[Shop-talk] Fuel Octanes

[Bob Spidell]

Depends on the wording in the manual.  If it says something to the 
effect of '91 or better is REQUIRED,' then you need 91.  If it says 
something to the effect of '91 or better is RECOMMENDED' then you can 
probably use a lesser grade.  Most cars designed in at least the last 
decade or so have knock sensors and are under computer control; if the 
computer detects knock it will retard timing and possibly enrichen the 
mixture--and change the cam timing if it's able--which could result in 
loss of mileage that might more than offset the cheaper price for the fuel.

My 2008 Mustang GT's manual says '91 octane or better recommended'--or 
similar, I'm too lazy to look--but does say that the car will run on 87 
octane.  My dad had a 500 gallon fuel tank of 87 for his tractors and I 
would fill the Mustang with it when I got the chance, and I didn't 
notice much difference in performance, but it did seem to have a very 
slight stumble when I punched it (I could very well have imagined it).  
The manual claimed a loss of about 10lb-ft of torque around 3,500RPM, on 
a 315HP engine with about 340lb-ft of torque.   I also tried 95-octane a 
couple times but it didn't make much if any difference in day-to-day 
driving.  I've never heard of a modern car being damaged by using a 
lesser fuel, and can't think of a mechanism that would cause damage 
given the computer 'has your back;' not like an older car with 
mechanical timing where you could very well blow a hole in a piston due 
to detonation.

I suspect, but have no proof, that the engineers who designed the engine 
would, as a matter of pride, like for their customers to get the best 
possible performance--esp. in a 'high end' vehicle like a MB--but unless 
you're really pushing the engine I don't think you can hurt it using 
lower grade fuel.

I'd run 91--you didn't buy an expensive German luxury SUV just to pinch 
pennies, did you?

Bob


On 5/7/2017 10:23 AM, eric at megageek.com wrote:
> At the risk of starting a thread to rival the 'synthetic vs. conventional oil' debate, I have a question.
>
> I always had trucks. I'm not a performance or luxury guy, so my fuel of choice was always go ole 87 octane.
>
> Two weeks ago, I got an amazing deal on a 2007 MB GL450 just has my GMC was getting ready to 'move on.'
>
> So, as all good geeks, I read the owners manual (not cover to cover yet, War and Peace was shorter.) But in there, they state I should only use 91 or higher octane.
>
> Well, I don't necessarily mind pay about $10 more a fill up, but it got me wondering.
>
> Does anyone know if this is REALLY necessary? Would 89 Octane do as well? What if I filled up the tank every time it hit a half tank and alternated between 89 and 93? (to average about 91)
>
> What if I just ran 87? Is it possible engine damage, or just reduced fuel economy?
>
> I started searching this on the web and I found out that the consensus is unanimous... Half agree, half dis-agree. 8>)
>
> Anyway, what does this think tank think of this tank issue? 8>)
>
>
> Sent from my Commodore 64 on a 2400 Baud Modem.
> Tech Viper
> "Be as beneficent as the sun or the sea, but if your rights as a rational being are trenched on, die on the first inch of your territory." Ralph Waldo Emerson
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