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#4689037 - 03/08/18 07:03 PM Question about TBN vs Oil Change Interval
Brody Offline


Registered: 02/13/18
Posts: 4
Loc: Maryland
Hi all, my current understanding is that TBN is a measurement of the alkaline additives in an oil which in the case of a UOA seems to be one factor used to determine how much longer the oil can remain in service. In almost every VOA or UOA I've seen TBN is always mentioned in reference to the oils useful life. I've seen the phrase "TBN is still strong at x.xx, re-sample in 3000 miles." in several Blackstone reports.

What I'm wondering is can the TBN of a new oil be used to gauge how many miles it should go before being sampled and tested to determine if it's still serviceable oil?

My situation is that I have an extensively modified bi-turbo Audi V6. It's a new build, never installed in a vehicle. Engine bearing clearances are intentionally looser than stock to the point that it's spec'd for a 50w oil per the engine builder. I can go into greater detail if need be but I figured for now lets keep it simple. So that's one part of the equation, I need a 50w oil. The other part is the turbocharger manufacturer has a specified list of oils that they recommend be used or else they will not uphold their warranty on the turbos. Plenty of options to choose from on their list so that isn't really a problem. It seems like they're just wanting an oil at or above 1300 ppm phosphorous to be used.

I looked up UOA's and VOA's for every oil on their list and the one I keep going back to is Mobil 1 Racing 0w-50. I like the add pack and the TBN comes in at 7.9, at least on the VOA I found.

From my limited knowledge a TBN of 7.9 doesn't seem catastrophically low like some of the VOA's I've seen for lets say, engine break in oils, which are obviously intended to be used on a very short term basis. So I'm wondering, would it be reasonable to run the oil for 3000 miles and then do a sample? Or should I be conservative and test sooner? Thanks for thoughts and opinions.

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#4689041 - 03/08/18 07:08 PM Re: Question about TBN vs Oil Change Interval [Re: Brody]
Donald Offline


Registered: 03/21/04
Posts: 20386
Loc: Upstate NY
TBN depletion is not linear.

However an oil with a VOA TBN of 10 will last longer than one with 8.


Edited by Donald (03/08/18 07:10 PM)
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#4689045 - 03/08/18 07:13 PM Re: Question about TBN vs Oil Change Interval [Re: Brody]
Quattro Pete Offline


Registered: 10/30/02
Posts: 35730
Loc: Great Lakes
Originally Posted By: Brody
From my limited knowledge a TBN of 7.9 doesn't seem catastrophically low like some of the VOA's I've seen for lets say, engine break in oils, which are obviously intended to be used on a very short term basis. So I'm wondering, would it be reasonable to run the oil for 3000 miles and then do a sample? Or should I be conservative and test sooner? Thanks for thoughts and opinions.
Starting TBN of 7.9 is pretty decent - about typical for a lot of off the shelf oils. From an additive depletion perspective, 3000 mile OCI would not be a problem at all, but there is more to determining useful OCI than just TBN alone.
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#4689052 - 03/08/18 07:16 PM Re: Question about TBN vs Oil Change Interval [Re: Brody]
Leo99 Offline


Registered: 03/30/14
Posts: 2968
Loc: NJ
How many miles are you going to be driving this custom engine? It's a special engine that you don't need to stretch OCI.

How hard are you going to be driving this engine?
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#4689062 - 03/08/18 07:22 PM Re: Question about TBN vs Oil Change Interval [Re: Brody]
Dyusik Online   content


Registered: 09/26/14
Posts: 1942
Loc: US-WA
This oil has a good slug of Mg, it will do fine for much more than 3K miles. It is not rated as an API-Sx, due to phos content and is why it is stated not to be used in XXX conditions.
Just like the CJ/CK with no API-SN/SM stuff that can and does work fine in gassers, but we are "warned" not to use them because of "not for gasoline engines". Why? Politics.
I'd tighten up the OCI due to not trusting it to stay in grade. Just look at the VOA, it's ridiculously loaded with anti wear, and Ca/Mg comparable to any street oil.
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#4689077 - 03/08/18 07:38 PM Re: Question about TBN vs Oil Change Interval [Re: Leo99]
Brody Offline


Registered: 02/13/18
Posts: 4
Loc: Maryland
Originally Posted By: Donald
TBN depletion is not linear.

