Is it true that it is good for a car to fill it up with higher grade gas once in a while?
December 22, 2007 9:50 PM   Subscribe

Is it true that it is good for a car to fill it up with higher grade gas once in a while? I usually fill up at Shell, and there is 87, 89 and 91 kinds of gas, with 87 being the cheapeast and 91 being the most expensive. I usually get 87. However, I heard that if you own a car and want it to last longer, it is beneficial to fill it up with the most expensive gas, to "clean the engine". Is that true? If so, what is the explanation behind it? I drive a 2002 Kia Rio, if that makes any difference.
posted by esolo to Travel & Transportation (22 answers total) 3 users marked this as a favorite
 
Do you believe all the marketing you see, especially where it's implied and actual promises aren't made?

No, it's not true. The only things that are different are the detergents, which you don't need all the time. The only difference the octane rating makes is if your car requires a higher octane fuel because it's a higher compression engine... you'd know if you'd need it, you'd hear pinging.

If you really want a boost to the detergents, throw a little jug of Techron or another detergent through your fuel system every once in a while.
posted by SpecialK at 9:56 PM on December 22, 2007


Only use higher grade gasoline if your car's manual tells you to use higher grade gasoline.

While I have never owned a Kia Rio, I am guessing it does not.
posted by almostmanda at 9:59 PM on December 22, 2007


"Is that true?"

No.
posted by majick at 10:03 PM on December 22, 2007


Your car more than likely takes 87 octane fuel. It was designed for that, and it runs like intended on that. I'm just guessing though, check your owners manual for your actual octane recommendation. If it really concerns you, drop in a can of fuel system cleaner every now and then.
posted by sanka at 10:04 PM on December 22, 2007


Two things going on here: octane and additives.

The Car Talk guys do a pretty good job discussing premium gas here.

In summary, as far as additives, they say you should run brand-name gas about as often as cheapo gas, because the major gas companies use better additives. They also used to recommend running Techron through the tank every few thousand miles -- dunno if they still do that but it doesn't hurt -- and using no other aftermarket additives.

The octane question -- well, read their explanations but the nickel summary is, unless you're seriously concerned about performance you might as well use the gas that maximizes value (fuel cost/mile) as long as it doesn't make your engine ping.
posted by Opposite George at 10:10 PM on December 22, 2007


Not all of the major gas companies use the good additives. (For instance, Exxon is not a "top tier" gas, even though it is brand name, so even sticking to brand name gas isn't quite the right strategy either.)

That Car Talk page that Opposite George linked to recommends that you switch back and forth between Tier 1 and Tier 2 gas. You might find this page helpful in determining which retailers have Tier 1 gas and which have Tier 2.

(And then just keep on getting the 87.)
posted by adiabat at 10:21 PM on December 22, 2007 [1 favorite]


Yeah, what adiabat said about the Tier 1 vs. Tier 2. I oversimplified in my summary, and ended up misstating the article's point. Sorry 'bout that.

And the 87 probably maxes out that value equation for your car but that isn't the case for every car (e.g., my car gets better mileage on premium, but that's because it's designed for premium and detunes itself when I feed it lower octane gas.) Cost per mile is the key, unless you're Joe Sportscar and have the ride to match.
posted by Opposite George at 10:39 PM on December 22, 2007


The octane question -- well, read their explanations but the nickel summary is, unless you're seriously concerned about performance you might as well use the gas that maximizes value (fuel cost/mile) as long as it doesn't make your engine ping.

Er no, gas with a higher octane rating will perform slightly worse in cars that are not designed for higher octane gas. The octane rating has nothing to do with the performance of the gas it has to do with what kind of engine you can put it into. High performance engines compress the gas more before igniting, and you need a higher octane rating to withstand the pressure without igniting prematurely. But, if your car does not compress the gas enough, then the higher octane rating does nothing. I think (but I'm not sure) that higher octane gas has a lower energy density, and therefore produces slightly less power per stroke.
posted by delmoi at 11:20 PM on December 22, 2007


Just came from the mechanic yesterday and we were having this conversation. I told him I'd been going by the Car Talk guys' philosophy and that's why I only used regular. He was saying that, having taken apart enough engines, he has seen a definite difference in the ones whose owners use premium (very clean on the inside). It sounded (to me, anyway) like it's not so much a case of premium cleaning crud as not creating/depositing the crud in the first place.

So add that to the opinion pile for whatever it's worth. I'm still a little reluctant. I've gotten premium before and have never been able to tell any difference in performance or smoothness or anything, which my mechanic seemed to indicate he could. Never heard any pinging either. Ultimately, if it doesn't take significant life off the car and doesn't necessitate extra repairs, I'm thinking I'll just save the money.
posted by kookoobirdz at 11:23 PM on December 22, 2007


Again, octane is not the same as the detergents in gas. Chevron, for instance, peppers their gas with Techron, a fuel detergent. This is present in each of their relative octane levels.

Octane levels dictate burning temperature/compression stuffs. They are independent of "cleanliness" in your car's engine.

