old 09-15-2008, 04:18 PM
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One of his creations. A ~400hp 300TD.
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Prechamber modifications-maurinew_24v_om606.jpg  
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  #6  
Old 09-15-2008, 04:35 PM
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Good enough for me!!

Thanks
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  #7  
Old 09-17-2008, 12:03 PM
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I've never understood why those holes are so small either. It must work out okay because the air is so dense that the small hole is not too restrictive. That is the only way I can rationalize it to myself. I know that someone one the Binderbulletin forum tried machining a larger hole in the prechambers of a Nissan diesel, and had difficulty with starting. That prechamber is completely different however.
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Old 09-17-2008, 12:10 PM
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Originally Posted by Bajaman View Post
I've never understood why those holes are so small either.
It is to generate high levels of turbulence by forcing the air out at a high velocity. The turbulence allows for a more complete burn of the diesel.
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  #9  
Old 09-17-2008, 12:11 PM
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Most auto diesels, especially VW, are more of a swirl chamber type. AFAIK, MB had the only true prechamber design.
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  #10  
Old 09-17-2008, 12:19 PM
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Ya. I couldn't believe the first MB prechamber I saw, being used to the VW single large hole units.

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  #11  
Old 09-17-2008, 03:54 PM
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Ok, is there a difference in the PCs for a 616, 617NA, and a 617 Turbo?
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  #12  
Old 09-17-2008, 10:19 PM
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yes
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  #13  
Old 09-17-2008, 11:19 PM
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If you were to take the total area of the holes and compare them to the cross sectional area at the end of the flame tube, you would probably find that they are almost the same. The holes by them selves are not to accelerate the gasses they are there to direct the gasses to create swirling and mixing in the main combustion chamber. The acceleration is supposed to be done by the flame tube.
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  #14  
Old 09-18-2008, 01:22 AM
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I shall rephrase the question. What are the differences in the PCs used in the 616, 617-NA, and 617 turbo.

For context purposes, if one was going to build a custom 616 (turbo), would there be a particular PC that would be better to make the most HP?
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  #15  
Old 10-29-2008, 01:20 AM
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OK, I just received my MB engine manual and after a quick look I see that MB made changes to the cam and the PC burn hole diameters and eventually the PC burn tubes & pistons to get more power out of the early 615 & 616 engines. They refer to the changes as uprated.

Now I am having a difficult time determining exactly the point in time each of the upgrades to the 616.912 were made because both the normal power and the uprated power engines are identified as 616.912.

As I can determine the early 615 and 616.912 engines used a type 1 PC with an angled & flat burn tube end. The early 615 had a burn hole dia @ 2.6mm that was increased to 3.0mm at some point to make power. I think that the 616.912 PC featured the increased burn hole dia (3.0mm) from the start. But at some point the 616.912 was uprated and given a different cam and type 2 PCs that have a rounded end and require a different piston. The type 2 (uprated) PC has even larger burn holes @ 3.2mm & 3.5mm. I do not see how to determine if a 616.912 is a normal power or an uprated engine other than pull the PCs or perhaps measure the cam lobes. They reference a chassis number but if the engine is out of the car or has been changed that is of little help.

I am planning on putting a GT2056V turbo on my 1978 616.912 and I am trying to figure out what my options are to get the most out of the engine. I could live with changing to the uprated camshaft, vrs having one reworked but the uprated cam needs shorter valve guides. So if I have to change the guides I might as well have the cam I want made, but if I have the flat ended PCs, I would need to change the pistons in order to allow for the uprated PCs.

Obviously it would make sense to start with an uprated engine. They are the higher power 616.912s.
Question: so if there were five engines lined up on the floor and one was uprated how would you pick it out?

Question: So, since MB's opening up the burn holes by .4mm provided a power increase, ( to address the early 615s low power) and since the type 2 PC have even larger burn holes ( and more power) than the bored type 1, could some one explain why increasing the hole dias another .5mm or so would not be beneficial?
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