I have been using 89 - and power always seems to be fine; but I heard others demand only using 90+.

Is it really necessary to get 92 for my 2.7T?

 

you will be able to see how much the timing is pulled back in response to the use of low octane fuel.

 

try 3-4 tanks of 92/93. There is a reason why audi wants you to use that, and it is not just to go fast

 

engine what you should be demanding. Follow the owners manual recommendations.

 

is having to detune your engine to compensate for your decision to run a less than adequate fuel octane.

 

i'd never get 87 regular for my 4.2 ; even though 91 premium (in CA) isn't much to cheer about!

 

I would recommend the 4.2 use at least 91. It has issues discussed below, and from an engineering point of view it is not wise to use lower octane fuel if it can be avoided. The 2.7t can get away with 87 octane, but as pointed out, it does not run near as well. The 2.7t just goes to sleep on lower octane fuels, not just in the full power, but on part throttle as well. Even 91 octane the engine is very sluggish compared to 93.

When the gas charge in the cylinder is ignited by the spark plug a gas expantion begins. The idea is to have a timing point that permits an constant level of pressure to build up, pushing the piston down with a near constant force. A number of considerations are also important to the timing relationsip for a given compression ratio. Combustion chamber shape, intake and exhaust cam timing are the main ones.

All fuels can self ignite locally in the combustion chamber after burning has started if the pressure/temprature level gets too high. This is deadly, like a hammer blow, to the pistons. Low octane fuel will self-ignite at lower temprature/pressure combinations. That is the problem with getting power and getting throttle response at low-medium rpm from an engine running low octane fuel. One has to have the timing back secondary ingnition does not occur.

Low octane fuels actually tend to burn faster, which only adds to the problem. Now the pressure rise in the cylinder is too quick for the design of the engine and timing must be held back for risk of a secondary ignition at low rpm. Throttle becomes very sluggish when you tip into it even at 2000 rpm.

The modern turbo motor engine management can adjust both the boost and the ignition timing. When it runs low octane gas it can lower first the timing, but it can then descide to also lower boost pressure and run a more efficient timing curve. Turbo motors have low enough compression ratios and run well on mild cam timing. Their engine managements sometimes self-tunes over a very wide band of fuel octane.

A normally aspirated, high compression engine does not have quite this lucury. It can only pull the timing back, but it can only pull it back so far. Now, down low rpm they can also have camshaft timing that lets the cylinder fill just so (i.e. poorly) to compensate for having cylinder pressure too high. But a car tuned for 91-94 octane often has trouble at lower rpm, part throttle running on 87 or even 89 octane fuel for this very reason. Their design is not compensated for the low rpm running of low octane fuels. The lower octane cannot be compensated for and the combustion process is not efficient nor in some cases safe for the motor.

==m==
Mike

 

Great explanation. Thanx..

 

depends on where you are placing your bet.