My car requires premium fuel. If I run 87 in it, it will start knocking. My knock sensor (a glorified microphone, for real) will detect the knock and over the course of a few startups, ratchet down the timing on the engine, reducing my performance but stopping the knocking. It will take a good chunk of time even back at premium again for it to push itself back up and take advantage of the higher octane fuel, which is a bad scene, since you can waste the gas without getting the performance.

So remember, a cleaner engine is not related to octanes. It's related to the quality of detergents included.
posted by disillusioned at 11:39 PM on December 22, 2007


Every car gets a buildup of carbon inside the engine. This will manifest itself as a "pinging" or fluttering noise when accellerating (as noted by disillusioned above). I have a 99 Toyota Corolla and the pinging noise manifested 18 months ago. One mechanic informed me that I should have the carbon removed (at a price) as the engine tries to compensate and performance is reduced. My current (and more reliable) mechanic suggested that I should run a premium petrol through the car occaisionally - the idea being that it burns cleaner and will remove some of the carbon as it does so.

Whether this is the case, I cannot tell you as I don't have comparison shots of the interior of the engine. What I can tell you is that I do run premium in the car as suggested (1 tank every 3 or 4 fill-ups), the knocking has been reduced down to almost nothing.
posted by ninazer0 at 12:50 AM on December 23, 2007


If your car is able to take low octane gas, try this:

next time you fill up, record your odometer reading. Then fill up the tank. After that, fill the tank, record how many gallons and dollars you took, and get the odometer again.

Fill up with the regular gas a third time, recording dollars, gallons, and odometer reading.

Now run two tanks of high octane gas, recording the odometer, gallons and dollars each time.

After the five tanks, compute your miles per gallon and dollars per gallon.

The last car I had that didn't insist on premium gas was an RX-7 (maybe it did, but the label may have fallen off by the time I owned it). When I ran this experiment, I found that premium gas was actually slightly cheaper on a mile-per-dollar basis. The RX-7 has a funky engine, though. I haven't done this with an ordinary piston engine.
posted by b1tr0t at 6:01 AM on December 23, 2007


Yes, running on higher octane can have an advantage in the long term for your engine. There is some confusion among people here as to the correct octane rating for your car, and separating that from the effects of a higher octane fuel that the engine is rated for. I am assuming that you are talking about running a higher octane than suggested in the manual.

Running a higher octane fuel than rated can have an effect on the long term performance of your engine.
Always running a higher octane fuel than rated can have a detrimental effect on the long term performance of your engine.
Running a lower octane fuel that rated WILL have a detrimental effect on the long term performance of your engine.

Over time, and irrespective of how many detergents you have in your fuel, your engine will build up carbon and related deposits from when your car is not running at optimum (when the engine is cold, for instance). If you do lots of short trips, you will build up these deposits at a greater rate then if each run gets the engine fully up to temperature. They build up because once they 'set' - when the engine cools off - it is difficult to get it hot enough again in normal driving to burn off any deposits other than those that were created by warming it up in the first place. All engines have these build-ups. All. The rate that these deposits build up is dependent on the way you use the car and the efficiency of your engine's burn cycle. The newer a car is, the less these deposits will be an issue as the management system is designed to function in a way to minimise these being formed.

Once these deposits are there? Forget about detergents. They are just there to try and reduce the deposits in the first place. Cleaner, better, fuel will produce a smaller percentage of deposits per engine heat cycle. Once the deposits are there, you need to get rid of them to maintain the flow of gasses through your engine as designed or (over a long period) your performance will start to deteriorate (MPG and power). But over time. Like ten thousand miles or so, maybe more.

No, you will never, in a modern car, notice a significant/tangible difference between different octanes - the ECU's and control devices will just compensate for the different fuel to try and maintain it's nominal performance. In an old car (like a 1970's carb car that is in a good state of tune) you WILL be able to feel the difference, as the difference in fuel is not compensated for in any way.

BUT. A higher octane fuel will burn hotter than lower octane fuel - despite the management system compensating for the different burn type of the higher octane, the actual burn will still be at a higher temperature. This will mean that, once the engine is warm, it will burn a greater percentage of the existing deposits off during the time the engine is hot. With modern fuels and clever ECU's a management systems, this percentage change will be small, but it will make a difference. The question is whether it makes a tangible difference, or whether the deposits will ever get big enough to actually matter. The newer a car in terms of design, the less this will make any difference. The control systems are so good that incomplete burning (assuming good quality fuel) is small enough that it may be 50,000 miles before deposits form to any degree.

So, to summarise:
Deposits will form at a certain rate for a given engine during cold (engine) running - possibly at a rate faster than the engine can burn them away during the hotter part of it's running. This used to be a problem for older cars - witness the '99 Corolla example. With a brand new, all clever engine management system, it is possible that these deposits won't form enough to be an issue, or be enough that the engine can't compensate for over time. It is possible that using the higher octane fuel won't help by any degree beyond taking enough deposits off that the effect isn't noticeable until 100,000 miles as opposed to 80,000. So is it worth it? Maybe, but it certainly wont hurt.

Will it make a difference?

