Lot of talk recently about how Americans waste money buying premium unleaded for cars that don't need it.

I've tried to run 87 and 89 through this car, and it doesn't like it one bit. Lots of rattle and knock taking off from red lights, especially when the engine has warmed up. So I put 93 back in it today for the first time in months, and it did great. All the noise went away.

I don't mind spending the extra $$ for a happy engine; just wanted to make sure I'm not doing anything wrong here.

Jess

 

We don't have 87 or 89 in our market. The cheap fuel is 91.
My car has the rattle and knock with 91 but runs well on 95 or 98 (Uusually run 95 for cost reasons). I run 91 in my Jeep and it seems fine with it.
I run 98 pulp (premium unleaded pertol) in the bike because it doesn't like 91 or 95 at max revs.
I try them all in vehicles when I get them, they tell you what they don't like running on.

 

That would be a marelli car, what is the status of your fuel jumper link?

 

The difference is Australia uses the EU RON and USA uses MON

Our 98 is equivalent to US 93

 

In the EU we have 98, 95, 95-E10 and 95-E85 (the last two having 10% and 85% of ethanol respectively).
On my old jags (XK and AJ6 engines), I always put 98.
On the XJ with a modern V8 4.2 SC that I drive more often, I use 98 or 95.
But in none do I put gas-ethanol mix, in the long term the engines, gas tanks and hoses will not like it

 

I like to use ethanol free in my cars. Not hard to find here in my town as we have a lot of boats, so stations like WaWa sell it. Not sure about how easy it is to find when traveling out of the area though.

 

worth checking out the advance of both mech. and vacuum I did my 88 which was a dog and what a difference it made both had failed .Repaired both and set timing it will run with the big boys now on low octane fuel. Wonder what it would do on high octane?

 

No, North America uses AKI which is RON+MON/2.

Octane rating has nothing to do with ethanol content. Let's not beat that old horse again.

Back to the OP- what does the owner's manual say about required octane rating?

 

If it goes well on the higher octane stuff and badly on the lower, use the higher!
Greg

 

US manual calls for "Unleaded" fuel. That translates to 87 here. The lowest octane available.

I do not run on that. I run 91. "Premium" here. I Probably run 3-4, maybe 5 more degrees Advance without detonation than I could on "Unleaded".

I'd suggest in the strongest possible way that you do whatever is required to avoid that rattle and knock. Retard your timing, accelerate slowly or just park it. Detonation can and will burn pistons.

 

Wow, that's a surprise. No mention of octane rating?

 

The OP has an 89, which may or may not be Marelli.

My Marelli I always run on 95 with the jumper installed you can feel the difference from 91. There is no noticeable difference between 95 and 98.

 

Here's the manual insert.




1986 Jaguar XJ-S manual rear insert page.

Specifically says "Octane rating 87 RON Unleded"

Crazy talk.

And while the US may supposedly use this AKI system I don't think I've ever seen a pump stating an AKI rating of its fuels. They all say ie.

97
Minimum octane rating
(R+M)/2 method

They are commonly 87, 89, 91, and occasionally 100. And trying to find ethanol free fuels is impossible when traveling but locally you can find one or two stations that sell ( Regular Unleaded 87 ) in an ethanol free version. I haven't seen e free higher grades in years now. Them corn farmers get subsidized heavily to grow corn for fuel additive. Gotta move a lot of ethanol to keep up.

 

I have to warn you: I may be on the verge of making an ignorant statement, but...

I don't think I have a Marelli car. I've got a single "ignition amplifier" with LUCAS written on it sitting on top of the driver's side (LH) bank of cylinders.

Jess

p.s.: I'm also watching the "my car is idling rough" thread because mine also has a shake when idling at low-rpm that goes away immediately upon adding throttle.

 

Thanks. AKI (anti knock index) is the (R+M)/2 method you have seen on pumps. It's also called PON (pump octane number) in some places. All very confusing. I have no idea why N. America switched to the AKI system back in the 70s. We should have stayed with the RON system.

I'm surprised by the stated 87 RON number. In the '80s, the difference between RON and MON numbers was usually around 10 points, so that would meant a fuel of around 82 AKI. I've never seen such a thing.

I wonder if it's a misprint and they actually meant 97 RON which would be around 92 AKI? That would match with the OP's observations.

 

I'm pretty sure they were working diligently for some time to make the high compression v12 capable of running on lower octane fuel. This wasn't long after the oil crisis and requiring premium back then would have been another sales hurdle.

All that vacuum contraption and lower than optimal timing recommendations might make for less power but better economy figures.

 

unleaded refers to the absence of lead additive.
nothing more, nothing less.
it has no bearing on octane in and of itself.

 

Good work. Failure of the advance systems often goes unnoticed and unrepaired


Assuming you make no other changes, it'll run exactly the same.

The only purpose of higher octane is to eliminate pinging/knocking. If it isn't pinging on whatever you're using now, that's as good as it gets. A higher octane rating won't do anything for you

Cheers
DD

 

Yeah, in that era the V12 used several different crazily complicated vacuum advance schemes ....electric timers, dump valves, delay valves, regulator valves, etc. I'm sure part of the reason was to allow use of 87 octane fuel. Probably worked "ok"...for as long as all the gizmos stayed operational.

Haven't driven my present V12 enough to experiment by my old '88 XJS V12 was fine on 87 octane unless the coolant temp got a bit high or I was pulling a long grade. The I'd get a trace of pinging. For that reason I usually used 89 or 92. But, too, I was running without all the gizmos....vacuum hoses connected directly to ported vacuum.

Cheers
DD

 

So possibly the manual should have said 87 AKI and not 87 RON.