Engine oil Viscosity

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Re: Engine oil Viscosity

Postby brian » August 25th, 2016, 6:03 pm

Crank Spec clearance is about .0014 new. It can range up to .0032 if both components are at max wear. History tells me that our cranks really flex at high rpm. Especially in the middle. One touch between crank and bearing means damage. We began to use larger clearances to avoid touches. Synthetic oils with higher film strength can help in reducing the effect of touches but only careful balancing can reduce but not eliminate flexing.

Another key factor in all this, is how round are the bearing bores in the case? Starting with a tight clearance on a case that my be out of round by a mere .001, translates to a 40% reduction of clearance since the case forces the new bearing to become egg shape. All cases will distort over time and that's why line boring was developed.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
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Re: Engine oil Viscosity

Postby cendiv37 » August 26th, 2016, 2:24 pm

Crank "Flex" might be the wrong word to use in regards to what is happening in an AC VW engine. Crank "whip" is the term used in the references I read years ago when racing a Type 1 1600 engine powered car. "Bowing" might be an even more descriptive term. It is caused by the inherent, dynamic imbalance of the combination of the crank AND rods as they spin. Even if the crank is perfectly balanced statically and all of the rods, pistons, etc. are perfectly balanced statically, once the engine is spinning, centrifugal force (I know it's not the perfect engineering term, but let's not split hairs) causes the crank to bend (or bow) as the weight of the crank throws and attached con rods pull it away from the center line of the case. The good thing is that the crank is not actually flexing once per rev (which would cause fatigue problems at the journal radius's, etc.). It is taking a RPM dependent, bowed, or curved, shape causing the center main journal to "orbit" around the bore of the center main bearing (in particular) rather than simply rotate within it. This causes the clearance within the center main to be reduced on the side that the crank is bowing towards and increased on the side away. The bowing is resisted first by the stiffness of the crank but also by the hydrodynamic forces within the oil film which, up to a point, will keep the crank from contacting the bearing surface. The thicker the oil, the higher the hydrodynamic forces for any given condition (rpm, static clearance, etc.). The oil viscosity must be high enough to prevent this contact. The larger the clearances, the lower the hydrodynamic forces developed within a bearing, but the more the crank bends, the lower the clearance on the critical side. Which wins out? I wish I had the patience to do all the necessary calculations...

In the end, we are lucky that 1200 engines use lighter components than 1600 engines but the same diameter bearing journals. This makes them less prone to the problems caused by crank whip. In 1600 engines, anything over 5500 RPM or so starts to cause problems. This is the reason 1600 engines NEED counterweighting on the crankshaft opposite the crank throws to partially offset the weight of the crank throws and rods if they are to be revved much higher than stock.

All this said, on my old 1600 stuff, even using a stock crank, it was never a main bearing that failed. It was a rod bearing that first seized if oil pressure was reduced or lost. What did happen, without catastrophic failure, was that the engine case was "hammered" out at the center main bearing so that the imprints from the oil grooves on the outside of the center main became significant depressions in the case bore. Having little money and often no time, new cases were out of the question so the "fix" was to use Loctite sleeve and bearing mount on the backs of the center main bearing halves. It seemed to work, and that old single relief case lasted a long, long time. Eventually we replaced the crank with a used roller crank which worked very well in our application though it was a challenge letting it breath AND keeping the oil in it. Eventually, I did build a "case up" engine using a new dual relief case and new counterweighted (welded Gene Berg) crank. It ended up in my street car because the cam recommended to me for the new engine was too mild and the old engine with the "too big" cam was faster...

YMMV...

Edited slightly to try to make it a bit more readable (if that's possible).
Last edited by cendiv37 on August 26th, 2016, 8:28 pm, edited 1 time in total.
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Re: Engine oil Viscosity

Postby FV80 » August 26th, 2016, 5:36 pm

Bruce,
You're making my head hurt again :mrgreen:
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Re: Engine oil Viscosity

Postby hardingfv32 » August 27th, 2016, 1:14 pm

cendiv37 wrote:....What did happen, without catastrophic failure, was that the engine case was "hammered" out at the center main bearing so that the imprints from the oil grooves on the outside of the center main became significant depressions in the case bore....


Is this not a clear indication of the strength of the oil film? The oil film clearly has the strength to transfer damaging forces to the bearing and case without actually failing (spinning).

The fact is that the oil film in a journal bearing becomes stronger the thinner it gets. All this fear of the crank bending beyond the bearing clearance is a myth. The oil film is more than strong enough to control the crankshaft's motions. Checkout the science if you doubt me.

