Piston Slap: Gassing up a Swedish Meatball

CAHIBOstep, TTAC’s biggest fan on our Facebook page, writes:

My owner’s manual recommends 91 octane gasoline for my turbocharged five-pot (Volvo T5). There’s no 91 octane gas in Illinois, only 87, 89 and 93. When I was at the gas station the other day, I decided that instead of buying 93, I would mix equal parts 89 and 93 to get 91 (89 + 93 = 182/2 = 91).

Does this actually work? Does this make me the cheapest person in the world?

Sajeev’s answer:

You can make 91 octane with high and low octane gas, since mid-grade gasoline is mixed inside the pump like that. The quality/calibration of the pump is always a concern. Since Chi-town has 93 octane, the Pistonhead answer is to get your ECU re-flashed to take advantage of the extra two octane points. Which is a nice upgrade for many cars, but huge step up for a turbocharged Swedish meatball that’ll crank up the boost accordingly.

I’m sure a Volvo tuner like “IPD” can hook you up, and I wouldn’t be surprised if there’s a more generic Euro tuner worthy of your hard earned cash too. Automakers dumb down their spark timing to compensate for regions that use 91 octane. The number of regional blends and octane ratings in this country are staggering.

Bonus! A Piston Slap nugget of wisdom from TTAC’s resident Volvo-guru, Alex Dykes:

All of Volvo’s turbo engines require a minimum of 87 octane gasoline but recommend 91 octane to achieve rated power figures. As a rule of thumb, the more modern the engine, the better the systems are able to adjust to varying fuel grades. The thing to remember when feeding your turbo engine a lower octane fuel is that because of the turbo the effective compression is fairly high and pre-ignition is a concern, the engines will retard timing to reduce this which in turn reduces power and fuel economy slightly. If you feel really cheap, the difference in fuel economy may not make up for the cheaper cost per gallon.

On octane, its important to remember that each state sets the definition of regular/midgrade/premium in their market. Some states follow the SAE guidelines of 87/89/91 and some do not. States like California have very strict standards on the actual vs advertised octane numbers and other states do not, so a 91 octane in CA is going to be very close to 91 but 93 in some other state could easily test at a lower octane number. Also, for our friends in the EU, the USA uses Pump Octane Number PON aka Anti-Knock Index or AKI while other countries use RON or MON, these numbers cannot be directly compared.

[Send your technical queries to sajeev.mehta@thetruthaboutcars.com]