Question about TBN vs Oil Change Interval

[Brody]

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.

 

[Donald]

TBN depletion is not linear.

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

 

[Quattro Pete]

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.

 

[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?

 

[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.

 

[Brody]

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.

 

[Brody]

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.

 

[i_hate_autofraud]

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!

 

[Linctex]

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.

 

[SonofJoe]

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/150°C 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.

 

[Linctex]

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.

 

[SonofJoe]

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.

 

[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?

 

[SonofJoe]

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.

 

[69GTX]

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?

 

[BrocLuno]

For a built engine with very substantial cylinder pressure, I would not even look at TBN (much anway). I'd be looking at fuel dilution as the key element. I'd go maybe 1,500 miles and pull a sample for testing. Go another 1,500 and do it again, and so on.

I'm' not looking at oil life per se. I'm looking at engine degradation. Fuel dilution can kill highly loaded bearings and it will show up as wear metals. So I want to see how that is progressing. Anything else is frosting on the cake

 

[Leo99]

Originally Posted By: BrocLuno
For a built engine with very substantial cylinder pressure, I would not even look at TBN (much anway). I'd be looking at fuel dilution as the key element. I'd go maybe 1,500 miles and pull a sample for testing. Go another 1,500 and do it again, and so on.

I'm' not looking at oil life per se. I'm looking at engine degradation. Fuel dilution can kill highly loaded bearings and it will show up as wear metals. So I want to see how that is progressing. Anything else is frosting on the cake

As little as I know about oil and engines, if I had a specially built engine, I'd change it every 1500 miles. Especially if I'm going to track it. Baby that engine. You paid a lot for it, right?

 

[SonofJoe]

Originally Posted By: 69GTX
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?


Analytical chemistry isn't my forte but what I can tell you is that TBN & TAN are a bit weird!

In inorganic aqueous chemistry, you have simple acids & bases. When you add an acid to a base, they react and the pH of the mix changes. Simple! However in oil chemistry, if you add something like Poly Iso-Butane Maleic Anhydride (PIBSA), which is pretty acidic, to an oil, the TAN of the oil goes up but the TBN doesn't drop. Strange...

I never checked this myself, but since looking at BITOG, it seems that most fresh oils have a 'natural' TAN somewhere between 1 & 2. I don't know but I suspect this comes from the ZDDP which confusingly should be 'neutral'. It's almost like TBN & TAN act independently of one another and I do wonder if this is an artifact of how the tests are carried out.

So with this in mind, having a TAN of say 4.5 while simultaneously having a TBN of 3.0 to 4.0, might not be totally inconsistent. Are the acids present 'free' or 'neutralised'? I think the answer must be neutralised but the tests don't necessarily reflect this.

Another question might be does a TAN of 4.5, when you still have some TBN reserve in play, point to the oil being overly aggressive? I'm pretty sure the answer is no. Looking back, I can only think of one engine test I ran, where a very high TAN of around 15 (with zero TBN and the oil completely shot to pieces) appeared to be actively stripping bearing metals & iron. So 4.5 should be way safe.