KPG

Owned and 2006 A6 4.2, have a 2004 Touareg with the 4.2, and now an S5. I've consistently filled up with 89 octane (mid-grade), heard no knocking, pinging, pre-ignition, and noticed no other adverse effects. Now, I've burned one tank of 89 on the S5, and it runs great. Am I causing hidden damage, or does the engine have the technology to compensate for the lower octane without ill effects? If not, what would the consequences be?
Advice?

ryoung

Up to a point, knock sensors can compensate by retarding ignition timing, which does result in lost power. Excessive detonation, however, can result in overheating and/or mechanical damage to spark plugs, valves, or pistons. I use 93 octane fuel.

From <a href="http://www.contactmagazine.com/Issue54/EngineBasics.html">Detonation and Pre-Ignition</a> ...

"<i>Production engines are optimized for the type or grade of fuel that the marketplace desires or offers. Engine designers use the term called MBT ( Minimum spark for Best Torque) for efficiency and maximum power; it is desirable to operate at MBT at all times. At that operating point with the engine on the dynamometer and using non-knocking fuel, we adjust the spark advance. There is going to be a point where the power is the greatest. Less spark than that, the power falls off, more spark advance than that, you don't get any additional power.

Now our engine was initially designed for premium fuel and was calibrated for 20 degrees of spark advance. Suppose we put regular fuel in the engine and it spark knocks at 20 degrees? We back off the timing down to 10 degrees to get the detonation to stop. It doesn't detonate any more, but with 10 degrees of spark retard, the engine is not optimized anymore. The engine now suffers about a 5-6 percent loss in torque output. That's an unacceptable situation. To optimize for regular fuel engine designers will lower the compression ratio to allow an increase in the spark advance to MBT. The result, typically, is only a 1-2 percent torque loss by lowering the compression. This is a better trade-off.</i>"

5timeaudiowner

the kind of money these cars cost, would not want the performance they have to offer.

Just Me

You are doing no damage, because your car is reading knocks, and before they become damaging or audible, it is retarding the timing to prevent knocking.

This also saps performance.

So you bought a performance car, saved a few pennies, and wound up with less of a performance car.

Hmmmmm.

Damage? No. Overall best choice? You decide.

When I can get 94, I buy it.

Grant

Mister Bally

I'm a Audi A8 demographic reject. I always put brand name 87 (or 89 when I'm in Iowa or Nebraska) fuel. The engine roars and the car get up to speed plenty fast for me.

20ยข over 300,000 miles @ 18 MPG means I'll save $3k which can be used to pay for a timing belt change and other Audi cost of ownership issues.

audikillsbmw

My car is driven HARD, and I still get 20-21mpg. If you believe that you're not losing any performance, then you should throw some 100 octane in the tank (which is pretty close to what I'm running right now.) I guarantee you'll notice the difference.