Jeep Garage  - Jeep Forum banner
Status
Not open for further replies.

V8 --- what grade of gas to use?

86K views 45 replies 31 participants last post by  Mongo53 
#1 ·
Just curious...what grade of gas is listed for the V8?

I have always used the regular unleaded gas in my 2005 V8 Hemi....then a few weeks ago Costco was out of it so I filled up with high grade (they only carry regular and high grade, no mid-grade). I quickly noticed a difference in engine performance...I was surprised at how noticeable it was. Runs a but smoother and seems quieter along with more power.

When I get my new WK2....finally....I want to use the right fuel from the start.
 
#2 ·
The 5.7 Hemi uses recommended mid grade 89 octane. Do not burn premium in the car is its not designed to run on that nor will it benefit anything. It can run on 87 and shouldnt cause any problems, but since mid grade is usually only 10 cents more than regular theres no reason not to just use what it recommends.

Answer: Mid grade 89 octane and if 89 is not available burn regular 87. If anyone says anything different its all in their head and nothing more than personal opinion.

James
 
#3 ·
The "do not burn premium" isn't so much a warning as much as a recommendation; there's just no point to it- you're only spending more for no additional benefit. Running regular (87 octane) will result in the engine de-tuning itself to prevent knock (since blowing holes in pistons is generally A Bad Thing), which is why you felt more power when you used a higher grade of gas- the engine could advance timing a bit for more power.
 
#4 ·
The only thing that higher octane gasoline does is help prevent pre-detonation (knocking) in higher compression engines. If the manufacturer says that it can run fine on 87 octane, then even 89 is a waste of money. Simply using higher octane gasoline will not change the performance of the engine. It is probable that the higher octane at your local pump also has less ethanol in it than the regular, and decreasing the amount of ethanol will definitely increase the performance of the engine.
 
#5 ·
James --- When you refer to the 5.7 Hemi are you referring to the V8 in the 2011 or to my 2005?
 
#6 ·
Just saw this online...I'm going to use whatever the manual says....that's mid-grade 89 octane in the 2011 V8 correct?

http://www.ftc.gov/bcp/edu/pubs/consumer/autos/aut12.shtm

The Low-Down on High Octane Gasoline
Are you tempted to buy a high octane gasoline for your car because you want to improve its performance? If so, take note: the recommended gasoline for most cars is regular octane. In fact, in most cases, using a higher octane gasoline than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage or run cleaner. Your best bet: listen to your owner's manual.

The only time you might need to switch to a higher octane level is if your car engine knocks when you use the recommended fuel. This happens to a small percentage of cars.

Unless your engine is knocking, buying higher octane gasoline is a waste of money, too. Premium gas costs 15 to 20 cents per gallon more than regular. That can add up to $100 or more a year in extra costs. Studies indicate that altogether, drivers may be spending hundreds of millions of dollars each year for higher octane gas than they need.

What are octane ratings?

Octane ratings measure a gasoline's ability to resist engine knock, a rattling or pinging sound that results from premature ignition of the compressed fuel-air mixture in one or more cylinders. Most gas stations offer three octane grades: regular (usually 87 octane), mid-grade (usually 89 octane) and premium (usually 92 or 93). The ratings must be posted on bright yellow stickers on each gasoline pump.

What's the right octane level for your car?

Check your owner's manual to determine the right octane level for your car. Regular octane is recommended for most cars. However, some cars with high compression engines, like sports cars and certain luxury cars, need mid-grade or premium gasoline to prevent knock.

How can you tell if you're using the right octane level? Listen to your car's engine. If it doesn't knock when you use the recommended octane, you're using the right grade of gasoline.

Will higher octane gasoline clean your engine better?

As a rule, high octane gasoline does not outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your car's engine. In fact, the U.S. Environmental Protection Agency requires that all octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against the build-up of harmful levels of engine deposits during the expected life of your car.

Should you ever switch to a higher octane gasoline?

A few car engines may knock or ping - even if you use the recommended octane. If this happens, try switching to the next highest octane grade. In many cases, switching to the mid-grade or premium-grade gasoline will eliminate the knock. If the knocking or pinging continues after one or two fill-ups, you may need a tune-up or some other repair. After that work is done, go back to the lowest octane grade at which your engine runs without knocking.

