Centistoke oil rating

[jsgarage]

Centistokes and centipoises are just two ways of expressing the thickness of a liquid at a certain temperature. There are converters on the Web to do this, as its a bit complicated mathematically. The calculations involve the material's density, conversions to/from the metric system to english units and back, plus there are multiple ways of actually measuring viscosity. Various methods are used both in chemistry and engineering.

One way (chemistry) involves the mechanical drag on a rotating mandrel in a container of liquid, and another (engineering) involves a liquid's flow thru a sized orifice vs time. The two results do not directly compare. If you're trying to compare one result to the other, I suggest a Google converter, or I'd find which is mainly used in the auto industry so you can deal with other data than just what you found.

[RCJ]

David brings up a good point. 212f is not a high oil temp and we measure at the sump. At actual point of contact temp is higher. I've used oils rated at the same wieght that acted very differently in a running engine.

[RDY4WAR]

A good estimate for bearing temperature in a performance engine is 40-60°F above sump temperature. High end endurance engines like NASCAR cup engines see upwards of 75°F rise through the bearings, especially stretched out at Daytona. A daily commuter cruising easy down the highway will be 20-40°F above sump. The higher the dynamic viscosity relative to bearing clearance, the higher the hydrodynamic friction, and higher the temperature rise.

The expected operating oil temp needs to match the expected operating engine temperature. This is how you balance out the viscosity. My drag car uses a 0W-12 oil, which is 31.03 cSt @ 40°C (104°F) and 6.01 cSt @ 100°C (212°F). The oil temp on the top end of the track is 60-65°C (140-145°F). That 0W-12 oil is in the 13-16 cSt range at that temperature, the equivalent viscosity of a 40 grade up at 212°F. If I was to change out the 0W-12 for a 5W-40, that's 90.6 cSt @ 104°F and 14.8 cSt @ 212°F, then my operating temp of 140-150°F would have the oil at 37-44 cSt which is more viscous than a Nitro SAE 70 oil at 212°F and way more viscous than the engine needs, contributing a lot of hydrodynamic friction and viscous power loss. The additive solubility and reactivity is also reduced in the higher viscosity oil. A 410ci Sprint engine, built to operate at 260-280°F, will love the 5W-40. It's about matching the oil to the application.

Excellent info. These engines are splash lubricated but also have an oil pump that sends oil to the valve train. While back we did try some 50 weight (Mobil 1 I believe) and honestly that showed the most piston skirt wear along with a slight but measurable rise in the average temp under a sustained load. These are warmed up gently and not hammered on until up to temp. At least 160F oil temp. Used to run Rotella but that showed the worst wear. A quality synthetic seems to be so much better. There’s such a temp difference from being cold to hot being air cooled that I’m not sure if opening up the piston to wall clearance would be a benefit. Too much and they seem to have excessive blow by until really hot and up to temp. Too little? We have all ready found out the limit there lol. Thanks for the info again.

The higher the oil viscosity, the higher the hydrodynamic friction, specific heat capacity, and entrained air capacity. The higher hydrodynamic friction creates more heat, the higher specific heat capacity makes it reach a higher operating temperature and more difficult to cool down, and the higher entrained air capacity makes it more prone to aeration problems. There's also other issues with the higher viscosity like reduced additive response, especially a low temperature, and reduced solubility which makes it more difficult to keep additives in solution and reduces cleaning ability. You ideally want to run the lowest viscosity you can get away with.

The piston scuffing could be a combination of additive response issues and excessive viscosity hindering splash coverage. Note that ZDDP won't help a huge amount with piston scuffing since the piston skirts are aluminum. The sulfur in ZDDP reacts with iron surfaces to form barriers of iron sulfide, with the phosphorus breaking down in heat and pressure to form tribofilms of polyphosphate glass. Given the piston skirts are non-ferrous, they don't get much help from ZDDP like the cylinder wall does. This is why coatings for skirts and rings are molybdenum based. An oil with higher amounts of molybdenum dithiocarbamate (MoDTC) and other moly-based AF/AW additives may be beneficial. High Performance Lubricants Bad Ass 5W-30 and Driven LS30 5W-30 are two good oils with 500+ ppm Mo from multiple sources. The lower viscosity will help with the tighter clearances.

[David Redszus]

As you know, the Sommerfeld number is often used to predict the required oil viscosity for a given engine design and
operation. It uses bearing dimensions, clearance, speed, temp, viscosity and SpG to predict engine wear.

