Ideal Oil Pressures

[kradicke]

What are the ideal oil pressures, or a way to calculate the ideal pressure, at particular points in the lubrication system? For example, what kind of pressure would be ideal for the rod bearings and main bearings?

I ask this because I recently read some information that reinforces what I have seen on my engine bearings that indicates that on Triumph inline 6 cylinder motors, the front of the crankshaft, and the bearings associated with the front of the crankshaft, see minimal to no oil pressure at high RPMs. GT6Steve, if you're reading this, please chime in... as it is your particular car that is featured in Kastner's book about this subject.

Thanks,

Kai

 

[willeng]

kradicke:
There is no real ideal oil pressure it's a matter of having enough to handle the application.
For the 6cyl triumph run 65lbs min to 75 lbs for racing purposes & run external pressure lines direct to the main gallery plugs along the side of the block.
Make sure the rear cam journal is drilled to relieve the pressure from the rear plug other wise it may blow out.
Don't over rev when cold the excess pressure has a habit of blowing original oil gauge line when cold.

 

[kradicke]

Willeng,

Thanks for the info, much of what you said is already pretty well set in stone as far as racing Triumph engines... I just had not known the severity of the issue. I was more or less looking to see if there is some minimum standards for the amount of oil to the bearings.

While I understand that this question is largely application/load specific, there must be some sort of minimum associated with the longevity and proper functioning of standard style half shell automotive bearings.

Kai

 

[Tmoose]

One of the requirements is to make the oil flow thru the crank, that is, from the main bearing, into the main journal, and out to the rod journal. The first inch or so of the journey in the crank main journal is against "centrifugally" pressurized oil. Higher crank speeds have higher "fighting" pressure. Look at how Porsche introduces oil into a small diameter journal at the front of a 911's crankshaft.

http://vista.pca.org/ksc/images/loos/crankshaft.jpg

Of course enough oil must flow thru the bearing to cool things off. big diameter bearings spinning fast can heat themselves up, requiring more oil flow for cooling. The extra flow can, within limits be obtained with any combination of higher pressure and internal streamlining. If the crank/block oil holes are too small, or contorted, then satisyingly high oil pressure may not be a good indicator of "enough" oil flow .

In a semi-modern Chevy V-8 engine 10 psi/1000 rpm was considered OK.

 

[clshore]

This relates somewhat to a thread about viscous flow through an orifice. I forget the 'Law' quoted, but the laminar flow model is used to derive a mathematical relationship between the orifice diameter, the viscosity of the fluid, the pressure, and the temperature.
But derived differently, you can find out the flow through a journal bearing. To be meaningful in the real world, you probably need to account for the fact that the journal surfaces are not concentric, and that they are in relative rotational motion.

You could also conduct experiments to obtain results.

In Triumph Spitfires, it's always conrod #3 that goes bad first. I'd love to be able to instrument and record once and for all what's happening at the bearing.

Carter Shore