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...