However an oil with a VOA TBN of 10 will last longer than one with 8.


I'm assuming non-linear because it's purpose is to counteract acid and the rate at which acid is produced varies?

Originally Posted By: Quattro Pete
Starting TBN of 7.9 is pretty decent - about typical for a lot of off the shelf oils. From an additive depletion perspective, 3000 mile OCI would not be a problem at all, but there is more to determining useful OCI than just TBN alone.


Good to know, thanks!

Originally Posted By: Leo99
How many miles are you going to be driving this custom engine? It's a special engine that you don't need to stretch OCI.

How hard are you going to be driving this engine?


I "intend" to drive it fairly regularly because I love fast cars, but my work schedule may dictate otherwise. If I have my way it would see about 90 miles a week mostly highway. If I could run a 4-5k mile or once a year OCI I'd be tickled pink. It will definitely see some abuse. Have several road course track days lined up already. From past experience with a similar setup in another car oil temps should be around 260 Fahrenheit under hard use.

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#4689081 - 03/08/18 07:43 PM Re: Question about TBN vs Oil Change Interval [Re: Dyusik]
Brody Offline


Registered: 02/13/18
Posts: 4
Loc: Maryland
Originally Posted By: Dyusik
This oil has a good slug of Mg, it will do fine for much more than 3K miles. It is not rated as an API-Sx, due to phos content and is why it is stated not to be used in XXX conditions.
Just like the CJ/CK with no API-SN/SM stuff that can and does work fine in gassers, but we are "warned" not to use them because of "not for gasoline engines". Why? Politics.
I'd tighten up the OCI due to not trusting it to stay in grade. Just look at the VOA, it's ridiculously loaded with anti wear, and Ca/Mg comparable to any street oil.


Thanks for your input. Very helpful. The shearing out of grade was on my mind as well.

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#4689096 - 03/08/18 07:59 PM Re: Question about TBN vs Oil Change Interval [Re: Brody]
i_hate_autofraud Offline


Registered: 05/19/16
Posts: 702
Loc: Canada


You can run an oil to a TBN of 2.0 - 1.0 then replace it. Without FilterMags, etc, oil turns
black well before the TBN hits 4 to 5, lots of life in the oil!

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#4689110 - 03/08/18 08:07 PM Re: Question about TBN vs Oil Change Interval [Re: Donald]
Linctex Offline


Registered: 12/31/16
Posts: 5969
Loc: Waco, TX
Originally Posted By: Donald
TBN depletion is not linear.


It gets even "weirder" when one considers European formula oils, that are often Low-SAPS or Mid-SAPS (like APEA C3)

They may start off with a "low" TBN of 5 or so.... but then will stay there, seemingly forever.

There's a lot to oil blending science that remains a mystery to many of us.
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#4689204 - 03/08/18 09:44 PM Re: Question about TBN vs Oil Change Interval [Re: Brody]
SonofJoe Offline


Registered: 07/23/16
Posts: 1052
Loc: Europe
TBN in an oil is primarily derived from the over based metallic detergents in an engine oil. One of functions of these detergents is to neutralise the acids formed by the oxidation of base oil.

And this is where TBN gets to be a poor predictor of oil life. 10 TBN's worth of detergent in a Group I oil (old fashioned mineral base oil) will be depleted much more quickly than 10 TBN's worth of detergent in a Group III based oil (severely hydrocracked base oil). Put simply, if the base oil resists oxidation, it doesn't form carboxylic acids and so makes no call on the TBN reserve.