While normally used for plane bearings, has anyone attempted to modify the Sommerfeld equation to accomodate
other engine lubrication points such as cams, gears, tappets, etc?

[Roundybout]

A good estimate for bearing temperature in a performance engine is 40-60°F above sump temperature. High end endurance engines like NASCAR cup engines see upwards of 75°F rise through the bearings, especially stretched out at Daytona. A daily commuter cruising easy down the highway will be 20-40°F above sump. The higher the dynamic viscosity relative to bearing clearance, the higher the hydrodynamic friction, and higher the temperature rise.

The expected operating oil temp needs to match the expected operating engine temperature. This is how you balance out the viscosity. My drag car uses a 0W-12 oil, which is 31.03 cSt @ 40°C (104°F) and 6.01 cSt @ 100°C (212°F). The oil temp on the top end of the track is 60-65°C (140-145°F). That 0W-12 oil is in the 13-16 cSt range at that temperature, the equivalent viscosity of a 40 grade up at 212°F. If I was to change out the 0W-12 for a 5W-40, that's 90.6 cSt @ 104°F and 14.8 cSt @ 212°F, then my operating temp of 140-150°F would have the oil at 37-44 cSt which is more viscous than a Nitro SAE 70 oil at 212°F and way more viscous than the engine needs, contributing a lot of hydrodynamic friction and viscous power loss. The additive solubility and reactivity is also reduced in the higher viscosity oil. A 410ci Sprint engine, built to operate at 260-280°F, will love the 5W-40. It's about matching the oil to the application.

Excellent info. These engines are splash lubricated but also have an oil pump that sends oil to the valve train. While back we did try some 50 weight (Mobil 1 I believe) and honestly that showed the most piston skirt wear along with a slight but measurable rise in the average temp under a sustained load. These are warmed up gently and not hammered on until up to temp. At least 160F oil temp. Used to run Rotella but that showed the worst wear. A quality synthetic seems to be so much better. There’s such a temp difference from being cold to hot being air cooled that I’m not sure if opening up the piston to wall clearance would be a benefit. Too much and they seem to have excessive blow by until really hot and up to temp. Too little? We have all ready found out the limit there lol. Thanks for the info again.

The higher the oil viscosity, the higher the hydrodynamic friction, specific heat capacity, and entrained air capacity. The higher hydrodynamic friction creates more heat, the higher specific heat capacity makes it reach a higher operating temperature and more difficult to cool down, and the higher entrained air capacity makes it more prone to aeration problems. There's also other issues with the higher viscosity like reduced additive response, especially a low temperature, and reduced solubility which makes it more difficult to keep additives in solution and reduces cleaning ability. You ideally want to run the lowest viscosity you can get away with.

The piston scuffing could be a combination of additive response issues and excessive viscosity hindering splash coverage. Note that ZDDP won't help a huge amount with piston scuffing since the piston skirts are aluminum. The sulfur in ZDDP reacts with iron surfaces to form barriers of iron sulfide, with the phosphorus breaking down in heat and pressure to form tribofilms of polyphosphate glass. Given the piston skirts are non-ferrous, they don't get much help from ZDDP like the cylinder wall does. This is why coatings for skirts and rings are molybdenum based. An oil with higher amounts of molybdenum dithiocarbamate (MoDTC) and other moly-based AF/AW additives may be beneficial. High Performance Lubricants Bad Ass 5W-30 and Driven LS30 5W-30 are two good oils with 500+ ppm Mo from multiple sources. The lower viscosity will help with the tighter clearances.

Thank you for the detailed response. Sorry to hijack the thread from the OP. Sounds like a change in oil brand and formulation is what I’ll pursue. We did notice a power loss with the 50W. I don’t really need to worry about cold oil performance as we only run in the summer months. It’s the heat. These are tiny sump engines with 750ml capacity and there’s a ton of windage that we’ve tried to tame. Everything you pointed out about entrained air ect. makes perfect sense. Unfortunately these are China clones of a Honda engine that’s long been out of production and quality parts are hard to come by. I get the pistons from Taiwan and may look into a skirt coating they offer. Along with the oil recommendations. If interested google “GY6 scooter engine” and you’ll get an idea what im working with here. Personally I wouldn’t touch these things but it’s a father and son deal so I have to help lol. Keeps him and his friends out of trouble at least. Not many 16 year olds can say they can tune a carb these days.