Reliability on turbo 86

[gtengr]

Originally Posted by 86MLR

What is the effect on bearings at high load and low RPM, lets say for example 3k rpm and 300nm, as apposed ti 6k rpm and 300nm.

Exponentially increased inertial forces at higher rpm in combination with a reduction in film strength due to the increased shear rate (bearing is spinning faster on the journal). The common rule of thumb with oil pressure is that the supply should be on the order of 10 psi per 1k rpm. What is the equivalent rule of thumb that relates high torque to oil pressure and low rpm?

[gtengr]

Originally Posted by Irace86.2.0

. In fact, Delicious Tuning has meantioned in threads that they tend to tune for less low end torque, even on the superchargers, in order to protect the rods. They offered the same to me or to go more aggressive, and I opted to be conservative for my SC. As mentioned before, the source of why low end torque is an issue is due to a higher likelihood of preignition and lower oil pressures. These are reliability concerns that transcend the track world into everyday driving situations.

I'm lost as to why you keep bringing the rods back into this bc i've not posted anything to the contrary to the above when it comes to rods. The motors that are breaking at low rpm high torque are generally breaking rods though, not spinning bearings.

[86MLR]

Originally Posted by gtengr

Exponentially increased inertial forces at higher rpm in combination with a reduction in film strength due to the increased shear rate (bearing is spinning faster on the journal). The common rule of thumb with oil pressure is that the supply should be on the order of 10 psi per 1k rpm. What is the equivalent rule of thumb that relates high torque to oil pressure and low rpm?

As you stated, lower rpm = lower oil pressure.

The journal at the rods, due to low pressure and high load, can "push through" the oil layer at the bearing.

Therefore under high load there is a draging effect at the bearing.

This dragging effect can and does contribute to wear.

Dragging at the bearing can and does contribute to rod failure through excessive plastic deformation of the rods.

By excessive I mean more plastic deformation that the rod was engineered for.

[gtengr]

Originally Posted by 86MLR

As you stated, lower rpm = lower oil pressure.

But you're saying it's the excessive torque. So again, what is the numerical rule of thumb that factors in the torque?

Originally Posted by 86MLR

Dragging at the bearing can and does contribute to rod failure through excessive plastic deformation of the rods.

By excessive I mean more plastic deformation that the rod was engineered for.

Rods are not designed to sustain any plastic deformation in service. Plastic deformation on a rod means it's buckled or the big end has gone out of round. The rods are not engineered to last under either condition in the context of high performance street engines.

Also, the overlay surface of rod bearings are made of a softer material than the rods, and would be toast before the rod if the oil film is compromised to the point that the bearing iould cause localized plastic deformation on the rod.

I understand what you're saying about skimming the bearings. I do. I still disagree citing the qualifiers I listed earlier about extreme torque/lugging the engine. There are a few people on here who have broken rods and had serviceable bearings. I maintain that in general, unless one is treating their engine like an idiot (e.g. lemme try to hitting max boost in 6th gear at 2000rpm), then the idea that low rpm torque is as significant as high rpm inertial loading when it comes to oil pressure is bunk.

[Grady]

Originally Posted by 86MLR

Dragging at the bearing can and does contribute to rod failure through excessive plastic deformation of the rods.

By excessive I mean more plastic deformation that the rod was engineered for.

I thought the rods were made of steel? No wonder they bend so easy!

[86MLR]

Originally Posted by Grady

I thought the rods were made of steel? No wonder they bend so easy!

Material testing is cool.