despite being smaller....i felt like the XF was more of a guzzler than my XJ's were. pros of being all aluminum

 

Since we are having a back and forth discussion on this, I will do a couple of tests in my car when it arrives starting this week. I will not run the car at less than 91 octane because it is a very bad idea to run a boosted engine at lower than recommended fuel octane. And honestly we get such a poor quality of gas in general here in the states that even running 91 octane makes me nervous. Remember you can't consecutively test different octane gas and expect to get accurate results, you're going to have to run through at least 2 tanks of gas to prevent any dilution. So likely what I will do is run 2 or 3 tanks of 91 and 2 or 3 tanks of 93. I can already guarantee that throttle response and power will be up on the 93 octane...I have felt it in every boosted car I have ever owned and even in my naturally aspirated direct injected CTS.

In the grand scheme of things premium gas is not that much more expensive than 91 and it helps the engine operate in a higher safety zone from detonation and delivers more power. I have been able to get away with 87 for a while in my old Cadillac but I will be switching back to higher octane fuel.

 

Yes- understood. My S-type with a much simpler system does exactly the same thing.

There's light years difference between occasional activation of the system in rain or snow under acceleration and what athompson is describing.

 

I think there is a lot of variability in mileage due to fuel quality, but the ECU will be programmed to offer maximum performance with true 91 or above octane gas. Above 91 should have a negligible effect on mileage or performance.

What will make a big difference is how much ethanol is in the gas. Most pumps in the US will be labelled as having "up to 10%" ethanol. If you happen to get a tank with the full 10% of ethanol, you are losing right off the bat about 4% of the total energy available from the fuel (gasoline has 1.5X the BTUs of ethanol), and therefore lose an equivalent amount of fuel economy for the same driving habits. I have read that some brands, in order to achieve the 93 octane rating, actually use the higher levels of ethanol, because it is higher octane (but lower energy). In that case, you would be better off with 91 octane and a lower ethanol content.

 

Bingo!!! That is very true. We live in the very competitive world, with lot of options, therefore i don't really know what you get, when you put 91 in the car.

I always put nothing but 93 octane in my na 5 liter XF. I average about 19. I don't drive easy and most of the commute is in stop and go traffic. Without the stop and go, i could easily reach 23mpg if not more.

The static compression ratio in the NA engine is about 11.5.
The dynamic compression of the SC engine in the XFR is 13.5 or 14.5 (can't remember which one). You will definitely feel the difference between the 91 and 93.

Keep us posted.

 

+1. All true.

 

This may be mostly true on the NA models but for any of the supercharged models it won't be the case. Anyone having driven a turbo or supercharged car for a period of time will tell you that the car is night and day different on different octanes of fuels. I think most here are smart enough for me not to have to go into the reasons why, but boosted cars are a whole different beast.

 

Forced induction is a completely separate issue from octane rating. Any higher performance engine that is rated for 91+ octane fuel is actually designed to run without preignition on fuel that is slightly below 91 to try to avoid situations of less than advertised octane rating of the fuel being used. Only in a situation where 91 octane is borderline, and therefore the ECU is retarding the timing to avoid knock, will greater than 91 octane fuel have impact. Of course, if you have altered the ECU for more boost than the factory intended, the forced induction engine will need proportionally increased octane to keep it in a safe preignition range.

 

I am not saying you can't run a boosted engine on 91 octane, manufacturers certainly tune ECUs accordingly because you can't get better than 91 in some areas of the country. You absolutely can still experience knocking on 91 octane fuel and your car will not make as much power as its potential...espeically when it gets hot out. I have first hand experience with this from several stock and modified boosted cars. I had one of my stock Subarus go into limp mode from a bad tank of "93 octane". I honestly feel bad for the people who live in California, Nevada, Arizona, etc. because the quality and detonation resistance of their 91 octane from what I have heard is so poor.

What I am saying is in the same manner when you run higher octane fuel the stock ECU will adjust positively instead of negatively. It does so by advancing timing, etc. and the car will make more power. It can be a slight or drastic change depending on what gas you were running before. You usually notice better throttle response, smoother power delivery and more torque under the curve. A boosted car might be tuned from the factory for 91 octane but that doesn't mean it won't exceed its advertised power potential on a stock tune with better gas.

Personally, knowing what I know about fuel quality variations I am nervous to run less than 93 octane in a car that requires premium fuel (especially a super/turbocharged vehicle). Those few cents per gallon are cheap insurance to ensure your car is running to the best of its potential and exceeding knock safety limits during all climate conditions. But that's just me

As a side note, I miss being able to run leaded fuel like I did in my old catless STi. Best knock resistance ever! I ran over 36psi of boost through a little 4 banger and made a reliable 510+ all wheel torque / 560+ all wheel horsepower on VP 110 leaded~ (don't try that with a stock catted exhaust by the way)

 

Just reached 26 mpg on an 80 mile highway trip

 

So I took the spare and the golf clubs out of the trunk... 22/25! I really never would have imagined that it would make that much of a difference.

Thanks for the help and all the responses!
JG

 

Wow you are welcome! I didn't think it would make that huge of a difference either! You must have had a lot of junk in your trunk haha !

 

There's absolutely no way removing items in the trunk would cause a 6-8mpg increase in fuel economy, unless those clubs were made of gold - and you had two sets! But, glad the fuel economy has improved. I get between 19-22 mpg with my NA 5.0 on a tank to tank basis, with about 26 mpg when on dedicated highway runs - which would be at a "spirited" pace, LOL.

 

True..I also believe the Placebo affect is in play along with perhaps a lighter foot on the go pedal...just saying

But all in all, your mileage improved and that is what counts