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Is there any point in multigrade oil IF the engine starts ok?

The quantity of research done in the automotive industry is several orders of magnitude more than in aviation, and nobody would consider single grade oils in a car.
I know people will say that aero engines are different, have a different use profile etc, but the days of oil changes every few thousand miles has long gone in cars, and the fundamental job of preventing wear is being done better and better as oils improve. In the finest traditions of aviation, i.e. remaining at least 40 years behind the automotive industry, multi grade oils have relatively recently been introduced. It is surely a matter of time before improved multi-grades will appear for aviation use.

My question would be why NOT use 15/50 ? I only use it because better fully synthetic oils are not yet available.

Darley Moor, Gamston (UK)

I am sure that’s all true but car engines are made to much tighter tolerances and the oil runs a lot cleaner. I work above a service workshop and see engines in pieces every day. The Koreans have even abandoned hydraulic tappets because they can make the simple mechanical ones retain their accuracy for long enough – probably until the car finance package runs out and it gets “exported” to Romania

When one drains the oil from a Lyco or Conti engine after say 25hrs, let alone 50, it comes out black as tar.

I also wonder what is the point of W100 when used on an engine with a thermostatically controlled oil cooler. Surely it is completely pointless compared to W80.

Administrator
Shoreham EGKA, United Kingdom

My last DOHC solid tappet Japanese car engine went 170,000 miles without a valve adjustment and didn’t need one when I sold it at that mileage. I think pushrods driven by hydraulic tappets on a single gear driven cam remains a better solution for aircraft engines given low rpm, light weight as the major driver and cam chains or belts (and their tensioners) being highly undesirable.

I change my O-320 oil and filter at 25 hrs in service, and at that point it is not black or close. This is aided by 8.5 qts in the system including oil contained in large external filter, oil cooler, lines etc. I could run it with up to 9.5 qts but prefer one quart low on the dipstick.

Last Edited by Silvaire at 23 Jan 17:25

This is relevant.

Administrator
Shoreham EGKA, United Kingdom

I’ve had my 6th engine on the trot not make TBO and this time not even close. I’ve never known anything like this. All either factory overhauled or overhauled at top quality shops.

We’ve used 80 weight monogrades for decades with the engines being tipped up with 80 or 100 in the hottest few months of the year. So just to try something different we have switched to 15W50 as suggested by A&C. I often think that over the years I should have listened to him more.

Personally I just think the engines are crap and the overhauled crankcases have welds upon welds on them which is why they keep failing.

Interestingly our Rotax which has done more hours than all the 6 failed engines put together has done so on nothing but supermarket semi synthetic oil. Which less face it is probably better quality than anything else that we put in aero engines.

The company hack of the Rare Bear Reno team was a simple Piper Warrior. The SOP was to run it full throttle in climb and cruise. The Lycoming 320 would consistently go well beyond TBO. At 3,000 hours plus they would OH and wonder why they did so, given in what good shape the engine was.

Engine longevity seems to be inversely proportional to how much babying the engine has received. A well used Lycoming run consist 75% or 65% should make TBO.



Oxford (EGTK), United Kingdom

Well, you should not weld crankcases.

Also most engine shops in Europe are of a “highly variable” reputation.

I don’t believe the type of oil makes any difference to whether a Lyco or Conti engine makes TBO. It will certainly make a difference to how worn the parts are at say 2k hrs; for example we have the proven halved wear of soft metals when Camguard is used (posted previously). But that just very slightly affects the overhaul cost, and only if overhauling to “new limits” which I suspect not many people in Europe do (I always did that; in effect you get a new engine, but with metallurgically proven parts).

Administrator
Shoreham EGKA, United Kingdom

And I know of a conti that went to 4000 hours on the cheapest overhaul you have ever seen. Sadly it wasn’t mine.

All of these overhauls were basically new engines. Certainly ontwo the data plate was about the only component remaining. All overhauled by two different shops both of which would be regarded as top UK shops. One of them had a reused known history crankcase, one was factory supplied and the others were supplied by divco. And I wonder if that’s the weak link as I’ve heard rumours that 90% of the crankcases they supply have had weld repairs. Although on the grounds that the factory rebuilt faired no better I’m not sure.

I’m not sure what’s happening. I know one thing I was doing nothing different 10 years ago and we never had these problems.

