Theory: Atlas lubrication system is marginal

limequat

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Dec 8, 2011
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Here's another just-for-fun thread.

I'm working on a theory and would like input and anecdotes from our local denizens.

First the background. After several years of abuse in my Supra, my 2006 4.2 finally gave up by spinning the #6 rod bearing. After tearing it down, I found that all rod bearings were scored and the main bearings didn't fare much better. Granted, my use case is not typical, but I want to know what NOT to do next time, so I started talking to many of you here and other people from the old Vortec4200.com board.

I found Marc (boosted 4.2 in a 51 GMC) has been battling lubrication issues with similar results as mine. I found Bruhaba had a similar issue, spinning the #3 rod bearing after installing a supercharger. And the Black Opel racing team finally retired their 4.2 after using it to push an Opel GT past 200 MPH. Also it seems like many of the turbo I5 builds over on 355nation.net end with rod knock. That's the bad.

On the flip side we have success stories, like Denny's Camaro that has been running 13+ psi on his 4.2 for many years. No issue. In fact Chevrolet themselves commissioned a one-off twin-turbo 4.2. So what am I missing?

Here's some data:
Per Consumer Reports, the 4.2 has above average reliability for all years.
Every used 4.2 I've ever seen has horrible varnishing
Lubrication-related failures are common on only modified engines
Of the few changes GM made when making the trailblazer TT, one of them was a higher flow oil pump.
Most 4.2 engine swaps require a custom rear-sump oil pan - usually with reduced capacity (5 or 6 versus 7 quarts stock).
Denny's camaro is the only 4.2 transplant I know that actually increased oil capacity with a massive oil pan.

So my hypothesis is that the factory lubrication system is marginal for standard use and inadequate for modified engines. Even a stock engine will overheat the oil to the point that the innards varnish. If a change is made, such as increasing power or reducing oil capacity, the oil degrades, loses its viscosity and the bottom end slowly eats itself until a rod finally lets go. GM knew this, and when tasked when bumping output up to a not-incredible 400 hp, went right to the oil pump.

This hypothesis will be difficult to prove unless some enterprising individuals here volunteer to start installing oil temp gauges. However, I can say that if the Supra ever lives again, it will be getting an oil cooler and more frequent changes!
 
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littleblazer

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Jul 6, 2014
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Is it top down lubrication or does the crank get lubed first? I don't think it is marginal, I think its flow capability is inadequate for a turbo setup. Unless you can register 300+ degrees of oil temp for extended periods of time, it isn't an oil temp problem. The truck runs 200 so chances are the oil is at 240. Most oils don't start to degrade until 270...

I'm curious, increased capacity doesn't help temps either, you're just increasing your delta some. You have some more data as it seems to be a bearing problem and if they're last in line, you can't fix that without increasing flow or changing the priority.
 
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mrrsm

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EDIT 12/16/2020:

Recently @efi-diy contributed MORE suggestions involving Racing Applications about adding in an Accumulator-External Oil Supply at the mid-block Plug-Threaded Sealed Ports accessing the Main cross-block Gallery feeding the Main Bearings.

This tactic will provide More Oil Flow into into these Magnificent Atlas Engines to survive in the nether-regions above the 6,500 RPM Redline. He also recommends using a Special Woodruff Keyed Harmonic Balancer too to mitigate the horrendous increase in Longitudinal Amplitude of the Vibrations that will accumulate within the Crankshaft and try to tear the Engines apart.

"Difficult to prove..." may wind up being an understatement. The GM Atlas LL8 Engine employs a Gerotor Oil Pump driven directly off of the nose of the Crankshaft and power positively moves over 11 Gallons of Oil per minute at 65 PSI. This system is SO good at preserving the Rotating Assembly that even with over 160,000 Miles of Engine Wear and Tear... the Crankshaft Main and Connecting Rod Bearings will show only the very slightest signs of visible wear after proper Synthetic Motor Oil and Regular Oil Filter Changes at 3,000 Miles...and no more.

The varnished up interiors of the engines that wind up being disassembled and shown on line invariably are those that are VERY High Mileage Motors (200,000 Miles at a minimum is NOT uncommon for these engines to achieve) and those that display that baked on Varnish do so as a result of Gasoline Vapors present below due to Worn Ring Compression Blow-By as opposed to having any problem with the Oiling System. That same said Varnish is ever so much more evident in an All Aluminum Block and Aluminum Oil Pan than would be obvious inside of any Cast Iron Engine with a Mild Steel Oil Pan Crankcase Cover.

The Oiling Channels in this motor are HUGE in their Inner Diameter and Patent beyond all expectations in comparison to those of the average Chevrolet Short Block V-8 Engine. The design of the Oil Delivery system could NOT be more straightforward and elegant than it already is inside of this Engine. This Diagram will illustrate just how clean the Tech Design for Oil Delivery is following the Oil Path from down inside the Oil Pan to the Oil Pick Up, the Gerotor Oil Pump, the Oil Filter and thence up and into the Oil Galleries to the Crankshaft Main Bearings and up into the DOHC Valve Train and finally... right back down to return by gravity into the Crankcase:

GM42LOILFLOW.jpg

Anyone who decides to Force Feed these engines at or near 7,000 RPM with either Nitrous Oxide, or by using a Super-Charger or via Turbo-Charging will need to follow the High Performance Engine Building Guidelines that will specify the proper clearance adjustments necessary for either Looser ...or Tighter Ring Gaps on Forged Pistons for the Top End ...and if called for possibly a greater or lesser amount of freedom between the Main Bearings and the Main Crankshaft Journals. Rods should be examined and Shot-Peened to relieve Hidden Stress Points... or rely upon more expensive Forged Connecting Rods designed for high Performance for that matter.

