Reader Question: The Volt and Octane?

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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!

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  1. There is a “local” station around here that sells ethanol-FREE premium.

    I say “local” because it is 25 miles away, the 2nd closest gas station. The other is about 15 miles away (regular and diesel only).

  2. Mainly spitballing here but the Volt motor is moderately high compression (10.5:1) with VVT designed to maximize torque and fuel economy. It’s possible that it will see excessive cylinder pressure at certain times that would call for the use of high(er) octane fuel. That said, I doubt you will blow up your motor running on a lesser grade of fuel and it may not be worth the expense for a 0-10% increase in MPG (haven’t priced premium in a few years – currently have nothing that needs it).
    If you really want to know you should data log it (ELM327 or similar) and watch for knock activity.
    FWIW I have found that Shell premium is superior to the others because they use toluene as the octane boosting additive (also a cleaning agent). I tested some BP premium once that wouldn’t tolerate any more timing than the cheapest 87 octane gas (pump gas is always a crap shoot…)

        • I talked to one of my UST installers a couple of weeks ago and he was saying that the main problem with the ethanol fuels is biologic in nature. Apparently ‘bugs’ like to eat this stuff (not unlike diesel) and will gum up the works. The (more) rapid corrosion is a function of ethanol being electrically conductive…he also claimed the middle blends (around 40%) were the most problematic in that respect.
          I haven’t had any issue, or know anyone (racers) that has had issues (in the vehicle) with ethanol fuels – but I am not storing/dispensing large quantities. All the problems I have seen/heard of are on the distribution side. Had some 87 octane E10 last for two years without it going bad in a pressure washer…
          It would be nice if some research grant money could go towards this rather than studying crap like how chewing sugar free gum may improve academic performance.

  3. Hi Eric,

    Thanks again for the help. The question was more out of curiosity than anything else. I bought the car specifically because, in my situation, I will almost never need to put gas in it, so worrying about paying extra for premium is kind of silly.


  4. Morning Eric,

    Thanks for the thorough response.

    “Engine designed for high-octane fuel generally have higher cylinder pressures – from high compression pistons or from turbo-supercharging”.

    This is what has the skeptics on the Volt forums confused as the ICE in the Volt is not high compression or is it turbo charged.



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