Is knocking harmful?

Occasional light knocking or pinging won't harm your engine, and doesn't indicate a need for higher octane. But don't ignore severe knocking. A heavy or persistent knock can lead to engine damage.

Is all "premium" or "regular" gasoline the same?

The octane rating of gasoline marked "premium" or "regular" is not consistent across the country. One state may require a minimum octane rating of 92 for all premium gasoline, while another may allow 90 octane to be called premium. To make sure you know what you're buying, check the octane rating on the yellow sticker on the gas pump instead of relying on the name "premium" or "regular."
 
#7 ·
Is all "premium" or "regular" gasoline the same?

The octane rating of gasoline marked "premium" or "regular" is not consistent across the country. One state may require a minimum octane rating of 92 for all premium gasoline, while another may allow 90 octane to be called premium. To make sure you know what you're buying, check the octane rating on the yellow sticker on the gas pump instead of relying on the name "premium" or "regular."
One additional item they don't say: the octane rating is a measurement of resistance to preignition (it isn't so much a measurement of the existence of the hydrocarbon molecule octane expressed as a percentage of the whole). At higher altitudes, in less dense air, fuel is less likely to preignite. Therefore, if you're in Denver, you may notice that the octane ratings on the fuels aren't what they are closer to sea level. They aren't cheating you; the fuel just doesn't preignite as readily.
 
#8 ·
I wasn't aware that the manual recommends 89 octane. I just bought my Overland a couple of weeks ago. I have only filled it once so far, and with 87 octane. I'll wait until it's close to empty before filling it next time and go with 89 to see if I notice any difference.
 
#9 ·
I fill it with 89. As per recommendations of the manual, and its usually only 5-10 cents more a gallon, so at an expense of ~$2.00 at most per fill up, I'd say why not. Considering my gas mileage is a lame 11-13 mpg usually, I guess $2 x 3 or 4 fill ups is 6 or 8 bucks more a month but still not a killer.
 
#11 · (Edited)
I would say - try 93 and see yourself. Nothing will happen to your engine with this action. In general I would expect slightly better performance and as result less pressings on gas pedal/better millage per gallon. 90% of my time I am in stop-go traffic with max 40mil/h and would be no point to burn expensive gasoline driving like this. Also don’t listen comments “car not designed…”, there is no such thing that car is not designed for higher octane gasoline. Also there is no design, it is ignition adjustments which are computerized these days, however I use to do them manually long ago on my first car (fiat) to prevent knocks.
 
#12 · (Edited)
Also don’t listen comments “car not designed…”, there is no such thing that car is not designed for higher octane gasoline. Also there is no design, it is ignition adjustments which are computerized these days, .
Not correct. The engine itself can be designed to require higher grades of fuel. It comes down to the amount of compression in the engine- the more the volume of air in the cylinder is compressed, the more prone to preignition it is. Boyle's Law and all that- temperature and pressure are the same thing; compress a mixture, you will raise the temperature. More volatile fuels (lower octane) WILL preignite in very high compression engines. There's a reason many forced induction engines require premium fuel- high compression.

The more compression, the more efficient the engine, but that comes at a price. You can squeeze a few more HP out of an engine by increasing the compression, but that will result in preignition with low grades of fuel. This is why race engines generally require VERY high octane (100+) fuel; they're set up to get the most power possible out of a given engine size.

You are correct that the electronic engine controls can adjust timing on the fly to reduce preignition or to take advantage of less volatile fuel, but that isn't the full story. Usually the engine computer will dial things back to prevent damage. However, if the manufacturer intended for lower grades to be used to give maximum performance out of the engine, a higher grade of fuel will NOT give more power. The engine controller is not capable of just cranking things up like that. It will go to what it is designed (both the engine combustion chamber and the computer) and no more.

Putting a higher grade of fuel into a vehicle than it is intended to require is wasting money; anywhere from ten to thirty cents per gallon, depending on the grades involved and the price differentials. If you're putting premium into a Grand Cherokee on a 20 gallon fill-up (I'm assuming you don't run it all the way dry), that's somewhere between $2 and $6 every tankful. Not going to break you, but it adds up.
 
