I thought I'd ask here. Will using a lesser grade, say 87 or even 85 damage the vehicle, result in a noticable drop in performance or cause any other unpleasantness. I filled my new A4 2.8QM up today for the first time and was hit with sticker shock? Thanks for the help

Yes, the prices are pretty high, however everything is higher with this car than I'm used to (payments, insurance, taxes/licensing)...so I am prepared to spend more on fuel also. I think others can attest to the fact that this engine needs and deserves the best fuel that is available. Myself, I've not experimented with using lower grades, certainly not the 87 octane variety.

Happy motoring!

Dave
98.5 QTS
Santorin

nt

We get better gas mileage with lower octane fuel.

The reasons are:

1. Higher octane fuel has less energy (BTUs) than
lower octane fuel. The stuff that's added to
increase octane has less energy than gas.

2. The engine has much more power with higher
octane fuel. So, we (my wife especially) tends to
do stronger accelerations - which simply means
much more fuel is used.

With the knock detectors and computer in most cars
today, it really doesn't matter if you use low
octance gas. There will be no engine damage.

You'll just get less peppy engine response.

Car manufacturers will recommend high octane gas
so that you'll feel better about your car.

the responses are mixed (big surprise) especially at altitude. I live in Colorado. I'm not as concerned with decreased performance or mileage as I am with carbon or other build up on the injectors, etc. Does anyone really and truly know the actual answer?

I've noticed no difference bt. 89 and 93.

You purchased a car with a certain level of performance. That level is assuming a high octane fuel. Your car has a high-compression engine, and thus requires a high-octane fuel to prevent detonation (knock).

Yes, you can run on a lower octane, and yes, your car's engine control computer should compensate. But it's not recommended. If the cost of high octane fuel is making you nervous, then I think you picked the wrong car. Economy and performance tend to be at different ends of the spectrum, and it sounds like you are more concerned with economy.

The car will run on low octane fuel, but will run more INEFFICIENTLY. The ECU will compensate
and change the timing. You will have less power, and burn MORE fuel. I did a test with my
GTI - VR6 and get about 0.5-0.75 miles per gallon MORE with premium fuel than with 87
octane. I think you will find the same with the audi

...by the fact that you even mentioned 85 octane. I have always use premium (91 in CO) in my 1989 100Q and I'm still going strong at 170k mi. In regards to the removal of carbon buildup on the injectors, I believe that is more related to the additives in the fuel. Most major fuel companies have the same additives across their gasoline offerings, whether premium or regular.

Perhaps someone else could provide information about how carbon buildup occurs in the first place.

Jason,
Boulder, CO
89 100q
saving my pennies for an A4 1.8t sans sunroof

Had a severe case of not engaging my brain before starting pumping. Didn't notice I was pumping wrong grade until tank was full.

Didn't notice any pinging! But used a light right foot.

I also noticed better gas mileage the one time I used 89 octane just to see what would happen.

I do use premium as a matter of course and probably have my foot into it more. Usually try to do a couple of red line shift runs (in 1st & 2nd at least) every tank.

Since we're on the topic of fuel, I had a question about octane ratings.

While on holiday last week in LA, I noticed you have a choice of 87? or 92 octane (premium)

Here in the UK the cheap stuff is 95 and "super" is 98, anyone know if they are just calcuated differently and are in fact the same or what?

Hanzhi

It occurs in all cars and engines. I can increase the compression ratio because it reduces the volume of the cylinder head. It get on the intake valves as they open early and can get exposed to some exhaust gasses. It get on the exhaust valves and keeps them from sealing tight.

Hello Hanzhi,
I partly remember reading in some magazine a discussion of the differences in Octane Ratings across the Atlantic. What follows is retrieved from behind the cobwebs in the attic of my brain.

In the US, the octane rating listed on every pump is an average of two different methods of measuring fuel. RON - Research Octane Number - and MON - Motor Octane Number. I think this averaged value may be referred to as the AKI - Anti Knock Index. I believe both values are measured on a one cylinder testing machine, however the RON tends to be higher than the MON. I believe that on the east side of the Atlantic, the octane rating used is only RON. This is the extent of my knowledge, and I must emphasize that it is all quite foggy.

Hope that helps,

Jason
Boulder, CO
89 100q
saving my pennies for an A4 1.8t sans sunroof