No4x4Yet

There is so much misinformation in this thread. The only difference between regular and premium fuel is the octane. And octane is the resistance to detonation or burning! Meaning its harder to burn high octane fuels! High octanes do tend to have more additives which may be why some people experience mild improvements. But the higher octane can cause issues with the engine running and burning the fuel efficiently

As for the valve train noise, thats just crazy. The fuel has nothing to do with the valvetrain. There may be other issues though if thats the case.

Red91XJ

Agreed..the octane rating is, more precisley, the percentage of the hydrocarbon chain octane in the fuel. So 87 octane has 87%octane/13%heptane, and so on. Octane is more resistant to detonation than heptane, so the higher the octane concentration, the more the fuel can be compressed before it combusts. For XJs, the compression ratio of a low-mileage or 'clean' engine is too low to really get any benefit from high octane gas. Older, gunked up engines will have a higher compression ratio with age and buildup of crap so you might see improvement using premium fuel, but using additives/detergents would actually then defeat the purpose of using higher octane fuel.

djb383

From what I've experienced, (done numerous compression tests and engine tear downs) the very small mathematical fractional increase in compression ratio that carbon build-up may cause is more than offset by reduced cylinder pressure as an engine ages.

irs009

No4x4yet, Your opening sentence made me think that you were going to point out some of the 'mis-information' in this thread. Instead you went on to repeat things that have already been said in so many different ways.

Red91XJ

Then it stands to reason, like I said, the compression ratio of an XJ is not high enough to really benefit from premium fuel. I wonder if some people may actually see a decrease in MPG using premium, if the fuel cannot be consumed efficiently.

superj

i don't think they will as it can not be consumed less efficiently hten the regular fuel.

Diesel

Of course it can. The compression ratio in a jeep is not near what it takes to properly compress and thoroughly combust high test fuel, therefore a loss in power, fuel mileage and a increase in carbon buildup will occur.

>87 octane fuel is a waste of money in a vehicle that requires 87 octane fuel to run at peak efficiency.

Vindicator9000

Thread winner right here ^^

cdeal28078

I can see a high mileage Xj benefiting from premium but only because of carbon build-up on tops of the pistons and valves causing it to have a higher than stock compression ratio. Seafoam or plain water will cure that problem in an hour though.
In NC the stations are supposed to label their gas as having 10% Ethanol. I know Exxon does but it just says "our gas has 10% Ethanol" I guess that means all of their gas.
I feel running premium in an engine made for regular is a waste of money but man has that not been said already in this thread. lol I know premium fuels usually have more additives I know additives can help quiet engine noises. My facts to back up that statement? Leaded fuel. Why did they use leaded fuel? To keep the valves from beating themselves into the valve seats.
CLint

Diesel

By what, .1 or .2? Not sufficient.

Additives only AID in cleaning, they do not quiet any mechanical part.

No4x4Yet

If you have build up that bad in your engine your have some serious problems. When the piston is at TDC there isnt much room left in the cylinder. If that were the case a few tanks of premium will clean you out but seaform or the like would prolly work better.

There is way too much misinformation to cover it all, but octane rating is alot simplier than people think. I really covered all there is to cover regarding octane ratings. And being that higher octane takes more heat and pressure to burn the low the octane you can run the better.

djb383

Go back to page 1 of this thread.......there's claims of 6-7 mpg increase when using premium. Everything I've read and info links I posted beg to differ. My post #13 on page 1.


Premium better than regular?.........the myths just keep on keepin' on. At only 8.8:1 compression for the XJ 4.0, a lawn mower probably has more. I'm still looking for valid info (not opinions) that says premium is better for, improves mileage, makes the engine quieter, even though the owners manual says regular is fine. Someone help me out here.

http://www.consumerenergycenter.org/...s_premium.html
http://www.cartalk.com/content/featu...questions.html
http://www.straightdope.com/columns/...nd-regular-gas
http://www.usatoday.com/money/autos/...emiumgas_x.htm
http://www.epinions.com/content_2346164356
http://www.edmunds.com/advice/fuelec...3/article.html
http://www.caranddriver.sg/features/...emium_-feature

http://www.cartalk.com/content/featu...ium/myths.html
http://www.associatedcontent.com/art...n_premium.html
http://en.wikipedia.org/wiki/Gasoline

cdeal28078

I don't know by how much it would affect it but I have torn down high mileage engines that were not taken care of and seen at least .020 or more build up in the combustion chambers and on the underside and tops of the valves. So build up in the head side and on the piston side I can see where it would add to the compression ratio. Then any parts of carbon that are sticking out so to speak will get hot and hold that heat until the next cycle will cause pre-ignition.
I can see where premium would lower the chances of the pre-igniton or detonation in an engine in this shape or condition.
I have used premium in engines with low compression from the factory and never saw any increase in fuel mileage.
Was the lead in the old leaded fuel called an additive? I don't know. I do know it aided a mechanical part by keeping the valves from beating themselves into the heads.
clint

truckntractorhick

my other jeep's owners manual says: when its humid, heavily loaded, or hilly terrain etc. to use mid grade! now idk if its the same for mine cause i dont have an owners manual but if someone does just see what it has to say!

MyNameIsNick

I've never heard of premium actually being "harder to burn". I'm pretty ignorant on the actual differences in fuel other than higher octane has a higher resistance to knock and detonation. Aside from that, other specifics are all new to me and it's hard to tell what's truth and what isn't in this thread. Everyone seems to think they're right and others are wrong so I'll hunt for answers elsewhere.