Yes. Just on a newer car, not one you'll ever feel, and one that may not even be worth the hassle.
posted by Brockles at 7:53 AM on December 23, 2007 [1 favorite]


Er. So to actually answer the question, Yes. It absolutely is true.
posted by Brockles at 7:58 AM on December 23, 2007


Sorry for the hijack, but is there any way for a hearing-impaired person to detect pinging? Will the car feel different? Will any sensor lights come on?
posted by desjardins at 9:26 AM on December 23, 2007


Pinging (I'm used to calling it 'pinking') is the extreme case and signifying a high compression issue - the noise is that of petrol pre-igniting through the effects of heat through compression alone (ie the explosion partially going off before the spark plug actually lights it). There's no light to signify it on your dash, and no real way of finding it other than hearing it - it's like a repeated 'pinkyety-pinkety' noise. Usually on acceleration with load with a hot engine - say, accelerating hard from 20mph in 4th gear or so. It'd be pretty hard to spot under normal driving unless it was severe, in which case your car will feel like a dog anyway. You'll possibly notice your car doesn't feel as fast as it used to, but you'll normally only realise this after you fix the issue, because the degradation would have been so slight. The step change afterwards (say with the engine being rebuilt and cleaned out) will be more obvious.
posted by Brockles at 9:34 AM on December 23, 2007


For reference, the carbon deposits make this concept occur not just in physically reducing the combustion chamber size, but they also retain an awful lot of the heat with the bulk of high deposit levels, and so keep the core temperature of the combustion chamber higher (between burns temp) so it is less of a 'jump' to increase the temperature of the fuel/air charge which can promote pinking.
posted by Brockles at 9:37 AM on December 23, 2007


I buy the more expensive gas because my car starts to shudder and ping if I don't. Maybe not all cars do that, but mine does. It is worth the extra 25 cents for me.
posted by 45moore45 at 10:48 AM on December 23, 2007


desjardins: "Sorry for the hijack, but is there any way for a hearing-impaired person to detect pinging? Will the car feel different? Will any sensor lights come on?"

Talking about computer-controlled cars here (most anything for sale new in the USA today): In modern cars, disillusioned's glorified microphone will hear the pinging and adjust ignition timing and fuel/air mixture on the fly to prevent it. You might feel the car bog down while this is going on. No sensor lights normally come on*. You can also *waves hands* tap into the car's data stream to if you want to see this information. You could also set up an aftermarket "knock light" if you're really, really interested in knowing when this happens (e.g., if you're a tuner.)

---
*If the pinging is an extraordinary event, the ECM makes an adjustment and (I think) records the fact somewhere. In pathological situations, where it has to keep doing this a lot, your check engine light will eventually go on.
posted by Opposite George at 12:00 PM on December 23, 2007


delmoi: "High performance engines compress the gas more before igniting, and you need a higher octane rating to withstand the pressure without igniting prematurely. But, if your car does not compress the gas enough, then the higher octane rating does nothing. I think (but I'm not sure) that higher octane gas has a lower energy density, and therefore produces slightly less power per stroke."

Er, no (sort of.) While what you say is true if the only variable is octane, modern cars can adjust ignition timing and mixture on the fly to deal with this. If the car doesn't compress the gas enough, then your car will make an adjustment to allow it to use the gas anyway, as long as your car was built in this century and the adjustment doesn't fall outside its available range. When this happens, your engine's performance (i.e., "zip") decreases. Your mileage will usually be worse too.

True, it shouldn't matter if you put in a gas rated higher than the manufacturer's recommended octane, but lots of folks don't even know what that is or put in lower-octane gas to "save money" (more on this in a minute.) So, their car might apparently run fine on lower-octane gas but if they put in higher-octane gas in they'll see an uptick in performance. This is the case in my car, at least. So to the driver ignorant of all the thermodynamic juggling going on, and the manufacturers specs, the higher octane rating does do something.

If you're saying high-octane fuel causes reduced mileage due to lower energy density, you're really just making my point about minimizing fuel cost/mile for me, no? I believe you are mostly correct for modern formulations, but I also believe that these days gas compositions vary so much by distributor and geography that you can't just make a blanket statement relating the two (that's what God made calorimeters for.)

It's certainly the case for my car that I could run low-octane gas all day and probably not even notice. But there is better performance on the higher-octane stuff (again, up to the manufacturer's recommendation) and the way it works out on my car is that, under current pricing, the mileage increase I get running a higher-octane gas more than offsets the higher cost per gallon. That's why you don't always really save money running lower octane gas, even if your car runs apparently fine.
posted by Opposite George at 12:35 PM on December 23, 2007


No. The fuel control programming will counteract any differences that the higher octane gas would produce meaning there would be no benefit whatsoever. For what it's worth, this has been discussed nine ways to Sunday on a Hyundai forum I post on; Hyundai owns Kia and they use the same scheme for fuel control.
posted by Doohickie at 3:57 PM on December 23, 2007


No. The fuel control programming will counteract any differences that the higher octane gas would produce meaning there would be no benefit whatsoever.

In terms of power and MPG, yes. In terms of the finer elements of inside the combustion chamber (and the heat of the reaction inside) it is a different matter.
posted by Brockles at 6:08 PM on December 24, 2007


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