Question For Barry: Fuel Pressure & Advance:

[Doktor Bert]

Barry,

Since you are the pressure king, I wanted to ask you this. John at Advanced Diesel Systems in Fresno, California, told me once that fuel pressure will not affect timing. They are a Bosch dealer and service center.

But, if you consider that a 135bar injektor, releasing or spraying at only 100bar, would, in theory, advance the timing so to speak, by introducing fuel earlier.

What are your thoughts on this???

I ask because I am getting an idle-only, clattery injektor on my #1 zylinder and I suspect it is discharging at a lower than specified pressure.

Thanks...Robert

P.S. I have 68,000 on my complete rebuild with Bosch nozzles and pressures set to 2000 psi.

[barry12345]

Well I am not the pressure king unless the pressure the wife generates from time to time is factored in. Actually she is a great wife yet there are some simular typical traits amongst all her specis that they share in common. For example . I have been the reciepient of several verbal reminders that I was negligent at taking a pill I am to take with supper. Each reminder seems more insistant. So I better go take the pill.

Back to your injector. What would be more concerning than just the the injector opening earlier. Is the possibility of more actual fuel being delivered from that element.

I assume and it is always dangerous that once an injector opens the fuel in the element is used until the pressure in the element drops below the close off pressure of the injector. If the injector is opening earlier and closing off later. This is factually what an injector with a signifigant lower pop pressure might do in my opinion.

I guess with a little effort I could still do all the math. But it would be a pain. Generally though any issue you have if from this source would be caused by the increased volume of fuel in combination with the earlier start of injection I suspect. Additional fuel being more problamatic than the timing shift perhaps or a major contributor to it. .

Could it cause a rattle or whatever is beyond an absolute answer by me. It might. What is certain. If it is as low popping off as you quote it is certainly not ideal.

I would also expect that cylinder to be running somewhat hotter. Simply because of the slight timing advance combined with more fuel being used. . The reduced injector spray pressure resulting may be detrimental to the actual spray pattern as well.

Things I do not know but might speculate on. Does a sub standard pop off injector open wider? If so does it dump the elements fuel quicker and shuts off earlier. If this were so it would tend to further increase the timing advance as more fuel would be processed before the mid point of injection.

Some other member out there will know far more than me I believe. They hopefully will post. The more we learn in this area the better.

[Doktor Bert]

Interesting....I am planning on swapping in a spare set of good used injektors and then send these out for balancing, etc...

[barry12345]

Originally Posted by Doktor Bert

Barry,

Since you are the pressure king, I wanted to ask you this. John at Advanced Diesel Systems in Fresno, California, told me once that fuel pressure will not affect timing. They are a Bosch dealer and service center.

But, if you consider that a 135bar injektor, releasing or spraying at only 100bar, would, in theory, advance the timing so to speak, by introducing fuel earlier.

What are your thoughts on this???

I ask because I am getting an idle-only, clattery injektor on my #1 zylinder and I suspect it is discharging at a lower than specified pressure.

Thanks...Robert

P.S. I have 68,000 on my complete rebuild with Bosch nozzles and pressures set to 2000 psi.

At this point in my mind higher base fuel pressure will not change the timing relationship much between the number one cylinder and the pump. Why I feel the area is still open to discussion.. I have no knowledge if a pump shop has actually compared everything at elevated pressures. I even wonder if they had a variable pressure feed pump in the old days. Even today these mechanical pumps are calibrated using the original lift pumps as the fuel pressure source. First they make sure the base pressure is in the ballpark before starting calibration is a reasonable assumption.

I suspect higher base pressure will change the sequential timing between elements or the amount of fuel produced will change on a per element basis a little. This is all based on the lift pump recharging only once per injection pump revolution. All the internal reactions mean that between lift pump cycles things change from a supply perspective. Changing the supply pressure changes other things. Those heavy element cut off pulses occuring inside the pump influence things a lot in my opinion. How much seems to be the issue with increased base fuel pressure.

