gsobol

It's not about more energy in the 91 vs 87. It's also not about 'I've ran 87 with no problems for years'. It's a question of HOW you run your engine and THEN can you do it on lower octane fuel.

Side note: Keep in mind there is a direct relationship of compression ratio to octane number of the fuel (there are additional parameters such as cooling, etc.). Engines that use a compressor (turbo or supercharger), usually run higher compression ratios. Audi 3.0L supercharged engines run 10.8:1 compression. Anything higher than 9.5:1 usually requires premium fuel to limit knocking. Yes, there are knock sensors that will cause the ECU to retard spark advance, but that doesn't mean you will get proper or complete fuel burn. Incomplete fuel burn can lead to un-burnt fuel ending up in your catalytic converters causing additional heating issue and possibly failures of converters. There are more systems affected by your choice of fuel as well as it can tax your cooling as well.

Audi's 3.0L supercharged engine has a magnetic clutch on the supercharger, which means it doesn't operate all the time. In fact unless you are at WOT (wide open throttle), or above 3K RPMs, the clutch stays disengaged, which means lower compression ratio. If you don't constantly lead foot it and you keep your RPMs below 3K, you may actually get away with running 87 without major issues for a long while. Issues will definitely creep up if you spent a lot of time at WOT or above 3K RPMs when engine does operate at higher compression and chances of knocking are much higher.

rsilvers129

I plan to keep my car for 10 years, and so if there was any chance of engine damage, I really would care. But I don't like innuendo and speculation. I would not consider 87 or 89 if the Q7 required 91 octane, but it doesn't, as far as I can tell.

It is not a factor because Audi says it is fine, and because it is designed to run on it without damage, and because no two used cars are the same, so I would get the one that didn't have a ding on the door.

This is right from the user manual:

rsilvers129

There is no evidence that there will ever be issues, as Audi says it is fine, with the possibility for some reduction in performance at some times, perhaps during WOT in 90+ degree weather?

Spaceframe

I have no plans to use 87. 89 is readily available and only 30-40 cents more than 87. If I can find 91 that is not outrageously priced I will use it, but otherwise I will fill up with 89. With you analysis I see that as a lesser evil.

rsilvers129

Funny that half the people say the opposite. Someone posted this on another forum:

"87 octane fuel burns faster than 89, 91 or 93 octane fuels. as you go up in octane the slower the fuel will ignite to burn. If you run 93 octane in an engine designed for 87 octane fuel you will get incomplete fuel burn which can lead to carbon build up in the intake and combustion chamber and you want to avoid that. The best rule of thumb is to run the lowest octane fuel in your engine that does not cause engine ping."

rsilvers129

This document says that in summer, you car may need four octane numbers more than in winter. http://www.bp.com/content/dam/bp-cou...-car-knock.pdf

Audi is not going to say that you may need 91 to get the stated EPA rating when it is above 90 degrees out, but 87 is going to give full power 99% of the time if it is less than 90 degrees out. And they have to recommend the fuel that they did the EPA testing with to follow the MPG rules. So they just say something like "We recommend 91, but your car can also use 87, with the possibility of some reduction in performance."

rsilvers129

It's not a problem if it requires it. I just want to fully understand what I am getting for the extra $13.50 USD per tank full ($360 a year). The fact that I have the money to buy any fuel is not the point. I want to know the tradeoff. If it is slow engine damage, then 91 should always be used. Audi would void warranty of that were the case though, and make 91 a requirement rather than a recommendation.

That document I cited says "Going from 40% to 50% relative humidity at 30C means the engine will require one less octane number."

So likewise, going from 50% humidity to lower humidity requires more octane. So if you are in a dry and hot area, then you need the 91 more than if you are in a not dry and cooler area.

Audi can't do one EPA test for people in New Mexico / Arizona and another for people in New England.

The engine is not going to engage the supercharger as often if you use 87 octane in dry areas at high temps at wide open throttle, so if you are ever drag racing in Death Valley, use 91 for sure.

But I am just testing. I don't know what I will conclude. The test may lead me to use 87 or 91. I don't know yet.

gsobol

Which is exactly the same thing, just sort of stated backwards. 87 IGNITES quicker, and yes, it's possible that feeding 93 in 87 designed engine will cause incomplete burn as it occurs later in the cycle. It's two sides of the same coin. Fact is running higher or lower octane rated fuel has it's consequences. Funny thing, there is really no formula for 'this is precisely the octane rated fuel you need'. The recommended process is: start with the higher rated octane rating, and keep going down until pinging occurs than start going back up until pinging disappears and viola! you arrived at your magic number. However, ping detectors can thwart your efforts.

If Audi is saying that using 87 will have no ill effect on the engine, then by all means use it. I see no reason why not. That goes for 89 or higher as well.

rsilvers129

I would love to know under what conditions the drop in performance kicks in. If we take extremes, we can assume the car won't drop performance at 40 degrees F and will at 110 degrees F.

The question is, will it drop HP at 90 degrees F? 80 degrees F? That would be great to know. The engine computer knows, and it would be awesome if it would display when it is retarding the ignition and supercharger so that you knew if you were giving up something by not using 91 on that specific tank of fuel.

heisnuts

I guess if you feel it is ok not to use the 91 octane fuel, why pay extra for the 89? Costco has 87 octane fuel that should work just as good and will save you even more every fill up.