Mid grade gasoline

Factual Questions
1

Most pumps in my area sell three grades of gasoline with increasing octane. Most cars are just fine with the low grade. The premium is required for some cars. My own car (a 2002 Jeep Grand Cherokee) recommends premium, but it’s not required (it has the high output engine and you just don’t get the extra horsepower if you use regular)

So are there any cars that need mid grade but not the premium? Or is it just a marketing thing, since people tend to choose the middle option when there are choices, and they’re hoping people “upgrade” from the regular since it doesn’t really cost them much to provide a mid-grade.

2

The mid grade is just a blend of the lower grade and the upper grade - that is the pump mixes equal flow of the two grades

3

What does your owner’s manual say about this? Octane doesn’t have any effect on horsepower. It is related to detonation. If a high-compression engine uses gas with octane that is too low, the pressure will cause the gas vapor to detonate before the spark fires. This is manifested by engine knocking. There is no benefit to using gasoline with a higher octane than what is needed to prevent knocking.

4

Gas is surprisingly inconsistent; for one, it’s refined regionally and you’ll get different gas depending on where you live, and the additives are different in winter versus summer. The short answer to your question is that no automaker is going to recommend mid-grade because the difference between mid-grade and regular (say, 89 octane to 87) is within the margin of error for the gas’s actual octane rating. In other words, since the “87 octane regular” might be, in reality, 85 octane (owing to refining tolerances), Chrysler isn’t going to engineer an engine that will run fine on 89 octane but knock on 87. In reality, they’re going to make sure the engine runs great on 84 octane.

In fact, most cars that require premium will run just fine on regular, and I don’t mean that they’ll detect the knock and retard the timing. I mean that they’ll literally run just fine. The advantage to running premium is that it has additives to increase the octane rating, and those additives can be more tightly controlled than the base gasoline itself. Premium is more consistent, which is why BMW is going to make it a requirement.

Back in the day, engines would require higher octane as they aged and carbon deposits built up. If that were the case, you could switch from regular to mid-grade when you first noticed knock, and if that didn’t work, you could go all the way to premium. With modern fuel injection systems, carbon deposits are much less of a problem, so you’re more likely to be able to run the “intended” gas for the life of the vehicle. However, mid-grade doesn’t actually exist. It’s just half regular and half premium, mixed at the pump, so there’s very little added overhead for the gas station. That, and tradition, are probably the only reasons mid-grade is still around.

5

Yes. And if your car’s engine does not require premium, there’s no reason to buy it.

6

My 2003 Dodge Ram pickup recommends using mid-grade gasoline.

7

Probably mostly a marketing thing, though FatBaldGuy’s info indicates it’s a legitimate option. It’s conceivable that some cars might run better on mid-grade than on regular, but not show any improvement between mid-grade and premium. I suspect, though, that most customers choose it as a lower cost upgrade than premium just because they think it’s better, not because they actually know it works better for their car.

It may be a holdover from Sunoco’s “custom blending,” which offered a choice of several octane ratings. I remember seeing five options, this article says it got up to eight. Same rationale, though – I’d imagine most customers bought the “best” they could afford without really knowing if they were getting more benefit.

8

this is only true if the pump is a single-nozzle “blend” pump where you push a button to select the grade. If the pump has one nozzle for each octane rating, then the station may still have three separate tanks. I’m pretty sure that setup is going the way of the dodo, though.

9

“Recommends” is very different from “requires”. Still a marketing ploy. Certain high performance engine manufacturers will “require” that for your warranty to stay intact, you must use premium octane gasoline.

10

The owners manual says something like
“All engines but 4.7 HO: the use of premium fuel is not required nor recommended”
“4.7 HO engine: The use of high quality premium fuel with an octane rating of 91 is recommended but not required”

But doesn’t say why. A Jeep enthusiest site (that is not Chrysler) alleges that it’s because you wont’ get the extra power that comes from the HO engine without premium fuel)

I put regular in it once when I was in the boondocks in Iowa and the station didn’t have premium. I don’t know if it actually had less power of if it seemed that way because I expected it, but regardless it didn’t knock.

I am aware the mid grade is usually a mix of the other two, that doesn’t change my question of why they sell it.

11

My old Dodge Shadow got a few more MPG using midgrade instead of lowgrade. With the added cost though it was about breakeven for MP$. My Neon had a different engine and mid- and low-grade had the same MPG. When they brought back the 2.4L Shelby engine for the SRT-4, it required premium.

12

Fabulous user name / post combination! :stuck_out_tongue:

13

Modern engine controls include knock detection. Ignition timing can be retarded, and mixture enriched slightly to prevent knocking with lower octane fuel. These measures typically reduce power and fuel economy. BMW made some motorcycles that did not include knock detection, but did have a plug in the fuse box that retarded the ignition if you removed it. This was in case you couldn’t get the recommended premium fuel.