Wow, a lot of stuff being said hangin from an oily thread

My view on some of the points:

- CRANKCASES For decades used cases have been rotated through the system. Cases are inspected at OH and often cracks are found, such is the nature of the material and processes used on them (even though processes have been improved over the years, like permold vs sandcast with Conti) . Sometimes case cracks are found in service. Either way they typically end up at one of a couple of shops in Tulsa , OK or thereabouts who have been repairing cases for decades under FAA-approved processes. I would trust them more than EU shops simply because they do thousands of cases every year. They weld cracks, heat-treat, machine-align mating surfaces and rebore bearing seats.If after all this process deck height is within limits for correct positioning of cylinders, then you have a good case for another run. If the history of the case is such that the repair will derive too low a deck height then your case is good for scrap. Every time you machine mating surfaces deck height is diminished so there is a limited number of times the process can be done depending on how much machining you need to do each time.
Unfortunately most shops and owners do not have the time to wait for their own case to be repaired so an exchange case is procured. As cases do not typically carry historical data, you will never know the total time in service of your exchange case or how many times it has been weld-repaired. If you insist, you will get details of the actual repairs made the last time. And yes, if your case has been reused a few times and deck height is at the limits, it is more likely to fail in service or to be rejected at the next OH, but you will never know with an exchange case (OEM, field or MRO, it does not matter, you will not know). The only way to know is if you trace your own units or you get a freshly cast (ie new) case.I do not see any relevance of oil type to crankcase life.
IN my case I have new cases in my mid-life engine (known history!) so my tendency would be to reuse the same cases at the next OH if at all possible. THis disincentivates a factory OH or remanufacture for me, since I would in such case be swapping cases with one OH-life for cases with unknown number, perhaps two or three OH-lives, on them.

- OIL single vs multi-grade oil
This discussion has been beaten to death here and elsewhere. The difference is the change in viscosity with temperature. Multi-grades have a lower viscosity change with temp, single-grade is simply much more viscous at low-temp than high-temp.

There is ONE ideal viscosity for each engine RPM and power setting, which is not achievable in practice. The higher the RPM, the lower oil viscosity should be. From that point of view, it is better to have higher viscosity at startup and lower at cruise, which kind of fits in with oil temp-viscosity variation. Unfortunately single-grade is simply too viscous when cold to reach all parts quickly and effectively during startup. This can be mitigated with a preoiler pump.

The three key factors separating aircraft engines from car engines as to oil are, in my view:

- Low usage
- Low-tech-steel cylinder barrels (hence nickel cyls!) and distribution systems, prone to corrosion
- Leaded fuels

A multi-grade, low-viscosity-at-low-temp oil will gravity-drain away from all surfaces between uses much sooner than your thick-as-grease-when-cold W100 oil, which has an effect on corrosion protection. To counter that, cold startup wear will be higher on the latter oil (which is why you want to use W80 in winter, not because of your climb/cruise temp, which is the one being regulated by your vernatherm). Which of the two factors (idle-use corrosion vs startup wear) bears higher weight? It depends on your usage: time between flights and startup temperatures. If you do a lot of cold startups without preheating and fly often, multigrade is better. If you fly once or twice every couple of weeks or worse then you are better off with single-grade plus camguard.

Non-synthetic AD oils are much better at keeping lead salts in suspension than synthetic oils, as Mobil expensively found out years ago on aircraft engines.

- HARD/SOFT USE
One very important factor as to ‘hard-use’ is turbocharged vs normally aspirated.
The former will typically never reach TBO without major cylinder-surgery. The latter mostly will, if used and maintained correctly (see above). And WOT use on NA engines in global-warming times is typically 60-65% pwr unless you fly really low, even less average power if FP vs VP prop (O320/360 in PA28 is FP) since you will always be taking off at less than 90% pwr
Turbo engines are typically flown 75-80% most of the time. Also temp control is more difficult at the higher altitudes they use.
LOP vs ROP is also instrumental. The latter is harder on everything bearing power loads on the engine except perhaps exhaust valves and the rest of the exhaust system (much more so on turbos).

Reach your own conclusions, but my summary is that it is not as simple as saying hard better than soft , single better than multi, or most everything else, for that matter, except perhaps right vs left wing (er I surely meant high vs low wing, right?)

Last Edited by Antonio at 14 Aug 08:32
Antonio
LESB, Spain
39 Posts
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