Octane rating can affect engine performance

AP FILE PHOTO

Ernesto Evangelista, 33, fills up his Nissan Titan with regular gasoline in Miami Beach Fla., on Friday, June 13, 2008.

Q: I just bought a used 2007 Lexus ES 350, and the owner's manual recommends 91 octane gasoline.

I know there are two different octane rating systems. Should I use premium 91 octane fuel or will regular 87 octane gasoline be okay?

I have talked to two people who drive the Nissan Murano and use 87 octane fuel, even though the manual for their vehicle also recommends 91 octane.

Technician Tom Zekveld replies:

If your vehicle is designed to run on premium fuel, you should use the 91 octane rated fuel.

Engineers have designed your engine to a higher performance standard than a similar engine of lower performance, which operates on fuel at an 87 octane rating.

The difference between the two engines is primarily compression ratio, valve timing and ignition timing.

Engines with higher compression ratios offer higher performance, but also more readily exhibit a problem called detonation (fuel knock) in the combustion chambers. Detonation occurs when two flame fronts collide in the combustion chambers. If not controlled, detonation can cause internal engine damage.

The engine management system, through the Powertrain Control Module (PCM), monitors and manages engine performance. It controls the fuel flow, ignition timing, and to a lesser degree, valve timing in some late model engines.

The PCM monitors the knock sensor, which listens to the engine and reports back any knock (detonation) activity. The instant the PCM hears knock activity, it begins to retard the ignition timing incrementally until the knock is eliminated.

This all happens continuously and so seamlessly that you as the driver will not notice the immediate adjustments being made.

When you use a fuel of 87 octane rating when 91 octane is prescribed, you are forcing the PCM to continually adjust the ignition timing outside its normally operating parameters.

Now here is the rub: Retarding the ignition timing will negatively affect performance and fuel economy, however, the difference will be marginal.

Q: I own a 2006 Pontiac Grand Prix 3.8 L with 49,000 km on it. It uses half a litre of oil in between changes every 5,000 km. The engine is not leaking.

Is this a cause for concern? I have 19,000 km left on the warranty and the dealer is doing an oil economy data sheet to keep on eye on usage.

Technician Peter Lokun replies:

During the normal operation of an internal combustion engine, some oil will be burned. This is due to a very thin film of engine oil lubricating the cylinder walls to reduce friction between the pistons, piston rings and cylinders.

The oil also seals the piston rings to the cylinder walls. Since a very small amount of oil is present on the cylinder walls during the combustion (power) stroke, some of this oil will be burned during the combustion process.

As engine components such as piston rings and cylinders become worn, or if oil control rings on the pistons become clogged with carbon, more oil will enter the combustion chamber, resulting in more oil usage between oil changes.

All new automotive engines will use oil between oil changes. Depending on the engine design, the amount may be very small and unnoticeable. High output/performance engines will tend to use more oil between changes.

A half-litre between oil changes is definitely nothing to be concerned about. I would start to be concerned if your engine uses more than a litre of oil between oil changes.

Keep in mind, constant full throttle acceleration or a driver with a heavy foot will both greatly increase oil useage.

Service Centre technician Tom Zekveld is an independent garage operator.

Peter Lokun is an automotive instructor at Centennial College, Scarborough.

Email your mechanical questions to wheels@thestar.ca.

State make, model, year, engine and kilometres, as well as your name and address. Letters may be edited. Letter volume prevents personal replies.

More videos from Wheels.ca and our partners