#13 ·
All modern engines have knock sensors and when the onboard computer detects knocking it will either retard timing and/or richen the air/fuel mixture slightly to correct for the problem.

If knocking is detected then richening the air/fuel mixture by adding a little more fuel will affect your fuel economy slightly and reduce power also.

That being said, it is only under heavy load where this is likely to occur. If you drive your car around like a "normal" person all day then higher octane fuel is waste of money. If you are towing a lot of weight, climbing steep grades, and putting the pedal down then a higher octane fuel might yield more power because the computer won't need to retard the ignition timing (less power) or richen the air/fuel mixture (also less power).
 
#18 ·
There is a higher BTU content, potential energy, in 87 octane compared to higher octane fuel. An 89 octane fuel is actually a blend-at-the-pump mix of 91/93 and 87 to yield the 89 rating. The Hemi in the WK2 is designed to maximize performance with 89 octane fuel and has the ability to run the lower octane albeit at a marginal performance reduction (may or may not be noticeable). The octane rating is resistance to detonation.

Some engines are designed to maximize power with higher compression and require premium. i.e. 91/93 octane. These engines CAN run 87 octane, but the ECM will be retarding the timing and adjusting the fuel mixture to avoid detonation which reduces the engine's power potential often to noticeable levels. Some engine designs have combustion chambers that reduce/eliminate hot spots so that a higher compression, i.e. 10:1 or 11:1, can be used with 87 octane fuel like the Pentastar.

For modern computer-controlled engines, running with a higher octane than what an engine is designed for yields zero improvement in performance; it could even be argued that power/economy may be marginally reduced due to the lower BTU content of the higher octane fuel. The contrary point is if an engine experiences detonation running on the fuel for which it was designed; if a particular Hemi had detonation running 89 octane, then it would behoove the owner to run 91/93 if that rectifies the problem. A possible explanation for this occuring would be either poor fuel quality at a particular station or the gas station owner is cheating.
 
#43 · (Edited)
4 pages of speculation, has anyone actually got their O.M. out and actually read it?
2011 Jeep Grand Cherokee Owner's Manual said:
This engine [5.7L V8] is designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high-quality unleaded gasoline having an octane range of 87 to 89. The manufacturer recommends the use of 89 octane for optimum performance. The use of premium gasoline is not recommended, as it will not provide any benefit over regular gasoline in these engines.
This is the 2011 O.M., so later years might be different.

This is not the only engine from FCA with this recommendation, and likely not the only Manufacturer either. The PCM recognizes what octane you're using and detunes the engine to run that octane (within a certain range, in this case 87-89). So you can safely run 87 Octane, with slightly less performance, but 89 Octane will produce the best performance.

And that recognizing and detuning is not simply the Knock Sensor retarding ignition timing a few seconds. Its an algorithm that does use the knock sensor, but more a testing of the indicators for which octane, and switching to alternate set of tables for the tune. Testing again after a few minutes and either staying with or switching to the other tables.

I think the BTU content, or Energy Density is negligible. Higher Octane fuels usually have a higher pressure vapor, making it harder to atomize and mix with air. That definitely can have an effect on cold weather starting and stalling while cold. Both Ford and FCA have TSB's out, for customers complaining of hard starting and stalling in cold weather, first ask if they are using higher than recommended Octane fuel.

Lower Octane fuel's better atomization and mixing, plus the slightly higher energy density, could provide a slight mileage increase.

Just as E10, 10% ethanol, since alcohol only has half the energy density of gasoline, using E10 results in a slight decrease in mileage.
So what are you saying? fill our Jeeps up with Jet A? Yeah that will work haha :lol:
Jet A is actually Diesel fuel, which your still right, if the engine runs at all, it will run horribly and need some expensive repair to get all the diesel out. Different additive packages. But Military Trucks now have filter and fuel control kit, that they can switch them over to running JP-5/JP-8 in an hour.

