Had a long convo with Amsoil tech support earlier today about using a low-ash engine oil in my RX-8. BIG kudos to them for spending as much time with me as they did.
They gave me a lot of good advice, but they also said one thing that I'm struggling with: they told me there's no reason to expect a lower-ash oil to leave this engine any cleaner than a higher-ash oil. The reason they gave is that most of the carbon deposits will be coming from fuel, and even at 1 qt/1000 miles, not much oil is getting burned.
This seems to make sense in isolation. But everything I've read elsewhere -- including in one of Amsoil's own TSBs on the RX-8 -- suggests that sulfated ash correlates significantly with deposit formation. I've also seen many suggestions that low ash content is extremely helpful if not mandatory for rotary engines.
Any thoughts? Am I missing something that'd reconcile all of this?
Sulfated ash and carbon deposits...
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Covering their fundament? I thought sulfate ash was more about catalytic converters than carbon deposits.
When in doubt I'd listen to the TSB which was probably written by an engineer
TSBs over phone Techs every day of the week.
For what it's worth, from my experience, Sulphated Ash categorically does NOT correlate with carbon deposit formation. In fact these days, particularly in the US, it's probably incorrect to even refer to Ash as being 'sulphated' as there's bugger all sulphur in engine oil and even less in gasoline.
Originally Posted By: SilverFusion2010
Covering their fundament? I thought sulfate ash was more about catalytic converters than carbon deposits.
When in doubt I'd listen to the TSB which was probably written by an engineer
Originally Posted By: Koz1
TSBs over phone Techs every day of the week.
Quote:
Carbon deposit formation may not be due to the type of base oil, but can also be related to the ash content of the oil additive package. Higher ash content oils can form more carbon deposits than lower ash content oils.
Quote:
In recent years, API and ILSAC have reduced the ash content in engine oil to below 0.80% (max) by weight to meet API SM, ILSAC GF-4 specifications.
Originally Posted By: SonofJoe
For what it's worth, from my experience, Sulphated Ash categorically does NOT correlate with carbon deposit formation. In fact these days, particularly in the US, it's probably incorrect to even refer to Ash as being 'sulphated' as there's bugger all sulphur in engine oil and even less in gasoline.
Originally Posted By: Shannow
I thought that sulfated ash was the residue that was left after burning all the oil off it, after reacting the metallic additive pack with sulfuric acid (making a sulfate solid ash after heating).
Amsoil's interpretation of a TSB is undoubtedly biased.
I don't see S.A. creating "carbon" deposits...but certainly "deposits"....which was why S.A. became an issue in the first place.
It's pretty funny that Amsoil use the fact that their flash point "isn't that much different to dino, so therefore isn't a problem", when their other advertorials include statements that their "superior" flashpoint" differentiates synthetics from regular oils.
Early wankel issues from NSU days demonstrate that deposits building up on apex seal leading to seals jumping and chatter marks...S.A. can do that.
Originally Posted By: Shannow
I don't see S.A. creating "carbon" deposits...but certainly "deposits"....which was why S.A. became an issue in the first place.
Originally Posted By: Shannow
It's pretty funny that Amsoil use the fact that their flash point "isn't that much different to dino, so therefore isn't a problem", when their other advertorials include statements that their "superior" flashpoint" differentiates synthetics from regular oils.
Back in the day, when it wasn't uncommon for Group I base oils to contain 1% sulphur and for Diesel fuels to contain maybe 0.8% sulphur, there was a prevailing view that metallic ash going down the exhaust pipe was in the 'sulphated' form. The Sulphated Ash test sort of reflected this bygone reality.
The bulk of the ash in any engine oil originates from the overbased metallic detergent (eg 300 TBN Calcium Sulphonate) with a lesser amount coming from ZDDP. On the basis that both of these additives improve the oxidative stability of oil, you would expect both to reduce deposits.
Makes sense! Thanks, SonofJoe.
More not-even-half-informed ramblings to follow:
One of the practical questions in all of this is whether I'm barking up the wrong tree by pursuing high-end low-ash synthetic oils. My thought had been that an oil meeting a spec like MB 229.51 or 229.52 would offer the best chance at keeping the engine clean while retaining performance at-or-above the SN/GF-5 level in all other respects (except mpg).
If I'm understanding better now, using a low ash oil would mean the oil itself would contribute minimally to ash formation. Such an oil might also contribute minimally to other kinds of deposit formation IF the finished oil were sufficiently oxidatively stable despite the light additive package (e.g. thanks to highly stable base stocks). Is that correct?
If so, I think I understand where the fellow at Amsoil was coming from. He kept stressing that the vast majority of the deposits will be coming from the fuel. Might a lower-ash oil be less well equipped to control those deposits, all else equal? If so, might I be able to compensate by just keeping my OCI short?
Tangential: I've also read that low-ash oils can reduce intake valve deposits in DI engines quite significantly. Is that mechanism sufficiently unique to DI engines that it's meaningless here? If not, how does it fit?
Really appreciate you guys wading through all this and helping me understand!
