Interesting points....

In a hydraulic system (which is how the IP operates) I am not convinced that the supply pressure has that much to do with the high pressure side of the pump. However, a reduction in available volume could indeed create low, high pressure delivery.

Now if I recall Pascal's law, all points in a hydraulic system will always see the same pressure and two-stage pumps can produce high pressure with relatively low volumes of fluid. However, an injektion pump, at least on the high pressure (delivery side) is a single stage unit.

Now, one could easily prove or disprove this theory by adding a pressure gauge to the low pressure (supply) side of the IP and then watch the timing (electronically) while you modulate both supply pressure and volume.

experiment anyone???

Now, in the case of the GM 5.7 litre and 6.2 litre diesels, both using the Roosa-Master Rotary Injektion Pumps, they will produce the same delivery pressure regardless of the presence of a lift pump or even any degree of supply pressure. I have verified this on the test stand during calibration.

These IP's use a 6 to 7 psi supply from a mechanical fuel pump. The test stand will allow you to supply the correct inlet pressure via and electric pump, but you can also run the pump by sticking the suction hose in a bottle of fuel and either way, the delivery pressure doesn't change, so long as the high pressure side of the pump has an adequate supply of fuel, free of bubbles.

Now, I am not certain that an inline IP will react the same, but it would be worth experimenting with. I have a very good friend who owns a shop that calibrates these MBZ/Bosch inline IP's, so perhaps I will ask him this question today and see what his answer is.

Now, onto the rod bearing failures....

Based on my extensive experience in competition motorsports and engine building, I would like to point out a few things here. While this applies directly to competition engines, the principles are basically the same.

Let us assume that a given connecting rod bore is indeed round and that the crankshaft journal is also round. Let us further assume that the working clearance between bearing and journal is within acceptable limits, which is generally .001" for every 1.00" of journal diameter as a rule.

Let us further assume that this bearing/journal interface will receive a continuous flow of clean oil, with sufficient hot viscosity to maintain acceptable pressure, which is generally 10psi for every 1000 rpm of expected engine operation.

remember, oil flowing across the bearing is what cools it. That's why we open up the rod to crank journal side clearances in racing engines!!!!

If these factors are satisfied, the journal, in theory, would last until normal wear increased the clearances, oil pressure/volume (cooling) was reduced and wear increased. Eventually, pressure, film strength would be reduced until metal-to-metal contact occurs, which quickly destroys the bearing surfaces.

Notwithstanding normal wear, what destroys rod bearings??? One of the #1 killers of rod bearings in a competition engine is heat and detonation. Of course, we are speaking of a gasoline engine her, but let’s compare the two anyways.

High temperature and/or premature detonation of the fuel/air mixture creates very high combustion temperatures. Detonation also ‘hammers’ on the head of the piston with tremendous force. This force and heat quickly defeat the oil ‘cushion’ and allow metal-to-metal contact to take place between the bearing and journal.

In competition events, I have seen people experience continued rod bearing failures and try increasing oil pressure and/or viscosity, never realizing the problem is a too lean air/fuel mixture, detonation and high piston crown/exhaust temperatures. Some people I have spoken to simply found it inconceivable that heat could travel down the connecting rod and kill the bearings!!!

I have seen a Duetz Air Cooled Diesel seize a piston because of a clogged injektor deprived the piston of colling!!!! Keep in mind that in a direct injektion engine, this is a large part of the piston's cooling!!!

In a gasoline engine, detonation is destructive. However, in the 617 engine for example, the fuel is detonated on a regular basis; hence the more rugged construction required for diesel engine operation. The spray jets cool the piston crown, which not only saves the piston, but keeps the rod bearings cool!!!

But what mechanical pitfall could contribute to a rod bearing failure??? Now, let me use the Pontiac V8 as an example. The Pontiac V8, which I still build from time to time (along with other makes of V8’s) is a very rugged engine. However, they have a nasty reputation for #1 and #2 rod bearing failures. This was blamed on the cast steel rod for years, but the problem is much deeper than simple metallurgy.

On the Pontiac V8, the rod bearings are fed by oil feed from the main bearings, much like the MBZ engine and this is common to most all engines, except for some competition only variants, like the Ford 427 ‘Side oiler’ which used special oiling system.

Most engines use a main bearing with ½ of the surfaced grooved for improved oiling to the rod bearings. As the crank is spinning, the oil feed hole to the rods only lines up with the oil groove for a given number of degrees in the crankshafts’ rotation. In theory, although the pressure is always the same, there is a reduction in volume during this period of time that the oil feed isn’t aligned with the oil groove. If everything is right mechanically, then this isn’t a problem because the rod bearing is oiled during the compression stroke and early part of the exhaust stroke, where the bearing sees the greatest load.

The problem comes in when the main bearing bores are out of round, which is a very common problem on many engines that have years and years of service. When the bearing bore is egg-shaped, it reduces the volume of oil allowed to the rod bearings. I have found this condition present in every engine where a rod bearing failed, if no other aggravating conditions, as previously described, were present.

When I built my 617.120, the #1 main bearing bore was over .003” out of round and every bore, except the rear main bearing bore, needed to be align honed!!!!

The load on the spinning crankshaft is not constant. In fact, the crank actually turns in an arc, if you consider the degrees of rotation where it sees load. The reason a V-12 sounds like the spark plugs have been removed when it cranks is due to the closeness of the firing pulses in degrees of rotation. It is also why they run so smoothly.

The load on the crankshaft eventually forms the main bearing bores out of round. The condition is much worse if the engine is ever allowed to ‘knock’ from a bad bearing. This works on the perfectly round bearing bore like a jack hammer. The same is true for connecting rods. I resized all of my rods and had the block align honed prior to assembly, because there was so much out-of-roundness found during inspection.

In a running engine, we cannot check for this, but we should check for it during inspection and prior to reassembly following w rebuild.

For the running engine, clean oil, preferably one with a high ZDDP content, which protects the bearing/journal interfaces during cold starts/metal-to-metal contact, good quality oil filters and control of engine temperatures and RPM are sensible precursors to long engine life.

I hope you find this information helpful…Robert