Just How do Auto Engineers figure out the maintenance Schedule??

[Petersubaru]

It got me thinking when reading about the required oil change for the turboed 2008 subi...for ex. the newest tech bull. states that the oil needs to be changed at 3750mi or 3.75 months (why not 4 months, or maybe 5 or 6 months)...another example would be the timing belt exchange, which I certainly don't understand..for ex...105,000mi or 105 months...I am not sure that my wifes car will have even 70,000mi on it when the 105 month interval comes around...does it really mean that if the timing belt is not changed at that time or shortly there after I am putting the car in danger or risking a broken timing belt...I have asked this question at the subi dealer in my town, but the mechanic here said that his personal opinion might come back to haunt him ...the safe course is to stay with subarus recommendations

[heartless]

)...another example would be the timing belt exchange, which I certainly don't understand..for ex...105,000mi or 105 months...I am not sure that my wifes car will have even 70,000mi on it when the 105 month interval comes around...

 

you mean to tell me they are saying 8.75 years???

 

also, the "wife's car" must not go much of anywhere to not be able to get 70K in 8.75 yrs...sitting around doing nothing isnt exactly good for them either...

[Reveeen]

1) oil change intervals:

"They" (being engineers) figure on "worst case situation". I do not run garbage oil in my Subaru, but I might, so to be safe they cite worst case. You would be surprised at the number of folks that ***************/outright refuse about running premium fuel in their turbo cars, so the short change interval comes as no surprise. (I change mine every 3 months, @300,000 miles and still counting)

 

2) timing belt change interval:

I am not sure that my wifes car will have even 70,000mi on it when the 105 month interval comes around

 

Ok, let's stop and consider the material that timing belts are made of. Material I may add, that falls prey to oil, and heating and cooling cycles. A car used for long trips/high mileage is easier on the belt material than a car used for short trips/low mileage. In your case your useage comes under "extrene service" and I would stick with the time, rather than the mileage.

We have to add that if the timing belt "goes wrong", and fails, you will be into a motor (as in "replacement"), as the valves WILL hit the pistons. I would *think* it wise to stick to the recommended replacement?

[idosubaru]

it's not nearly as scientific as you would think. it starts out that way, but in the end there are marketing, reliability, and safety factors that contribute just as much to the final answer. sounds like they're final result is 1,000 miles per month..which is 12,000 miles per year which is a figure that's been around for decades in the automotive industry. it's best to stick with a quantifiable result...no matter how convoluted that "quantification' process was! so, they stick with 1,000 miles per month as a guide. realize that risk is a very "loose" term....yes you are putting yoruself in risk....but it's a very small risk and the risk would increase over time...it would be a curve, non linear for sure. that subaru chooses to pick a point on that curve that's very conservative shouldn't be that surprising. a "low" risk..pick a number...isn't likely to affect you personally, but could easily affect their reputation, sales, warranty costs...etc when multiplied out by thousands and millions of vehicles on the road. so subaru choose to manage that risk in a certain way....of course most of the time a belt it replaced it's not close to breaking. but that's the mark of a well designed product and maintenance schedule.

 

if you owned a company that relied heavily on your reputation you would certainly err on the side of caution...particularly when it adds money to your bottom line! most (the fact that i said most should be nearly humerous) people don't make decisions based on maintenance intervals, so there isn't a huge incentive to make them as long as possible. they would rather make the intervals shorter and add a little to maintenance costs...which most don't look at anyway, than even come close to maxing out the change intervals at the expense of high expensive engine work or replacement of an interference engine.

 

also...maintenance adds significant revenue.

 

also...the more often they are in there, the easier it is to spot leak, find a failed tensioner...etc. maybe the belt is okay for 10 years and 150,000 miles...but that's assuming the belt is a stand alone item, unaffected by anything else. not the case, engines are very dynamic. what one part does isn't independent of every other.

 

and in reality if you're making a one-size-fits-all product...speaking of maintenance schedules, there will certainly be lots of safety margin in the figures. people running in very hot or cold climates may not be the majority but those environments make a difference as well.

 

after 8 years i'd want to replace a timing belt and check everything else behind it. if i get a subaru that's 8 years old or more and the timing belt was "recently replaced", i'll still pull it apart to check the tensioners and seals since many mechanics and all dealers aren't very prudent in checking/replacing those.

