tjohnson1147

It should be easy enough to test... take someone with you and do a couple runs from 0-100MPH with the 95 octane, also noting the outside temperature.

Run that tank of fuel completely out and put regular 91 back in... wait a few days, and then have the same person go with you again (on the same stretch of road, with the same outside temperature) and see if there is a difference.

I know many cars will get better MPG with higher octane fuel, but I have never been able to tell a power difference on any cars that I have driven... including BMW, Porsche, Audi, Chevy, Ford or Dodge.

DirtyVegasTT

LOL, I'm not trying to convince anyone here. I'm just passing the information along... You got a 4.0T and you want to try it, then you'll like it after a tank to adapt. You don't have a 4.0T, then you won't know.

Old beater MB V12TT or old experiences in general don't apply...

MP4.2+6.0

I followed post w/ some interest and figured I would poke at it a little technically too.

So, we already know many of the chip sites show a difference from 91 to 93, and they have tunes for either. Some like APR have fairly complete dyno runs. The 91 vs. 93 choice is common for chip mods regardless of Audi or a variety of others.. But I figured, let me look at the best source for straight stock, not just tuned. How about...the manufacturer? Well, U.S. specs don't work because if they said you "need" 93, that takes out a bunch of the country, including their #1 market (CA). So, I can guess that is why they (and others) say 91.

Well, what does Audi say from the Heimat? See below. Directly off the German Audi site for the 4.0T detailed specs. They are still back at 435HP 4.0T flavor BTW. FWIW I checked the S8 plus and it says the same thing. 98 ROZ is 93 here. 95 there--found most everywhere--would be 91 here. See: https://en.wikipedia.org/wiki/Octane_rating Thus, they could have spec'ed our 91 if that were their view of the optimum. But, it's not.

DirtyVegasTT

I doubt manufacturers would give you different performance numbers based on octane rating. They don't have to. Reported HP and TQ numbers are made up these days. Case in point the Audi, BMW, and MB engines which are all underrated. So they are already starting from made up numbers to begin with... u really think that they would care to continue making numbers up further for octane rating? Let's even assume for a second you believe in the 435 number, they will then tell you 435 HP, on any octane, any brand of fuel, any temperature, and any elevation. You really believe that? We all know that all these variables greatly affect the vehicle's every day performance.

My point is this: At high elevation, extreme desert heat, and with poor 91 fuel I am seeing a difference with 95 octane. Maybe given all the factors I listed robbed my car of power, and all I am seeing now is more of the 'intended performance'... But that is still better than what I started with. Given the ability of modern direct injected turbo engines to advance / retard timing as to adjust to aforementioned conditions, the benefit of higher octane should not seem that far fetched.

Examples of old NA or even old turbo engines is irrelevant when compared to today's engines. This is not rocket science...

MP4.2+6.0

FWIW, not even slightly what I was trying to say. I was tending toward agreeing with your themes underneath about the octane booster. A little disappointed by the tone as a result. FWIW as to the rocket science line BTW, there are some subtle things in here--conservative nature of HP ratings, how timing advance/retard can overcome octane delta but affect feel, how boost can overcome altitude but affect sense of lag, etc.

Yes, I know the HP #'s are sandbagged. No news there. And they will still be sandbagged almost no matter what octane you run. Audi doesn't even bother to distinguish any more between SAE HP number and Euro PS type numbers. They are not identical on a measurement basis, but Audi has just merged them basically on their newer specs whether Euro or U.S. Enough margin it doesn't even matter any more to them.

Underneath you likely get less ignition advance and in extreme maybe a bit of boost pullback as you drop the octane. What you are probably sensing with the heat and other conditions most likely where the knock point comes into play more easily. Not directly on altitude BTW, since that would lower effective compression until boost makes it up in absolute terms; changes in air density though are relevant for hot vs. cold in a given locale. May be felt as somewhat increased lag though for altitude generally. In winter or a low stress drive like hours of highway cruising at an ambient of 50-70 F, heck mid grade does it for me historically, on a car with good modern (and multiple) knock sensors. Tahoe at 6000 ft plus, the same and a big benefit of a modern turbo vs. an NA.

What I was saying was where both fuels are available from the same station all the time (95 and 98 ROZ in Europe for super), Audi says choose the equivalent to our 93. That market set up in Europe is unlike here where depending on states' smog rules and how that state's gas supply flows in pipelines relative to neighboring states and refineries, you only get 91 or 93, and not typically both, and not in the same station. And with enough data monitoring with VCDS I expect you would find at least some timing differences between the two octanes that the electronics just sort behind the scenes. And in there will be some feel of power deltas, more so if it gets to boost. What I expect you are sensing informally, and then agreeing with that. But motor rated numbers are sandbagged enough, even down at 91 room to spare against quoted spec. and with the electronics room to spare as far as safe engine performance in widely varying locales and seasons. Nonetheless on its home turf and with a choice of fuels at the same pump, Audi says equivalent to our 93.

Jack88

I take offense to that...

No need for insulting comments, my intent with bringing up the placebo effect is that it very much applies here. If you put in higher octane fuel looking for performance differences, you may find them more because you are looking rather than there being an appreciable difference. I would expect there to be a difference, yes, but I wouldn't expect it to be detectable by the seat of your pants. Just because you think you feel a difference doesn't mean there is really much of one. I'd be surprised if there was more than a tenth or two to 60 between the the different octanes.

DirtyVegasTT

Thank you for informing me of what a placebo effect is by copying and pasting its definition. Until your further explanations I had no idea of what I was experiencing! This explains everything.

A quick question: Do you have a 4.0T, live in a high altitude desert, and tried 95 to come to your conclusion above? Because if not, then I can start throwing some vocab words with their definitions too... but I'd rather you guess

DirtyVegasTT

I'm sorry.

the_duke