Why would I need higher then 30 grade synthetic?

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Originally Posted By: bbhero
I like these pictures and the graph Shannow. Quick question I have is this.... What about a rather short duration of high rpms?? Let's say like 6 seconds worth of high rpms. How much would that raise oil temps up too? And how long would it take for them to return back to normal afterwards given rpms being around say 2k??



Good question BBhero, and thanks for the input Shannow. Yes, 6 seconds is about right for general driving with a bit of traffic on the road and heavily enforced speed limits.
 
Originally Posted By: bbhero
I like these pictures and the graph Shannow. Quick question I have is this.... What about a rather short duration of high rpms?? Let's say like 6 seconds worth of high rpms. How much would that raise oil temps up too? And how long would it take for them to return back to normal afterwards given rpms being around say 2k??


A good way to look at it is to express HP in terms of BTU

A 290 HP engine will put out "about" 12,300 BTU of heat per minute into the system.

1 btu heats one pound of water one degree.

So "standing on it" at max output for 6 seconds puts about 1,230 BTU's of heat into mainly the oil, but lots is absorbed by the assembly

The second you put your foot in it the top of the bearing shell is subjected to the majority of that and the oil will run out of those crank journals smoking hot.

As the temp of rotating assembly and surrounding bearing journals rise the system expands clearances open up and pressure across the board drops as the double whammy of both expansion and the oil thinning out from being blasted with heat and pressure.

This is where the thicker oil helps you out by keeping pressure higher, and a larger boundary layer between the moving parts.

Very rare passenger auto are wide open for even 1 full minute at a time, where with trucks towing its a normal to operate that way for 15 minutes at a time or more.

UD
 
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Originally Posted By: UncleDave
The second you put your foot in it the top of the bearing shell is subjected to the majority of that and the oil will run out of those crank journals smoking hot.
As the temp of rotating assembly and surrounding bearing journals rise the system expands clearances open up and pressure across the board drops as the double whammy of both expansion and the oil thinning out from being blasted with heat and pressure.

Not to burst your air, it's the reverse that clearances tighten up instead.
 
Originally Posted By: zeng
Originally Posted By: UncleDave
The second you put your foot in it the top of the bearing shell is subjected to the majority of that and the oil will run out of those crank journals smoking hot.
As the temp of rotating assembly and surrounding bearing journals rise the system expands clearances open up and pressure across the board drops as the double whammy of both expansion and the oil thinning out from being blasted with heat and pressure.

Not to burst your air, it's the reverse that clearances tighten up instead.


No burst necessary.

I guess I always saw it as It as a mix with things like pistons growing and other things moving apart with different metals expanding at different rates.

Net net is after a 15 minute wide open run when the system has been saturated with heat at idle watch the gauge sink.
You will practically set off the low pressure warning

Seems counterintuitive that the tolerances all shrank, but maybe so.

UD
 
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Thermal expansion in similar metals is is a product of cross-section. So a crank journal is 2" across. The rod big end is maybe 1" counting both sides of the loop and adding them together. The journal will "grow" faster than the rod big end, so things get tighter.

That's why we make main and rod bearing clearance's bigger for race engines so they have room to grow w/o inducing so much drag they spin a bearing ... The only thing holding that bearing shell are the two little tabs in the pockets. Not much strength against many square inches of drag when things get tight ...

But, that same larger clearance needs more and thicker oil to fill the void when cold. So staying away from 5W is a good idea (for race motors). Many race engines rely on straight weight oils because they cushion well when cold and fill well when hot
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0.003 on the big ends in Nitro motor. 70W oil pre-heated to 150 before it goes into the oil tank. Life expectancy including burn-out, backing, staging, staging games, R's up (for auto classes) and run to the end - 2 minutes. Oil temp in the tank at shut down 275*F or there abouts ... That's a gain of 125*F in 5 gallons of oil in 2 minutes. Never did the math, but that is mostly bearing heat ...
 
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Ive always been amazed the little keyways on the bearings hold the whole thing in place.

Totally aware that the rotating assembly will need to be clearanced larger in a race/performance engine and that steel will grow faster than cast iron and the the main caps and block under load

3/4 of a thou to a thousandth of clearance is what I hear the typically street engine heat clearance growth is under load.

Wether your crank is cast or steel makes a difference here - I dont think of guys moving to steel under 500HP but I guess it happens more these days and the poster VQ is forged.

from 70 to 220 the parts expansion looks like this in an aluminum block with steel caps

Block - Aluminum grows by 0.0051" from say 2.7500 to 2.7551
Main cap - Steel grows by 0.0030" from 2.7500 to 2.7530
Crank shaft - cast iron grows by 0.0025" from 2.7470 to 2.7495

If the crank was steel rather than cast iron it grows by 0.0030" from 2.7470 to 2.7500 tightening the gap.

A cast crank wont grow as fast as the clearance under load.

In the truck after your wide open run at the top of the hill once you crest the incoming load drops - the rotating assembly gets the cool oil flow first and shrinks back down quicker than the heated block and caps and now the the clearance is open up and the pressure drops more than the thinned oil alone would normally cause.

or at least that how heard it went.....





UD
 
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Originally Posted By: Shannow
Originally Posted By: 4evrnyt
After reading the Oil 101 article I came away wondering; if synthetic oil doesn't change grade over time why would anyone need to use higher then 30 grade. If most engines require 10cs at operating temp and that can be achieved with 30 grade oil what benefit is there to running a higher grade synthetic oil then 30 grade?


I've repeatedly asked the author of 101 where he gets the understanding that engines are designed for 10Cst KV100, and hear chirping crickets every time, then some time later he'll point newbies back at it.

Manufacturers don't spec a KV100 directly, but they DO spec it indirectly by speccing different grades, the latter number of which has a range of KV100s.

More recently, focus has been on the viscosity characteristic that actually DOES most of the protection, and that's the HTHS...again, misconstrued in 101 by interchanging Cp and Cst.

Lots of manufacturers have set minimums on HTHS, and seeing as you have specifically mentioned "30s", a straigh mineral SAE30 dino will have an HTHS in the range of 3.5, and the multigrade synthetics can go from 2.9 (ILSAC economy grades) to 3.6+ (ACEA A3/B4, C3 type oils).


I think that you are right, the assumption of a required 10cSt oil is based on the recommendation of an XW30 and to keep the numbers simple. This is why I kept it that way in the revised and expanded version of Oil 101 last week. I posted them in the Interesting Articles forum for feedback from the group. If you have a recommendation for a paragraph change or would like to reword something then feel free to comment on those threads. I'm hoping that but updating and correcting some of the things discussed in Oil 101 will lead to less confusion in the future. Hopefully this will grab the attention of the powers that be and we can replace the motor oil university with more relevant info on the main page.
 
Have read and commented. All good goals
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We need to get Oil U fixed ...

In all cases it is the presence of oil in adequate amounts including film strength and depth that is lubrication, no matter how it gets there; pump, splash, sling, drip, gravity return, etc. Just like lubing a squeaky hinge, put a big enough drop on there and it's lubricated, you do not need to "pump" more on ...
 
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