Your Thoughts about 89 and 93-Octane Gas

[JCCADILLACMOBILE]

Hi All. I have the 1985 Jaguar, and, much to my surprise.. The Owners Manual says "87" is OK!! .. Weird thing about my car is, it seems to run -BETTER- on 89 and 93!! I think my "Ignition Timing" is Very Advanced.. but nevertheless, it DOES Run Smoother.

I also heard t makes the engine "Hotter" .. Which is a good thing when burning off Impurities in Oil and Fuel, Right?

I also found out that "Summer Blend" Gas Now has 10% Ethanol/Methanol, and the Higher Octanes "Compensate" for thsi percentage that "Isnt Gas?"

Im wondering if Enlightenment can be shed.. I currently try to switch Tansk between 89 and 93, but im wonderign WHY my car runs better on the Higher Octanes.

This is the 1985 Jaguar XJ6 4.2L Inline-6 "Leyland JagGOOOar" with 152,110 Miles.. Thanks! - Joseph

P.S.: I have 10W-40 in there.. seems OK in place of 20W-50.. though i made my "10W-40" by Blendign Viscosities of the same kind of Oil, including 2 Quarts 20W-50.

 

[GROUCHO MARX]

I've always thought the 89 was over priced. If I wanted an octane boost I'd add a few gallons of 93 octane to the 87.

 

[IndyIan]

Old beaters with out of tune carboned up engines may need higher octane to avoid preignition or knock. It's counter intuitive but well worth the extra buck or two on a fill up. Higher octane burns slower(allows advanced timing) and is more "stable" lets say, so a hot bit of carbon in the combustion chamber might not light it off vs 87 octane.
I'd check the timing to see if its in spec, if it is, you've probably got a carbon problem.

 

[lexus114]

valve train wear is also a need for the higher octane.So what you feel is probably correct.

 

[rclint]

You should really study running higher octane gas than is recommended by the manufacture. If the engine is computer controlled it's set up to run on a specific octane rating. Using the wrong grade gas can cause poor fuel economy, damage the pistons, and that's just what I know about. The owners manual for your car is the best thing to follow when choose grade of fuel/octane rating, OCI, and fluid specifications.

 

[Schmoe]

Shouldn't it have a knock sensor that would adjust timing to the fuel used?

 

[lexus114]

In 85? not sure about that one.

 

[brandini]

Originally Posted By: GROUCHO MARX
I've always thought the 89 was over priced. If I wanted an octane boost I'd add a few gallons of 93 octane to the 87.


Not worth it with current prices.

Ex my tank 18 gallons with 2.799 87 and 20 cent diff in grade going up it would save maybe a dollar a fill up to get the perfect mix of 87 and 93 to make 89. Then it throws all your nice clean mpg calculations out when you get the error of pumping twice.

 

[jmac]

First, an '85 XJ6 is hardly going to be computer controlled anything, especially spark timing. You want to set the timing on these, get out your old timing light and loosen up that distributor and find the timing marks and have at it. Knock sensors? Hah!
Next, As I recall, these engines always ran better on premium, since timing from the factory was slightly more advanced (in time from TDC, not in technology), but not so far that if you wanted to be more economical and use regular you could, some slight pinging was not considered to be very detrimental. Or if you want to run regular gas always just back off on the timing a degree and eliminate any pinging altogether, it will just cost a few hp which you may not even notice depending on your driving.
Anyway your experiences sound exactly like what is normal for your engine.
As far as the ethanol gas, as far as your timing goes, it actually helps you a little as pure ethanol has a higher octane rating to begin with.

 

[mechtech2]

Higher octane gas does not burn slower.
It is more resistant to ignition and detonation.
Carbon deposits will cause you to need higher octane.
So will advanced ignition timing.
If you have a knock sensor, it may have been activating with the 87, and the computer will pull a LOT of timing.
so the higher octane will show a nice boost.

 

[Scott_Tucker]

Originally Posted By: mechtech2
Higher octane gas does not burn slower.
It is more resistant to ignition and detonation.
Carbon deposits will cause you to need higher octane.
So will advanced ignition timing.
If you have a knock sensor, it may have been activating with the 87, and the computer will pull a LOT of timing.
so the higher octane will show a nice boost.


Agreed

 

[Vikas]

Anybody know the algorithm used in typical engine when it pulls the timing due to knock? When does computer decide to "try" again?

- Vikas

 

[bluestream]

Originally Posted By: rclint
The owners manual for your car is the best thing to follow when choose grade of fuel/octane rating, OCI, and fluid specifications.


Not going to help much here. No E10 gas listed in owners manual in 1985. Use what works best. MY 92 XJ6 called for premium, but it ran great on Regular.

 

[TurboJim]

The XJ6 has two gas tanks, use that to your advantage. I think you'll find that it MMO (4 oz per 10 gallons of gas) will make it run smoother at idle. Unless the timing is horribly advanced it should not ~ping~ on regular gas. If it does then you've got clogged catalytic converters most likely.

 

[Scott_Tucker]

Why would a plugged catalytic converter cause it to ping?

 

[TurboJim]

Originally Posted By: Scott_Tucker
Why would a plugged catalytic converter cause it to ping?


Excessive back pressure can cause that. Clogged catalytic converters is a common problem with XJ6's. I speak from personal experience.

 

[jmac]

Originally Posted By: mechtech2
Higher octane gas does not burn slower.
It is more resistant to ignition and detonation.
Carbon deposits will cause you to need higher octane.
So will advanced ignition timing.
If you have a knock sensor, it may have been activating with the 87, and the computer will pull a LOT of timing.
so the higher octane will show a nice boost.


Show where I once said higher octane gas burns slower?
Fact is car in question has no knock sensors, spark advance has no computer controls and you have no proof that it has any carbon deposits, and no experience with the car in question. It is a fact that factory new these cars always exhibited better performance on premium gas.
Forget replying to me, I am done with you.

 

[mechtech2]

Vikas - A KS is a microphone which activates from certain frequencies that engine spark knock are at.
Unfortunately, they go off at other times.
I yanked mine and use good fuel - this is a common mod on performance car forums.

Anyways, the average computer will pull 4-6 deg or so, not 1 or 2.
It will do it as long as the signal is received.

 

[TurboJim]

The XJ6 engine has a 8.1 to 1 compression ratio. Not likely it needs anything beyond regular gas. http://www.hemmings.com/hsx/stories/2010/01/01/hmn_buyers_guide1.html

 

[rshunter]

Originally Posted By: jmac
Originally Posted By: mechtech2
Higher octane gas does not burn slower.
It is more resistant to ignition and detonation.
Carbon deposits will cause you to need higher octane.
So will advanced ignition timing.
If you have a knock sensor, it may have been activating with the 87, and the computer will pull a LOT of timing.
so the higher octane will show a nice boost.


Show where I once said higher octane gas burns slower?
Fact is car in question has no knock sensors, spark advance has no computer controls and you have no proof that it has any carbon deposits, and no experience with the car in question. It is a fact that factory new these cars always exhibited better performance on premium gas.
Forget replying to me, I am done with you.

First off, RELAX.

He was replying to an earlier post. Whenever a post is made, without using the "quote" button on a specific one, it is automatically allocated to the last post. In this case, it just happened to be yours.

JEEZ...