[hohieu]

Maintenance guidelines tend to be ultra conservative to account for the mutiple variables that contribute to engine life or death as it were. Much of these change intervals depends a good deal on your driving style.

 

Timing belt:

City or highway is one example, as the belt is under more stress under the load of frequent acceleration and deceleration than when it is not loaded at constant highway speeds. In stop and go driving, the engine also idles for longer periods of time while no miles are being registered on the odometer, and engine temperatures are also higher in city driving. Also if your wife has a heavy foot and floors the car from red light to red light around town, this also puts an unnecessary heavy load on the engine, transmission, and everything else, including the belt. If you have logged only 70K miles in 8.75 years, the assumption is that you doing a lot of city driving and not too much on the highway.

 

Timing belts on all the cars that my family has owned have owned have far surpassed the factory maintenance guidelines in terms of both mileage and time, but I changed the belt out of my interference 2.5 SOHC at 105K miles for peace of mind.

 

Oil is often changed prematurely. BMW recommends 15K-intervals, but they also charge you $100 so the dealerships are not losing out.

[nipper]

you mean to tell me they are saying 8.75 years???

 

also, the "wife's car" must not go much of anywhere to not be able to get 70K in 8.75 yrs...sitting around doing nothing isnt exactly good for them either...

 

Timing belt, re read the schedual, its mileage OR time, whichever comes first. Its always listed that way assuming 1000 miles a month on avg

 

www.cars101.com.

[nipper]

It got me thinking when reading about the required oil change for the turboed 2008 subi...for ex. the newest tech bull. states that the oil needs to be changed at 3750mi or 3.75 months (why not 4 months, or maybe 5 or 6 months)...another example would be the timing belt exchange, which I certainly don't understand..for ex...105,000mi or 105 months...I am not sure that my wifes car will have even 70,000mi on it when the 105 month interval comes around...does it really mean that if the timing belt is not changed at that time or shortly there after I am putting the car in danger or risking a broken timing belt...I have asked this question at the subi dealer in my town, but the mechanic here said that his personal opinion might come back to haunt him ...the safe course is to stay with subarus recommendations

 

We sit around the bar after work dooing shooters tryhing to figure out how to annoy new car owners, how else do we liven up our dreary cubical based lives?

 

snicker....

 

Intervals are based on a few things. one of them is what managment wants them to be. Another is how hard is the part to replace. Next is what emission/saftey rules do we need to follow, and finally what is customery in the business, or at that one particular employer.

 

Next we look at the materials themselves, and what the cost basis is for the particular part, and can it even be done.

 

Then there is extensive testing. We build maint schedules for worst case scenario on some things (fluids), and other things (timing belts) are really pushing the limits of the materials.

 

Finally there are some things that we cant change. Rubber breaks down with time, even if the car is never started. Same with plastics. Those are finite limits.

 

Everything is tested and analyzed, and data is carried over from year to year. We know that antifreeze and tranny fluid lasts about 3 years, thats an industrial standard. If we wish to design for longer, then an extensive (in theory) amount of testing is done with that new fluid to make sure it meets expectations, and to see when total failure happens.

 

Same with engine oil. the one thing i do love from GM (everyone gets it right every so often) is the oil quality monitor. it works extreemly well, and i wish every car had one.

 

Now let me get back to my shooters as we try to develop better blinker fluid.

 

nipper

SAE member

Automotive engineer and technician for more years then i like to admit too.

[Petersubaru]

you mean to tell me they are saying 8.75 years???

 

also, the "wife's car" must not go much of anywhere to not be able to get 70K in 8.75 yrs...sitting around doing nothing isnt exactly good for them either...

...yes, 8.75 yrs...the car now will start to put more miles on it..the original owner hardly ever drove the car and the car did have issues because of it not being driven enough(sat around for 1.5yr)... the problems are resolved now

[OB99W]

Just How do Auto Engineers figure out the maintenance Schedule??

Much of it is based on past experience with particular systems or materials. If something is so "new" that there isn't much previous data, one technique that's used is "accelerated life testing"; do a Google search on that to learn more.

[porcupine73]

I think Calif. emissions requirements say something about emissions related components can't require maintenance for at least 100,000 miles. Hence the timing belt interval of 105k miles. Heat is a big enemy of timing belts too.