SamFromTX

I agree with you. Higher octane gas require higher compression ratio and if one uses a 100 octane gas, for example, in a car that specifies 93, it will actually reduce performance. If one uses leaded race fuel, it will kill the O2 sensors. I chuckle every time I see drivers putting 110 octane gas in their street cars at the track.

frayed

Burns hot? Octane ratings have nothing to do with energy content, just resistance to pre-ignition.

frayed

Other way around. High compression + advanced timing requires high octane.

High octane fuels used to have lower energy content. I don't believe that's true anymore.

Bottom line is that if your rig is old school and is not designed for high octane, or you have a modern car that cannot adjust timing to take advantage of high octane, then high octane will just lighten your wallet with no benefit.

Macca

Sam. This is all wrong mate. High octane fuel will improve combustion in any engine. Its not the o2 sensors that will fail with leaded fuel but the catalyst material. My car was designed for 98 RON (93 MON in USA) and it runs much better with 100 AvGAs on the track. I have cat delete of course but regardless the engine has improved efficiency.

Mike in CA

Macca, I agree with frayed. If your car truly performs better with 100 octane then it's because it allows your specific engine to take advantage of advanced timing and higher-than-stock compression without pre-detonation due to some mod you've made to it. From everything I've read, higher octane fuel is formulated to be less likely to ignite spontaneously under compression without spark, but there's nothing in it that produces more energy or improves combustion per se.

Assuming no aftermarket changes, if Porsche has tuned a car like yours or the 991 GT3 to run optimally on 98 RON (93 MON US) then using higher octane fuel is a waste of money. That's my understanding anyway.

ShakeNBake

Higher compression (safe) combustion is more efficient. Without safe combustion an engine management system will back off on fuel delivery for a given scenario.

Higher octane produces (effectively) a more predictable combustion process, the extent to which can be taken advantage of by the programming and mechanical limits of ignition timing, and valve lift and duration timing. As mike says, unless the system is design to handle a higher octane (adjustibility in timing, ultimate peak pressure ability, etc), it's a waste of money - BUT, when engines run at higher temps in the summer, it can be way to offset the negative effects of engine temps progressing premature ignition. ....I'd guess non of us know what Porsche has calibrated their engines to accept.

Macca

Hi Mike. I have no idea about more modern machinery, all I know is that lower octane leads to timing retardation on the track than higher octane. Timing retardation leads to less power. Overall the 993 feels better with higher octane fuel. My understanding of the Motronic system is that it was designed to handle fuel from 91 MON to 96 MON no problem. That said I now use Motec/Link tuned for 98 RON with 100 RON maps downloadable via PC. I guess higher octane reduces Knock and timing retardation. I probably didnt choose my words right before. I do agree there isnt much point on modern cars like the 991 GT3 to waste money buying 100 RON when its rated for 98 RON but with my old girl she does seem to like the stuff...

Macca

Thanks SkaenBake. You said this how I was thinking it but not writing it. Im having a brain fart day today....

Mike in CA

IF an engine is suffering from premature ignition, it can obviously benefit from higher octane fuel. OTOH, if fuel of a given octane rating allows full ignition advance under all temperature ranges and operating conditions without knocking, then using a higher octane fuel offers no additional benefit. I think we may each be saying that in slightly different ways.

It sounds, Macca, like you have upgraded your Motronic with a more aggressive mapping that may very well benefit from higher octane. In general though, at least according to the owner's manuals in my 997.2 S and Cayenne S, modern Porsches are tuned to deliver maximum performance under all conditions with 98 RON/93 AKI octane fuel. If the manual is right, I don't believe there's any reason to use fuel with a rating higher than that unless the engine has been modified.

SamFromTX

You're right, not sure what was I thinking when I wrote this, perhaps time of night and quality of my own octane drink then...

ShakeNBake

Yes - but it's a function of the range (headroom) of tuning Porsche enables through their ECU programming. Just because they recommend a specific gas in their user manual does not mean that is all that will work, nor even optimized for. Those are generally low-end numbers to make sure you don't fall off the lower end of the range the engine can handle/make it's power and EPA ratings. IT could very well be that magic happens when you throw 100 in the tank.

And assuming the whole electro/mechanical system can go even further, tuners will push even more to find more HP.

Tacet-Conundrum

A!n't that the sh!t5?

Zeus

I completely understand what you are all saying in regards to fuel & octane ratings/management. It just further shows how much this board is a wealth of information...

So I guess the only questions left, if you are **** about what you put into your car, would be:

1.) Driving off the lot in your new 991 GT3 with it's 12.9 : 1 compression ratio would the stock Porsche ECU software be able to detect if you put 91 octane, 93 or 100 unleaded in the tank - and - be able to compensate/advance the timing for a little more power, or does it not matter since it was "tuned" by the factory to run most efficiently on 93?

2.) I guess if it made no difference in power whether you ran 93 or 100 unleaded, then yeah, it would be a waste of money to buy the higher octane fuel. But is the stuff out of the pump "better" in terms of less contaminants, less moisture, better "ingredients" per se? Is the pump gas essentially a $5 dinner wine vs. $120 Merlot with the same alcohol content?

Thanks for all the input gents...damn.

Mike in CA

I assume that Porsche has already built headroom into their octane recommendation which makes sense to me from an engineering standpoint. I would never design a system that didn't have a margin for error built in. You assume there's a possibility they haven't accounted for headroom and that using higher octane may provide a benefit. We can each make assumptions all over the place.

The fact is, the manual states specifically that the "engine is designed to provide optimum performance and fuel economy using unleaded premium fuel with an octane rating of 98 RON/93 AKI." To your point about the lower end of the range, the manual actually specifies what that lower end is; 95RON/90AKI.

All of this is in reference to an unmodified car. If you have an aftermarket tuned engine, then fuel requirements obviously may change. Short of that, if you think the manual is inaccurate and magic may happen in an OEM engine using higher octane fuel, then by all means buy it if you can find it. More magic; there are magnets you can attach to your fuel line that will dramatically improve fuel economy.....and engine ionizers......

Just kidding, S&B....

neanicu

If you read your owner's manual you'll see it also states that if you're ever in a situation where you can't find 93 premium gas it is ok to use regular 87 for short periods of time,because the computer is designed to retard timing with inputs provided by the knock sensor. It will just run rougher and have less power,but nothing will happen to the engine. I believe the same applies if you're using higher octane gas,the computer will probably advance timing...