I saw a Brit Car show that actually tried this. Surprisingly, both Diesel and Gasoline engines ran, although poorly, on the wrong fuel. Eventually the engines quit after a while, the gas engine ran for hours on diesel. The Diesels died in a few minutes to nearly an hour, and the diesels suffered very expensive damage. Diesels need extreme high pressure fuel pumps, and those pumps are lubricated by the thicker more oily diesel. Gasoline can't lube a pump nearly as well, and thus the pumps burn up right away and need to be replaced, and they are expensive.
I fill mine up with the mid-grade.. whatever octane that may be. Here where I live, it's 87.. up in the mountain towns, it could be 85.
People have been talking about Compression Ratios determining the Octane needed. Actually it's the peak pressure and temperatures in the cylinder, which the Compression Ratio has an enormous effect on that. But so does cam timing, duration, lift and overlap. So does intake, exhaust tuning and ignition timing.

So if you live in the mountains, where ambient air pressure is lower, you're starting with a lower pressure and thus less of a peak pressure and temperature. So you do not need as high an octane. That is why they sell lower octane fuel in the mountain communities.
I like to keep things simple and it looks like the conclusions we can draw for the 5.7L engine are:

1. The manual recommends 89 octane.
2. Many people here run 87 octane without any apparent problems.
3. If you live at higher altitude, like Denver, the 87 octane may be the essential equivalent of 89 octane at sea level.
4. Running 91 or 93 octane doesn't add any value.
5. Ethanol is bad because it corrosive and Benzene is bad because it's carcinogenic, so pick your poison.

Bottomline for me is I will run 89 octane from a reputable gas company as long as it's available, if not I'll run 87 octane. I'll run premium if nothing else is available. Tell me if I've gotten something wrong.
That's not what the manual recommends, see above. Sure you're not far off, but why pass on something kinda of right, instead of actually reading what the O.M. really recommends?

And someone was talking about the knock sensor is there to let you use lower octane? Ummm, no. Sure in the Hemi you can, but it's designed to use the lower 87, and its not simply the knock sensor retarding timing.

To get the most efficiency out of modern engines they're pushing the tune closer to the limits, so knock is more likely. In the past, they just detuned the motor to make sure knock would not happen under any circumstance, then dealt with the lower mileage or if you wanted more power, just make a bigger and more fuel hungry engine. There are hundreds of variable to determine the best fuel and ignition timing, and they can change in an instant, so even with a computer processing all the calculations, you're still going to get knock once in a while. The engine is designed and tested to use the recommended octane, and it will still get knock in some circumstances, that is what the knock sensor is for.

In some cases yea, you could use lower than recommended octane and the knock sensor retarding timing would still protect the motor.
In some cases no, you use lower octane, get knock and the knock sensor retards ignition and that still won't be enough to stop the knock and you'll get engine damage. They didn't test and design that motor to use a lower octane and have the knock sensor protect it for knock from lower octanes.
In some cases yes/no, you use lower than recommended octane, the knock sensor retarding timing is enough to protect the motor from damage, but the constant retarding of the ignition reduces mileage so much, you would have saved more money if you just bought the correct octane.
And in the case of living in the mountains, you could use lower than recommended octane and never get knock and thus not have the knock sensor retard ignition.
 
#25 ·
I have the exact same issue (question) as the original poster. I live in Denver, bought a 2011 Overland with the Hemi engine and purchase fuel at Costco, where only 87 and 91 octane are available. The 91 octane at Costco is usually priced the same or lower than 89 at "regular" gas stations, so if there are no issues with using it with the Hemi (even if there is no benefit) from a purely economical perspective it makes sense....

Just my $.02
 
#26 ·
OP most likely still had some regular gas in his tank and when filled up with premium ended up with a mid grade blend.

If it ran better that way, then it is a strong indication that he might consider running mid grade from now on as recomended in his owners manual and fuel filler sticker.

If always buying from Costco, consider switching back and forth between regular and premium to optimize costs. (Filling up around 1/2 tank each time).

If you run the cheaper gas, the computer will pull timing to stop any knock. This will reduce fuel economy and torque. Some might not notice it, but it will happen.

Jeep has designed this car to be able to run on regular, without damage, in a pinch if mid-grade is unavailable, but to perform at it's best with mid grade in the tank.
 