Otherwise why does the Injection pump manufacturer want the pump calibrated to a specific base or feed pressure? What I do not know and so far only can speculate is that much higher base pressure may modify the sequential timing profile between the elements somewhat. Or change the amount of fuel delivered by each element in comparison to the others. Or even change the profile of the fuel release by the injector. It does and has to change something. One has to allow not withstanding how expensive the equipment is to deal with these mechanical pumps. The equipment may stil be somewhat crude by todays technology standards perhaps. These older indirect injection engines had a lot more lattitude to operate fairly well in comparison to a truly modern direct injection diesel.

There may be enough element adjustment range to compansate properly for this higher pressure feed. Setting up and calibrating an injection pump at say thirty pounds base supply pressures has not been done yet to my knowledge by any member. The pump manufacturer suggests around 15 pounds supply pressure with 19 pounds being both benificial and used by some calibration shops apparently.

The point? I have not put everything in this area to bed yet in my mind. These car are quite perky at thirty pounds supply pressure some have reported. If nothing is changed profile and fuel per element we should all be there. Unfortunatly even with nothing conclusive at this time it is suspected that things are changed a little at least.

Or if thirty pounds supply pressure is effective. Fourty or fifty pounds might even be more effective. What I really should do is get involved with a pump rebuild shop. Compare all the elements output and timing at various feed pressures. Calibrate the pump for thirty pounds pressure and take it home. Hopefully if not driven harder the fuel milage would remain the same.

The harmonic balancer design is too weak as is to tollerate much increase safely in timing. I again suspect without knowing the european diesel hot rodders are doing something to overcome this limitation. Nobody in their right mind wants a spun balancer on a 617. I just hope they have not developed a quality repair or easy modification that we in north america know nothing about...

[bdblk12v]

I'm no injector expert. I belong to a forum competition diesel. I have a 1994 dodge cummins. I know that setting pop pressure does in fact change timing. Most everyone in the diesel performance side does up the pressure. I can say from experience changing pop pressure is a good thing. I had some stock injectors with low miles at 260 bar. They idled horrible and wasn't running good. I had a new set of nozzles 200hp over stock set at 290 bar. It idles way smooth and runs great. Fuel pressure does play a large role too in the cummins inline pump. Stock pressure is around 20psi. The higher pressure helps fill the pump plungers. General 50psi or so at wot is a good rule of thumb. Hope this helps.

[barry12345]

Originally Posted by Doktor Bert

Interesting....I am planning on swapping in a spare set of good used injektors and then send these out for balancing, etc...

Why just not change out the number one injector with one of your spares and see how things sound first? I was thinking that being so close to the front of the engine it may be something else in that area. Sounds can be deceptive sometimes. Although you have a lot of experience.

[barry12345]

[QUOTE=bdblk12v;3053565]I'm no injector expert. I belong to a forum competition diesel. I have a 1994 dodge cummins. I know that setting pop pressure does in fact change timing. Most everyone in the diesel performance side does up the pressure. I can say from experience changing pop pressure is a good thing. I had some stock injectors with low miles at 260 bar. They idled horrible and wasn't running good. I had a new set of nozzles 200hp over stock set at 290 bar. It idles way smooth and runs great. Fuel pressure does play a large role too in the cummins inline pump. Stock pressure is around 20psi. The higher pressure helps fill the pump plungers. General 50psi or so at wot is a good rule of thumb. Hope this helps.[/QUOTE

Depending on the dodges injector pump design may make a signifigant differance. Personally I have not advocated going beyond nineteen pounds supply pressure with these older mercedes diesels.Since there are still a couple of unknowns.

If they are signifigant or not is still the question. We have a lot of new site members that would perhaps do things without thinking them through. I still think the time will come when we regularily increase the fuel supply pressure. It is pretty certain that really low fuel supply pressure can hurt things. Especially on the 616 four cylinder engine over time. That engine will run quite well at very low substandard base fuel pressures. Unfortunatly doing it can have a serious price attached to it over the long term. I have to add that is my opinion. As times move along I feel more and more certain about this. Thanks for your input. The more people share things the more we learn. These cars are very old in comparison to the dodge trucks with their cummings engines. So a lot more effort is stressed on more fuel system maintenance types of issues.