This engine is internally balanced and therefore very special attention should be applied to chamfer and polish (...and I Do Mean Ultra-Smooth and Spit-Shined!) Crankshaft Journals and Oiling Holes and the Oil Passages that Cross Lubricate the Con-Rod Bearings made 'Klean as Kleenex'. And EVERY SINGLE COMPONENT needs to be Balanced and Blue Printed --> Gram Scale Weighed First ---> and then have the Weights of the Rotating Components applied while turning the Crankshaft to Dynamically Balance and Match their counter-weight opposites to whatever bolts up to it in a Machine Shop before these components ever get near the inside of that engine block.

If the Engine Tolerances are simply held to the GM Stock Standard Catalog.... you will be looking for trouble when Oil at ANY Pressure... simply will not pass around journals that are much too near in clearance differences to their Aluminum Silicate Bearing Counter-Parts.

In that over-looked scenario...if those components meet up and touch metal-to-metal conditions and then rapidly overheat and expand after losing lubrication even for the slightest moment. In short order... this problem will invite Main Bearings to Seize to the Crankshaft Journals and Spin...wallowing out the inside Crank Bores inside of the lower engine block. You can have the Best in Show where New High Performance Engine Parts are concerned ...and still suffer an Engine Failure if the Racing Tolerances are not completely factored into the build.

This last thing concerns whether or not you are using an Aluminum Crankcase that has a Factory Oil Baffle Plate installed. The presence of this device may... or may not... hinder or help how quickly the returning oil can reach the bottom of the Pan and become an asset or a liability when trying to keep the the Oil Reservoir filled enough to avoid having the Oil Pick Up Tube suck in Air if the Lurch Off of the Line in a Drag race causes a Tidal Wave of Oil to push itself up the ramp to the back of the Case...or when making harsh left and right slalom turns that leave the Pick Up Tube exposed to the open atmosphere... just long enough to starve the Engine of Oil at Higher RPM. This is what that Two Part Baffle looks like when in place. Not many of the later model engines seem to have these installed:

GM42LOILPANWITHBAFFLE.jpg
 
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limequat

Original poster
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Dec 8, 2011
520
Thanks for posting that lube diagram. I had it in my head, but not on my PC.
In terms of high performance, let's separate high engine speed (7000 rpm) from high engine load (boost, nitrous). I did both, but most people do one or the other. And for the sake of discussion, I'm most concerned about boost.
So for boost, we should be messing with bearing clearances. We are concerned about maybe cylinder pressure and heat.
Cylinder pressure: I can see how adding 50% more bang in the cylinder might stress the bearings...but in fact, the highest stress is incurred during the exhaust stroke. Here the piston is going up, the crank is head down and there's no pressure in the cylinder to counteract it. So I don't see boost being hard on the bearings.

HEAT on the other hand...turbos do nothing but make heat. Compressed air => hot air. More charge, more heat from combustion. Not to mention all the heat radiating from the turbine housing. And heat causes thermal breakdown in the oil. So it's only a hop, skip, and a jump to get from boost to scored bearings.
 
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littleblazer

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Jul 6, 2014
9,265
Thanks for posting that lube diagram. I had it in my head, but not on my PC.
In terms of high performance, let's separate high engine speed (7000 rpm) from high engine load (boost, nitrous). I did both, but most people do one or the other. And for the sake of discussion, I'm most concerned about boost.
So for boost, we should be messing with bearing clearances. We are concerned about maybe cylinder pressure and heat.
Cylinder pressure: I can see how adding 50% more bang in the cylinder might stress the bearings...but in fact, the highest stress is incurred during the exhaust stroke. Here the piston is going up, the crank is head down and there's no pressure in the cylinder to counteract it. So I don't see boost being hard on the bearings.

HEAT on the other hand...turbos do nothing but make heat. Compressed air => hot air. More charge, more heat from combustion. Not to mention all the heat radiating from the turbine housing. And heat causes thermal breakdown in the oil. So it's only a hop, skip, and a jump to get from boost to scored bearings.
I'd think the boost would be worse, you're saying exhaust because theres no pressure to counteract it but on the combust stroke it smashes down to BDC and the crank swings around just as hard. Also I'd imagine the cylinder pressure across the piston face is greater than the reciprocating mass swinging back and forth, which should be well within the film strength of the oil.

You gap the bearings a little large so that the oil film is at its maximum loading capacity vs film thickness. Just my theory.
 
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mrrsm

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--^-- +1 ....and the biggest threat to the Bearing surfaces besides Oil Starvation in an N2O --> Fuel --> Air... set up ...is Detonation and the inability of the Piston Head to dissipate the added Heat. If the Ring Gaps are not generous enough to deal with the expansion that comes Kash and Karry when running N2O... They can meet at their ends and place so much pressure upon the Compression Rings from below as to cause the Upper Piston Head to separate and pop right off.

Also... the amount of sudden, downward pounding force experienced by the Bearings during these explosive events is sufficient to asymmetrically squash them concentrically in places ...Flatter than Dick's Hatband. So Forged Pistons are the order of the day. Too many of these repetitive Detonations can also melt holes in the tops of the Pistons as well. Having a decent Boroscope to poke down into the Spark Plug Holes and examine the inner cylinders and pistons during post-race tune ups will reveal any such problems.
 
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