#27 · (Edited)
This debate never dies. You can run 87 ALL THE TIME without issue. You don't need to feel as if you can run it in a "pinch" and otherwise need to run 89. The vehicle runs fine on 87. I have a HEMI in my 2009 Dodge Ram Laramie and I EXCLUSIVELY used 87 with NO problems and have over 51K on the odometer. If you want to run 89 to optimize the performance, go ahead but if you want to save money use 87. There's ZERO reason to be concerned with using 87. Not trying to pick you on Uncle Paul but I want to make sure we aren't needless scaring people into believe using 87 will in some way harm their vehicle. I have NEVER noticed ANY knocking whatsoever in my HEMI on my Ram and not in my HEMI on my Jeep after one tank either. I'm speaking from experience here.
 
#30 ·
IMHO, you are better off worrying about the overall quality of the fuel you use. 87 Mobil, Chevron, etc.. would be preferable to say, ARCO 91. Really what I think it comes down to is benzene vs. Ethanol content. The lefties want more ethanol and less of the "carcinogenic type" components in gasoline. I live in CA currently, and can tell you that any brand or octane of gasoline is garbage here compared to my home state of CO. Why? Because of CARB, EPA and the lefties. WA state just adopted the CA laws and their gas is weak now too. I can smell the difference. Benzene is what gives gas that heavy, sweet aromatic smell. Ethanol, in a word, just smells "bad". It kind of reminds of grease. Ethanol and benzene accomplish basically the same task of limiting predetonation. Ethanol is corrosive and bad for fuel system components and also causes gas to go "stale" a lot faster. Benzene is pretty much awesome but its bad for humans and extremely carcinogenic. The lefties have made your choice for you on what you buy. Standby all other states for the same crap gas and donkey dick "vapor recovery" pump handles we have in CA. CA residents you know what I mean if you ever have filled a motorcycle in CA. Lame. Sorry for my rant. CA sucks. It should be given to Mexico in Exchange for sealed borders. My .02.
 
#33 ·
I was very low on gas and late for an appointment the other week. I had an old 5 gallon gas can with 50:1 premix (which was probably a year old and only 87 octane) from my dirtbike. I threw that in and then filled up later that day. I do love the smell of premix. :)

I usually put 89 octane in anymore. My wife still puts 87 in it when she fills it up.

I really don't think it matters either way. As long at whatever you put in still burns, you're golden. :)
 
#34 ·
hi all jeepies . here in Australia we have 4 different octanes of petrol which are 91 e10 ( ethanol ) 91 octane 95 octane and 98 octane . for the first 6 months of owning my jeep 2014 5.7 v8 overland I ran 98 octane with no deteramental effects except a very noticeable increase in performance and fuel ec0nomy when compared to 91 octane which I am running now which leaves it running like a tired snail ( comparatively ) . if I am not having any issues with knocking or other problems would it be ok to use it ?
 
#35 ·
If I remember correctly, you folks down under use RON ratings for fuels. The best power (and thereby economy) you can get out of any engine is by running the lowest octane the engine will handle without preigniting the charge. Food alcohol (ethanol) contains less energy than gasoline by volume. About 40% less I believe. However, ethanol is about 110 octane so it does increase the RON. Even so, I would choose alcohol free fuel any day of the week. Being in your shoes, I would run the 91 E0 all day long. Your economy will come up and so will the power as soon as the ECM learns the timing and ignition.
As a back story: I used to believe that you had to run the octane recommended for best performance. It is a lie. The higher recommended ratings are for emissions purposes only. Since getting into the performance motorcycle world years ago, I have found this out. We can consistently get 2 more horsepower by simply running 87 AKI instead of 93 AKI (that is US has grades) as the manufacturer recommends. Fiddling with the tuning can net more. Even with tuning, the lower AKI will produce more power. These are engines operating at 13:1 compression. Much more sophisticated than a pushrod Hemi is.
 
#36 · (Edited)
Evil', in the US, Jeep "recommends" 89 octane (mid-grade) for best performance, not 91/93. But as I've noted in previous threads, I've only ever run 87 for all but one half tank over 106K miles with no disappointment. If I were doing heavy towing or had a lead foot, I'd use the 89 for sure, however.

I do agree about avoiding the Ethanol-based stuff with more than 10% for the V8 as it's not (at least in North America) setup for flex fuel like the V6 is.
 
Status
Not open for further replies.
You have insufficient privileges to reply here.
Top