GregBBRD

I'm guessing the budget for Mercedes to design, develop, and test this engine might be larger than most 928 owners are willing to spend?

Actually, the tool the builder used to check the two connecting rod's size and side play probably would exceed most everyone's budget....

Strosek Ultra

I have no practical experience from piston guided rods but doubt increased big end clearance will have an adverse effect on low rpm oil pressure. Like to see any tests telling us otherwise. Main bearings except the thrust bearing are all open at both sides having no restriction for the oil to "leak" out. I have heard but I am not sure Mercedes has used piston guided rods in many engines going way back as far as the 60´s. Here are some quotes from Speedtalk on this matter.

The flow through the bearing is determined by the radial clearance. Axial clearance just has to be sufficient to not restrict the exit. Piston-guided rods with very large side clearances are a common friction reduction trick in some applications. (Do a SEARCH for "piston guided rods".)

I have an old federal mogul bearing handbook and they explain normal oil flow with normal clearance, excess flow with excess clearance then also, excess flow with normal top to bottom clearnace but eliptical in shape and no where do they mention side clearance as it is largely a non-factor to oil flow unless dead tight or loose in combination with a very loose radial clearance.

Everyone worries about side clearance on rods and it letting all the oil out of the bearing, while completely ignoring that every main bearing except for the thrust has a metric ton of open space on either side and yet the rods are still getting oil to them.

Mr. Brown you being a very able guy I suggest you build an engine with piston guided rods and tell us how it works. We sure will get the result from Slate Blue when his engine is finished running.

Åke

Strosek Ultra

John, good to hear the hard bearings have worked well for you. Bad the company has closed down. Probably it is still possible to source your bearings for still some time at several distributors/dealers throughout the world.
http://www.aclperformance.com.au/stockists_w.htm
Åke

slate blue

I am pretty busy with our move, so I will keep the comments pretty brief and try to add to this in a week or so. As far as low oil pressure goes at lower revs/idle. There should be no effect on gauge oil pressure. Why? Well the oil pressure is not measured at the rods and we know that oil pressure is just resistance to flow. In the case of the bottom end, if you make the resistance higher in the rods, it stands to reason there will be less flow at the rods and more flow at the mains.

What we want is enough volume to lubricate and cool the bearings. What some call oil pressure, that is the pressure registered on the gauge, it is really just telling us indirectly that we have flow. Because we know if we have pressure we must have the flow. The critical lubrication/pressure happens when the oil is forming a hydrodynamic wedge. This oil film is very thin. Now what we do know is that engine oil viscosity has declined as better machining tolerances have been achieved and better engine designs implemented. Go back to the beginning of the ICE and the clearances were very large and the oil extremely thick to keep moving metal parts apart.

Then step right up to high end motor racing and you have Nascar engines running 20 weight oil in 870+ HP engines. They use precision bearings and super finished crankshafts. Because the surfaces of the bearing and crank can be separated by a very thin film, the engine lives. Albeit it would live longer with a thicker oil as the oil is designed more on a performance footing than a longevity basis.

In this metal oil metal sandwich, the oil pressure from memory is measured in thousands of PSI. It is how well you can make this work will drive efficiency in the friction levels in the main bearings. One thing I would ask people to think about is when a dry sump engine pulls a large vacuum what happens to the oil pressure? In this situation you have the pump both pushing and pulling the oil.

GregBBRD

I think the percentage of difference in power output between a piston guided rod and a crankshaft guided rod is very moot, at the level of tune we are dealing with, in our 928 engines.

Interesting thought about rod side clearance, on your part.

I wonder why Mercedes would even bother to pull up a multi-thousand dollar dedicated digital custom machine to specifically measure that side clearance, if it made absolutely no difference?

I wonder why every engine ever made that has multiple rods on a common journal spells out a maximum amount of side clearance?

I'll reword my statement:

I've always assumed that rod side clearance was a factor in oil pressure and delivery, however, I've never built an engine with "over specification" rod side clearance to verify this....and I will continue to specify and monitor this clearance.

Strosek Ultra

I am in the process of fixing up the engine of my wifes old Volvo 745 GLE 1986. From 720.000 km (450.000 miles) of service the engine internal wear has been found to be surprisingly limited. I was also surprised to see the engine do have piston guided connecting rods. The side clearance between the piston and the con rod small end is 0,25 to 0,45mm (.0010 to .018"). The total end play at the con rod big end is very large appr. 5mm (.200") and it has no influence on the oil pressure. This engine has always had very little oil consumption. Like for all CIS engines the camshaft has almost no overlap. It is well known that running a camshaft with much overlap does not work for a CIS engine like older 928 2V, at low speed and under low load pressure pulses from the exhaust side will interfere with the air metering plate making the engine to jerk and pull unevenly.
Åke

V2Rocket

Ake - is the pin press-fit to the rod in that engine, or floating?
Did Volvo use "pin buttons" to keep the pin centered in the cylinder?

Strosek Ultra

Spencer, the wrist pin is floating in both the con rod as well in the piston. The pin is kept in position by normal wire circlips. In picture standard parts to the right. By the way the stock 23mm wrist pin is very heavy and by turning it out internally I reduced the weight from 144 grams to 84 grams. The con rod small end weight was reduced by appr. 45 grams. Performed a proper balance job on the engine.
Åke

slate blue