Originally Posted By: SonofJoe
Back in the day, when it wasn't uncommon for Group I base oils to contain 1% sulphur and for Diesel fuels to contain maybe 0.8% sulphur, there was a prevailing view that metallic ash going down the exhaust pipe was in the 'sulphated' form. The Sulphated Ash test sort of reflected this bygone reality.
The bulk of the ash in any engine oil originates from the overbased metallic detergent (eg 300 TBN Calcium Sulphonate) with a lesser amount coming from ZDDP. On the basis that both of these additives improve the oxidative stability of oil, you would expect both to reduce deposits.
Originally Posted By: SonofJoe
Back in the day, when it wasn't uncommon for Group I base oils to contain 1% sulphur and for Diesel fuels to contain maybe 0.8% sulphur, there was a prevailing view that metallic ash going down the exhaust pipe was in the 'sulphated' form. The Sulphated Ash test sort of reflected this bygone reality.
The bulk of the ash in any engine oil originates from the overbased metallic detergent (eg 300 TBN Calcium Sulphonate) with a lesser amount coming from ZDDP. On the basis that both of these additives improve the oxidative stability of oil, you would expect both to reduce deposits.
Originally Posted By: d00df00d
Makes sense! Thanks, SonofJoe.
More not-even-half-informed ramblings to follow:
One of the practical questions in all of this is whether I'm barking up the wrong tree by pursuing high-end low-ash synthetic oils. My thought had been that an oil meeting a spec like MB 229.51 or 229.52 would offer the best chance at keeping the engine clean while retaining performance at-or-above the SN/GF-5 level in all other respects (except mpg).
If I'm understanding better now, using a low ash oil would mean the oil itself would contribute minimally to ash formation. Such an oil might also contribute minimally to other kinds of deposit formation IF the finished oil were sufficiently oxidatively stable despite the light additive package (e.g. thanks to highly stable base stocks). Is that correct?
If so, I think I understand where the fellow at Amsoil was coming from. He kept stressing that the vast majority of the deposits will be coming from the fuel. Might a lower-ash oil be less well equipped to control those deposits, all else equal? If so, might I be able to compensate by just keeping my OCI short?
Tangential: I've also read that low-ash oils can reduce intake valve deposits in DI engines quite significantly. Is that mechanism sufficiently unique to DI engines that it's meaningless here? If not, how does it fit?
Really appreciate you guys wading through all this and helping me understand!
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I do believe that you “might” get oil introduced to the CC via the PCV ? Just a thought tho, maybe put a catch can on like I did and solve the whole issue ...... plus it might be better for HP due to not lowering the octane in the fuel at the same time. I don't think the RX engine has a PCV but all other engines do.
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The Ring-Dings purposely inject oil into the combustion chamber to lubricate the Apex seals.
Mercedes used a special porous floor to the intake manifold so that there was a thin "sheen" of oil that was introduced only to the apex seal area, but they had peripheral ports, so it worked with tiny quantities.
RX8 is a special animal.
What you want is non PAO, high spec C3/229.51-52, LL04. Preferably GTL based oil.
Originally Posted By: chrisri
RX8 is a special animal.
What you want is non PAO, high spec C3/229.51-52, LL04. Preferably GTL based oil.
Ed,
In answer to the points you raised...
First off, the Sulphated Ash test is a really old test. It harks back to a time when there was a EXCESS of sulphur in fuel and oil. I have a vague recollection that SA was originally limited to 1.5% max because of spark plug 'whiskering' (I might be wrong on this because I heard this story when I was still working in Fuels).
TBH, when I first started out formulating oils, no-one paid much attention to SA other than as a tick-box exercise. There were lots of tests where oil related deposits needed to be considered but none related directly to ash. Gasoline sludge, piston ring/groove deposits and Teost deposits all figured prominently. Interestingly, combustion chamber deposits (which I think is what this thread might be focussed on) do not, as far as I know, figure in any engine oil specification.
Regarding the issue of whether there is enough 'bonded' sulphur in Detergent & ZDDP to ensure that ash is naturally 'sulphated' (even when both the base oil and gasoline are essentially sulphur-free), I am not sure this as clear cut as you suggest.
For starters, the great bulk of Calcium in overbased Calcium Sulphonate exists as Calcium Carbonate; not Calcium Sulphate. There is a bit of so-called 'neutral' Calcium Alkyl Sulphonate present but this is primarily there to ensure the Calcium Carbonate is kept in colloidal suspension. You should also bear in mind that some Detergents (Salicylates and Calixerates) are entirely sulphur-free.
Ash from burnt ZDDP could be sulphated but could just as easily exist as Zinc Sulphide or even Zinc Oxide (remember that ZnO is what's used to make ZDDP in the first place so at high temperatures, it might just 'revert').
To be clear, I'm not saying you're wrong. I'm saying that in the olden days, when you had a massive excess of sulphur swirling around the combustion chambers, it was easy to think of ash being in the Sulphated form. Take away that excess sulphur, and things become less certain. TBH, I suspect no-one probably knows the answer as all of the analysis will be of ash laid down in the DPF itself, which of course could very likely change the chemical nature of said ash.
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