The rest of the motor racing industry is on board with low viscosity lubricants. Why are the FV engine builders still operating in the dark ages?

OEM's are going to 0w-0/5/10 oil in the next year or two. There are some exciting things going on with oil formulations that will have direct application to FV engines.... if your engine is properly prepared.

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Re: Engine oil Viscosity

Postby brian » August 27th, 2016, 7:39 pm

Now my head hurts. It's amazing that years of study and experience across the industry suddenly become myth. Yes OEM has gone to thinner oil; mostly to improve mileage and they are able to do so with modern machining and control. Try that on a nearly 60 year old engine design. Don't go racing without making sure that OEM will stand up to the high rpm. Most of the 0 weight engines are qualies or get rebuilt after every race. Wonder what weight they run at the 24 hour.

I can guarantee if I put a vee engine with .0015 run out in the case, you have well over a 50/50 chance of seizing the rear main bearing. Thicker oil, and to a lessor extent, synthetic oil can minimize but not really change the outcome.

Now there are a lot of myths in this class, some are perpetrated by engine builders. The worst is that their 500 dollar dynos can not consistently measure .5 or .75 hp differences. That's not to say there's a couple of shops that don't have excellent dynos. It's virtually impossible for a human or conditions to be that repeatable. Environmentally controlled dyno rooms, and automated load control can get closer but costs in the tens of thousands.

Don't be hooked into thinking that teeny tiny hp differences will make you faster. Yes a $1500 dollar manifold or exhaust will make your engine better but it's because it's more efficient in producing essentially the same hp, but over a wider range of rpm.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
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Re: Engine oil Viscosity

Postby hardingfv32 » August 28th, 2016, 12:33 pm

brian wrote:I can guarantee if I put a vee engine with .0015 run out in the case, you have well over a 50/50 chance of seizing the rear main bearing. Thicker oil, and to a lessor extent, synthetic oil can minimize but not really change the outcome.


What are you talking about? What does this have to do with using low viscosity oil? Are you implying that a .0015 run out case is the best you can do? Is the design of the lubrication system limit by poor case preparation?

Try to stay on point.....

Yes... years of experience become can become useless if the science and technology change. You have to keep up with the current science.

This has nothing to do with dynos. Why is that even brought up?

NO... we are not talking about tiny improvements. A modern lubrication system in a FV can produce 2+ HP. Switching from an old manifold to a current spec manifold is good for 1.5+ HP. Those are relevant numbers for a 60 HP engine or someone trying to be competitive.

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Re: Engine oil Viscosity

Postby brian » August 28th, 2016, 6:41 pm

Brian, trying to talk to you is worse than herding cats. My last try, honest!

"What are you talking about? What does this have to do with using low viscosity oil? Are you implying that a .0015 run out case is the best you can do? Is the design of the lubrication system limit by poor case preparation?" No implication about prep. When viscosity goes down, so does the effective film strength, any weakness,like oval main bearings, only makes matters worse. Only used cases get out of round due to horizonally opposed design and time. It's up to the rebuilder to decide if it's time for a line bore.

"Yes... years of experience become can become useless if the science and technology change. You have to keep up with the current science." only if you can work with luxury of contemporary components. You can't make a pig fly


"This has nothing to do with dynos. Why is that even brought up?" Cause you said,"Generally speaking when you lower the viscosity by 10 (from 40w to 30w) you increase power by about .75 HP." Can you measure that on your dyno? I can't.

"NO... we are not talking about tiny improvements. A modern lubrication system in a FV can produce 2+ HP. Switching from an old manifold to a current spec manifold is good for 1.5+ HP. Those are relevant numbers for a 60 HP engine or someone trying to be competitive." Your not going to see those kind of numbers unless you're moving from a defective component to a state of the art component. I ran the same manifold from the 90's with good success. This whole deal reminds me of the famous back tp back spark plug test, "just plug these babies in and you'll get 5 hp." Ask them if the plugs taken out were new or junk.

We've know each other, what 25 years? I'll compare my results and junk pile to yours anytime, I'm out (mike drop)
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
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Re: Engine oil Viscosity

Postby hardingfv32 » August 29th, 2016, 12:22 pm

[quote="brian"We've know each other, what 25 years? I'll compare my results and junk pile to yours anytime, I'm out (mike drop)[/quote]

Well now thats how you end a technical discussion. Your years of experience and success are a clear indicator that your engine expertise has assimilated all the latest performance trends.

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