Con rods

[DAVE BRADY]

I must admit that I am not an engineer but I think I have grasp of things and can understand principles when explained for the layman.  So here is something that is quite interesting and I think I understand it.

"Oil Pressure Vs Flow

Optimal oil flow with the correct pressure ensures optimal cooling and lubrication.
Oil pressure is to be understood as the resistance in the engine which the oil overcomes to establish a certain flow rate.
The most important criterion for good oil lubrication is optimal oil flow and not oil pressure. Low oil pressure is not necessarily bad, in fact it can be beneficial to some extent. High oil pressure is not necessarily good, in fact it can be a cause of concern.
Oil pressure is not built by the oil pump, but by forcing oil through small passages. Small oil passages with high viscosity oil will have low oil flow rate, whereas large oil passages and low viscosity oil will have a higher flow rate.
A fast flow rate with lower oil pressure will result in minimal wear and optimal cooling. A low flow rate with high pressure will result in a less efficient lubrication and cooling and therefore more wear on the engine.
Too high an oil pressure can result in failure of seals and leaking of oil and should therefore be considered as an indication of poor flow.On a cold start the oil viscosity is high (oil is thick) and the oil pressure is therefore higher. As the engine warms up, the oil viscosity decreases (oil becomes thinner) and the flow rate therefore increases while the oil pressure drops and stabilises. When the oil pressure is stabilised, the oil flow is at its optimal rate and the wear on the engine is minimal.
Under ideal circumstances, the oil pressure should stabilise soon after startup. Any sudden larger increase or decrease should be investigated."

"How Fluid Film Bearings Work

Operating Principle

In a fluid film bearing, the stationary and rotating surfaces are separated by a thin film of lubricant, such as oil, air, water or process fluid. In a hydrodynamic fluid film bearing, the film pressure that separates the surfaces is created by the relative motion (rotation) of the surfaces as the lubricant is pulled into a converging geometry between the surfaces.
No contact of the surfaces takes place except during start-up and shutdown."

Are the big ends 'fluid film' or 'hydrodynamic film'?  Also, it looks like the most vulnerable time is on startup especially after a time of inactivity.

Dave.

[Bess]

Hi Dave,

Interesting read and you haven't convinced me. Bearing in mind we are talking about a motorbike whose oil passages are as designed and matched to its pump. It's also fitted with an OPRV to stop high pressure. I guess that passage is for design engineers not operational engineers.

Here's my thinking:

The size of the oil pump is designed to supply the correct amount of oil to meet the engines needs. I believe the standard A65 is 1.4l/s (quite low compared to other marques), the SRM unit delivers more.

When the engine is new, the clearances are tight and the pressure is as specified and you will get fluid film. As filters block, OPRV galls, oil pump wears and the bearings wear, clearances increase and resistance to flow decreases dropping oil pressure.

When the pressure drops you get an idea something is wrong with the engine. With the increased clearances the pressure drops and the flow from the pump increases until it eventually reach its maximum delivery. You need indication something is wrong hence the need for a pressure switch or better still a gauge. A flow meter is not practical on the A65, also some people will think the engine is getting better when the flow increases because they believe flow is more important than pressure.

An example, if the main bush is worn the flow under pressure will find the least line of resistance along its edges, the pressure will drop and starve the big end journals. In this case the journals are subject to centrifugal forces so the oil is flung out you will get some hydrodynamic film before the bearings fail.

In the engine, generally flow stays the same but reduction or loss of pressure has many influences all of them likely to damage the engine.

Therefore I still believe pressure is more important than volume as volume is the constant, pressure is the variable. You need pressure to push the volume to the bearing.

Best wishes...

[DAVE BRADY]

Hi,

Perhaps a bit of dead thread but I did put the oil pressure gauge on my high mileage A65 and I went out on a ride today.
Only about 40 miles and not the hottest of days but interesting non the less.

Starting pressure 55psi so quite good and almost certainly activating the OPRV when revs increased.
Once warmed holding steady 50psi but increasing with revs to OPRV activation
Little drop in pressure at slower speeds
Tick over pressure back home of 25psi

So on a really hot day and after a longer distance the pressures may be less but not bad for a bottom end that has done probably over 120,000 miles.
My conclusion is that I have the best of both flow and pressure for the big ends.  Flow from the end feed and pressure from what must be good pump.
How long will it go on for?  We will have to see as I do not intend to strip the bottom end unless something tells me I really need to.

Dave.

[Bess]

Hi Dave,
             

We will have to see as I do not intend to strip the bottom end unless something tells me I really need to.

   Like low pressure? 

Regards Bess...

[DAVE BRADY]

Perhaps low pressure but I am taking the pressure gauge off again.  I wonder how many miles more?

Dave.

[LightningAndStarfire]

Really impressive readings for such a high mileage.

[DAVE BRADY]

Definitely and I cannot claim any credit as I did not build the engine.  The head has been overhauled and the carb replaced as the Amal concentric completely wore out.  other than that all I have done is change the oil and filter and routine maintenance.  I have replaced the primary chain once and the clutch friction plates once.  I suspect that I will be replacing these items again quite soon.

Dave.