Using 91 in 95 or 98 engines can cause costly damage, AA advisor says

As high fuel prices continue to make dents in Kiwis’ wallets, many people may be wondering if they could get away with putting 91 octane petrol in their car if it takes 95 or 98.

But that short-term gain may lead to long-term pain, warned New Zealand Automobile Association (AA) motoring advisor Ryan Bos.

The low-octane 91 fuel has an average price of $3.44 per litre, while 95 averages $3.63 and 98 at $3.75 according to Gaspy.

In early March, before the war in the Middle East began, 91 was $2.49 per litre on average.

“It’s really tempting to use 91 at the moment. It is the cheapest fuel currently, but engines that aren’t designed to run on it, shouldn’t be run on it,” Bos told the Herald.

Doing so can cause performance issues in the short term and engine damage, resulting in costly repairs after a few months to a year of constant use.

Standard 91 petrol burns faster and is less stable than higher octane fuels, so when put into a higher-pressure engine, it ignites sooner than it should.

“That then leads to what we call knocking or pinging. And that basically is localised shockwaves and intense heat.”

Using a lower octane fuel than is meant for your car can cause performance issues and engine damage.

The knocking and pinging can melt and crack internal engine components.

“So pistons, cylinder walls and all that can get damaged,” said Bos.

“Because it’s right in the bowels of the engine, it’s really expensive to repair, and on most occasions it does actually lead to a complete engine overhaul or replacement.”

Severe damage would only happen after a few months to a year, depending on the vehicle, so if you fill up with lower octane fuel once, there’s no need to worry, said Bos.

Initial effects you would notice after filling up would be the car being slower than usual and idling rougher.

“The car will ultimately feel a little bit sluggish, it’ll not feel as quick off the line, so you’ll have to be pushing the car harder.

“That will actually increase your fuel consumption as you have to accelerate harder.”

The car would use notably more fuel because of the need for harder acceleration, said Bos.

A similar effect would be noticed when filling a car meant for 98 fuel with 95 petrol, he said.

In Bos’ past life as a mechanic, it was common to see people complain about performance issues after filling their car with a lower octane fuel.

“A Volkswagen Golf might seem like a cheap car, and it’s got low running costs, but some of the engines actually need to be run on 95 minimum.”

The recommended fuel type is usually displayed on the filler cap or the owner’s manual, but if it can’t be found, Bos recommended calling a local dealership to ask.