Nissan 370Z Forum - View Single Post - Potential issues for hot upcoming track day?

clintfocus · 07-01-2013, 01:48 AM

Quote:

Originally Posted by cossie1600

I don't doubt you one bit that the rod bearings might be soft, but I highly doubt it is as critical as you make it sound. More people suffered from fuel starvation/clutch,ps,brake fade than rod bearings failure. I am guessing 50 cars on this forum had gone to the track more than once, the failure rate is less than 5 cars?!? I will take that odd anytime.

threeseventy, mine might fail, but it hasn't after 2000+ miles on the track and 500 autocross runs. I will keep going back at it unless something proves otherwise. I have never hit things at the track either, but it doesn't mean I won't. I hope it doesn't happen, but you don't know.

You guys with modified cars/engines have different requirements than the guys with stock cars. What's to say the failure was caused by the rod bearing and not your mods or tunes. Track conditions are rough on the engines, but most modern cars are smart enough to prevent itself from blowing up.

I snagged this from a write up my good friend Mike Kojima wrote, since he explains it better then i do regarding Nissan's engine bearings because of his race engineering backround. He used to work for Nissan and has tons of motorsports experience with Nissan platforms.

Also, mine and threeseventy's cars are no more modified then most of the people here. Simple bolt on power mods on stock motors.

"The most oil temperature susceptible parts are the engines crank and rod bearings. The engines bearings are made of soft metals, such as aluminum, tin and zinc with other trace alloying agents such as indium which refines the metals grain for better mechanical properties. The reason why soft metals are used is that they offer good embedability. If hard contaminates are present in the engines oil such as casting sand, metallic particles, hard carbon bits and dirt, a soft bearing surface will allow the particles to become embedded into the surface of the bearing where the damage to the bearing and crankshaft can be limited. The contaminating particle will be soaked up by the soft bearing instead of being ground into the hard steel journal surface of the crank with damaging effect.

Soft metals can be used as engine bearings because under normal conditions, the crankshafts journals never touch the bearings surface. Since the liquid oil layer is not compressible, the crank rides on a pressurized hydrodynamic film of oil a few thousands of an inch thick that is maintained on the bearings surface by the engines oil pump. The oil pump must maintain pressure (this varies for as low as 5 psi at hot idle to more than 60 psi at higher rpm) and continually replenish the oil because the oil leaks out at the edges of the bearing and is flung out by the centrifugal pumping action of the rotating rod journals. This circulation is necessary because the shearing action of the oil in the boundary layer between the rotating crank journal and the static bearing surface creates heat and this heat must be dissipated. Most of the heat is removed by the oil as it passes though the interface between the bearing and crank.

The bearings, although soft still have to bear a heavy load because the incompressible liquid oil film transfers the forces acting upon the engines reciprocating parts to the bearings, thus the bearings have to withstand thousands of pounds of force even though direct contact does not happen. The best engine bearing have high embedability with a high load bearing capacity.

Usually everything works fine until a couple of things happen. In the case of modern late model engines, the green movement is to blame for part of the problem. In the last few years, Nissan has worked hard to make their cars green and more recyclable. In an engine traditionally one of the most toxic areas was the bearings. A few years ago, many Nissan bearings were made of trimetal construction using layers of lead, zinc and tin alloys of different percentages. This tri metal construction has been a mainstay of heavy duty bearing construction and composition for decades. The old Nissan bearings were very strong, heat resistant and durable. It’s a little known secret that old L-Series Nissan bearings are so strong and durable that many race engine builders use them, adapting them to other engines. The Infiniti IRL engine used in Indy cars used off the shelf L-Series bearings for this reason.
Sometime in the last few years, in an effort to be greener, Nissan discontinued the use of lead in all engine bearings. We know of this through discussions of sources deep within Nissan which will have to remain unnamed. At this time the engineers at Nissan noticed that warranty claims for bearing failure in the RB26 GT-R engine, another Nissan engine noted for running high oil temps, spiked. Nissan launched an intensive study on the reasons of this bearing failure and discovered that the new environmentally friendly bearings started to loose their load bearing capacity at temperatures as low as 260 degrees. Although there was plenty of reserve strength for normal passenger cars, the turbo high output RB motor pushed many sets of bearings to the failure point. Although you think of lead as a very soft metal with a low melting point, it was still a major contributor to the bearings mechanical properties and resistance to heat.

So as the oil temperatures climb, the bearings soften and loose their load bearing capacity, if the temperature climbs past 300 degrees and the engine is being pushed hard the bearing material can start to flake and spall. This disrupts the hydrodynamic film allowing metal to metal contact, resulting in catastrophic failure of the bearings, the crank, rods and even the entire engine often within seconds. Oil also begins to deteriorate quickly at temperatures above 260 degrees. It starts to oxidize and thicken, loosing its lubricity while forming sludge and varnish. This further compounds the problem, generating more drag and heat in the bearing/crank interface area.