14

I observed this on a 1988.5 (yes, it really was a fractional model year) Ford Escort that I used to own. This thing had one big fuel injector for all four cylinders, right at the throttle body. (This was the 1.9L CVH engine, which was originally carbureted but switched to fuel injection in later model years.)

(This was in southern California, where the octane ratings commonly seen are 87, 89, and 91). The owner’s manual called for 87 or above, but after a while it started knocking when I gave it 87. That knocking went away if I put 89 in, so that’s what I used from then on.

I don’t have the records in front of me, but I don’t recall noticing a change in fuel economy.

15

[mini rant mode]
First of all, “grade” and “octane” are two different things. The octane rating is just how much you can compress the gasoline before it spontaneously combusts all on its own just from the compression. The way gasoline is supposed to work is that you compress it, then you ignite it with a spark, and that’s what makes your engine go. If you have too low of an octane though it may ignite just from the compression, which is too early in the cycle. The engine will knock and may even be damaged. Fortunately, most modern engines can compensate for this and while they may not work as efficiently, they will at least detect the knocking and will adjust themselves so that you don’t damage the engine.

None of this has anything to do with how much energy you have per gallon of gasoline. High octane gas isn’t necessarily “better” and in fact may or may not have less energy per gallon than the low octane stuff. People think that high octane is better because high performance engines need it. That’s only because high performance engines also tend to have higher compression ratios. They need the higher octane in order to run properly. That doesn’t mean the fuel itself is any better.

The fact that they refer to the octane rating as “grades” is all just marketing BS.

I also like “enriched with up to 15 percent ethanol” that I see on pumps around here. Excuse me, but “enriched” usually means adding something to make it better. Making my gas mileage drop by a couple of percent isn’t “enriching”. Bastards.

[/mini rant mode]

Ok, that said, while many cars will run on 87 octane (I’m intentionally not using the term “low grade”) a lot of them only run without any noticeable problems because they have properly functioning knock sensors and systems that compensate for it. If you actually look in the manual you’ll find that many of these cars actually require 89 or better.

In my own personal experience, my mother’s old Toyota Camry used to require 89 octane. You could put 87 in it without it knocking, but your mileage would go down. Also, if you put 91 into it the mileage would also go down, though not as much. So it definitely ran best on 89 octane.

My Cadillac recommends 91 octane or higher, but requires 89 or higher. The mileage is best with 91 octane but it’s not enough better than 89 to justify the extra cost. The best cost per mile comes from 89 octane. It will technically run on 87 without noticeable knocking (I’ve done it in a pinch when I was out of gas and the only pump working where I stopped was 87 octane) but the manual says that it requires 89 or better.

My Dodge pickup truck requires 89 or better. It pings a bit with 87 octane.

You should always go by what the manual says. Just because you can stuff 87 octane into the tank and you don’t hear it pinging doesn’t mean it’s good for the engine.

16

I had a truck engine that pinged on low grade and needed the mid grade to not ping. I didn’t need to buy the highest octane at the service station and spend more. Enough is as good as a feast and cheaper.

17

Interesting…my car (Cadillac DeVille 4.6 liter Northstar V-8) is supposed to get premium. It runs fine on regular-except last week (I bought some regular at a different station)-it pinged.
So I suspect that most of what is sold as regular is actually 89 octane.

18

As another data point, I drove a 94 Astro Van for a while that called for 89 octane, although I don’t remember if it was required or recommended.

19

My old truck’s manual recommended 87 octane as a minimum. My new truck recommends 87 for most applications, but under heavy use (towing) they recommend 89 octane.

The engine manual on the boat I just sold was very specific that 89 octane was the absolute minimum and warned of dire consequences should one cheap out with lower grade. I just bought a new (to me) boat and its manual says the engine is just fine on 86 octane. FWIW: both boats have the smaller V8 (5.7L) but the first one was rated at 310 horsepower, while the newer one is only 260 hp. I assume this is based on differing compression and engine timing, but I’ll leave this to the experts here.

Anyway, two data points on vehicles that need mid-grade, but not premium gasoline.

20

Just in case any Europeans start reading this thread, the US & Europe use different methods of measuring a fuel’s octane rating. The US uses the AKI method, and Europe uses the RON method; for the same fuel the RON score will be 4-5 points higher than the AKI score.

Anyways, in my part of the country (eastern South Dakota) 89 octane fuel is usually 10 cents cheaper per gallon than 87 octane fuel. That is because the 89 octane fuel is mixed with 10% ethanol, while most stations have ethanol free 87 octane fuel. I use the 87 octane fuel in my 2009 Mazda 5, since I get slightly better gas mileage with the ethanol free fuel, enough to justify paying 10 cents more per gallon for it.