I do advocate checking the base fuel pressure on the 616 engines as a cost effective method of reducing the risk on the number one rod bearing parting company. A permanent fuel pressure gauge may be that models best friend. A twenty dollar expendature for all the parts and gauge. Less so on the five cylinder 617s. Yet to me still a factor.

I am also far from being an injector literate person. We on this site have not yet discussed upping the pop pressures. We have examined increasing the fuel supply pressures some..Actually it is still at the stage of suggesting that people just check what they have for being adaquate. Low substandard fuel supply pressure existing gives many issues.

[Doktor Bert]

Originally Posted by barry12345

Why just not change out the number one injector with one of your spares and see how things sound first? I was thinking that being so close to the front of the engine it may be something else in that area. Sounds can be deceptive sometimes. Although you have a lot of experience.

It is definitely combustion related. I have noticed the noise is more prevalent on the 'no name' diesel I sometimes have to buy. The noise disappears completely at about 950 rpm.

[Doktor Bert]

Barry,

What is the specified length for the relief spring????

Do you have a revised or preferred setting based on your work????

[qwerty]

Originally Posted by bdblk12v

I had some stock injectors with low miles at 260 bar. They idled horrible and wasn't running good. I had a new set of nozzles 200hp over stock set at 290 bar. It idles way smooth and runs great.

So…does the credit go to the higher pressure or the new nozzles?

[barry12345]

Originally Posted by Doktor Bert

Barry,

What is the specified length for the relief spring????

Do you have a revised or preferred setting based on your work????

I believe the springs might have originated from the suppliers of the relief valves in two lengths. Perhaps two suppliers? So the length to seek depends. I am not positive of this. Yet there seems some conflict.

I cannot believe that some of the springs sag to the exact same length. I guess it is possible though.

Actual pressure readings obtained with a gauge are a better methology. If one is aware of basically how everything impacts each function. I think the typical case is a person might stretch the spring back to whatever normal is. Without observing there was fuel overflow first or after.

The valve was not active because the system operational pressure was not adaquate. So no change is observed. As nothing has really changed.

Or the system may process enough fuel volume at idle and fall much flatter at higher speeds. So we in fact have absolutly no ideal of what some of the stretched springs release pressure really is.

Otherwise just stretch the spring to the manufacturers suggested length to compensate for the ageing sag. You should really have a gauge installed to go beyond normal.

[suginami]

I just have to say......I graduated Summa Cum Laude with one undergraduate degree with two minors, along with a graduate degree, and you guys are smarter than me.

How did you guys learn all this stuff?

[Doktor Bert]

Originally Posted by barry12345

I believe the springs might have originated from the suppliers of the relief valves in two lengths. Perhaps two suppliers? So the length to seek depends. I am not positive of this. Yet there seems some conflict.

I cannot believe that some of the springs sag to the exact same length. I guess it is possible though.

Actual pressure readings obtained with a gauge are a better methology. If one is aware of basically how everything impacts each function. I think the typical case is a person might stretch the spring back to whatever normal is. Without observing there was fuel overflow first or after.

The valve was not active because the system operational pressure was not adaquate. So no change is observed. As nothing has really changed.

Or the system may process enough fuel volume at idle and fall much flatter at higher speeds. So we in fact have absolutly no ideal of what some of the stretched springs release pressure really is.

Otherwise just stretch the spring to the manufacturers suggested length to compensate for the ageing sag. You should really have a gauge installed to go beyond normal.

Where are you plumbing in for the readings?????

[Doktor Bert]

Originally Posted by suginami

I just have to say......I graduated Summa Cum Laude with one undergraduate degree with two minors, along with a graduate degree, and you guys are smarter than me.

How did you guys learn all this stuff?

Not smarter, just lots of trial and error. The first Mercedes-Benz diesel I worked on was given up on by another shop and I became enchanted with the idea of fixing it. I bought one soon thereafter....