Do I have to run premium fuel(91 octane) in my 2002 xtype jag or can I run 89 octane middle of the road fuel?

 

prash, some members have run 87 octane in their cars with no dramatic effects. Most do say that they see a slight loss of power. But, keep in mind that the computer is having to work harder to overcome this lower grade gas as the computer is programmed assuming you are using the higher grade gas. Stepping down to the mid grade isn't going to be as bad, but can still lead to issues.

All I can say is if you want to give it a try, run the tank to near empty, put in a half tank and see how the car runs. If you don't like how it runs, fill the tank up with premium and all should be good. If you like how the car runs, continue with the mid grade. But, you are doing this at your own choosing/risk.

 

Hey Prash

Here's what happened to me. I ran 87 octane, then had a ton of issues (do a search on my username), the last of which ended up being traced back to the throttle position sensor. So I took the Throttle body apart to clean and it was filthy. All of the filth can be traced back to low octane crap gas (probably a lot of it from a station in Bellingham)

Go with better gas, better hotter burn rate.

 

The fuel is injected far downstream from the throttle body.

Premium gas does not burn hotter/colder/faster/slower than regular grade. All modern gasoline has more than sufficient cleaning additives, mandated by law. Premium does not premium quality, no connection.

Let's not drag out all the old myths for yet another painful thrash, please?

 

Its a Jaguar. Don't cheap out, but the good stuff!

 

When I first got my 05 jag I tossed in a tank of 87 octane. Shortly after the car began knocking. After letting the gas get down to 3/4 tank I put in 93. The knocking was much less. When I was able to fill it with 93 completely the knocking went away and I've never put crap in my tank again.
Don't risk it.

 

I alternate between tankfuls of mid and premium, since I use exactly one tank (well, 14.5 gal.) every week.

No problems/noises/issues here.

 

Here's my 2 cents. First a little background. My family owns a small gasoline distribution business in NE. I grew up working in the shop, and have been in the automotive business for 8 years now. I'm not an expert on this, but Ive been around quite a few.

First, octane ratings. The listed octane rating on the pumps is the "minimum" allowable octane for that pump. Chances are actually pretty good that "87" is actually "88" or "89". This can also apply to "89", and "93" octanes. Second, octanes' significant purpose is to reduce knock. It also burns better in forced induction engines like turbos and superchargers. Also in high compression engines such as NASCAR and F1 series engines. Basically engines that run at higher pressures, higher rpms, and higher temps than ours.

Second, brands of fuel. The single biggest difference in brands of fuel is the detergents that are added at the the distribution stations. All the fuels are refined at the same locations, and pumped through the same pipelines. Therefore, since all the fuel is from the same basic sources the only difference is perhaps a few tablespoons worth of chemical additives added into the big tanker trucks for distribution.

This all boils down to this. First, use a quality fuel source. Yes Top Tier fuels exists for a reason. You'd be surprised what a tablespoon of chemical can do for a few 1000 gallons of fuel. Second, look for "high volume" or busy fuel stations. The busier the station, the more fuel they go through, the more frequent the fuel deliveries. Which means the less likely to get to water in the tanks, "bad or old gas", or other nasties. Third, try all 3 grades on your own and see what you think. In NE we are forced to have ethanol in our fuel in the winter. Oddly I find 89 in the winter and 93 in the summer work best for me. 87 works just fine as well, though she's not as "snappy". I've checked the mileage differences and for me I figure I spend about 50 to 100 a year extra for the response I prefer. Cheap investment for the return.

 

Again, lets leave the myths and misunderstandings where they belong. Gasoline with a higher octane rating has only one characteristic and only one purpose- it quells detonation (pinging, knocking) through the use of additives and blending at the refinery. It does not burn 'better' in any sense of the word.

An engine may or may not require high octane fuel to achieve maximum designed performance under specific conditions but relating it use in racing engines is not really a good indicator. If a car (any car, including racing cars) is driven in manner that does not induce detonation, then low octane gas is all that's required. Higher octane would be a waste of money.

Jaguar's engine management system automatically detects detonation and reduces spark advance to eliminate it. This has the side effect of reducing maximum engine power and fuel economy. Whether a driver ever operates a car in a manner that induces detonation is a very individual thing. Most don't.

 

Mikey, what I think that Pillow was trying to get at is that with the use of octane, it reduces the likelihood that the gasoline in the cylinder is going to undergo pre-detonation (through the diesel effect which is really a process by which compressing air with a flammable substance, you can cause a rapid heating of the air to the point that the temps will cause the flammable substance to auto ignite). But, because this diesel effect has been retarded, you have higher temperatures and pressures which is raising the activity rate of the fuel. So, once the fuel is allowed to burn through the introduction of a spark, the fuel is closer together and heated to a higher temperature so the chemical reaction that occurs will progress faster. But, if you have a motor that is not suffering from pre-detonation, the difference between the two fuels is negligible as the compression of the air is not what is causing the fuel to burn.

Something that might help clarify things a little bit is a thing from the FTC. I know it is a government site, but it may put things into perspective and keep things on a non-technical level for most to have something to believe in. Check out The Low-Down on High Octane Gasoline. The summary of the site is, if your car calls for a certain grade gas, put it in. If it calls for something lower than what you are putting in, you are simply wasting your money because there are no performance gains to get.

 

My 2 cents here also, but this works for me. With the lower octane I don't get as good of mileage that I get with higher octane. Lower octane up here in the mountain of Pa. I get around 17 to 22 miles a gallon. Higher octane I get 20 to 29 miles to a gallon. Even on the highway, I get the same results, lower octane I don't any better than 25 mile to the gallon, higher octane I have gotten 34 miles to the gallon. At least 4 times a year I add a bottle of octane & cleaner. Around here there aren't to many stations that have 93 octane, when I get to one that has 93 octane my 02' cat has a lot spunk, and the milleage is on the higher side. To me 2 to 3 dollars more for a tank full of gas is worth the better mileage and the car has a lot better feel and better responce and I like that.

 

Thanks for that link- it's got some good info and confirms my statement above regarding cleaning additives.

As a side issue and not that it's all that relevant but the condition is not called 'pre-detonation'. There is no doubt that an engine actually suffering from detonation would be operating inefficiently and may suffer permanent damage, but these conditions are actually quite rare. For those of us old enough to remember carbureted muscle or sports cars from the 60s, detonation was only a concern with either heavy use of the throttle at high RPM or when lugging an engine in the wrong gear up a hill. I have an older Corvette that requires at least 91 octane as a result.

On all but a very few ultra exotics, light/medium throttle or steady state cruise (usually 95% of driving ) creates conditions that are far removed from the detonation threshold and the engine will purr along very nicely on concentrated maiden's water or gnat's pee. This means that the extra octane we've all payed for only has benefit 5% of the time, the remainder being a complete waste.

 

I switched from 91 oct to 87 oct and gained about 1 mpg (from ~26 to ~27). Odd, however my observation is based on a 140 mi. round trip daily commute on a rural mountain highway with little variance on engine/vehicle demands. I haven't noted any knocking under load, so I'm sticking with the 87.