We are in the process of buying a 1986 xj6 with 78k miles on it. The car drives and feels great!
One thing that concerns me a little is the type of fuel being used in it at the moment. The seller had to add gas to the tank for the test drive, and he put 85 octane in it. Is this OK? or could this lead to a future problem with the filters and such?

Thanks!
USdragoon

 

I didn't know you could even buy 85 octane fuel . I'll be darned.

If it's a USA car the spec is 87 octane so it's not that far off and probably OK in gentle driving. So long as the engine doesn't ping/knock, you're ok.

As far as filters go, that really depends on fuel quality, not octane rating....two different things

Cheers
DD

 

Here in Colorado, 85 octane is normal for most cars. I read somewhere that super unleaded was the prefered fuel for the cats.
Anyone else use 85/ 87 in their Jaguar's? If so, what the results?

USdragoon

 

I cannot believe US fuel has such low octane ratings, here in Europe it is 96 and 97. Fuel this low normally demands lower compression pistons.
Is a different standard in use in the US ?

 

We use a different rating method here in the 'States so the numbers are uniformly lower. And, yes, Jags sold here had lower compression.

Even with that said, I had never known about 85 octane.

Cheers
DD

 

I used 87 octane in my Ser III 95% of the time. Only in very hot ambient temps...which drove the engine temp up a bit...did I hear any pinging and temporarily switch to 92 octane

Cheers
DD

 

I hadn't seen 85 until I lived out in Denver, CO for awhile. All the levels of gas are a couple of octane points lower than what i was used to. Since i was in Denver, I assumed it had something to do with the altitude. Can anybody else confirm this is the reason? Just curious.

 

I know Denver call itself the 'Mile High City', but I don't think that height has anything to do with the octane rating. In the days of carburettors, height caused weak running, but with fuel injection and mass air flow metering, this is automatically compensated for. I think it has more to do with the fuel company wanting more profit, because higher octane ratings cost more to produce.

 

Yes, that's exactly right. High altitude equals air that's less dense , less air in turn means less compression and less need for higher octane gas to avoid pre-ignition. Your car was built in an era when high octane gas did not (yet) exist, so 87 sea level, 85 at altitude is just fine. As always, higher octane levels that that required to avoid pre-ignition are a complete waste of money.

Octane ratings have been different in North America than those in Europe for several decades due to a change on the way we calculate the rating. There's usually 4-5 points difference due to this. 98 octane in Europe is the same as 93 here for example.

 

Here in the U.S. we use the (R+M)/2 method. In Europe they use the Research Octane number, which as a rule of thumb is about 5 points higher then the (R+M)/2 method. 87 research octane (RON) has the same anti-knock ability as 87% isooctane and 13% n-heptane. The Motor octane number (MON)is not quite so straight forward. This number is derived while putting the fuel under a load, like it would be in an engine to arrive at its MON. So take these two numbers and avg them. That is the number posted on the pump. And as stated earlier the higher altitude has less dense air, therefore the engine is able to run on the lower octane fuel.

Remember the Octane rating has absolutely nothing to do with the "power" in the fuel. It is just simply a rating of the fuels ability to resist detination.

 

Thanks for the responses and education! I now understand why we have 85 octane here in Colorado. One would think it would be cheaper to produce and thus be cheaper at the pump.

Here's another question....
Would it be prefered to use high 92 octane over the 85 octane?
Would it be better for the engine life, or is the expensive stuff just that- expensive?

Also, we are planning to take a road trip this summer to Seattle to visit family. Since I will be going from 9200 feet to sea level, what should I expect in fuel/ engine performance? This is for a 1986 XJ6 with 78k miles.

Thanks Much!

USdragoon

 

The sole purpose of increased octane is to eliminate engine knock/ping. If you don't suffer from knock/ping, there is no advantage to the higher octane. A high quality 87 octane should be fine.

Not sure how to answer about performance. Could you clarify the question?

Cheers
DD

 

What I was meaning was going from an altitude to sea level, would the engine require better fuel for compression.

I think I got it figured out though. The lower in altitude I go, the market is only going to provide higher octane fuel.

