Reader Question: The Volt and Octane?

Here’s the latest reader question, along with my reply!

Jeremy asks: As you know, I picked up a used Volt over the holidays. It calls for premium fuel. I’ve read conflicting reports of whether it’s really necessary. All agreeing that gas performance will decrease slightly but some taking seriously Chevy’s claim that it could cause engine damage and others dismissing Chevy’s claim as hyperbole. The latter believe that Chevy is actually more concerned with gas life than damage and exaggerated the need for premium on that account. What are your thoughts?

My reply: This is intriguing!

As a general rule, it’s good to abide by the manufacturer’s recommendations regarding octane – “premium” being synonymous with higher octane; “regular” with lower octane – for the sake of optimum performance and gas mileage. The reason for that being an engine designed to use premium will operate most efficiently on high-octane fuel while an engine designed for regular will operate most efficiently on lower-octane fuel.

Engine designed for high-octane fuel generally have higher cylinder pressures – from high compression pistons or from turbo-supercharging. The higher octane fuel has a greater resistance to premature ignition – the fuel/air mix exploding in an uncontrolled manner before the spark plug fires and before the piston is where it should be. If the fuel/air mix ignites while the piston is still coming up on compression stroke, the force of the explosion exerts tremendous downward pressure, resulting in mechanical duress – which can often be heard in the form of “engine knock.”

Very bad.

But one doesn’t hear knock much these days – unless one drives an old, pre-computer car like my ’76 TA, which doesn’t have the ability to sense incipient knock resulting from gas with too low an octane rating for the engine and which can then dial back certain parameters such as ignition timing or turbo-boost in order to compensate for the lower-octane fuel.

The Volt, being a very modern car, should be equipped with knock sensors and without question has an ECU – electronic control unit. Using regular or mid-grade gas rather than premium may result in a slight – probably not noticeable – dip in mileage (power/performance shouldn’t be affected at all in this case because the Volt’s engine serves primarily as a generator rather than a motivator) but I hugely doubt you risk any mechanical damage.

As far as gas life: I’m not sure why Chevy would consider premium to have longer shelf life as it probably has at least as much ethanol (and sometimes, more) as regular. Ethanol being an inexpensive octane enhancer. There may be some difference in additive packages – one brand of premium vs. another – but unless there is a specific recommendation from Chevy to use Exxon premium rather than Shell, I’d just use whatever brand premium is the most convenient and least expensive!

. . .

Got a question about cars, Libertarian politics – or anything else? Click on the “ask Eric” link and send ’em in!