The other thing is that TBN depletion isn't usually linear. On industry standard tests like the 100 hour/150C Sequence IIIG, TBN usually depletes exponentially and in severe cases you might find that half way through the test, the entire TBN reserve has be used up, such that oxidation resistance is carried by supplementary Antioxidants in the oil.


Edited by SonofJoe (03/08/18 09:45 PM)

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#4689247 - 03/08/18 11:07 PM Re: Question about TBN vs Oil Change Interval [Re: SonofJoe]
Linctex Offline


Registered: 12/31/16
Posts: 5969
Loc: Waco, TX
Originally Posted By: SonofJoe
Put simply, if the base oil resists oxidation, it doesn't form carboxylic acids and so makes no call on the TBN reserve


Thank for that, Joe.

That would explain why I have seen some very long OCI UOA's of "Euro" low-saps oils that still had decent TBN left over.

There's been so much "It's Low Saps!!!!! The Sky is Falling!! Change the oil soon!!!" hype on here before... Thanks for dispelling the myths.
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(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4689297 - 03/09/18 03:03 AM Re: Question about TBN vs Oil Change Interval [Re: Brody]
SonofJoe Offline


Registered: 07/23/16
Posts: 1052
Loc: Europe
One other thing...

Most folks would consider an engine oil to be shot when the TBN reaches 2 - 3. This isn't actually the case. An oil can function quite happily with a TBN of zero.

The thing you have to remember is that oil formulators don't tend to focus directly on stuff like TBN depletion, acid formation or oil oxidation. What they're really concerned with is the onset of exponential viscosity increase. For many of the lengthy, industry standard engine based oxidation tests, it's both impractical and uneconomic to just use over based detergent to control oxidation. Given how fast it can deplete, you might need an oil with a starting TBN of around 20 to keep the TBN above 2 at the end of the test!

For this reason, modern oils contain a cocktail of various Antioxidant chemistries, each of which plays a role in keeping the oil from 'breaking' long after the TBN reserve has been exhausted.

Just to be clear, there's nothing wrong per se with changing out your oil when the TBN hits 2 - 3 but if you do a UOA, and find the oil's TBN is zero, it's not something to freak out about.


Edited by SonofJoe (03/09/18 03:04 AM)

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#4689445 - 03/09/18 09:28 AM Re: Question about TBN vs Oil Change Interval [Re: SonofJoe]
CT8 Offline


Registered: 10/09/14
Posts: 10746
Loc: Idaho
Originally Posted By: SonofJoe
...The thing you have to remember is that oil formulators don't tend to focus directly on stuff like TBN depletion, acid formation or oil oxidation. What they're really concerned with is the onset of exponential viscosity increase.
Would the exponential viscosity increase have to do with the NOACK testing?
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#4689460 - 03/09/18 09:53 AM Re: Question about TBN vs Oil Change Interval [Re: CT8]
SonofJoe Offline


Registered: 07/23/16
Posts: 1052
Loc: Europe
Originally Posted By: CT8
Originally Posted By: SonofJoe
...The thing you have to remember is that oil formulators don't tend to focus directly on stuff like TBN depletion, acid formation or oil oxidation. What they're really concerned with is the onset of exponential viscosity increase.
Would the exponential viscosity increase have to do with the NOACK testing?



Not really...

Oxidation, oil volatility & viscosity increase do interact to a certain degree but things get too complex to explain in this forum. For the purposes of clarity, you should treat the onset of exponential viscocity increase & Noack volatility as independent variables.

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#4689494 - 03/09/18 10:31 AM Re: Question about TBN vs Oil Change Interval [Re: Brody]
69GTX Offline


Registered: 09/23/15
Posts: 3783
Loc: Connecticut
TBN of 0-4 could be an issue depending on where the TAN is at. I've seen some UOA's posted here with TBN of 3.0-4.0, yet the TAN was up at 4.5. Now what?
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