Thanks for the response, Doug


On a side note...
I may be picking up another Jaguar of about the same year XJ6 thats in need of alot of cosmetic attention. The engine and trans are in good shape and the body is in good shape. It has 89K miles on it. I hope to be picking y'alls brains later this summer if, and when I get it, as its missing some vitals.

Thanks again,

USdragoon

 

Probably better going, than when returning. It's downhill;-)

Look forward to the write-up about the new' car.

LOL

 

See answers already provided. The common myth is that higher octane gas is 'better quality' gas and will do all sorts of marvellous things for engines. It's easy to convince consumers of this being true- as expensive things are always better than less expensive. Aren't they?

 

Cosmetic repairs? Like paint, chrome, leather?

Unsolicited advice: be careful about cosmetic repairs. They are hugely expensive. A Jaguar-quality repaint can easily run $3000-$5000. New leather can be nearly as much if professionally installed. And we haven't even touched wood, chrome, headliner, carpets...

I'm not really sure what you had in mind but it is MUCH cheaper to buy a car that already has good cosmetics.

Cheers
DD

 

Here is a brief run down about octane of fuel and how we got to where we are today starting with "Oxygenated" fuels. Actually "Oxygenated" fuels have been around since the demise of leaded gasoline. Lead was a cheap way to boost the octane rating of fuel. With lead no longer an option MTBE was developed to raise the octane in fuel. It added oxygen to the fuel reducing the amount of carbon monoxide and unburned fuel in the exhaust gas, thus reducing smog. The problem with MTBE is it causes cancer and is 3 times more attracted to water than soil. So all this MTBE is now leaching into the water table. Then the next great thing. Remember the introduction of Gasohol (E10, 10% ethanol, 90% gasoline) in the 80's. This was another option to MTBE. The problem was that it was a very good cleaning agent. So you put gasohol in an older engine and it would clean everything out of the tank and fuel lines and shove it forward, clogging up filters, carbs and injectors. Giving it an instant bad reputation. Of course they do not call fuel gasohol anymore because of its bad reputation.
Todays "oxygenated fuels" do not have the problems of old. Todays engines are designed to run off of "oxygenated fuels" with better sealers and gaskets to stand up to the use of ethanol.

Here is the biggest confusion of all. All gasoline is the same. Depending on what time of year it is and what part of the country you are in (U.S.A.). It has to meet several different criteria to meet federal law. The difference between brand "a" and brand "b" is the cleaning additive package (DC - Deposit Control additive) that is added to it for that retailer. By law all fuels have to have a certain amount of cleaning additives in them (LAC - Lowest-additive concentration).

So if all gasoline starts the same what is the difference. Well like i stated the cleaning package differs from brand to brand, plus the quality of the distribution also difers and can contribute to a better fuel.

So why is high octane fuel better? Actually it is not. 91-92 octane fuel costs around 4-6 cents more per gallon to produce than 87. But the retail price difference is more like 15 to 20 cents more per gallon. At first the high octane fuel did have more DC additives but this is no longer the case. Back in the late 70's early 80's this started a multi-million dollar push by gasoline retailers to sell more "premium" fuel. They did this by pushing the myth that Premium produces more power and was better for your engine, both myths.

 

Usdragoon, I live in South West Denver and have an 82 XJ6 for sale. New (cloth/Vynil) seats, headliner, tires.... 2K, come and drive it!

 

In the old days of carburettors, a car set up for somewhere like Denver, (the Mile-High City, I believe), and going down to sea level would expect to be running too rich there, but with fuel injection, and measurement of air mass flow using meters, this no longer applies, and you will not really notice much difference, I would have thought. Certainly on my own car in 2007 (a Rover 75), I noticed no difference at the top of the Furka Pass in Switzerland and that is about 2400 metres above sea level.

 

While it is true that todays vehicles compensate the lean/rich mixture there is no cheating physics and there will be a loss of power the higher you go. Granted most people will probably not notice it, but the higher you go the "thinner" the air. The o2 sensors pick up the rich condition and lean out the mixture accordingly. Less o2 and fuel going into the engine at higher altitude = less power.