Technical What causes an engine to fail?

high performance engines just need better stuff to survive the increased output.

You could build an engine following an exact High performance recipe but using stock or cheaper parts and it will give increased output but will self destruct in short order.

 

But you need to be careful about using multi-viscosity oil in older motors that weren't designed for it or anything that's air-cooled. Remember, a 10W30 or 20W50 oil is still just 10W or 20W and in some motors the too-thin oil will do as much or more damage as the thicker oil. Clearances on many older motors are too big for thinner oil and you'll get leaks and rattles with multigrades (seen it personally). Air-cooled are particularly susceptible to damage with thin oil, put 10W-30 in a old VW or a Harley and you'll scuff the pistons/cylinders to the point of failure.

 

Multi-vis oils are only lower in viscosity at start-up. At operating temp they are the same viscosity, and because of the higher viscosity index they are often even higher viscosity at operating temperature. Also, a multi-vis oil, let's say 10W-30, has a higher viscosity when cold (at start up) then straight grade SAE 30 has when at operating temp. So you can't really say it's "too thin". If SAE 30 is not too thin, then 10W-30 will not be either.

 

I can't believe that that bit of misinformation is still being presented... That is absolutely false. A 10W30 is a 10 weight oil that has been bolstered with additives to give it the load-carrying capabilities of a 30W, but it's still a 10W oil, that's why the 'W' for weight is after the 10, not the 30. You don't have to believe me, you can check it yourself if you pick up a cheap viscometer from a paint store. Pour a 10W30 and a 30W oil through the viscometer when they're cold and note the times. Heat the oil to about 200 degrees (operating temp) and try it again.

Multigrade oils DO NOT increase in viscosity when heated... or even maintain their cold viscosity. They'll thin just like a single grade.

 

Steve, I'm a Certified Lubrication Specialist with the STLE, I know what I'm talking about. The "W" stands for "Winter" not weight. In order to meet the spec for 10W the oil has to meet a specification for cold oil flow.

You are right, multi-viscosity oils do not increase in viscosity when heated, that is true. All oils decrease when heated, the amount they heat up varies from oil to oil, and that difference in viscosity change relative to temperature change is measured and reported as the "Viscosity Index", or "VI". The more the viscosity changes relative to temperature, the lower the VI. The less it changes relative to temp, the higher the VI. As an example, take that Crisco shortening you have in the cupboard, at room temperature it's thick like grease, but take some out and put it in a pan on the stove and heat it up and it flows easily. That is an example of a very low VI. In contrast thinbk about a very high performance synthetic oil, you can put some into a freezer and leave it overnight and when you take it out it will still pour just as easily as when it went in. That is an example of a very high VI.

Multi-grade oils have higher VI than straight grade oils, so they don't thin out as much as they heat up. The effect is that at start up the multi-grade oil will be lower in viscosity and will flow easier than the straight grade oil. Then as they both heat up the straight grade oil will start to loose it's viscosity, while the multi-grade oil maintains it's viscosity, At operating temperature they will both have the same viscosity, or the multi-grade may be even thicker. (I really don't like using the terms "thinner" and "thicker" when talking about oil, because they aren't accurate, but it makes it easier to explain, so I'll go with them). If needed I can draw this out on a viscosity chart to illustrate the effect.

 

This is the statement I take issue with. I've heard all the scientific gobbly-gook about this, but the fact remains if two cars come in off the highway for an oil change at full operating temp, one with 10W30 and one with 30W, the multigrade will come out like water and the straight-grade will come out like the 10W30 when cold. I've seen it, no amount of explanations or charts will convince me otherwise. I've also personally seen the cylinder/piston damage from multigrade oil (or the wrong multigrade) in some air-cooled motors.

Don't get me wrong, multigrade is a great choice in the right application, but it's not for just any motor, the motor needs to be designed for it.

 

Why then does all your air cooled-honda, briggs, kohler,husky,ect. equipment engines that run at full throttle recommend 10w-30 oil from the factory .Multigrades were not used in older engines, with the exception of slinger type oiling, not due to sloppier tolerances but because it wasnt available at the time. They may have rattled because sucking 30w oil to the top of an engine at -10 degree morning was a slow process. With multi grade oil, engines now see 200k+ mi.with out overhaul. Older cars were lucky to see 50k.

 

This can occur with cheap oils that use VI improvers that are not shear stable, but the current API service classifications require stable polymers, if they shear out of grade they will fail the testing required to get API licensing. If the oil carries the API donut on the bottle indicating SM, then it's passed a shear stability test. Viscometers don't lie. The cylinder/piston damage you've seen may have had other causes than viscosity.

 

Well, Crazy Steve may not be interested, but I thought I would go ahead and post this graph for anyone else that may be, that illustrates how the SAE viscosity grading system works, comparing a mulit-viscosity oil to a straight grade. The vertical axis is viscosity measured in centistokes (cSt), the horizontal axis is temperature in Celsius. I just went out on the net and grabbed a data sheet from Valvoline for their conventional motor oil and used the typical viscosities at 40C and 100C and plotted them out, then added the SAE limits for the 2 grades. YOu can see the 10W-30 cannot have more than 7000 centistokes at -25C, and there is a range from 9.5 - <12.5 centistokes at 100C for SAE 30. Both oils measure the same at 100C, at 40C the 10W-30 is quite a bit lower in viscosity, and at -25C the difference is substantial. This is what allows you to get your engine started when it's cold, and let's the oil pump out to the bearings and everywhere else much faster at start up. By the time the oil is up to working temp the viscosity is the same between them.

 

That is a real difficult question to answer with only a little space, but there are answers depending on how it is built and what are the application. You can build a motor with all stock parts and still run the piss out of it and it can still last a long time. Likewise you can build a motor with all the best parts and it still can fail. Attention to machining,cleaning,balancing, break-in, rings, bearings- full groove,3/4 groove, cross drilled crank or not,oil pump,side clearance on rods,oil used and changed, block sonic checked before boring, valve spring pressure, cam profile,crank indexed,and I could go on but it would be too much to list.

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