Glycerine filled 0-30 pound mechanical gauge used at idle on base fuel in injection pump. Fairly steady but needle vibrates usually. It is doing a good job at averaging things. The high pressure spikes are so fast an event in duration it can almost totally ignore them.

Mechanical 0-30 pound non dampened gauge at idle.
it does a better job of tracking actual activity in the base of the injector pump. It will try to destroy itself. It of course is still trying just to average activity as well. You cannot usually read it as it tries to destroy the needle against the stops. If nothing else this proves there is not one constant pressure in there. Instead there are quite dynamic events happening. I was getting a little concerned the common viewpoint was that of a simple fixed pressure.

Lift pump is a constant pressure pump. Pressure it supplies to the mix still falls away through the injection pumps cycle until it restores itself again after all the elements have been serviced sequentially. We cannot measure this event. The higher the indicated base pressure the higher the flow/ steeper decline profile as injection pump overflow is greater.

If this is true the injection pump will not remain properly calibrated with a fixed maintained source pressure replacement for the lift pump. It must be calibrated to existing conditions.

When this injection pump is calibrated at a service shop I would think either its own lift pump or a substitute of the same item is probably used.

If the lift pump is substandard. It to me is reasonable to assume the injection pump calibration is no longer on its original lift pump profile. Or that portion missing of the actual real senario in the base of the injection pump is changing things.

Up to 19 pounds lift pump pressure all is well or close enough. Injection pump is still close enough to its probable originaly intended calabrated profile. Higher pressures may change that to some extent.

There is either no more or less smoke usually reported from people elevating their lift pump pressure to 19 pounds and verifying feed system has no abnormal restrictions.

This is a solid indication we are just not dumping more fuel in. The smoke would be greater. We are in fact of course getting more fuel in though. My suspicion is we are getting it in more equal quantities per injector as well.

Now we come to the real issues. Done properly fuel milage is increased over previous much lower feed pressures. Same or less smoke is produced, power is up. So it is a given that efficiency is really improved. . This of course has to mean the apparent timing between elements combined with/or overall timing has been improved.

When trying to understand what is really occuring in the base of the injection pump there are lots of events. Intense pressure spikes. Integrated with the lift pump supply that is not in itself of constant pressure. Combine this with almost instantainious reflected pulse waves etc.

So if all this is so it is perhaps better just to get the lift pump operation and any fuel supply restrictions dealt with by installing a pressure gauge and call it a day.

I feel and have felt that what is occuring in the base of the injection pump may be too complex to easily understand totally. The need to know is not absolute to our needs either.

If you cannot sustain a proper base pressure at cruise. Either the lift pump is tired or restrictions exist.
if these are dealt with properly the car should produce less smoke. Get the optimum fuel milage possible. Allowing for the other conditions that may be present and typical of a very old mechanical piece of equipment. The same logic applies to equalising power strokes as much as practical in a given situation.

My viewpoint the number one rod bearing is worked harder at substandard injection pump base pressures is still a healthy suspicion of mine and will be proven one way or another by testing eventually I believe.

These same engines may have been pushed harder in certain european countries and the intense heat of the middle east. The 616 and 617 engines where in production too long for mercedes to have not improved the oiling supply to the number one bearings if it was indicated. Back then their engineers where pretty conciencious. They suggested their engines could be run wide open without harm.

Their high quality oiling system was not modified to my knowledge from the introduction of these 616 and 617 engines from start to finish of production.

The 616 seems to experiece a greater percentagewise number one rod failures than 617s. Yet the oiling path to their number one bearing is shorter. Now if we can believe that apparent element timing/or and/fuel quantity in relationship to base pressure is involved. Combined with fewer power strokes/ Less frequent events. It is starting to become interesting.

The design stress is almost certainly less with no turbo and normal loading on the 616 per cylinder. The rear end ratio is higher to compensate somewhat for a simular weight car as the 617.

The 617 has a turbo usually, longer oil feed path to the number one bearing. Higher average power loading per cylinder still with the lower rear end ratios. Yet the number one rod events seem infrequent compared to the 240d on site and the 616 certainly is found in much lesser numbers.

More of my speculations prove nothing really on the rod issue. There are more. I just preffer to make sure any 616 I own is going to have proper functioning injector pump base pressure. It costs next to nothing to have and has no downside I am aware of.